Corneal physiology & Corneal physiology & contact lens-2contact lens-2

Corneal physiology & Corneal physiology & contact lens-2contact lens-2

oRGP lenses & astigmatismRGP lenses & astigmatismoToric contact lensesToric contact lenses

Instructor: Areej OkashahInstructor: Areej Okashah10/12/200910/12/200924/12/200924/12/2009

• The goal of contact lens is to meet the physical & physiologic requirements of the eye, & to address the visual requirements of the eye

• Corneal toricity is a major component that contributes to the patient total astigmatism..

• Refractive astigmatism that manifested in the subjective refraction.. could be more or less than the corneal astigmatism.. & it could be on different axis..

• Employ the best choice of contact lens to meet the refractive status of the patient in addition to physical & physiologic features of the eye

Residual astigmatism

• Residual astigmatism: is the amount of astigmatic refractive error that remains uncorrected when a contact lens (spheric; toric; soft or rigid) is applied to the cornea to correct the refractive error

• For example; the residual astigmatism with a spherical rigid lens on the eye is the difference between the corneal astigmatism & the refractive astigmatism this is only if the corneal astigmatism less than 2.5D

• The residual astigmatism of a spherical soft lens is the same as the refractive astigmatism

Sources of residual astigmatism

1. Physiologic residual astigmatism i.e. total physiologic residual astigmatism (cornea, lens , vitreous, retina,…); this arise from:

- Contributed power from the different meridians of the crystalline lens (ant. & post. Interfaces)

- Contributed power from the posterior cornea-aqueous interface….note the different refractive index

- Variations in refractive index of the cornea, lens, …..

- Obliquity of light incidence to the cornea- Eccentricity or irregularity of the fovea

Sources of residual astigmatism

2. Induced residual astigmatism: this could increase or decrease the physiologic residual astigmatism

Induced residual astigmatism is the amount of astigmatism which is contributed by the presence of the lens on the eye because of lens flexure (i.e. fold), decentration, Induced residual astigmatism, warpage (i.e. is a distortion where the surfaces of the molded part do not follow the intended shape of the design), tilt of CL (especially for high power lenses, or toric designs)

• Residual astigmatism may affect the visual performance of the contact lens; either reduction of the visual acuity or exhibiting asthenopia if this is the case try to correct the residual astigmatism or choose a different type of CLs

SO WHAT?!

CAN WE PREDICT THE RESIDUAL AMOUNT OF ASTIGMATISM FOR A PATIENT WEARING A RIGID LENS?

…….calculation of CRA• YES, if you have the K readings & spectacle refraction

• Calculated Residual astigmatism (CRA)= Total refractive astigmatism TRA (i.e. spectacle cyl.) – Keratometric cylinder

• Examples : Spec Rx -1.00 -2.00X 90 “K” 44.00@90/ 45.00@180 CRA for a rigid Spherical lens = -2.00X90 – (-1.00X90) CRA for a rigid Spherical lens = -1.00X90Note that we solve on inspection that there is no CYL in the tear

lensNote: for soft CL CRA= TRA in this example -2.00X90, guess why?

…… calculation of CRA• Note that for if the spectacle Rx is high i.e. > or =

±4.00D you should solve (adjust) for the vertex distance in each major meridian before the calculation of the CRA… look for the following example:

Spec Rx +12.00 -3.00X180Spec vertex distance 12mm ..you should measure this for

every patient“K” 42.00@180/ 45.00@90 The adjusted effective power at the corneal plane at

180= +14.00 D & at 90= +10.00 DSo, the RX at the corneal plane is +14.00 -4.00X180CRA=TRA-”K” cyl CRA= -1.00X180

Measured residual astigmatism (MRA) versus CRA

• There is as significant variance between CRA & the actual (i.e. measured) residual astigmatism… which has been found in some studies; MRA is found to be 0.25D less than CRA

• It’s clinically preferable to perform over-refraction i.e. to find MRA

• Although it’s acceptable method to find the residual astigmatism; CRA is not an accurate tool because of the inaccuracy of keratometry

• The residual astigmatism have higher value among patients who wear spherical contact lenses

• The acceptable value of residual astigmatism; when spherical rigid lens on; is < 0.75 D; if >= 0.75 D consider an alternative type of lens; since vision may be reduced & a patient may exhibit asthenopia…. Cont…

……cont• Or..spectacles over the spherical rigid lens to correct

the residual cyl.,

• Special lens design instead. E.g. toric rigid lens, or toric soft lens

• Manipulate lens thickness to increase or decrease lens flexure.. > flexure (thin lens) less cyl can be corrected; you can perform over-keratometry

Residual astigmatism with spherical soft lenses

• CRA=TRA; so as the refractive astigmatism becomes close to zero the patient becomes a good candidate for spherical soft lenses; because sharper vision can be achieved…

• AS a clue note this example:Spec Rx -2.00 -0.25X180, “K” 43.00@180/ 44.50@90For a rigid lens CRA= -0.25X180 – (-1.50X180)=

+1.25X180 ????!!!!!On the other hand for a soft lens CRA= TRA=

-0.25X180 ???!!!!

Soft toric lenses• It could be the best choice today to correct refractive

astigmatism….; a soft toric lens provides good comfort, sharp optics, its parameters can be easily modified by manufacturers

• A large range of stock parameters i.e. large trial set is available

• Can correct low as well as high astigmatism• Can be applied for therapeutic purposes e.g.

postsurgical HOWEVER• Soft toric lenses: as the lens power becomes higher, it

becomes more difficult to control variations in meridional edge thickness this could mislocate the lens from the wanted axis causes blurred vision or instability of vision

……

• Because of these rigid toric lenses are superior for the correction of high corneal astigmatism

Front toric lenses• It is a rigid lens used usually to correct the residual

astigmatism when the soft toric lens used or other spherical lenses are not useful or not applicable ….

• The aim of this lens is only optical i.e. to provide better vision; because it provides spherical base curve radii

• Are indicated for low to moderate corneal cyl <= 2.00D, or when there is a significant amount of residual cyl with spherical lenses

• This lens has spherical back curvature (BCR), base down prism, toric front surface that correct the required degree of cyl

• The base down prism is used to reduce the lens rotation when blinking i.e. the lens kept in a stable position

cont• ..so, prism ballast produces different thickness across

the lens surface.. In the front toric rigid lens, the superior of the lens (apex) is thinnest while its inferior is thickest

• front toric rigid lens (i.e. front toric lens) may be either circular (circular prism-ballast lens) or truncated (truncated prism-ballast lens)

Truncated prism-ballast lens• Are indicated for patients with normal to high

positioning of the lower lid; & upper lid at or near the limbus

• The OAD, BCR, PC radii are similar to those selected for spherical RGP lenses, but you can’t choose a large lens diameter because the lens should lift with the blink & settle inferocentrally , the lens also shouldn’t be in contact with the upper lid

• The vertical diameter typically is 8.7-9.2mm (larger than the pupil diameter in the dim light); the horizontal diameter usually 0.4-0.5mm greater than the vertical diameter

cont• The upper edge of the lens should be well rounded and

polished to not irritate the limbus • The truncation should be properly shaped to meet the

contour of the lower lid• Truncated prism usually is 1.50-1.75 prism diopter, but

should be more for high minus lenses as the edge thickness increases

• To achieve maximum fitting with truncation the optic zone of the lens should be decentered upward by 0.5mm to neutralize the geometric lowering of the OZ created by truncation to avoid flare & to maintain better centration of the lens…… ALSO

cont• Order the lens so the truncation is temporal to the

base-apex line by 15-20 degrees; because prism ballasted lenses ride with their base-apex line rotated nasally (i.e. to neutralize the amount of kick towards the nose which is 15-20 degrees)

• The truncated prism ballast should rest evenly on the lower lid; and the lens will still rotate nasally

The circular prism ballasted lens

• Is much simpler than truncated prism-ballast lens• It is indicated if the patient has lower lid at or below the lower

limbus; large palpebral fissure, loose lid, if also has discomfort with the previous design (i.e. truncated ballast)

• For this design the lens has a centred optic zone & less prism is needed than with truncated lenses… require 0.75-1.00 prism diopter of ballast for moderate to high powered minus lenses; for plus lenses & low power minus lenses which they have thinner edges require 1.25-1.5 prism diopter..

• Notice that using lower amounts of prism make this design more comfortable

cont• For each design perform spherocylindrical overrefraction • Lens rotation & tilt same as before• E.g. if the right lens were found to rotate 15 degrees toward

the nose (counterclockwise) & the subjective refraction axis is 90 degrees the right lens would be decreased by 15 degrees, lens ordered at 75 degrees

• The optic zone is 1.2mm smaller than the OAD• Diameter range from 8.8 (for steeper curve) to 9.2 mm (for

flatter curve)• Lens should be centred inferocentrally but should lift with

blink (means slightly flat)

• Note that as the lens thickness changes from the apex to the base there is a gradual change in power; e.g. prism lenses increase in plus power ~ 0.25D per 0.1mm increase in thickness

Lens tilt & rotation…….• Examples: • For the Rt eye; If the subjective

refraction axis is 70and 15 rotation to right; subtract order 55 axis

• For the Lt eye; if subjective refraction axis is 110 and 20 rotation to left (clockwise); add order 130 axis

Prism ballast & binocularity• Front toric lenses with prism ballast produce vertical

prism power before the eye (verification of thickness; apex base) however;

• Most residual astigmatism cases are bilateral so the bilateral vertical prism have no significant effect on the binocular system; but

• If the front toric lens is used monocular; in this case you should corporate a prism in the fellow eye as well to avoid inducing or increasing the vertical phoria

• Prism magnitude & type (effect) can be determined by lens position on the eye

Non-prismatic lens designs• Peri-ballasted lens & double truncation are two methods used

to stabilize toric lenses while on the eye without incorporating prisms

• Periballasted lenses: it’s a lenticulated lens design with high minus carrier; i.e. the superior portion of the lens has been largely removed= this gives a heavy inferior portion of the lens that help stabilizes the lens without using prism throughout the optical portion of the lens

• Periballasted lenses: is used for monocular residual astigmatism i.e. avoid the effect on binocular vision; this type of lens also has an advantage as the diopteric power of the lens is unchanging throughout the optical zone; unlike prism ballasted lenses; also this lens is thinner than ballasted lenses (reduced mass lenses) which makes it more comfortable & improve tolerance by patients

• For periballasted lenses; lens tilt & access rotation effect should be compensated in most cases

cont…• Double truncation: using double truncation

(upper & lower) to stabilize the cylinder lens; in this case no compensation for axis is made; the best fit in this design is to use base curve radii with minimal apical clearance; H diameter (9.0-9.6mm) V diameter (7.7-8.2mm)depending on the pupil size & palpebral aperture size; small optical zone (6.8-7.0mm); because truncation removes parts of the lens

toric back surface

Correction of HIGH astigmatism (cyl.)

• Toric back surface lenses are indicated in cases where a spherical lens used on toroidal cornea is unable to provide acceptable lens-cornea bearing or an acceptable physiologic results…….so

• The use of toric back surface lenses is recommended to improve the physical fit on a toric cornea and in some cases for optical correction of residual cyl..

back toric surafec

Using spherical lenses

• Spherical rigid lenses correct low degrees of corneal toricity;;;;;; but if high degrees of corneal cyl present

Instead of paralleling the corneal

contour; a spherical lens aligns with the flat meridian; so it leaves excessive clearance in the steep meridian

back toric surafec

cont• Good visual acuity is possible with spherical rigid

lenses; however if these are used on highly toric cornea some complications (e.g. limbal & conjunctival injections; unnatural blinking pattern; 3&9-staining; visual disturbances due to flare & lens decentration) are possible…

• So; don’t depend strictly on K-readings to wether use a spherical rigid lens or not; note that peripheral corneal toricity may differ from central corneal toricity== very important to evaluate fluorescein pattern

Horizontal bearing with-the-rule toric corneal

cont• Using spherical BCR steeper than K to improve

centration or to compensate for corneal cyl. may cause limbal hyperemia; inadequate tear exchange; corneal staining due to corneal compression by the lens.

• Note that moderately toric cornea (keratometry) may have less toricity of the cornea at the periphery

• If the spherical rigid lenses are not useful/meaningful for your patient; or whenever there is a doubt about the efficiency or safety of a spheric CL; consider other types of CLs such as lenses with aspheric BCR (toric back-surface lens);

Back surface toric lenses• To align the corneal contour more closely in

moderate- high corneal astigmatism

• To best fit toric lenses: diagnostic fitting is superior to empirical fitting (i.e. keratometry & refraction only)

• Lid tension; lid position; CL edge thickness & clearance; etc… all could greatly affect lens centration & movement

Back surface toric designToric base curve initial design; based on K readings

Corneal toricity (cyl) Flat meridian compared to K

Steep meridian compared to K

Remba 2.00D 0.25D flatter 0.25D flatter

3.00D 0.25D flatter 0.50D flatter

4.00D 0.25D flatter 0.75D flatter

5.00D 0.50D flatter 0.75D flatter

Goldberg 2.00-4.00D 0.50D steeper Steeper curve is flattened by ½ the difference of the meridians

4.25-6.00D 0.75D steeper

> 6.00D 1.00D steeper

contSelection of OAD/OZD based on lens toricity, using Goldberg’s technique:

Lens base curve in diopters

Lens toricity 40.00-41.75D 42.00-43.75D 44.00-45.75D

2.00D 9.4/8.0mm 9.2/8.0mm 9.00/7.6mm

3.00D 9.2/7.8mm 9.0/7.8mm 8.8/7.4mm

4.00-6.00D 9.0/7.6mm 8.8/7.6mm 8.6/7.2mm

cont• Inherently; back-surface toric lenses

will fit more tightly than spherical lenses on toric cornea, with less rotation & lag… so the lens should be designed to ensure tear circulation as well as correcting enough corneal toricity to improve lens centration & overall bearing relationship

Toric-base with spherical front & bitoric lenses

• The design principles of the posterior or fitting curvature are the same for toric-base lenses with spherical front surface; or bitoric lenses (i.e. toric-base with toric-front surface)

• Two options (saddle fit & low toric simulation) to design the back (i.e. rear) surface curvature in these designs

Saddle fit design• i.e. full alignment of the principle corneal meridians; fit

on-K for each meridian

makes the lens tight; so smaller lens is recommended

• Saddle fit is best used with corneal toricity of =< 2.50D; in cases where toric lenses is superior to spherical lenses

• OAD : 8.5-9.00mm• OZD: 7.2-7.6mm

Low toric simulation• Based on under-correction of the corneal cyl.

produce fluorescein pattern aligning the flat corneal meridian but allows for mild clearance in the steeper meridian

• i.e. the flat meridian of the lens should be fitted on k or within ±0.25D of the flat K. the steeper meridian is under-corrected (fit flatter than the steep corneal meridian) by 1/3 of the difference between the major corneal meridians.

• For this design OAD should be large enough; because the fitting pattern is not tight as in saddle fit design.. OAD/OZD= (9.0-9.4mm/7.6-8.0mm); a typical lens has

OAD/OZD of 9.2/7.8mm

Peripheral curve radii (PCRs)• Wether to choose a spherical or an aspherical PCRs is a

controversial however for ease of use follow this philosophy:

• Use spherical PCRs when designing saddle fit & when corneal toricity is < 3.00D; because it will has smaller diameters

• Use toric (aspherical) PCRs if larger diameter & if corneal toricity is >= 3.00D as in low toric simulation design= this is to prevent ovalization of the optic zone….. If this case to be used a tricurve design should be employed (BCR,SCR,PCR); edge lift shouldn’t exceed 0.12mm

• When ordering toric PCRs the same toric difference should be maintained between SCR & PCR principle meridians as it’s the case in the BCR principle meridians

Examples

*This case the lens has toric BCR & toric PCR; the same difference is kept in the PCRs as in BCR: the lens have a circular shaped optic zone

BCR SCR(W) PCR(W) OAD OZD Rx CT

8.0/7.5 9.0/8.5 (0.5) 11.0/10.5 (0.3)

9.2 7.6 -1.00/-4.00 0.17

cont

*This case the lens has toric BCR & spherical PCRs; the lens have an oval-shaped optical zone

• Note that, normally, the pupil size should be round & not oval

BCR SCR(W) PCR(W) OAD OZD Rx CT

8.0/7.5 9.0 (0.4) 11.0 (0.3)

9.0 7.6 -1.00/-4.00 0.17

cont• Back-surface toric (toric-base) lens could

provide excellent alignment to cornea but not necessarily adequate visual acuity; because of inherent properties of this lens e.g. induced cyl. & air cyl (lensmeter readings).

Induced cylinder• Pre-corneal fluid (tear film) refractive index= 1.336• Refractive index of PMMA (plastic)= 1.49• Refractive index of GP= 1.47-1.48• Refractive index of air=1.0• Refractive index of keratometer=1.3375

• Induced cyl. is the unwanted cyl that results from the difference of refractive indices of the lens material & the pre-corneal fluid; which acts as an additional refractive error regardless of the physiologic residual astigmatism….Induced cyl. acts to combined whatever physiologic astigmatism, which may or may not present; thus it affects the visual acuity

cont• The induced cyl is always minus of the same

axis as the flatter principle meridian of the toric-base lens

• Induced cyl equation:Induced cyl= 0.456X corneal cylCorneal cyl= the difference between K readingsn tears – n plastic/ n air- n keratometer= 0.456

cont• The induced cylinder mostly has harmful effects on the

visual acuity; so back-toric lenses should be compensated to counter these effects….. For example by applying a front surface cylinder in addition to the existed back-toric surface leading to bitoric-lens design

• Simply optics of back-surface toric lenses have 1:2:3 ratio== induced cyl may be considered to equal 1 unit; tear surface toricity equals 2 units; the air cylinder equals 3 units

Bitoric lenses• Note that back-toric lenses failed to correct

cyl. Adequately; so we need a front surface cyl to correct the induced cyl in addition to correct any residual astigmatism….

• The power of bitoric lens can be determined by empirical as well as diagnostic calculation:

Bitoric lenses: empirical calculation

• Treat each meridian separately:• Example: K readings 42.00 @ 180/ 46.00 @ 90 Spec. Rx -2.00 -5.00X180

The patient is being fitted with toric lens; it is determined that a lens with toric base curve diopteric equivalent of 42.00/46.00 is required (saddle fit i.e. fitted on K)… Determine the final lens power?

Step 1: adjust each meridian of spec.Rx for the vertex distance Power at the corneal plane is -2.00 -4.50X180 -7.00 -6.50 -2.00 -2.00

Spec Rx Vertex adjusted RX

contStep2: calculate the lacrimal lens by relating the BCRs (in D) to

the K diopteric readings:

46.00 46.00 PL

42.00 - 42.00 PL

“K” Ordered BC lacrimal

lens (in this case saddle fit i.e. on K )

cont• Step3: determine the final bitoric lens prescription (add

the refractive power at the corneal plane to the compensatory powers of the lacrimal lens; in this example the lens fitted on K (saddle fit)in both meridians)

• -6.50 PL -6.50 -2.00 + PL -2.00

Spex Rx Lacrimal lens Final CL RX(vertex adjusted ) compensation

cont• Note that the flat meridian is -2.00& the steep meridian is -6.50

Bitoric lenses: diagnostic calculation

• To determine the final required power of the CL; you can use sphero-cylindrical over-refraction while the back-toric lens on the eye.

However this is a laborious technique.

Spherical power effect (SPE) Bitoric lens

• Has a toric BC; but the induced cylinder that is normally produced on the eye is countered by an equal but opposite (plus cylinder) generated on the front surface of the lens; the axis of the cylinder on the front surface is coincident with the axis of the flatter toric meridian

• The back surface toricity is known; the laboratory knows the amount of the induced cyl & they will compensate to cancel it…..so we talk about optically spherical system………………………………

Cont.• E.g. corneal cyl -4.00D refractive cyl -4.00D at the same axis >>> conventional back-toric lens will produce

poor acuity due to induced cyl>>> with spherical lens the patient would see

will if the effect of lens centration, & flare ignored

>>> SPE bitoric is ideal because it allows the patient to be fitted with toric posterior curvature while producing excellent acuity expected with a spherical optical system

cont• The plus cylinder in this lens continues to

neutralize the effect of induced cyl regardless of the rotational position of the lens; & the rotation of the lens on the cornea doesn’t

change the patient’s vision

Spherical power effect

diagnostic sets • SPE bitoric lenses have simple & elegant

design ; the diagnostic set of these has spherical power set but varies only in the BCR or toricity which makes it easy to apply….; over-refraction can be easily performed

• If you have SPE bitoric in your practice you should follow these steps to order the lens

………cont• Obtain K & refraction• Decide on Saddle fit or low toric simulation for the

patient & select the closest diagnostic set available• Select from the diagnostic set a lens with flat meridian

closest to the patient’s flat K• Evaluate fluorescein pattern (work until there is a

general alignment with adequate peripheral clearance) • Spherical over-refraction (the resulted power should be

added to each meridian)• Vertex adjustment if the spherical overrefraction >

4.00D• Spherocylindrical overrefraction if visual acuity is poor

with spherical overrefraction