nclerigid gas permeable. rigid gas permeable. rigid gas permeable (rgp) is the term used to describe...

NCLESTUDY MATERIAL

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Medical Factors

Patient SelectionSystemic drugs. Drugs for gastric ulcers (atropine-like drugs). Birth control pills which can cause rigid lenses to be intolerable and soft lenses to be rapidly covered with protein deposits. Pregnancy and menopause also have similar effects.

Handling problems due to Arthritis and other physical conditions.

Allergies. A history of hay fever, drug reaction, or skin reactions to cosmetics or perfumes may be a warning to later sensitivity to preservatives in contact lens solutions.

Nystagmus is an involuntary, rapid movement or rotation of the eye. Nystagmus is often associated with photophobia. Patients with a significant refractive error may profit from contact lenses because the lenses move with the eye.

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Introductions

Anatomical Considerations

Correcting refractive error is the result of the entire intraocular system of the eye combining with an extraneous lens to provide good vision. Successful contact lens wear is dependent on this as well, but the relationship of the contact with the cornea, eyelids, limbus, and tear film, must continue to have a balanced interaction to ultimately provide the vision and comfort the patient expects.

This section will give insight into anatomical considerations when fitting contact lenses.

Cornea

When light strikes the cornea, it bends--or refracts--the incoming light onto the lens. The lens further refocuses that light onto the retina, a layer of light sensing cells lining the back of the eye that starts the translation of light into vision. For you to see clearly, light rays must be focused by the cornea and lens to fall precisely on the retina. The retina converts the light rays into impulses that are sent through the optic nerve to the brain, which interprets them as images.

Because the cornea is as smooth and clear as glass, but is strong and durable, it helps the eye in two ways:

1.It helps to shield the rest of the eye from germs, dust, and other harmful matter. The cornea shares this protective task with the eyelids, the eye socket, tears, and the white part of the eye (sclera).

2.The cornea acts as the eye's outermost lens. It functions like a window that controls and focuses the entry of light into the eye. The cornea contributes between 65-75 percent of the eye's total focusing power.

The five layers of the cornea are:

Anterior EpitheliumBowman’s MembraneCorneal Stroma Decements Membrane Endothelium

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EPITHELIUMCornea (continued)

The epithelium is the cornea's outermost region, comprising about 10 percent of the tissue's thickness. The epithelium functions primarily to: (1) block the passage of foreign material, such as dust, water, and bacteria, into the eye and other layers of the cornea; and (2) provide a smooth surface that absorbs oxygen and cell nutrients from tears, then distributes these nutrients to the rest of the cornea. The epithelium is filled with thousands of tiny nerve endings that make the cornea extremely sensitive to pain when rubbed or scratched. The part of the epithelium that serves as the foundation on which the epithelial cells anchor and organize themselves is called the basement membrane.

BOWMAN’S MEMBRANE

The acellular second layer, which provides strength to the cornea, is Bowman’s Membrane. Because this layer has no regenerative properties, scarring may occur as a result of trauma.

STROMA

Beneath Bowman's layer is the stroma, which comprises about 90 percent of the cornea's thickness. It consists primarily of water (78 percent) and collagen (16 percent), and does not contain any blood vessels. Collagen gives the cornea its strength, elasticity, and form. The collagen's unique shape, arrangement, and spacing are essential in producing the cornea's light-conducting transparency.

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Anatomical ConsiderationsCornea

(continued)DESCEMET’S MEMBRANEUnder the stroma is Descemet's membrane, a thin but strong sheet of tissue that serves as a protective barrier against infection and injuries. Descemet's membrane is composed of collagen fibers (different from those of the stroma) and is made by the endothelial cells that lie below it.

ENDOTHELIUM

The endothelium is the very back, or posterior, layer of the cornea, consisting of a single layer of flattened cells. These cells are very susceptible to trauma and pathology, and very rarely regenerate.

Eyelids

The palpebra, or eyelids, primarily function to protect the anterior surface of the eye from injury. They also aid in regulation of light reaching the eye. The eyelids also distribute the protective tear film over the cornea during blinking.

The innermost layer of the tear film is the glycocalyx layer. It consists mainly of carbohydrates extending from corneal and conjunctival epithelia.

The lipid layer is cecreted by the meibomian glands lining the upper and lower eyelid margins. These glands secrete a mixture of proteins and lipids that forma a liquid at body temperatures. The eyelids spread lipids throughout the tear film during upward blinking. Downward blinking returns the lipids to the inferior reservoir.

The third layer of the tear film the aqueous layer is formed by aqueous secretions of the lacrimal glands. Stimulation of the cornea typically increases lacrimation and is know as ‘reflexive lacrimation.’ (Crying in response to burning or painful sensations).

The final layer of tear film is the mucoid which forms stabilizes the trar film and prevents bacteria and debris from adhering to the eye.

The layering is not fixed and distinctive, but rater blends into a flowing liquid that coats the corneal surface.

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Eyelids (continued)

Anatomical ConsiderationsTo provide proper tear flow, the patient must blink at least12 times per minute. An incomplete, or false blink, occurs when the eyelids do not meet during the blinking process. This may result in a dry lens.

The opening between the eyelids is the palpebral fissure.

Limbus The boundary, about 1mm in width, between the cornea and sclera, is the limbus. Part of the corneal nutrients pass through this area. Because of its close proximity to the cornea, the contact lenses must not bear directly on the limbus. This must be considered for most lens fittings.

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Tear Film


The tear film is vital for normal corneal function. It has a number of important roles.

• It moistens and lubricates the anterior surface of the globe.• Provides a smooth optical surface, which allows a sharp image

to be focused on the retina.• Removes dead corneal cells and bacteria.• Protects the cornea against infection, since it contains an

enzyme called lysozyme that can destroy bacteria by acting on their cell walls.

• Provides a slight amount of nourishment to the corneal epithelium.

Perhaps the two most important factors in successfully adapting to any contact lens is keeping the lens and cornea sufficiently wet and supplying a sufficient amount of oxygen to the epithelium. A stable and adequate tear film, distributed by the eyelids, fulfills these needs.

The tear film, which has a ph of about 7.6.

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Anatomical ConsiderationsTear Film (continued)

LIPID LAYERLipids are a fatty hydrophobic material. The lipid layer forms a thin, top layer over the tear film. Lipids help prevent rapid tear evaporation, which could result in keratoconjunctivitis sicca (KCS) or dry eye.

AQUEOUS LAYERThe middle layer, or aqueous layer contains ions, sodium, potassium and a concentration of protein.

MUCOID LAYERThe innermost, or mucoid, layer of the tear contacts the corneal epithelium.

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Tear Film (continued)

Anatomical ConsiderationsBUT, or Break Up Time, indicates the amount of time after a blink it takes for a tear to breakdown. A BUT shorter than 10-12 seconds is considered abnormal, may be a contraindication to contact lens wear, and may mean there is a deficiency in the mucoid layer.

The Schirmer test is a method for evaluation of tear production. Disposable pieces of dry filter paper (Schirmer strips) are placed on the eye, and absorb tears. Based on the amount of wicking occurring on the strip, a normal or abnormal tear production can be determined.

Deturgence Normally, the cornea is in a state of partial dehydration known as deturgence. Water is moved through the cornea by the endothelium and evaporated from the epithelium layer.

Isotonicity When an equal amount of water flows into the cornea as out, the cornea is isotonic. This is normal and allows the cornea to maintain its natural thickness.

Hypertonicity Hypertonicity occurs when more water flows out of the cornea than in, causing corneal thinning. This condition is often due to a high salt concentration on the cornea.

Hypotonicity The cornea is hypotonic when more water flows in than out, resulting in corneal swelling. This typically happens when normal evaporation does not occur.

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Contact Lenses Design

Introduction In this section we will be discussing the three basic categories that contact lens designs fall into, as well as, all the basic terminology associated with contact lens design.

Scleral (Haptic) Lens Were designed to conform to the shape of the cornea. These lenses are very difficult to tolerate. This lens is used when other types of lenses fail, such as in cases of severe keratoconus, severe lids problems, and great amounts of astigmatism.

Corneal Lens Corneal lenses are the most common design for rigid gas permeable lenses. The diameter of this lens is smaller than that of the cornea. The diameter of the cornea ranges form 10.5mm –12.5mm.

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Semi-Scleral Lens

Contact Lenses Design

This type of lens bridges the limbus and lays partially on the conjunctiva tissue overlying the sclera and adjacent to the cornea.

Hydrophobic Lenses Lenses that are composed of materials which will repel water. An example of these lenses are PMMA lenses.

Hydrophilic Lenses Lenses that are made of materials that will naturally absorb water. Soft contact lenses are hydrophilic.

Wetting Angles The angle that the edge of a bead of water makes with the surface of a given material. The smaller the angle,

.

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Rigid Gas Permeable

Rigid Gas PermeableRigid gas permeable (RGP) is the term used to describe lenses that are manufactured of a material that is more rigid than soft contact lenses. They are usually worn on a daily wear basis.

Today’s rigid gas permeable lenses are much more technologically advanced than the previous polymethylmethacrylate (PMMA) hard lenses. Although there are benefits associated with these lenses, there will also be compromises.

RGP lenses are most effective for correcting astigmatism and producing sharper optics. These lenses can be easily handled and may have a longer life span than soft lenses.

Due to the rigidity of the lens, there is a longer adaptation period. Initial insertion causes more tearing and blinking than experienced with a soft lens.

PMMA The PMMA lens was the first hard transparent material to be usedas a contact lens. This lens was known as the “Conventional HardLens” having a long history of acceptance.

PMMA lenses gave consistent vision, durability, were easily fabricated, and spherical lenses could correct up to 3.00 diopters of corneal astigmatism.

The disadvantage to this lens was its lack of comfort. This lens only absorbed 1.5% water. There was a poor exchange of gas permeability causing symptoms such as corneal edema or spectacle blur.

Rigid Gas Permeable

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CAB Cellulose Acetate Butyrate was originally developed in 1938 by Eastman Kodak as a photographic material. It was not until 1974 that it was actually used as a contact lens material.

The CAB lens wet easier than the PMMA, allowing better tear flow under the lens. It was a more comfortable lens to wear allowing for a longer wearing schedule. The edge flare, common with the PMMA lens, was eliminated due to the larger fit.

The most significant disadvantage to this lens was the poor shape retention it exhibited. After one year or after hydration the lens could warp.

Silicone-Acrylate This material was the next generation of rigid gas permeable contact lens materials. A combination of silicone (38%) and PMMA (65%) was the material composition of this lens. It was developed in 1979 by Syntex Ophthalmics. These lenses are clinically known as Polycon I or II, Boston Lenses I or II, Menicon O2, Optocryl 60, Paraperm, and B&L Gas Perm.

The principal advantage over CAB lenses was its stability enablingthe lens to be made thinner, increasing patient comfort, and oxygentransmission.

Fluorocarbon/ Fluorocarbon Silicone Acrylate

This rigid gas permeable lens is made of either pure floropolymer or fluoropolymer combined with silicone.Examples of these lenses are Boston Equalens, Fluoperm 30, 60, 90, and Fluorex.

Introduction

Soft Contact Lenses

In this section we will discuss the various soft contact lenses available.

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The Food and Drug Administration (FDA) has jurisdiction over the contact lens industry. The first soft contact lens that was approved by the FDA in 1971 Initially, soft contact lenses were produced from a gel-like material called hydroxyethylmethacrylate HEMMA.

There were many advantages to this material such as, greater comfort, rapid adaptation, lack of spectacle blur, minimal over wear reaction, less flare, less photophobia, and more corneal protection.

Although there are many advantages there are a few disadvantages. Some of these include lack of durability, susceptibility to protein buildup, impossibility to modify and difficulty to verify.

Soft contact lenses are classified as either high or low water content. Low water content contact lenses contain less than 39%. Theses lenses provide sufficient oxygen for daily wear. Contact lenses that contain 79% are considered high water content lenses and allow even more oxygen to be transmitted to the cornea.These lenses are usually worn as extended wear.

Daily Wear The daily wear lens is worn on a daily basis, but is removed nightly to be cleaned. Since the lens is removed nightly, the risk of over wearing and eye infections is greatly reduced. This lens will last six months to a year, although the frequent handling of the lens increases the risk of torn or lost lenses.

Extended Wear

Soft Contact Lenses

Extended wear lenses are thinner than a daily wear lens and allow more oxygen to permeate the cornea. This lens may be worn while

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sleeping. These lenses require more follow-up care since they are left on the eye for longer periods of time.

Disposable Soft Contact Lenses

Disposable contact lenses are the current preferred lens in the industry today. Due to the multiple benefits that these lenses offer, many patients are converting to them.

Disposable contact lenses are discarded every one to two weeks, depending on the wearing schedule established by the eye care practitioner. Approximately 10% of conventional contact lens wearers purchase disposable lenses each year, due to the many benefits associated with them.

The primary benefits are outlined below:

CONVENIENCE:Disposable lenses, are able to be worn and discarded as indicated by the doctor. The patient always has a “spare pair” of lenses should anything happen to their current pair.

OCULAR HEALTH:With conventional contact lenses, the incidence of an eye infection is more frequent due to the accumulation of various deposits forming on the lenses. With disposable lenses, the incidence of an eye infection is significantly reduced since a new pair of lenses is worn according to the wearing schedule.

VISUAL ACUITY:Visual acuity is optimized because the lenses are discarded before deposits form on them.

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Soft Contact LensesDisposable SoftContact Lenses(continued)

COMFORT:Disposable contact lenses provide the comfort of a brand new pair of lenses each time they are replaced.

VALUE AND AFFORDABILITY:Technology has dramatically improved enabling manufactures to mass-produce high quality contact lenses at a fraction of the cost. The patient receives all the benefits of disposable contact lenses at a price comparable to conventional contact lenses.

Programmed Replacement

Programmed replacement lenses are also referred to as frequent or program replacement lenses. The lenses are used on a daily wear basis, but are replaced every one to three months. The doctor determines the length of time they are used. Programmed Replacement lenses are grouped into the disposable category.

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Handling Visibility

Tinted Contact Lenses

A “VISI” or visibility tinted lens was not designed to enhance or change the eye color, but to make the lens more visible to aid in handling and cleaning. If the lens is dropped or lost, the handling tint makes the lens easier to locate.

Tinted Lenses Tinted lenses are for patients who desire to change their eye color. Prior to ordering these lenses, a trial lens should be fitted so that the patient can select the desired color. Although there are visuals available depicting the colors, results may vary with each individual’s eye color.

ENHANCING TINT:An enhancing tint is for use by patients who have lighter eyes and want to enhance their natural eye color. The degree of color change depends on the individuals own eye color and the lens color they select. This type of lens generally does not change the color of dark eyes. This lens will create a more natural color as opposed to an opaque lens.

OPAQUE TINT:Opaque lenses were designed to truly turn brown eyes to blue. This lens can cause changes in eye color. These lenses utilize a dot/matrix pattern as part of the lens material. At close inspection the natural eye color can be seen through the dot/matrix. From a distance, the observer is actually seeing the color of the dot/matrix.

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Curves & More Curves

Introduction In the process of manufacturing contact lenses many curves are involved. In this section we will review all the various curves and their importance. Each plays an intricate part in the design of contacts.

Bicurve As the name indicates, a bicurve lens contains two curves. They consist of a primary base curve and a flatter peripheral curve. The junction of the two curves is generally blended to permit greater comfort. The total diameter of the lens is equal to the diameter of the optic zone plus the peripheral curve widths.

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Tricurve


A tricurve contains two peripheral curves, of which the intermediate curve may be very narrow. Generally tricurve lenses are relatively large, 9.5mm or greater, with an optic zone of 6.5 to 7.5mm, just large enough to clear the maximum pupil diameter.

The peripheral curves are slightly flatter than the base curve by 0.4 to 0.8mm, or 2 to 4 diopters, and contain a width of approximately 1.3mm. The intermediate curves of a standard tricurve lens are 1mm flatter than the base curve.

Base Curve

The base curve of a lens is the central portion of the back surfaceof the lens. It may also be referred to as the

.The base curve is designed to conform to the optic zone of thecornea and is measured in millimeters of radius of an arc, or indiopters (i.e. 45.00D = 7.50mm).

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Base Curve (continued)

Every circle or sphere contains a radius of curvature. The radius of curvature is the distance from the geometric center of the circle to its periphery where the line is drawn. As the radius of curvature becomes larger the circle gets larger. As the circle gets larger, its curve gets flatter, as the curve gets flatter on a transparent material such as a lens, the power in diopters of that surface becomes less.

The base curve of contact lenses may be expressed in terms of radius of curvature in . The lower the radius of curvature, the steeper the base curve.

.

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This is a rough rule of thumb. Simply remember that 46.00 diopters is 1.00 greater than 45.00 diopters and use it as a starting point. This rule can be used to approximate the radius of curvature in millimeters when given the power in diopters, or to approximate the power when given the radius of curvature.

Example: What is the radius of curvature of a 46.00 diopter lens?

Keeping in mind the rule of thumb, 46.00 diopters is 1.00 diopter greater than 45.00 diopters. This indicates an approximate radius of curvature change of 0.20mm. In this case the radius of curvature is approximately 7.30mm.

Example: What is the diopter power of a lens that has a 7.18mm radius of curvature?

7.18 is 0.32mm variance from the rule of thumb. Using the rule of thumb, we can asses that the dioptric power difference is approximately 1.50 diopters. In this case we see that the dioptric power is 46.50. Now if we look at the table and look up 46.50D we see that the radius of curvature is 7.26mm. As stated the rule of thumb is an approximation only. On the NCLE exam they will not provide both 46.50 and 47.00 as choices for an answer. They would only provide one of the two. Remember always select the closest parameter.

Example: What is the radius of curvature of a 40.75D lens?

40.75D has a 4.25D variance from the rule of thumb. Using the rule of thumb, we can asses that the radius of curvature difference is approximately 0.85mm. In this case the radius of curvature is approximately 8.35mm.

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The following is a diopter to millimeter conversion table. This table is used by many and although it is a great tool, it is not allowed to be used during the NCLE test.

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Quiz

Curves & More CurvesConvert the following diopters and millimeters:

43.00D48.50D38.75D7.0mm8.3mm6.85mm

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Sagittal Depth or Height

Curves & More CurvesThis is the distance between a flat surface and the back surfaceof the central portion of the lens. A greater sagittal depth producesgreater “vaulting” of the lens and in effect would be steeper.

Loosening/ Tightening a Lens

Two important variables are diameter and radius of curvature.As the diameter gets greater, the sagittal depth or “vaulting” effectof the lens increases. The greater the sagittal depth, the

the fit of the lens.

The above diagram demonstrates the radius of curvature while the diameter changes. As the diameter gets greater, the sagittal depth or vaulting of the lens increases. The greater the sagittal depth, the tighter the fit of the lens.

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Loosening/ Tightening a Lens (continued)

Curves & More CurvesIn the diagram below the diameter is constant, but the radius of curvature changes, the sagittal depth will be effected. The shorter the radius, the shorter the sagittal depth.

As shown below, if the diameter is kept constant, the sagittal depth will decrease with an increasing radius of curvature as demonstrated from A to C.

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Basic Optical Principles

Introduction This chapter will introduce basic optical principles in order to fit rigid and soft contact lenses. It will give a better understanding of why a contact lens prescription can differ slightly from an eyeglass prescription.

Vertex Distance The distance from the front of the cornea to the back surface of the lens is called the vertex distance. When a spectacle lens with power is moved either closer or farther away from the eye, the position of the focal point, relative to the eye, also changes.

As the vertex distance of the lens changes, the power of the lens is changed. As the effective power of the lens changes, its ability to correct any ametropia is effected. Although the effective power of any lens with power will change with variations in vertex distance, the change only becomes significant for contact lenses when the lens power exceeds .

: The effective power of a 10.00 diopter lens moved a distance of 5mm will change by approximately 0.50 diopters.

The sign of the lens, as well as, the direction it is moved will determine whether the power changes by ± 0.50 diopters. The illustrations below will demonstrate the changes.

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Vertex Distance (continued)

Basic Optical PrinciplesThe table below is a Vertex Distance Conversion Table and willassist in the conversion of prescriptions according to the vertexdistance.

Keratometer

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Keratometer The Keratometer is primarily used to measure the curvature of the cornea. The measurements are limited to a circular area of about 2-4mm of the corneal cap. The corneal cap itself generally measures 4-5mm in diameter. Measurements of corneal curvature may be expressed either in diopters of power or radius of curvature, directions of principal meridians, degree of corneal astigmatism, and the presence of any corneal distortion. The various controls of the Bausch & Lomb Keratometer serve to position it properly before the patient’s eye. Power readings are taken from the scale imprinted on each knob. The Keratometer may have an attachment called a topogometer that is designed to pinpoint the specific location of the corneal cap to be measured by the Keratometer. Another instrument that is helpful in assessing the regularity of the cornea is the placido’s disk. When this instrument is used, the shape of the concentric circles, which are reflected off the corneal surface, can indicate the presence of corneal or irregular astigmatism.

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How to Use the Keratometer

KeratometerThe following steps outline the basic procedure for the correct use of the Keratometer.

Step 1:Adjust the eyepiece. Place a white sheet of paper in front of the eyepiece and turn the corneal cap counterclockwise all the way. Looking through the eyepiece, a blurred cross will be seen. Rotate the eyepiece clockwise until the cross first comes to sharp focus.Adjusting the eyepiece is critical in getting accurate readings, especially when more than one individual uses the instrument.

Step 2:Position the patient so that their head is firmly against the headrest.

Step 3:Occlude the eye that is not being measured. Fixation should be accurate, with the other eye occluded.

Step 4:Position the instrument in front of the eye and have the patient look into the light of the ocular. A bright ring will be seen if the operator looks at the cornea from the side. The patient will see a reflection of their eye. To help line up the patient’s cornea use sights attached to the instrument light from a penlight directed through the eyepiece, looking for corneal reflection.

Step 5:Initially, the instrument is positioned at a greater distance from the cornea than necessary therefore, slowly move forward until the mire images come into view and are sharply focused.

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Keratometer

Step 6:Align the crosshair into the lower right circle.

How to Use the Keratometer (continued)

Step 7:There should be four images, two of each mire on either side of thecenter. With one hand on the focus and the other on the horizontal(left) drum rotate to superimpose the plus signs.

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KeratometerHow to Use the Keratometer (continued)

Step 8:With one hand on the focus and the other on the vertical (right) drum superimpose the minus signs. It is important to realize that the first focus (horizontal) will blur when attempting to superimpose the vertical meridian.

Step 9:Record the readings from the two drums. For example, the reading could be recorded as:43.00D at 90 45.00D at 180

or 43.00/45.00X180

The difference between the two readings is the corneal astigmatism. In the example above, there is a 2.00D of corneal astigmatism.

In with-the rule astigmatism, the power in the vertical meridian is greater than the power in the horizontal meridian.

For example: 43.00/45.00X90

45.00D

180 43.00D

90

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How to Use the Keratometer (continued)

KeratometerIn against-the-rule astigmatism, the power in the horizontalmeridian is greater than the power in the vertical meridian.For example: 45.00/43.00X90

43.00D

180 45.00D

90

Perfect Mires The Keratometer in figure A below indicates the correct alignment of axis but the power is not yet correctly indicated. The mires in figure B indicate the correct setting for both the power and the axis.

Extending the Range At times it is necessary to extend the range of the Keratometer to higher than diopter range. This may be necessary during a case of Keratoconus or traumatic corneal scarring. This is accomplished by attaching a +1.25 diopter trial case lens over the aperture (the engraved side of the lens mount facing the mire). This may extend the upper range to 61.00 diopters.

Keratometer

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Lens Power When determining the power of a spherical rigid contact lens on a spherical cornea the first step is to compensate the power for vertex distance at the corneal plane, if necessary. The resultant compensated power of the spectacle Rx at the corneal plane will be equal to the combined power of the rigid contact lens plus the tear film.

Example 1:Spectacle Rx: -3.25K: 43.50 Vertex Distance: 12mm

If the lens being designed were to be fit “on K”, it is indicated that the base curve is designed to parallel the flattest meridian. The illustration below demonstrates this relationship.

In the example above, the cornea is “spherical”. A lens fit “on K” would contain a base curve of 43.50. As demonstrated on the above illustration, the back curve of the contact lens parallels the front curve of the cornea producing a tear film that is plano or zero. The Rx for example 1 is –3.25. Since the spectacle power is lower than –4.00, compensating for the vertex distance is not needed.The total power of the lens plus the tear film needs to equal –3.25. Since the tear film is plano, the power of the lens is –3.25.

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Keratometer

Lens Power (continued) K: 43.50D

Example 2:Spectacle Rx: -3.25 Vertex distance: 12mm

If the lens being designed were to be fit 0.50D steeper than K, the base curve would be 44.00 diopters. The illustration below demonstrates this relationship.

In the example above, the tear lens is thicker in the center and thinner on the edges. This creates a tear film of plus power. Being that the lens is fit 0.50D steeper then K the tear film is approximately equal to a +0.50D. Since the total power needed is–3.25D, combining the power of the contact lens plus the tear film.The tear film containing a power of +0.50D will result in the contact lens power of –3.75.

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Keratometer

Lens Power (continued)

Example 3:Spectacle Rx: -3.25K: 43.50D Vertex Distance: 12mmIf the lens being designed were to be fit 0.25D flatter than K. The illustration below demonstrates this relationship.

In the example above, the tear film is thinner in the center and thicker on the edges. This creates a tear film of minus power. Being that the lens is fit 0.25 diopter flatter than K, the tear film is approximately equal to a –0.25D. Since the combined power of both the contact lens and tear film needs to be –3.25D, the contact would require a power of –3.00. So –3.00 (contact lens power) plus –0.25 (tear lens power) results in the needed –3.25D.

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Keratometer


Example 4:Spectacle Rx: -6.75-1.00X180K: 44.50\45.50D Vertex Distance: 12mm

When presented with prescribed astigmatism, the first step is to determine if the astigmatism is corneal or residual. Residual astigmatism may be caused by a toricity in the crystalline lens, posterior chamber, or by some other reason. By comparing the K readings and the amount of cylinder included in the spectacle prescription, it indicates that one diopter of toricity is present in both. Upon this observation, all the astigmatism is corneal.

Step 2:Transpose the Rx to minus cylinder, if applicable. The spectacle Rx above already indicates this.

Step 3:Drop the cylinder and axis-6.75

Step 4:Compensate for vertex distance of power greater than +4.00 diopters, using the vertex distance formula.

Diopters2 X millimeters of movement 1000

6.752 X 12 = .54D1000

-6.75 +.54 =-6.21D

Step 5:Now we have to determine if the lens is fit on K, steeper, or flatter than K.

If fit on K, the tear lens will be plano and the contact lens Rx would be –6.25D. If fit steeper than K, the tear lens contains a plus power which would require more minus power in the contact lens.

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KeratometerIf the lens is fit flatter than K the tear lens would contain minuspower thereby requiring less minus (or more plus power) in thecontact lens.

As an example, if the lens were fit on a 44.75 diopter base curve, what contact lens power would be required?

44.75D is 0.25D steeper than K, the tear lens contains a plus power of +0.25D. Therefore, the contact lens power must be-6.50, which when combined with the tear lens power of +0.25D, it yields the necessary –6.25D.

Practice

Fill in each chart using the information on the previous pages.

Example 1:Rx: -7.50-1.50X180 K’s: 44.00/45.50 VD: 10mm

Lens Base CurvePower of Tear Lens

Power of Contact

On K

0.50D steeper than K

0.50D flatter than K

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KeratometerPractice (continued)

Example 2:Rx: -3.50-0.75X05 K’s: 43.00/43.75 VD: 10mm


Power of Contact

On K



Example 3:Rx: +3.75+0.50X90K’s: 46.75/47.25VD: 8mm


Power of Contact

On K



Contact Lens Fitting

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Introduction In this section we will be answering questions such as; How big should a lens be? When should a contact lens be fit on K, steeper, or flatter then K? Although PMMA lenses are rarely fit today, the principles one needs to apply in fitting them are fundamental to all rigid gas permeable lenses. Fitting goals of soft contact lenses will also be covered.

Palpebral Lens This lens, as well as, the Lid Attachment lens are two broad categories of rigid gas permeable lenses. The palpebral lens is small and thin. They are small enough to be contained within the cornea with little or no contact with the upper and lower lids.

The diameters usually range from 7.8mm to 8.6mm. During blinking the lens moves up and approaches the superior limbus. In ideal cases between blinks, the lens falls slowly towards the inferior limbus lowering to just below the center. Thickness’ are generally between 0.08mm and 0.12mm and the peripheral curves are generally steep.

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Palpebral Lens (continued)

Contact Lens FittingIn order to produce as little glare as possible, the lenses should be large enough to provide a minimal sense of motion during blinking. A lens that is too large however, can reduce comfort. Tear exchange is helped by the thinness of the lens that produces flexure.

ADVANTAGES:• The lens provides excellent tear exchange resulting in minimal

corneal edema and spectacle blur.• Being that the lens is smaller than the palpebral fissure, eye

movement is free from lid irritation.• The thinness of the lens allows for greater comfort than

conventional lenses and can be a possible alternative to soft lenses. This is especially true with a patient that has a significant amount of corneal astigmatism.

• A high riding lens problem may be solved through the use of small microthin lenses, which may center better.

• A smaller microthin lens may be beneficial to patients who have induced corneal astigmatism caused by the “molding” effect of larger lenses.

DISADVANTAGES:• The interpalpebral lens should not be used where the lens is

likely to be high riding, such as with severe myopia. The thinness of the lens can actually carry the lens higher.

• The lens can be difficult to remove. Their thin edge makes it difficult for the upper lid to dislodge the lens.

• In cases of high corneal astigmatism, they don’t center well.• They have a tendency to warp easily, are difficult to modify

because of their size, and due to their thinness, they are easier to damage or break in handling.

• Patients with large pupils often complain of “flare”.

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Lid Attachment Lens

Contact Lens FittingThe lens diameter is approximately 9mm – 10mm and contains a large optic zone of 7-8mm. This type of lens assumes the shape of a with a radius of the intermediate curve 1mm flatter than the base curve. With blinking the lens attaches to the upper lid and is lifted high and may even override the superior limbus. A high riding lens may also be achieved through the use of a .

A larger lens of this type may be indicated by a flat, large cornea over 11mm or flatter than 44.00 diopters, or by a large palpebral aperture of 10mm or greater. It can also be useful when smaller lenses result in poor centration. When made of PMMA, these lenses can only be tolerated by about 20-30% of patients.However, considerably greater success is achieved when the newer gas permeable materials are used.

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Contact Lens FittingADVANTAGES:Lid Attachment

Lens (continued)

• The larger optic zone can minimize or eliminate the annoying flare often seen with smaller lenses.

• The wider peripheral curves hold a larger reservoir of tear fluid that aids in cushioning the lens and assists in tear exchange.

• The larger size enables the lens edge to remain under the upper lid during blinking.

• Spherical lenses can correct up to 3.00 to 4.00 diopters of corneal astigmatism.

• The lenses center well, are stable and are easy to handle.

DISADVANTAGES:• There is an increased risk of corneal molding due to the bulk of

the lens that can induce up to 5.00 to 6.00 diopters of corneal astigmatism.

• Not practical for patients with small palpebral fissures, or small, steep corneas.

• Lenses can only be tolerated by about 20-30 percent of patients.

• Peripheral curves must be precisely designed or lenses can become very loose.

We should note that the newer gas permeable materials have solved many of the problems associated with large PMMA lenses. The advantages though, still remain the same.

Fitting Methods There are multiple methods for fitting contact lenses. We will be discussing the Trial Lens Method, Dyer Chart Method, and the Harstein Modification Method. All are unique and we will discover which method is most suitable in various situations.

Trial Lens Method

This method is also known as the “diagnostic lens” method and is the most accurate for fitting contact lenses. It allows for a close estimate of proper lens size, power, and curvature. This method enables the fitter to evaluate if any edge modifications are necessary. Most of these lenses are composed of PMMA and can give an approximate fitting characteristic of most gas permeable rigid lenses. The difference of each needs to be taken into consideration.

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Contact Lens FittingThe chart below is a suggested standard for hard lens trial sets.Trial Lens Method

(continued)

Dyer Chart Method

This method, as the name indicates, uses charts to fit contact lenses. Here the fitter would use the following charts to find the appropriate base curve, diameter, optical zone, peripheral curve, and power.

BASE CURVE:First locate the flattest corneal meridian in the left hand column of the chart. Then locate the amount of corneal astigmatism in the column to the right. Note the base curve indicated.

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Contact Lens FittingUsing the instructions on the previous page, use the chart below.Dyer Chart Method

(continued)

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Dyer Chart Method (continued)

Diameter, Optical Zone, and Peripheral Curves:Once the base curve is found using the chart on the previous page, refer to the Dyer chart below for the lens parameters. These can be found in the left hand column.

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Contact Lens FittingDyer Chart Method (continued)

POWER:To calculate the correct power of a rigid gas permeable contact lens, first convert the spectacle prescription to minus cylinder form, if applicable. Then drop the cylinder and axis completely. When necessary, compensate for vertex distance. This resultant sphere power will correspond to the power of the lens. The power calculation assumes the lens is being fit “on K”. The difference in diopters between base curves that vary from the flatter corneal meridian must be taken into account when considering the power of the contact lens.

Examples

Now let’s put together all the material that has been introduced.The following are examples of how to use the Dyer Chart Method.

Example: Spectacle Rx –3.25-1.00X180 Vertex Distance = 12mmK: 43.50/44.50

Given the data above and referring back to the Dyer charts, determine the following rigid contact lens parameters: base curve, optical zone size, peripheral curve size, surface power, overall diameter, and power.

Step 1: Determine of there is any residual astigmatism, or if it is all corneal astigmatism. When we compare the astigmatism in the spectacle Rx versus the astigmatism in the K readings it is determined that there is no residual astigmatism. All of the astigmatism is found in the cornea. This indicates that a spherical rigid lens should correct this patient’s ametropia.

Step 2: After transposing the Rx to minus cylinder form, if applicable, it is now possible to drop the cylinder and the axis. Since the power is less than +4.00 diopters there is no need to compensate for vertex distance. The combined power of the tear lens and contact lens will need to equal –3.25D.

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Examples (continued)

Contact Lens FittingStep 3: To locate the base curve, we need to determine the flattest corneal meridian. In this case, the flattest corneal meridian is43.50. Using the Dyer Chart and finding the existing 1.00D ofastigmatism we can see that the indicated base curve is equal to 44.00D.

Step 4: For the other parameters such as diameter, peripheral curve, and thickness, the Dyer Chart is also used. Finding 44.00 diopters in the left hand column of the chart, it is found that the diameter is 8.4mm, with a 7.0 optical zone. If the lens contains only one peripheral curve, its curvature would be 40.50 and a width of 0.2mm. If the lens has two peripheral curves, the second curve would be 36.50D while a possible third curve would be the flattest at 30.00D with a width of 0.3mm.

Step 5: The recommended base curve according to the Dyer Chart is 44.00D. The base curve is 0.50D steeper than 43.50D, the flatter of the two corneal meridians. The approximate power of the tear film is +0.50D. The power needed to add to the +0.50 tear film to acquire the –3.25D is –3.75D.

Step 6: In this example, the base curve is 8.4mm and the power is–3.75D. Using these figures and the Dyer Chart, it yields a center thickness of 0.12mm.

Practice Problems

Practice: Spectacle Rx: +3.25+1.75X010 Vertex Distance = 10mmK: 44.75/46.50

Given the data above, indicate power, base curve, total diameter, optical zone, peripheral curve, peripheral curve width, and thickness.

Step 1: Determine if there is any residual astigmatism, or is it all corneal astigmatism. When we compare the astigmatism in the spectacle Rx versus the astigmatism in the K readings, it is determined that there is no residual astigmatism. All of the astigmatism is found it the cornea. This indicates that a spherical rigid lens should complete this patient’s ametropia.

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Practice Problems (continued)

Contact Lens FittingStep 2: Transpose the Rx to minus cylinder form,

.

Step 3: It is now possible to drop the cylinder and the axis, .

Step 4: Compensate for vertex distance. The combined power of the tear lens and contact lens will need to equal, .

Step 5: To locate the base curve, we need to determine the flattest corneal meridian. In this case, the flattest corneal meridian is

. Using the Dyer Chart and finding the existing 1.75D of astigmatism, we can see that the indicated base curve is equal to,

.

Step 6: Power of the tear lens is approximately .

Step 7: The combined power of the contact lens and tear lens needs to equal +5.25. Since the tear film is +0.75D, the power of the contact lens, in this case, needs to be .

Step 8: As we look at the Dyer Chart, we find: Total Diameter:Optical Zone Diameter:Peripheral Curve: Peripheral Curve Width: Thickness:

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Harstein Modification The value of taking measurements to determine final dimensions of a contact lens is strongly stressed in this method. The corneal diameter and the pupillary distance are used to arrive at the necessary dimensions.

LENS DIAMETER:To determine the lens diameter, add 4mm to the size of the pupillary diameter. For example, a pupil with a diameter of 4.0mm would require a lens size of 8.0mm. If a cornea is 12mm or larger, add 0.5mm to the overall diameter. Corneas that measure 10mm or less, reduce the diameter by 0.5mm.

OPTICAL ZONE:The optical zone is simply 1.5mm smaller than the overall lens diameter.

SECONDARY CURVE:This curve should be 0.75mm wider than the base curve and 5.00D flatter.

THICKNESS:Standard thickness for a plano lens is 0.16mm. For each diopter of minus power, 0.01mm is subtracted from the center thickness. For each diopter of plus power, 0.02mm is added to the center thickness.

BASE CURVE:Using this method, the base curve is fit 0.25 to 0.50D “steeper than K”. If corneal astigmatism of between 0.50 to 2.00 diopter exists, a spherical lens is ordered parallel to the flattest corneal meridian, or “on K”. If the corneal astigmatism falls between 2.00 to 3.00 diopters, it would require a lens 0.50D steeper that K. Any astigmatism over 3.00D would indicate the use of a toric contact lens.

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Fluorescein Patterns

Contact Lens FittingFluorescein patterns are an important indication of contact lens fit. The illustration below shows an ideal fluorescein pattern. There is even distribution of fluorescein, which is an indication of good support of the lens by the cornea, and an even flow of tears under the lens. The flatter peripheral curve results in the added thickness of film around the lens.

The central pooling in the diagram below is an indication of a lens fit too steep. Intermediate touch is the result of a relative lack of fluorescein just outside the region of pooling. The thin peripheral curve indicates a diminished lift of the lens edge.

Central touch and diminished dye indicates a flat lens. There is a greater amount of fluorescein found under the peripheral rim of the lens. This is shown in the diagram below.


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Astigmatism There are other fluorescein patterns that have to be taken into consideration, with the rule and against the rule astigmatism patterns. With the rule astigmatism indicates that the meridian has the steeper curve. Against the rule astigmatism is present when the meridian has the steeper curve. The illustrations below illustrate the variation for each.

The following figures demonstrate the various fluorescein patterns of the various types of astigmatism.

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Fitting Goals

Contact Lens FittingThe contact lens should not fit too tight on the eye. A tight fit could result in pain and redness within just a few minutes. This causes the limbal vessels to become compressed due to tissue edema at the edge of the lens. The lens should center well allowing the limbus to be completely covered at all times during blinking. Spin cast lenses generally fit about 2mm larger than the horizontal visible iris diameter (HVID).

Upon insertion, the lens should be left on the eye for a minimum of 15 minutes prior to making a final decision upon the fit of the lens. This enables the lens to conform to the pH, temperature, and osmolarity of the eye. A small bubble or collection of tears at the apex of a newly inserted lens can temporarily effect its curvature.

Keratometry should be taken into consideration. Although it is not as important as with hard lenses, it provides a baseline measurement of corneal curvature. This also provides a rough basis as to whether a flatter or steeper lens should be tried first.Soft lenses are generally available in three base curves, which will be suitable for the majority of corneas. Base curves may be indicted in millimeters of radius of curvature, as flat, median, and steep, or as Vault I, Vault II, and Vault III. They can also beindicated by sagittal depth such as: Sag I or Sag II.

Spherical soft lenses are often fit even when a small amount of astigmatism is present. A spectacle prescription of –3.50-0.50 X 180, a –3.75 contact lens could be prescribed. This power is known as the spherical equivalent and is derived by algebraically adding half the cylinder power to the sphere.

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Good Fitting Lens Characteristics

Contact Lens FittingThree point touch is indicative of a good fitting lens. The lens should rest lightly on the apex and the peripheral areas of the cornea. All soft lenses regardless of manufacturer, power, or lens size should obtain the three-point touch fit.

Good centration is also essential. After a blink the rim of the lens should not show more on one side of the cornea than on the other.

Adequate movement is also important upon fitting. A standard thickness should move about 0.5mm-1.0mm while looking upward after a blink. Thinner lenses are generally fit tighter and less than 0.5mm of movement is acceptable due to the greater oxygen permeability of the thinner lens.

Visual acuity should be as sharp as possible and remain equally clear before and immediately after the blink.

A soft lens has been fitted well when the retinscope streak or spot is as sharp and crisp before and after blinking as it would of no lens were in place.

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Undistorted keratometry mires before and after the blink are also necessary.

Good-Fitting Lens Characteristics (continued)

Mires B and C are reversed in the sample above.

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Introduction

The Slit LampThe slit lamp, or biomicroscope, is an instrument used to view the cornea with a magnification from 10 to 50 times, and is very useful in detecting symptoms of poor fitting contacts.

The slit lamp is comprised of two principal parts:

• A lamp, with an accompanying optical system designed to provide a variable amount of light on the eye.

• A stereomicroscope mounted horizontally, to provide a directview of the patient’s cornea.

Combining the varied light and microscope positions will allow different views of the transparent structures of the eye.

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Illumination Techniques

The Slit LampTo assist in viewing the eye, there are six illumination positions with the slit lamp:

•

•

•

•

•

•

Diffuse Illumination When diffuse illumination is used, the instrument is adjusted to project a wide beam of light, with no attempt to focus, on the cornea. This method provides a good overall view, but no fine details can be seen, and is used for a general observation of the eye. Among the problems which can be seen are:

• , along with folds or striae on Descemet’sMembrane.

• will appear hazy, grey and somewhat granular in appearance.

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Direct Focal Illumination

The Slit LampWhen direct focal illumination is used, the microscope and beam oflight are focused on the same area. There are three types of directfocal illumination:

• Optic section• Parallelepiped• Conical beam

OPTIC SECTIONUsing optic section, it is possible to see all layers of the cornea.

Depth of foreign bodies, as well as, corneal contour distortion, canbe determined.

PARALLELEPIPEDA broader view of the anterior and posterior corneal surface is

obtained using parallelepiped illumination. Surface irregularities can be detected and the endothelium can be examined.Parallelepiped illumination can be used with flourescein to evaluate contact lens fit.

CONICAL BEAMThe conical beam, using high magnification, is best for observing flare and clouding in normal aqueous humor in the anterior chamber, which will resemble a light, penetrating fog.

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Indirect Illumination

The Slit LampIndirect illumination is especially suited for studying the iris for pathology. The biomicroscope must be focused on an area immediately adjacent to the illumination position.

Retro Illumination Retro illumination focuses on the interior structures of the eye, such as the iris, crystalline lens or retina, while the microscope is focused to view the anterior section of the eye in the reflected light. Usually, light is reflected from the iris to study the cornea, in evaluating corneal edema or neovascularization. Deposits on Descemet’s Membrane can also be observed.

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Specular Reflection

The Slit LampWhen using specular reflection, the illuminating arm and microscope are positioned so that the beam of light will pass through one of the oculars of the microscope when reflected from the corneal surface. This position indicates that the angle of incidence of the light is equal to the angle of reflection. Specular reflection can be used for observing:

• of the front surface of the cornea or a contact lens.

• on contacts.• condition of contact lenses.• to include mucus and Meibomian

secretions.

Sclerotic Scatter Sclerotic scatter focuses a broad beam of light at the temporal limbus to transilluminate through to the nasal limbus. Using this technique, without the microscope, with the unaided eye, is the best method to detect corneal edema, which will appear as a foggy patch of cotton.

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Filters

The Slit LampThere are four filters that can be used with the slit lamp during examinations:

• Cobalt Blue Filter• White Filter• Neutral Density Filter• Green Filter (red free)

Using these filters can aid in observation, patient comfort, or to enhance the presence of certain conditions.

COBALT BLUE FILTERThe cobalt blue filter is used with fluorescein to observe staining patterns and contact lens fit.

WHITE FILTERThe white filter, used in routine exams, will decrease ultraviolet rays and decrease light intensity somewhat.

NEUTRAL DENSITY FILTERUse of the neutral density filter decreases light intensity about 10%, to increase comfort for photophobic patients. This filter is used during the examination of the eyelids and conjunctiva.

GREEN FILTERThe green filter, red free, will make red or brown objects, such as blood vessels, appear blacker.

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Burton Lamp

The Slit LampAn alternate method of observing fluorescein patterns with the naked eye is with the hand held Burton Lamp. This device is used instead of the slit lamp and consists of an ultraviolet light, which will represent corneal touch as dark blue and corneal clearance as green.

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Introduction

Contact Lens Wear:Adverse EffectsWith contact lens wear, many adverse conditions may occur, whichin reality are medical problems. Whenever this situation exists, it isbest to recommend

.

This section will explore some of the problems and their symptoms often associated with contacts, such as:

• Corneal edema• Corneal striae• Conjunctival and episcleral injection• Giant papillary conjunctivitis• Corneal vascularization• Keratometer mire distortion• Corneal epithelial straining patterns• Defective tear film distribution

Corneal Edema Corneal edema, or swelling, may occur when the hydrating balancing mechanism fails, resulting in too much water being retained in the epithelium.

An edema may also occur, known as , when the cornea lacks an adequate supply of oxygen. When this condition exists, microcysts may develop and will result in trauma to the cornea when ruptured.

Corneal edema resulting from hypoxia may affect corneal topography and reduce the clearance between the epithelium and posterior contact lens surface. Corneal abrasions may result from lens movement over these areas, often leading to drying.

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Lens Type and Edema

Contact Lens Wear: Adverse EffectsThere are common causes of edema associated with different lens types and materials.

PMMA LENSESWith PMMA lenses, the amount of oxygen delivered to the cornea is . When tear flow is inadequate, corneal edema often results. Common causes of edema with PMMA lenses are:

•

•

•

•

•

RGP LENSESAll the causes of edema in PMMA lenses apply to RGP lenses, but only if the cornea is deprived of oxygen. Due to the oxygen permeability of this lens material, this will not occur unless the lens is heavily deposited from lack of proper maintenance.

SOFT LENSESTypically, corneal edema associated with soft lenses is caused by interference with oxygen transmitting through the lens. Common causes of this occurrence may include:

• Inadequate lid closure.• Excessive lens thickness.• Relatively low lens water content.• Poor lens maintenance.

These factors are especially critical when fitting extended wear contacts.

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Epithelial Edema

Contact Lens Wear: Adverse EffectsCorneal epithelial edema appears as a . This type of edema is classified from +1 (barely perceptible) to +4 (densely circumscribed area) and is observed using a slit lamp, utilizing

.

Microcystic Edema Microcystic edema is caused by the rupture of the epithelial cell membrane resulting from . Microcysts are formed from the accumulation of fluid in the spaces resulting from the ruptured cells. A slit lamp, using , is the best method to observe microcystic edema. Fluorescein may be used to identify the presence of microcysts and stain the areas where they may have ruptured. This type of edema is also classified from +1, slight, to +4, severe.

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Corneal Striae

Contact Lens Wear: Adverse EffectsLinear opacities in the cornea are known as corneal striae, or simply striae, folds, wrinkles, striate keratopathy, striate corneal lines and are typically found in about 50% of . These opacities are vertical, subtle, translucent lines located near Descemet’s membrane, are rated in severity from +1 to +4, and rarely occur with patients who wear hard lenses.

Corneal striae is usually asymptomatic. Typically caused by hypoxic edema during the first six hours of lens wear, corneal striae usually dissipates within forty-eight hours after removal of the contacts.

Corneal striae can be seen using a slit lamp with.

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Conjunctival and Episcleral Injection

Contact Lens Wear: Adverse EffectsEpiscleral injection is a nonspecific manifestation of an inflammation of the conjunctiva, rated in severity from +1 to +4, occurring when the conjunctival blood vessels become red.Causes of episcleral injection related to contact lens wear may be due to:

• An allergic or toxic reaction to .• , although this is usually

coincidental and unrelated to contact lens wear (if soft contact lenses are worn when the infection is present, they must be discarded and replaced or the infection will likely return).

• and drying of the corneal epithelium.

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GPC

Contact Lens Wear: Adverse EffectsGPC, or Giant Papillary Conjunctivitis, is usually seen with

wearers, and is a response to a conjunctival insult, resulting in the formation or papillae, or small bumps (cobblestone effect), about 1mm in diameter, on the palpebral conjunctiva. This condition typically manifests after several months of contact lens wear.

Common symptoms of GPC include:

•

•

•

•

Depending in the number, distribution and size of the papillae, GPC is ranked from +1 to +4.

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Corneal Vascularization

Contact Lens Wear: Adverse EffectsA normal cornea has no blood vessels. When the cornea is deprived of , blood vessels will invade the cornea. This condition is known as corneal vascularization and can be caused by:

• Epithelial infection• Corneal degeneration• Exposure to toxins• Noninfectious inflammatory conditions• Traumas

Contact lens wear may cause , (an abnormal formation of new blood vessels), to occur due to:

• Persistent epithelial hypoxia• Recurrent epithelial defects• Improper lens insertion and removal techniques• A damaged or poorly designed lens edge• Surgical, traumatic, or other pathological conditions for which a

contact lens is worn

When the cause of the condition is removed, the blood vessels will usually regress, although “ghost” (empty) vessels may remain and be visible with close examination.

The number of blood vessels present determines the severity of the neovascularization, ranked from +1 to +4, with +4 being the worst condition.

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Keratometer Mire Distortions

Contact Lens Wear: Adverse EffectsDistorted Keratometry mires may be an indication of irregular astigmatism or apical displacement.

This also emphasizes the need for Keratometry readings to be taken for all patients so that a baseline of corneal curvature can be established.

Some common causes of distorted Keratometry mires include:

• Alterations in corneal shape from contact lens wear• Preocular tear film abnormalities• Indicatives of a if clear mires are

retained before and after the blink• Indicative of a if mires are only clear

when the wearer blinks, and then blurs• Indicative of a if mires blur

immediately following the blink, then clear as the lens settles

Distorted Keratometry mires are graded for +1 to +4.

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Contact Lens Wear: Adverse Effects

Corneal Epithelial Staining Patterns

With contact lens wear, a common adverse effect is cell loss in the corneal epithelial layer. This cell loss can be seen with a slit lamp, however, the use of fluorescein dye can provide a more exact location and give indications of probable causes. Fluorescein will either pool in the area of the defect, or stain the remaining basement membrane of the epithelium or Bowman’s Membrane.Another dye, , stains damaged cells of the conjunctiva and cornea prior to full degeneration and loss.

Corneal epithelium damage can range from slight, with the loss of a few cells, to severe, with deep crater-like lesions, at times to the level of Bowman’s Membrane. Epithelial damage can be caused by either direct trauma or a defective tear film distribution over the cornea.

Common causes of epithelium cell loss from direct trauma include:

•

••

•••

SWIRL LENSESA semicircular, swirl like strain may include:

•

•

•

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Staining Patterns (continued)

Contact Lens Wear: Adverse EffectsSNAKE OR CRATER LIKE STAINSThin, snakelike tracks on the cornea may indicate the presence of a , such as dust, make up or mucous particles trapped under the lens.

A small, crater like lesion is symptomatic of the intrusive particle having imbedded itself in the cornea.

SCRATCH STAINSScratches or deposited mucus not cleaned from the posterior side of a contact lens can cause irritation and the appearance of scratch-like stains.

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Contact Lens Wear: Adverse EffectsFATIGUE STAINSA fatigue stain is characterized by a dull, hazy appearance, possibly accompanied by folds or wrinkles. This condition is usually associated with:

•

•

•

•

•

ARC STAINAn arc stain is crescent shaped, and typically does not result in trauma to the eye. Correction of this problem usually requires a change in lens design and/or fit. An arc stain can be caused by a number of conditions:

•

•

•

•

•

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Contact Lens Wear: Adverse EffectsRE-CENTERING STAINA re-centering stain is arc shaped and linear and may be caused by the edge of a hard contact lens. There may be one, or several, unbroken stains. These do not reoccur unless the lens continues to be displaced. Other possible causes may be a flat lens fit, or too much and too flat a peripheral posterior curve.

3 AND 9 O’CLOCKSTAININGAbnormalities in the quantity or composition of the tear film, an

incomplete blink, or decreased blink rate may result in corneal exposure, drying and/or hypoxia under the contact lens. Staining at 3 and 9 o’clock indicates drying of the epithelium. Movement of the lens may dislodge cells in these areas.

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Contact Lens Wear: Adverse EffectsPERIPHERAL STAININGA stain encircling the entire lens may be caused by a:

• Small diameter soft lens.• Wrinkled or dry lens edge.• Damaged lens edge.• Lens deposit or dried mucus.

DRY SPOTSDry spots appear as dark areas of the cornea, rather than stained,and retain their appearance during blink. Dry spots may be visibledue to:

•

•

•

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Contact Lens Wear: Adverse EffectsROUND DEPRESSIONSA series of round depressions indicate the presence of

trapped beneath the lens. There is usually no trauma to the epithelium, and the depressions disappear after the lens removal.

DIFFUSE PUNCATE STAININGToxic epithelial injury typically produces a diffuse puncate-staining

pattern. Usual causes of this will include:

•

•

•

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Evaluating Fit

Contact Lens Wear: Adverse EffectsFluorescein pooling is also useful in evaluating rigid lens fit. Common staining patterns and their indications include:

• Apical pooling: (the patient maycomplain the eye feels “hot”).

• Inferior and superior concentration (touch on the horizontal meridian): .

• Touch showing on the vertical meridian:.

• Apical touch with little fluorescein in the area of contact:.

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Introduction

Advanced TechniquesThis section will provide an overview of contact lens fitting methodsand philosophies involving more challenging patients. The types ofproblems discussed will be:

• Stabilization techniques• High myopia• High hyperopia• Presbyopia• Astigmatism• Aphakia• Keratoconus

Fitting these patients usually requires a more experienced technician.

Stabilization Techniques

Lens rotation on the eye, to maintain good optics, needs to be minimized with toric, as well as with segment style multifocal, contact lenses. There are several methods available to achieve lens stability:

• Prism ballast• Truncation• Double slab-off• Posterior toric• Aspheric lens surface

Prism Ballast One of the most common techniques used for stabilization, this method involves grinding between

into the lens, although tight lids, flat corneas or oblique astigmatism may require a greater amount. The added thickness from the prism will tend to position the lens inferiorly on cornea. While improving lenspositioning, the added thickness may create a hypoxic conditionunder the prism area due to reduced oxygen permeability.

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Truncation

Advanced TechniquesOften used with prism ballast, truncation, or cutting off, 0.50mm to 1.50mm of the bottom edge of a contact lens will provide stability when the lower flat edge of the lens lies adjacent to the lower eyelid. Occasionally, the superior edge of the lens is sectioned off as well, resulting in double truncation.

Truncation reduces lens diameter, resulting in a looser fit. This may be remedied by steepening the base curve.

Double Slab-off A lens fabricated using the double slab-off technique is thicker centrally and thinner in its upper and lower areas that come to lie beneath the eyelids. This lens design, many times incorporating a prism ballast, offers increased comfort due to decreased lid sensation, however it does not offer as much lens stability as the truncated or front toric technique.

Posterior Toric Lenses Back surface toric lenses are made to match corneal curvature, thus stabilizing the lens and reducing rotation.

Aspheric Lens Surface An aspheric lens design, typically used with, aids in lens-

axis positioning by adding drag to the motion of the lens.

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Reference Markings

Advanced TechniquesTo determine the amount of lens rotation that exists; most toric lenses are manufactured with reference markings, circles or lines at six o’clock or at three o’clock and nine o’clock, allowing the fitter to observe lens orientation on the cornea. Rotation measurement can be taken using a slit lamp equipped with a protractor.

If the lens rotates to the fitters left, the number of degrees of rotation is added to the prescribed axis. If rotation is to the fitter’s right, the rotation degrees are subtracted from the prescribed axis. An acronym used to assist in remembering this technique is

.

The use of trial lenses is important when fitting toric contact lenses so that lens rotation can be determined, and the final prescribed axis can be accurately ordered.

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High Myopia

Advanced TechniquesA common problem encountered fitting patients with a high degree of myopia is a lens, which does not center properly and/or rides high due to the interaction of the thick contact lens edge with the upper lid. A hyperflange lens design is usually successful dealing with the problem.

. The thinner edge allows the lens to move more easily from beneath the upper lid, creating better centering.

High Hyperopia High plus lenses often ride low due to the weight of the central portion of the lens, combined with the thin edge having little interaction with the upper lid.

, creating more upper lid interaction and improving lens centration.

Presbyopia The onset of presbyopia with age is inevitable and necessitates the addition of a reading correction. Contact lenses can be used by the presbyope, implementing one of two methods:

•

•

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Advanced TechniquesSingle Vision Contacts READING GLASSES

If the presbyope also wears contacts for distance viewing, the near point problem can be overcome by wearing a near-only correction with the contact lenses.

This method provides the wearer with normal distance vision,however, it does necessitate the use of glasses in some form,which may be objectionable to some patients.

TWO PAIR OF CONTACT LENSESThis impractical method entails the use of one pair of contact for distance and the other for near, requiring the patient to change lenses frequently.

MONOVISIONMonovision is a popular, simple, and generally inexpensive method of correcting presbyopia for the contact lens wearer. One lens is worn for distance vision, usually on the dominant eye, and one lens is worn for near vision. This means that, depending on the gaze, one eye’s vision is always blurred. The brain automatically used the eye which is able to see, and after an adaptive period most patients are quite comfortable using monovision.

There is some loss of depth perception using this method, so it is recommended that for any activity requiring prolonged use of far vision, that two contacts for distance correction be used, or a bifocal contact lens is used in the near eye.

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Bifocal Contacts

Advanced TechniquesBifocal contacts are divided into two broad categories:

•

•

CONCENTRICThe concentric bifocal can be used interchangeably with the terms

bifocal.

The basic concept of this lens design is that the distance portion of the prescription is located in the contact and the near vision is contained in the lens . When gazing downward to read, the lens naturally raises upward, allowing the patient to see clearly for near.

There are several lens designs using the concentric/target/annular theory:

• : two separate powers for distance and near achieved by grinding curves or fusing a portion with a higher index of refraction on the lens periphery. Figure A.• : the anterior lens surface is aspheric and results in a gradual power change, with the most plus power on the edge. Figure B.• : the reading and distance prescriptions are both contained in the central portion of the lens. This method presents two images at once, and as with monovision, the patient subconsciously selects the image. To position the distance portion of the lens properly, this design requires a tight fit.

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Advanced TechniquesBifocal Contacts (continued)

SEGMENTThe segment bifocal contact lens design uses the same concept employed with spectacle lenses, using a reading segment in the inferior portion of the contact. The segment may provide improved near acuity, however, the lens is thicker and will give the wearer greater lower lid awareness.

In order to maintain horizontal stability, the contact must be prism ballast and/or truncated.

The reading segment is most often crescent shaped, although a flat top or circle may be used, and is either fused on the posterior of the lens or ground on the front. This bifocal style is also known as a Black lens. Figure A.

A bifocal is a one piece design, truncated atthe bottom. The optics are calculated so that no jump occurs at thesegment line. Also known as the Mandell lens.

A crescent shaped fused segment. Figure E.

Another variation of the fused segment bifocal contact is the, which contains a very

large reading segment. It is prism ballast, custom made, and available with both variable seg heights and optic zones.

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Diffraction Bifocals

Advanced TechniquesThe diffraction bifocal, first introduced by Pilkington Glass called theDiffrax™, is made up of concentric rings with small facets, similar toa prism, that diffract 50% of the light rays into a separate point.The higher the addition, the more rings required.

• The diffraction type of bifocal lenses provides simultaneous vision. The facet on the back surface diffracts some of the rays of light to a second focal point.

Good Candidates The most likely patients to succeed with bifocal contact lenses are those who:

• Were successful single vision wearers and highly motivated• Have an occupational need• Have a reading addition between +1.00D and +1.75D• Have no more than 1.00D of cylinder, requiring a spherical

equivalent between -4.00 and +2.00• Understand that visual acuity may be compromised, especially

in dim lighting conditions.

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Toric Lenses

Advanced Techniques

Advanced TechniquesA toric back corneal surface, and/or a toric or partially dislocated crystalline lens may result in residual astigmatism. If this residual astigmatism results in vision that is compromised, toric contact lenses may provide a solution. Toric lenses may also be necessary when irritation or centration problems with rigid lenses exist in patients with moderate to high corneal astigmatism. Hard and soft contact lenses are both available in toric design, and fall into two basic classifications:

• Anterior (front surface), back spherical surface• Posterior (back surface), toric base curves

1. posterior toric only2. bitoric

ANTERIOR (FRONT) TORIC WITH SPHERICAL BACKSURFACES

Front toric lenses are prescribed for patients with a significant amount, more than 0.75D, of uncorrected residual astigmatism. Available in both hard and soft lenses, the cylinder power is on the front side of the lens with the meridians of power 90° apart, while the back curve remains spherical.

Example: spectacle Rx: -4.00 -3.00 x180 Keratometry: 44.00@180, 45.00@90

From the Rx it is evident the patient has 3.00D of refractive cylinder. The K readings only indicate 1.00D of corneal astigmatism, leaving 2.00D of residual or lenticular astigmatism. The corneal astigmatism will typically be corrected by the tear layer under the back lens surface, with the residual astigmatism corrected by the toricity of the front surface.

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POSTERIOR BASE CURVE TORIC LENSESToric Lenses (continued)

Typically used in hard contacts when a spherical lens is unstable and at least 1.50D of corneal astigmatism is present. The front lens surface is spherical and the back surface has two principal meridians of power 90° apart. The back surface curvature closely matches the corneal curvature to maintain stability. This lens may also be used when irritation is experienced due to inadequate clearance of a spherical lens on a toric cornea.

BITORIC LENSES

Bitoric lenses are fit on patients with at least 1.50D of corneal astigmatism and significant residual astigmatism. The posterior toricity corrects the corneal astigmatism and the anterior curves correct the residual.

Advanced Techniques

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Aphakia As with any contact lens patient, Aphakics need to be assessed as to their likelihood of successful contact lens wear. Included in the areas to be evaluated are the ability to:

• handle lenses properly• comply with the required cleaning regimen• supply sufficient tear film and have a healthy cornea• have someone available for assistance if necessary.

As most aphakic candidates are elderly, these areas of concern are more likely to exist than with a younger patient. Tremors and /or loss of central visual acuity due to macular degeneration may inhibit insertion techniques. Also, being unable to remove lenses, or being unable to find lenses for insertion, especially with bilateral aphakes, are common fears which may exist for this patient.

Due to the popularity and success of intraocular lens implants, contact lens fittings for aphakics are decreasing drastically in frequency.

Aphakic contact lenses may be fitted using rigid or soft lenses.

In the rigid aphakic contact lens family, there are basically two lens designs available:

•

•

Advanced Techniques

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MINUS CARRIER LENTICULARRigid Aphakic Lenses (continued)

Because of the steep posterior curvature necessary to achieve the high plus power required by most aphakes, conventional lenticular lenses are rarely used as the lens thickness leads to problems with inferior lens positioning and lid irritation.

The solutions to the problems associated with conventional lenticular lenses are found in the minus carrier lens. This lens design has a small optic zone, about 7.0mm, reducing center thickness and weight. The lens maintains its positioning better, about 1mm above the corneal center and has less tendency to drop over the limbus.

SINGLE-CUT

The single-cut aphakic lens is an alternate lens design used when one or more of the following conditions are present:

• Steep corneas (over 45.00D)• Small palpebral apertures• Other lenses ride low

Compared to lenticular designs, the single cut edge is thinner, making the them more comfortable. Although the lens is smaller, the dimensions of the optic zone are comparable to a lenticular design. With the absence of the lens carrier, their is no sharp juncture between the two posterior curves, thus eliminating “lid bump”.

A disadvantage to the single-cut design may are difficulties which may be encountered when removing the small, steep lens from the cornea, especially for the elderly.

Advanced Techniques

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Aphakic Soft Lenses Aphakic contacts are the thickest lenses used. Due to the high plus power, the thickest portion of the lens exists over the central corneal area, meaning any corneal edema which may develop is critical to vision and corneal stability. Even with the highest DK lenses available, permeability with aphakic lenses is relatively low. An aphakic soft lens can cause a 4% to 8% increase in corneal thickness. The life expectancy of this lens is generally 6 months to a year.

Keratoconus

Keratoconus is a degenerative, hereditary condition of the cornea.

There are two primary classifications of kerataconus:

• Round or nipple shaped cone.• Oval or sagging cone.

ROUND CONE

The round cone is the most common keratoconus and usually is inthe lower nasal quadrant. Round cone keratoconus is usually lessthan 65.00D and is more easily corrected with contact lenses.

SAGGING CONE

The oval or sagging cone is usually rounder and steeper than round cone keratoconus. Sagging generally occurs in the inferotemporal quadrant with an average of 68.00D of corneal curvature.Indentation of the lower lid by the cornea (Munson’s sign) may alsooccur with inferior gaze.

A localized iron deposit, Fleischer’s Ring, may also be visiblearound the base of the cone in kerataconus.

While not curing nor stopping the progression of the disease, rigid contact lenses are most often used to successfully treat keratoconus. The tear layer between the posterior lens surface and the mishapen cornea create a spherical refracting surface to improve visual acuity.

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Advanced Techniques

A, Three-point fit-apical touch to the cone plus peripheral touch. Ideal for keratconus because of the distribution of weight of the lens, B, Flat fit-apical touch but poor centration because of rocking on the corneal cap and edge stand-off. C, Steep fit two—point touch with an air bubble between the lens and the cone. The apical one is cleared.

There are several lens designs used in treating keratoconus:Keratoconus (continued)

• Small, steep, single cut PMMA or RGP lenses• Soper two-curved vaulted lenses• Thin lenses• Soft lenses• Piggyback soft and rigid lenses

SINGLE CUT LENSES

Ideally, single cut lenses will exhibit light apical touch and rest on the corneal edge where little or no thinning exists. A flat fit may result in corneal abrasion. A steep fit may result in tears pooling around the periphery of the cone, potentially leading to discomfort, hazing and bubble formation.

SOPER LENSES

Soper lenses, with a lens diameter ranging from 7.5 to 9.5mm, have a steep base curve to match the steep central cone area and a much flatter outer curve to rest on the surrounding cornea. Trial fittings are recommended with Soper lenses. A good fit should exhibit:

Apical clearanceTear circulation between the corneal apex and posterior of the lens Good centrationSome lens movement with blink

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Advanced Techniques

THIN LENSESKeratoconus (continued)

Often referred to as Dura-T-Lenses, the primary benefit of this contact is its thinness, reducing the lens weight by approximately 30% and improving centration and comfort.

SOFT (HYDROPHYLLIC) LENSES

While not able to provide the visual acuity of rigid lenses, soft lenses are more comfortable and may be used in combination with spectacles. The lenses are relatively flat, with base curves of 8.1 to 8.4mm, with a 13 to 14mm diameter to provide stability.

PIGGYBACK LENSES

This fitting method involves the use of a soft lens, with a rigid lens riding on the anterior surface, usually used when patients are intolerant of rigid lenses.

• A soft lens of approximately 14.0mm diameter is placed on the cornea. A rigid lens, with a diameter or 8.5 to 9.5mm, is placed on the soft lens or in a depression in the soft lens designed to hold the rigid lens in place.

•

•

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Advanced Techniques

Kerataconus is classified in severity by the dioptric powers at the corneal apex:

• : less than 45.00D• : greater than 45.00D• : greater than 52.00D

:

X-Chrom Lens The X-Chrom corneal lens transmits light in the red zone from 590nm to 700nm. This lens can improve color discrimination for the individual who is partially color-blind in the red-green area. A red lens is fitted to the non-dominant eye of a person with red- green partially defective vision; the other eye remains uncovered. The uncovered eye will perceive the red or green object as usual, but the eye with the red lens will perceive red wavelengths of light and absorb the green wavelengths. The brain now receives two different intensities; by a rapid self-learning process, the patient can identify both colors properly.

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Flare or Streaming of Lights

Fitting Problems and SolutionsFrequently this is caused by a lens with a small optic zone thatthat can be overshadowed by a large pupil. The typical story is that the person cannot stand driving at night because of the streaming that is perceived around lights. When the pupil is dilated under dim illumination, there is a prismatic displacement of light coming in from the periphery of the lens. The same effect can occur from a lens that is decentered. In such a case the optic zone is displaced and the pupil will not be fully covered. Corneal edema from any cause can also create the same symptom.

Blurred Vision The most obvious cause of blurred vision is “incorrect power” of thelens and is simply handled by refracting over the lenses.

At times hazy vision may be caused by a “grease smudge or coated lens.”

The most worrisome cause of hazy vision is “corneal edema.” Corneal edema always means a poorly fitting lens. When the edema is severe enough to create epithelial erosions that pick up fluorescein stain, it indicates severely decompensated corneal epithelium.

The best antidote to corneal edema is a gas permeable lens. If a gas permeable lens has resulted in blurred vision, the following points must be checked:• Is the lens warped? Check the lens on a Radiuscope.• Is the lens centered? Displacement can occur with blinking or

eye movement. When this occurs, the lens, which normally rests on the central spheric portion of the cornea, is shifted to the flatter more ellipsoid section. The fit is then imperfect and molding of the cornea is a common complication.

• Is the lens tight? Small thin lenses are designed to be a little tight. A different lens may be needed.

• Any change greater than 0.50 in comparison with the amount found before a contact lens fitting is considered significant. Usually K readings are steeper and may be blurred.

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Fitting Problems and Solutions

Burning If burning occurs soon after the lenses have been inserted, it probably means that the lens is dirty or that the wetting agent used is irritating the eye. However, if the burning occurs after several hours of wear, it invariably suggests a poorly fitted lens or faulty blinking patterns. Burning is the symptom of equivalent corneal edema and is a complaint to which one must pay attention.

Corneal Edema The symptoms of corneal edema, which occur with any poorly fitting lens whether flat or steep, are photophobia, burning, hazy vision, and excessive spectacle blur.

Spectacle Blur This is such a common finding after wearing a rigid lens it has been accepted as normal. However, many authorities now believe that spectacle blur in excess of 0.50D is symptomatic of a marginal fit. The usual duration is variable. It may be present for only a few minutes to a half hour, or may persist for hours after the lenses have been removed.

Spectacle blur from corneal edema can be resolved with the use of a higher-DK gas permeable lens. The wearer can read or watch television within minutes of removing the lenses. This asset is one of the main features of a gas permeable lens.

Mucus Formation This is a common complaint and has a variety of causes. Eyeliner, mascara and lash thickeners, especially those that are water-insoluble, can cake on the surface of the lens.

Imperfect lens edges account for more problems than any other defect. They can cause corneal abrasions, lid irritation, excessive Meibomian gland secretion resulting in dirty lenses, and decreased lens tolerance.

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Fitting Problems and Solutions

Tight Lenses A tight lens is a lens that hugs the cornea too tenaciously and shows little movement with blinking. The flourescein pattern typically shows a dark band around or near the periphery of the lens with pooling in the center. A person wearing such a lens may be comfortable in the morning, but as the day progresses they develop hazy vision, burning sensations, and an inability to tolerate the lens during the day. Removing the lens is frequently a struggle because it clings strongly to the cornea. A tight lens can be corrected by:

•

•

•

•Loose Lenses A loose lens does not conform to the cornea because it is either too

flat or too small in diameter and as a result, moves too freely on the cornea. Such a lens may be tolerated initially, but eventually the excessive movement causes problems. Excessive lens movement leads to flare, intermittent hazy vision and frustration from a lens frequently decentering or actually popping out of the eye. A loose lens may be corrected by:

•

•

•

•Low-Riding Lenses A low-riding lens may cause tight lens symptoms because of the

inferior displacement of the optic zone if there is a lack of movement. The lens can also irritate the lower lid by mechanically abutting against it with each successive blink.

The most common cause of low-riding lenses is a thick, heavy, plus lens such as that prescribed for the ordinary aphakic patient. The lens diameter must be increased to permit more upper lid traction.

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High-Riding Lenses

Fitting Problems and SolutionsLike all displaced lenses a high riding lens suffers from a steep optic zone of a lens resting on a relatively flatter corneal periphery or even a limbal zone.

The most common cause of a high-riding lens is a high-minus lens that has a relatively prominent concave anterior surface. The periphery of the lens presents a ridge that is easily grasped by the upper lid and raised. Incorporating a prism ballast while reducing the bevel on the periphery of a minus lens is a good solution. A minus edge can be converted to a plus edge using a plus-carrier lenticular lens.

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Introduction

Contact Lens VerificationTo verify contact lens power, radius and curvature, there are several instruments which are used commonly and are discussed in this section. They are the:

• Radiuscope• Lensometer• Measuring magnifier• V/diameter gauge• Projection magnifier• Thickness gauge• Shadowgraph• Con-ta-check

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Rigid Lens Verification

Contact Lens VerificationUpon receiving rigid gas permeable from the laboratory, the following parameters should be inspected for accuracy.

DIAMETER:This measurement can be verified with a hand held magnifier, a diameter gauge, or one of the various types of projector inspection devices. Tolerance is within 0.05mm.

SURFACE QUALITY AND EDGES:Utilizing a measuring magnifier, projector inception device, or through the use of a slit lamp an assessment of quality can be made.

BASE CURVE:The radiuscope is the most common tool used to obtain this measurement. They can also be obtained using a Keratometer with a special holder which allows the contact lens to be held horizontally while the Keratometer remains in its normal position. This holder is called a Con-ta-check. The base curve should be accurate to within 0.025mm of specifications.

POWER:Back vertex power is measured with a lensometer. This measurement should be within 0.25D of the specified power and less than 0.12D of any unprescribed astigmatism.

Thickness:A thickness gauge is used to obtain this measurement.

BLEND & TRANSITION ZONES:A hand magnifier, projector inspection device, or a slit lamp are all instruments that enable the examination of these zones.

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Contact Lens VerificationRigid Lens Verification Tools

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Soft Lens Verification

Contact Lens VerificationThe methods which can be utilized for verification of soft lenses cannot be relied upon for complete accuracy. As a result there is a risk of premature rejection due to inaccurate or invalid test results. The only valid method of inspection is once they are in the eyes.Any problems can be detected and dealt with at that time.

DIAMETER:The hand held magnifier may be used to measure the diameter. The soft lens will contour itself to the magnifier.

BASE CURVE:Templates of a known radii of curvature may be used. A Soft Lens Analyzer can also be used to measure diameter center thickness, and provide close surface and edge inspection.

POWER:A lensometer can be used. A water cell may also be used to measure the power of a soft lens in the hydrated state. The soft lens is floated in normal saline and measured in the lensometer. The lensometer reading is multiplied by a factor of four. This is not a reliable method since the power can vary with the thickness of the lens which varies from high plus to low minus.

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Radiuscope

Contact Lens VerificationThe radiuscope is used to measure the radius of curvature of the anterior and posterior surfaces of rigid contact lenses. While there may be some minor differences among radiuscope manufacturers, most are operated using these common procedures to measure spherical rigid lens.

• Place a drop of water on the lens mount, then the contact lenson the water, front side, or convex surface, down.

• Looking into the instrument, center the spot of light from theradiuscope and raise the body of the microscope as high aspossible.

• Focus the microscope downward until a spoke pattern becomes clear. This is the aerial image and is the reflection from the back, or posterior, surface of the lens. Bringing the spoke pattern to as sharp a focus as possible, set the pointer to zero on the scale.

• Continue to move the body of the radiuscope downward. The next image to appear will be a bulb filament.

• Progress beyond this image to the next spoke pattern. When this pattern is in sharp focus, the scale on the right will represent the radius of curvature of the lens surface in millimeters.

• To verify the final reading, move the microscope back to the aerial image. If the reading is still zero, the radius reading obtained is correct. Otherwise, reset the scale to zero, and remeasure the final reading.

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Contact Lens VerificationRadiuscope (continued)

Toric Curves Certain designs of rigid lenses, such as front surface torics, contain more than one radius of curvature. Similar to using a lensometer, all spokes will not focus simultaneously. When the radius is normally recorded, note the reading when the first spoke is clear, then continue to move the microscope downward until the next spoke, 90 degrees away from the first spoke, is clear and record the reading.

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Lensometer

Contact Lens VerificationThe lensometer is useful in measuring contact lens power, especially on rigid lenses, using a contact lens holding device. Soft lenses can be measured if they are dry. Another method for measuring soft lenses employs the use of a “wet cell” and multiplying the result by a factor of approximately 4. This factor will vary somewhat depending on the index of refraction of the lens material.

Verification Guidelines The following guidelines are helpful for interpreting results when verifying lenses on the radiuscope and lensometer.

Lens Type Radiuscope Lensometer

Spherical Warped Front Toric Back Toric Bi-Toric

SphericalSphericalSphero-cylindricalSphero-cylindricalSphero-cylindrical

Measuring Magnifier Also know as a hand magnifier, this instrument provides 7x magnification to measure

.

V/Diameter Gauge Used to measure total lens diameter, this instrument is a ruler with a V-shaped channel graduated between 6.0mm and 12.5mm.

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Projection Magnifier

Contact Lens VerificationThis instrument is also used to measure lens diameter by projecting a magnified image onto a calibrated screen.

Thickness Gauge Operating on a principle similar to that of lens calipers used to measure ophthalmic lens thickness, the thickness gauge provides an accurate assessment of lens thickness.

Shadowgraph The shadowgraph is used

Profile Analyzer While the Shadowgraph, Keratometer, and Burton lamp all show rigid contact lens blends, only the Profile Analyzer can adequately assess the quality of blends between peripheral posterior curve (PPC) and the intermediate posterior curve (IPC) and central posterior curve (CPC or BC).

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Con-ta-check

Contact Lens VerificationAn attachment to the Keratometer, this instrument can be used to measure in the absence of a radiuscope.

ANSI Standards Following are the ANSI Standards for PMMA (conventional hard), RGP and soft contact lenses.

PMMA RGP Soft

Diameter (mm) +0.05 +0.05 +0.25

Base Curve (mm) +0.025 +0.050 +0.20

Center Thickness (mm) +0.02 +0.02 +0.03

Refractive (diopters)

0.00 to 5.00 +0.12D +0.12D +0.25D5.12 to 10.00 +0.18D +0.18D +0.25D10.12 to 15.00 +0.25D +0.25D +0.50D15.12 to 20.00 +0.37D +0.37D +0.50Dover 20.00 +0.50D +0.50D +0.75D

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Introduction

Rigid Lens ModificationSome modifications of conventional hard and rigid gas permeable contact lenses are able to be performed in the office. These adjustments are limited to those which require a looser fitting lens and/or increasing the tear flow beneath the lens. A new lens needs to be manufactured to achieve a . The in office modifications possible for rigid lenses include:

• Blending the transition or junction zones• Reducing the optic zone diameter• Reducing overall lens diameter• Flattening intermediate curves• Adding minus power• Adding plus power• Removing scratches• Polishing and refinishing lens edges• Adjusting peripheral curves• Identifying the lens• Fenstrating• Flattening the base curve

The most common of these procedures performed in the office are:

•

•

•

•

This section will provide a description of the above adjustments and the resultant effects on lens prescription and/or fit.

POLISHING COMPOUNDS

Mild abrasive agents, such as Silvo or Xpal, mixed with oil or water, may be used to polish rigid contact lenses. Usually Xpal is more compatible with the softer gas permeable lens material, as it tends to scratch less than Silvo. Alox PG or Sil O2 care are specifically designed for use with gas permeable lenses.

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Rigid Lens ModificationBLENDING TRANSITION OR JUNCTION ZONES

Rigid Lens Modification (continued)

The junction zone, or area where the CPC and IPC or IPC and PPC meet, should be as smooth as possible. If this condition does not exist, there may be improper tear exchange and/or corneal irritation. The transition zone may be blended by using a tool with a radius of the average of the two curves it is blending.

REDUCING THE OPTIC ZONE DIAMETER

Increasing the width of the PPC or IPC will effectively reduce the optic zone diameter, thus creating a looser fit. This is accomplished using a diamond impregnated tool of the same radius of curvature.

REDUCING OVERALL LENS DIAMETER

Using a razor blade, emery board or a cone shaped stone will reduce the overall lens diameter and loosen lens fit. Very simply, the lens is centered on a spindle, concave side up. While the lens spins, one of the tools mentioned above is applied to the lens edge to decrease the diameter.

FLATTENING THE INTERMEDIATE CURVES

A felt covered tool of a flatter radius of curvature can be used to flatten the intermediate curve.

ADDING MINUS POWER

Up to one diopter of minus power may be added to a lens byflattening the front curve using a piece of velvateen or sponge.

ADDING PLUS POWER

Up to one diopter of plus power may be added to a lens using a technique similar to that used to add minus lens power, except the edge is steepened by polishing the lens periphery.

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Rigid Lens ModificationREMOVING SCRATCHESRigid Lens Modification

(continued)Scratches may be removed from the front and back lens surfaces of rigid contact lenses using a soft foam pad or soft, cloth covered drum. Care must be taken not to alter the curve structure, which would result in a power change.

POLISHING AND REFINISHING LENS EDGES

Although there are several techniques used to polish and refinish lens edges, the most common method utilizes a felt disc.

ADJUSTING PERIPHERAL CURVES

Using a diamond impregnated tool of correct radius of curvature, peripheral curves may be flattened, blended or made wider.

FLATTENING BASE CURVES

Although usually not performed in the office, the base curve of a lens may be modified by as much as diopter.

IDENTIFYING THE LENS

Lens dotting to identify the right lens may be performed in office using several techniques.

FENSTRATING THE LENS

Fenstrating has the effect of loosening lens fit and is accomplished by drilling small holes. Fenstrating does not significantly effect oxygen flow through the lens.

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Rigid Lenses

ManufacturingRigid lenses are manufactured from individual pieces of plastic material called “buttons”. These “buttons” are generally lathe cut similar to the way a semi finished lens is manufactured. Molding technology has also been developed for PMMA material with the benefit of increased curve consistency.

Soft Lenses Like rigid lenses, soft lenses are also manufactured by sophisticated lathe cutting technology. The lenses are manufactured in a hard state and then hydrated with water. The spin-cast process developed by Otto Wichterle is also used in the manufacturing of soft lenses. The major advantage of spin-cast technology is high reproducibility and consistency.

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Lens Inspection

Inverted Lenses A lens has an inside and an outside surface. The contact lens wearer must be able to determine that the lens is right side out prior to lens insertion. During dispensing training, the patient must demonstrate an understanding of this concept. There are several methods to illustrate this point. Use whichever method appears to work best for the particular type of lens being dispensed. It may be necessary to demonstrate several methods to ensure complete understanding.

Inspection Method Carefully examine the lens to ensure that it is clean and free of blemishes. Never put a damaged or torn contact lens on the eye.

Rinse the lens and drain the excess solution. If the lens is dehydrated, visual acuity will be poor. Observe the lens on the tip of the finger at eye level. The correct lens will appear to be a somewhat warped bowl shape. The inverted lens will be a flatter saucer shape with a sharper edge. Observe the edge of the lens closely. Sometimes there is only a subtle difference in the way the edge of the lens curves.

Explanation

Lens “A” Lens “B”

Lens “A” illustrates a lens that is right side out.

Lens “B” is the clearest illustration of an inverted lens. There is a lip at the edge of the lens and the lens edge appears sharp and thin. Notice the difference in the curvature of lens “B” compared to the other lens. When a lens is right side out, the sides will form an angle almost perpendicular to your finger as they approach the edge of the lens.

Lens Inspection

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“Taco Test” Method For successful application of this method the lens must be freshly wetted and drained of excess saline. Your fingers must be dried of excess saline to prevent the lens from sticking to your wet fingers.

Place the lens on the index finger of one hand, and use the thumb and index finger of the other hand and attempt to lightly roll the lens edges in toward each other. The correct lens will roll in on itself and away from your fingers. The inverted lens will tend to flatten back against your fingers, and roll towards them.� �

Correct Inverted

Lens Comfort Method If all else fails, it may be necessary to resort to trial and error. An inverted lens placed on the eye may result in the following symptoms:

• Discomfort• Increased lens awareness• Blurry, unstable, or distorted vision• Excessive lens movement• The lens folding up and popping out of the eye

If the lens is comfortable, then the lens is probably right side out. If there is no noticeable difference, ask your practitioner to evaluate the lens.

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Folded Lenses

Lens InspectionIf a lens becomes folded and sticks together, do not grasp the edges and pull them apart! This will tear the lens. Instead, place the lens into your open palm in a pool of saline (rinsing solution), and using the forefinger of the opposite hand, gently tease the lens open by rolling it in the direction of the fold. Keep it in a pool of saline solution, and be patient.

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Lens Insertion Preparation -Soft Lenses

Introduction Many patients experience difficulty with lens insertion in the beginning. Informing the patient of this can help instill confidence in the event the first few attempts at lens insertion are not met with success. Let them know that lens insertion is a skill that needs to be learned. The following training techniques will prove helpful in assisting the new patient.

Follow these steps to prepare for lens insertion:Preparation for Insertion

1. Remove the right lens from the storage case, placing it in the palm of your hand.

2. Apply rinsing solution to the lens.3. Inspect the lens for foreign particles or any damage before

placing it on the eye. The lens should be moist, clear, clean and intact.

4. Grasp the lens gently and place it on the tip of the index finger of your inserting hand.

5. Check to make sure the lens is not inside out.6. Repeat Steps 1 through 6 for the left lens.7. Discard the solution in the storage container.

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Insertion Method

Lens Insertion1. Keep the elbows out while inserting the lens. If they are

positioned too far in, toward the body, it may restrict the movement of the index finger into the space between the lids.

2. The lower lid should be held down with the middle finger of the dominant hand while the upper lid is held with the middle finger of the other hand. Holding the lids apart creates an opening large enough to accommodate the lens and it delays the normal blinking reflex.

3. While inserting the lens the patient should look straight ahead. At first, a mirror should be used so the patient can actually watch the lens as it is applied to the cornea. Eventually, if the patient continues to look straight ahead during the insertion process, a mirror will not be necessary.

4. If after several tries, your patient continues to experience difficulty inserting the lenses, have the patient use the two-handed technique described above. But this time to touch the sclera with the index finger with no lens present. This can sometimes help to instill confidence.

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Removal

Lens Removal- Soft Lenses1. The patient should look straight ahead at a spot directly in front.

2. The gaze should then be directed upward, being sure not to move the head.

3. Pull the lids apart in a manner similar to the technique used to insert the lens.

4. Gently slide the lens straight down from the cornea to the sclera with the index finger of the dominant hand.

5. While the lens is being held on the sclera by the index finger, bring the thumb onto the eye and gently pinch the lens out.

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Introduction

Lens Insertion Preparation -Gas Permeable

The rigidness of the lens elevates the level of care needed when inserting rigid gas permeable contact lenses. Proper insertion of gas permeable contact lenses is pertinent, to ensure that the individual will not scratch their cornea.

Preparation for Insertion

1. First, remove the right lens from the storage case placing it in the palm of your hand.

2. Next rinse the lens with fresh wetting solution. Use an extra drop of solution on the concave surface to cushion the placement on the eye.

Waiting 20 - 30 minutes after waking to insert rigid lenses will increase the level of comfort. If lens sensation is increased or vision is blurred immediately after insertion the lenses should be removed, cleaned, rinsed, and re-inserted.

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Re-Centering

Centering: Gas PermeableOccasionally, a lens may be displaced onto the sclera when inserting, with a sudden eye movement, or with increased tearing. The lens may actually be left on the sclera for several hours without injury. In any case, the lens cannot slip behind the eye. To re-center a displaced lens, follow Method 1or Method 2 below.

Method 1 1. Have the patient lean over a table with their face parallel to it.2. With the middle finger of each hand on the center of the lid

margins, gently pull the lids off the eyeball.3. The patient should look in all directions as far as possible.4. If the lens slides onto the cornea, they should look straight

down, release the lower lid, and then release the upper lid. If the lens positions itself out of sight under the top lid, have them hold their head erect, pull the upper lid well up off the eyeball, look down as far as possible, and move the eye from side to side until the lens slides down into view. If movement seems difficult, flood the area with drops of conditioning solution.

Method 2 1. Locate the lens by inspection in a mirror.2. Have the patient turn their eye in the direction opposite to the

lens position.3. Instruct the patient to use their fingertip to press the lid margin

gently against the sclera beyond the lens edge, and slide it onto the cornea.

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Lens Removal - Gas Permeable

Removing First, clean and dry your hands thoroughly. Then, use Method 1, described below, to remove your lenses. Try to remove them a minimum of 20-30 minutes prior to sleeping.

Method 1 1. The lens should be centered on the cornea.2. Open the eye wide enough so that the edges of the lens are not

in contact with the eyelids. Then pull the outer corner of the eyelids slightly to make them tight.

3. Quickly blink in an effort to remove the lens from the cornea as the result of a bumping action created by the eyelids. While the opposite hand covers the eye to catch the lens, it is recommended that a tissue or soft cloth is used to cushion the lens in the event it falls.

ADDITIONAL TIPS

• The lens must be on the cornea for Method 1 to be effective.• The lid margins must clear the top and bottom of the lens until

the blink.• Do not pull too hard or let your fingertips slip off the lids.• Do not release pressure on lids until the blink is completed.• If the lens does not come out with the first blink, relax, reposition

eye and fingers, and repeat. Looking towards the nose when blinking may help.

• A DMV, suction holder, may be used by the patient only in cases where the gas permeable lens does not dislodge after multiple attempts.

Proficiency The patient must be able to display a proficiency in insertion and removal of their lenses before he/she leaves the store with the lenses. If the patient is having difficulty learning insertion, it may be necessary to schedule a second appointment to teach insertion and removal. Be diplomatic and professional. Do not tell the patient that you are rescheduling because you have run out of time. Inform the patient that it often helps to take a break and continue training at a later date.

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Trial Period

Dispensing: Follow-Up CareThe prescribed lens undergoes a trial period during which time the doctor and the patient determine if the lens is acceptable. A lens which looks satisfactory at the initial dispensing may tighten or loosen after continuous wear causing discomfort or blurred vision. This information cannot always be determined at the time of the original fitting.

Normal TEARING: This is a natural response by eyes that are not used to having foreign objects on them. This symptom subsides rapidly as wearing time increases.

MINOR IRRITATION: This tickling or awareness sensation gradually disappears with wear. This symptom may cause mild discomfort on upward gaze due to the lens coming in contact with the upper lid.

BLURRED VISION: You may experience intermittent blurred vision due to excess tears.

SENSITIVITY: Light sensitivity, as well as, extra sensitivity to wind, smoke, and dust may occur with contact lens wear.

REDNESS: This symptom may occur due to minor eye irritation. Your lenses may bother you in the morning, in smog, and in smoke- filled rooms. Colds, hay fever, and other illnesses can also affect comfort. Alcohol and certain medications may upset the metabolism of the eye and cause irritation if contact lenses are worn.

Abnormal There are certain symptoms, that although are minimum may beexperienced by the patient. If a sudden sharp pain is ever felt bythe patient, most likely dirt or a lash is lodged under the lens.Remove, clean, and reinsert the lens. If pain or blurring persists, call the office for an appointment. If any of the following symptoms are ever experienced by the patient direct them to call the office for an appointment severe or persistent haze or halo around lights, severe redness or irritation, or blurred vision.

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Wearing Schedule

Dispensing: Follow-Up CarePMMA: PMMA lenses should be worn no more than three hours the first day with the wearing time gradually increased one hour per day. They should be worn no more than eight hours in a single day until the patient can be seen for a follow up visit. However, the lenses should only be worn for as long as they are comfortable.

GAS PERMEABLE: The lenses can generally be worn for about four hours the first day, increasing the wearing schedule two hours each day for as long as they remain comfortable. If prolonged wearing time results in decreased comfort, the wearing time should be limited so that it does not exceed the point of comfort. In many cases the lenses can eventually be worn from 12 to 18 hours per day, provided they remain comfortable and the eyes are not red.

SOFT CONTACT LENSES: Initial wearing schedules can vary. However, on a healthy eye, a well fit lens can sometimes be worn for a full day right from the start. A more conservative schedule might indicate six hours the first day, with an increase of four hours each day until a maximum of eighteen hours is reached. Again, the lenses should never be worn beyond the period of comfort. Should the lenses become very uncomfortable, they should be removed and a follow up visit scheduled.

Every Six Months It is the doctor’s recommendation that all contact lens wearers have their eyes checked every six months. Optimally, after the initial follow-up visit, the patient should be seen in about six months while wearing the lenses to see how the lenses are performing, as well as, monitoring the health of the eyes.

The frequency of these exams are to ensure the health of the patient’s eyes ,as well as, monitor the integrity of the lenses. A change in either could potentially cause injury or unwanted changes to the patient’s eyes.

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Introduction

Contact Lens CareSoft contact lenses must be disinfected if they are to be reused. Disinfection is the term used to describe the procedure that kill common pathogenic organisms, although other nonpathogenic organisms may survive. Disinfection should not be confused with sterilization, which implies that the complete elimination of all bacteria, spores and fungi has occurred.

Described below are the two basic methods used to disinfect soft lenses, thermal and chemical, and the most common systems available.

THERMAL DISINFECTIONIn this now seldom used method of disinfection, lenses were boiled in saline solution.

CHLORHEXIDINE-THIMEROSOLThis disinfection system, known commonly as Flexsol and Normol, was the first of its type approved for chemical contact lens disinfection. The lens is soaked in the solution overnight, and rinsed with saline prior to insertion into the eye. Allergic reactions to the Thimerisol, a mercury based compound, were common.

IODINE SOLUTIONSA low water lens (less than 45%) may be disinfected in Pliacide, which contains 0.1% iodine.

QUATERNARY AMMONIUM SOLUTIONSThese systems, marketed by Hydrocare, Allergan, and Bausch & Lomb among others, are a combination of cleaning and storage solutions.

HYDROGEN PEROXIDEBrand names for this type of disinfection system include AOSept, Lensept and Septicon. The lens is soaked in a 3% hydrogen peroxide solution for ten minutes, and then stored in saline solution and a catalyst for at least six hours. Prior to lens insertion, the lens is soaked in fresh saline for at least three minutes. The steps following the hydrogen peroxide storage are mandatory, or corneal damage may occur.

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Cleaning & Disinfection

Contact Lens SolutionsThe proper cleaning and disinfection of lenses is very crucial to the ultimate success of the contact lens wearer. Improper care of the lenses highly increases the risk of eye infections.

Basic Components There are several basic components of lens care that are common to all types. For certain types there may be specific steps for each.

Saline For some soft contact lens types a saline solution (a sterile isotonic solution containing sodium chloride and an antimicrobial buffer system) is used to rinse and store contact lenses. Saline DOES NOT disinfect from bacteria. Tap water should NEVER be used in place of saline. , a micro-organism, could be present in tap water and cause a serious eye infection.

Surfactant Cleaner A surfactant cleaner is used to clean the surface of a lens from deposits, dust, make-up etc. It is very important to always use this in conjunction with a disinfectant or conditioning solution. A surfactant cleaner should be used on all types of contact lenses, including disposable contact lenses as needed.

Disinfecting Disinfecting solution is used to remove bacteria from the lenses. Unless it is an “all in one” solution, disinfecting solution may need to be used in conjunction with a saline solution. The lenses must be thoroughly rinsed, otherwise irritation and possible injury could occur.

Wetting/Conditioner With all rigid gas permeable lenses, a “wetting or conditioning” solution is used. When used with a “surfactant” cleaner, this solution helps destroy bacteria. It also helps to cushion the lens at insertion, and helps to preserve the wettability of the lens.

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Enzymatic Cleaning

Contact Lens SolutionsEnzyming lenses is widely used with soft contact lenses but is recommended for use with gas permeable contact lenses as well. This step is necessary to remove residual protein, lipid and other types of deposits, which can make the lenses more comfortable, provide sharper vision, cleaner lenses and possibly extend the life of the lens. Different types of enzymatic cleaners will determine soaking time.

Enzymatic cleaning can come in tablet form (most common), to be used with some type of saline, or in liquid form.

Expiration When a cleaning system is dispensed the dispenser should ALWAYS check the dates and determine if it is too close to the expiration date. For both dispenser and patient, expired solutions should be discarded.

nclerigid gas permeable. rigid gas permeable. rigid gas permeable (rgp) is the term used to describe...

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