the challenge of fitting astigmatic eyes: rigid gas-permeable toric lenses

Contact Lens and Anterior Eye, (Supplement), 22, pp. $2-S13, 1999 © 1999 British Contact Lens Association Printed in Great Britain

THE CHALLENGE OF FITTING ASTIGMATIC EYES: RIGID GAS-PERMEABLE TORIC LENSES

David M. Ruston *

Abstract - - It is generally acknowledged that rigid gas-permeable lenses (RGP) afford superior visual correction of the ametropic eye compared with hydrophilic lenses. However, there are occasions when a spherical rigid lens will not provide an acceptable result in terms offitting or vision on astigmatic eyes. In these instances a toric RGP lens will be required. This paper deals with the definitions, indications, fitting technique, calculations and designs relating to this modality of fitting. Whilst often perceived as a complex area within contact lens practice, the basic optics and fitting oftoric RGP lenses is not overly difficult. A simple empirical means offitting the

lenses will be described which has a high first-fit' success rate.

KEY WORDS: back surface toric, f ront surface toric, bi-toric, ast igmatism, rigid gas-permeable , contact lens

Introduction

T here is little argument amongst contact lens practitioners that rigid gas-permeable (RGP)

lenses generally correct patients' ametropia better than soft contact lenses2 -~ In addition, RGP lenses are associated with a lower incidence of sight-threatening microbial keratitiC -~° than soft contact lenses as well as being more economical They also deliver more oxygen to the cornea during both open and closed eye wearing situations than currently available hydrogels, n-~8 How- ever, whilst a very high percentage of potential patients can be fitted with spherical or aspheric lens forms, there remains a significant percentage who can only attain optimal vision and/or mechanical fitting with a toric lens. ~9-22 Whilst it is not necessary to be fully conversant with the optical theory to design lenses for this important patient group, it is highly desirable to be aware of the concepts involved as this aids greatly in problem solving. In this paper, the definitions and basic optical concepts of toric RGP lenses will be considered in general. Then the different approaches to attaining a satisfactory fitting and visual outcome will be described and illustrated by use of examples and images of RGP lenses on astigmatic eyes. Finally, problem solving, verification and basic modification will be reviewed.

Definitions One needs to be acquainted with several terms to have an understanding of this subject. These are:

Spectacle Refraction: The combination of spherical and cylindrical lenses required to correct the patient's ametropia at a stated vertex distance.

Ocular Refraction: The spectacle refraction modified to be effective at the ocular surface. This is calculated

*BSC FCOptom DCLP FAAO

in both principal below:

meridians by using the expression

F K - - - -

(1 - df)

where K is the ocular refraction (dioptres), F the spectacle refraction (dioptres) and d the vertex distance (metres). It follows that for a myope, both the sphere and cylinder powers are reduced when one considers the ocular refraction compared to the spectacle. An opposite effect applies to hyperopes.

Spectacle astigmatism: cylinder measured in the spectacle plane. It can arise from corneal and/or lenticular astigmatism.

Ocular astigmatism: cylinder measured in the corneal plane after allowance for vertex distance. As already stated, it is lower for myopes and higher for hyperopes.

Corneal astigmatism: this is the astigmatism arising from the cornea only. When measured by the keratometer it can be calculated approximately by using the clinical rule that a 0.1 mm difference in radii corre- sponds to 0.50 D.

Lenticular astigmatism: this is total ocular astigmatism less the corneal astigmatism. When a spherical RGP lens is placed on the eye, the corneal astigmatism is corrected and lenticular astigmatism becomes known as residual astigmatism.

Induced astigmatism: this is the astigmatism which is introduced into the overall optical system when a toric back surface lens is fitted. It arises as a result of the toric lens back surface being in contact with a tear film of differing refractive index. It is defined optically as:

n tears - n lens n tears - n lens BOZR steep BOZR fiat

where BOZR is the back optic zone radius.

$2

THE CHALLENGE OF FITTING ASTIGMATIC EYES: RIGID GAS-PERMEABLE TOPIC LENSES

The refractive index (n) of the tears is 1.336 and a typical value for the refractive index of a RGP material is 1.47, so substituting these values gives:

Induced astigmatism = (1.336- 1.470) 1.336 - 1.470) BOZR steep BOZR flat.

134 134 This reduces to

BOZR steep BOZR fiat

It is very important to realise that induced astigmatism is always opposite in sign to corneal astigmatism.

Incidence of astigmatism Bennett and Rabbetts 23 analysed the incidence of ocular astigmatism in 1984 and Table I shows its distribution in the population.

If one accepts that a toric soft lens is generally required when there is more than 0.75 to 1.00 dioptres of ocular astigmatism, then it is apparent that approximately a third of all potential soft lens fittings ought to be toric. In the case of rigid lenses it is generally considered that a toric lens needs to be fitted on mechanical grounds when the corneal astigmatism exceeds 2.50 to 3.00 dioptres. 19-22 Whilst the figures for ocular astigmatism are only a guide, about 6% of RGP fittings need to be back surface toric.

RGP Toric Lenses: Indications There are just two indications for the use of toric RGP lenses rather than spherical. These are: • Poor mechanical fit with spherical RGP lenses.

This will typically occur with corneai cylinders of more than 2.50 to 3.00 D. A back surface (BS Toric) or bi-toric will be employed.

• Poor optical result with spherical RGP lenses. Here, a spherical lens gives a good fit, but residual astigmatism is present. The lens used will, therefore, have a toric front surface (FS Toric).

Patient Selection Matching the individual patient to the appropriate lens option is always a primary objective in professional contact lens practice. The most important considera- tions are:

1. Wearing time. Toric RGP lenses, rather than toric soft lenses, are ideal for those wanting full-time

wear. Toric soft lenses suffer from a number of disadvantages amongst which are: lower oxygen transmissibility, poorer likely visual outcome, greater ongoing costs and poorer reproducibility.

2. Present Correction. Existing RGP wearers who have poor vision or an inadequate fitting with spherical lenses are also ideal candidates.

3. Hygiene. Where the practitioner has concerns regarding the patient's standards of hygiene or likely compliance with cleaning procedures, RGP lenses are considered a safer option than soft lenses.

4. High Astigmatism. If the cylinder is greater than 2 to 3 D, the availability of toric disposable soft contact lenses (SCLs) is more limited and likely visual success reduced.

5. Difficulties with soft contact lenses. Many patients experience problems such as deposition and desic- cation with SCLs and are good candidates for RGP lenses if the vision with SCLs has also been poor.

Problems induced by spherical RGP lenses on astigmatic corneas The inappropriate fitting of a spherical lens on an eye having a significant degree of astigmatism may result in the following: • Corneal moulding/spectacle blur. The differential

beating effect of a spherical lens on a significantly astigmatic cornea can lead to unintentional astigmatic orthokeratology. Refraction following lens wear may indicate a reduction in the magnitude of the cylinder by as much as 50%. This will lead to spectacle blur and is generally regarded as unac- ceptable.

• The comfort of a RGP lens is reduced when the area of alignment is reduced and the lens movement dynamics compromised.

• It is possible to cause some mid-peripheral mechanical stain along the flattest corneal meridian (see Figure 1).

Table 1. The distribution of astigmatism in the population of the UK

Amount of ocular astigmatism Percentage of total (D) population

0 32.0 0.25 to 0.50 34.6 0.75 to 1.00 17.7 1.25 to 2.00 9.8 2.25 to 3.00 3.8 3.25 to 4.00 1.5 More than 4.00 0.6 Figure 1. Staining of the cornea arising from excessive

pressure along the flattest meridian.

$3

DAVID M. RUSTON

• Poor centration is an inevitable result of a compromised RGP fitting (see Figure 2).

• Lens fexure will often occur if the lens is not fitted in alignment with the flattest corneal meridian. This will lead to unstable vision and only partial correction of the corneal astigmatism? 4

• Three and 9 o'clock fluorescein staining of the cornea is frequently observed in patients having both with- and against-the-rule astigmatism. In each case this can arise due to lack of lens movement due to reduced edge clearance and decentration. It may also arise due to excessive edge clearance along the steepest corneal meridian if this is horizontal. 25

• Residual astigmatism can arise with spherical lenses and be corrected by either a back surface or front surface cylinder.

Selecting the back optic zone radius when fitting spherical lenses on an astigmatic eye As stated above, there is now very good evidence that the best fitting of a spherical lens on an astigmatic eye is close to the flattest keratometry reading. In the sequence of images shown in Figure 3 the same eye is fitted with a range of back optic zone radii from the same h-i-curve fitting set. It can be seen that the best classical 'dumbbell' fit occurs with the radius 7.90 ram. However, on an eye with this degree of astigmatism, it is unlikely that a spherical lens will give a stable fitting showing good movement and centration and therefore a toric back surface will be desirable on mechanical grounds. However, on an eye with this degree of corneal astigmatism (3.50 dioptres) there was no spherical lens of this size that gave a satisfactory dynamic fitting.

Options to improve the physical lens fit There are several possible options to improve the alignment of the lens with the corneal surface. These are:

Reduction of total diameter. Smaller lenses (<9.00 mm) will show less difference in fitting between the principal meridians, ff lens centration is

good and no corneal distortion occurs, then this is an acceptable approach, except that flare may occur with the small back optic zone diameter (BOZD) which is necessary. Aspheric lenses also show improvement in the area of alignment ~6, but both methods can really only be satisfactorily used up to about 3 D of corneal astigmatism. 2°,21 The flattest meridian should generally be fitted to avoid flexure. This will result in poorer performance at more modest levels of against-the-rule compared to with- the-rule astigmatism, due to lens decentration. A toric periphery can be used to avoid the problem of induced astigmatism and yet still improve the peripheral alignment. This will be particularly useful when the corneal cylinder and the ocular cylinder are equal and relatively modest (<3 D) and there is no residual astigmatism. 2°-2a27 A fully aligned toric back surface which is one in which the two principal meridians are fitted in close alignment. This is the method of choice. A toric back surface where both meridians are not in alignment. This 'compromise' arrangement is usual- ly used where the amount of astigmatism induced by the toric back surface needs to be controlled.

How to fit spherical back optic zone/toric peripheral zone RGP lenses This type of lens is only indicated when use of a toric back surface would introduce unwanted induced astigmatism into the overall system and reduce the quality of the visual result. Indications are when the ocular astigmatism and corneal astigmatism are approximately equal and the magnitude of the latter is relatively modest (<3.00 D). The stages of the fitting process are:

Insert a spherical RGP trial lens to give alignment fit along the flattest corneal meridian. Assess fitting with fluorescein in the normal way, making a judgement as to how flat the fit is along the periphery in the steeper meridian. The peripheral design is then steepened to reduce the corneal clearance along the steeper meridian and the final lens ordered.

Example 1: Spectacle Refraction: - 5 .00/ - 2.50 x 180 Back vertex distance=12 mm Ocular Refraction: - 4 .72 / - 2.16 × 180 Keratometry: 7.80 along 180; 7.40 along 90 Trial lens inserted: C3 7.80:7.50/8.65:8.50/10.00:9.50 BVP- 3.00 Over-refraction: - 2.75 sph VA=6/6. Fluorescein assessment: Good alignment fit along horizontal, too flat along vertical. Final Order: Toric Periphery:

Figure 2. Poor centration resulting from the use of a spherical lens on a grossly astigmatic eye.

C3 7.80 : 7.50/8.65 : 8.50/10.00 : 9.50. BVP - 5.75 8.00 9.00

$4

THE CHALLENGE OF FITHNG ASTIGMATIC EYES: RIGID GAS-PERMEABLE TORIC LENSES

i! C

~!~ i ,_:Li::;:Siiiii;i6:i;ili:ii:iFi:! iii̧ ¸̧ i̧:̧ ?̧̧ ¸̧ ¸̧

b d

Figure 3. A series of tri-curve RGP lenses fitted to an eye having keratometry readings of 8.15 x 7. 45 mm. (a) BOZR 7. 60 mm. Note gross central clearance and mid-peripheral bearing and inferior centration (b) BOZR 7. 75 mm. Note hard mid-peripheral bearing along horizontal and improved centration. Central clearance still excessive. (c) BOZR 7.90 mm. Classical 'steeper than flattest k by one third the degree of astigmatism' type fit. Centration is improved but central clearance increases chances of lens flexure. (d) BOZR 8.05 mm. Improved alignment horizontally and better 'dumbbell' pattern but superior decentration. Static fitting is acceptable, but dynamic fi t will show unstable movement and lens positioning. (e) BOZR 8.20 mm. Flatter fitting has resulted in a lens which is even more unstable and shows grossly excessive clearance along the vertical meridian. Decentration is worsened.

$5

DAVID M. RUSTON

Determination of the back peripheral radii (a) Axial edge lift tables are used to determine what the

peripheral curves for each of the principal meridians would be and then these are combined as shown above. Back peripheral radii (bold print below) giving equal axial edge lifts of 0.14 mm at the total diameter in each meridian are:

7.80:7.50/8.65:8.50/10.00:9.50 7.40:7/50/8.00:8.50/9.00:9.50

(b) Use of traditional 'fit x mm flatter than keratometry (K)' methods in each meridian. So that the following flattening factors are applied in each meridian:

First back peripheral curve = K+0.6 mm Second back peripheral curve = K+l.4 mm Third back peripheral curve = K+2.6 mm

(c) Potentially the best method is the use of a computer program. A 'guestimate' has to be made as to the corneal eccentricity and apical radius, unless this can be measured by use of a videokeratoscope. The computer can calculate the exact back peripheral curves needed to produce a given degree of corneal clearance along each meridian.

Tips for toric back peripheral zone RGP lenses Experience indicates that the following guidelines may be useful: • The difference in the peripheral curves ought to be

at least 0.6 mm to help stabilise the lens. • It is recommended that there is at least a 2 mm

difference between the BOZD and the TD, so that there is enough room for the toric periphery to be able to 'lock onto' the correct orientation.

• It is possible that lenses can stabilise 90 ° off axis. This will have no visual affect, but can reduce comfort!

It has been noted 27 that there are manufacturing and reproducibility problems inherent in toric periphery lenses. This theoretical analysis indicates that, in many typical orders, it is impossible to manufacture the lens requested due to the steeper radii producing an optic zone bigger than the total diameter of the lens. The laboratory modify the radii thus reducing the potential for corneal alignment and leading to variations each time the lens is ordered. Unfortunately, it is not possible in practice to check the toric peripheral curves, although the BOZD should appear oval in all toxic periphery lenses.

The advantages of toric periphery lenses are: • There is no optical complication arising from the use

of a toric optic zone. • A wider range of adjustments are possible after

manufacture (see later). • There may be a cost saving over full toric lenses.

The disadvantages of toric periphery lenses are: • They are difficult to manufacture and the ordered

curves may have to be altered by the laboratory. • They are difficult to check. • The lens may not 'lock on' and stabilise easily.

• They are only useful for modest degrees of corneal astigmatism.

Types of toric back optic zones The jargon surrounding the description of toric RGP lenses can be confusing for the beginner. For complete- ness it is set out below:

• Back surface toric (BS Toric). These lenses have a toric back surface and a spherical front surface.

• Parallel bi-torics. These lenses have a toric surface on front and back surfaces with both cylinder axes being parallel.

• Oblique bi-torics. The two cylinder axes are not parallel. They are extremely difficult to manufacture and are rarely required. Generally, if the ocular cylinder is less than 20 ° different in orientation to the corneal cylinder, an oblique bi-toric will not be required. No further mention of this type of lens will be made in this paper.

• Compensated bi-toric. In this type the front surface (FS Toric) cylinder only compensates for the astigmatism induced by the back surface. Any rotation will not lead to a reduction in visual acuity.

• Full bi-toric. Here, the front surface cylinder compensates for both induced and residual astigmatism. Any rotation will lead to some reduction in visual acuity.

However, whilst these terms can cause confusion, in practice the design of toric RGP lenses is relatively straight forward as will be shown below.

Toric Optic Zones - deriving the radii There are three possible methods: The empirical method. In this technique no trial lens is employed. Instead, the lens is designed by consideration of the refractive error and keratometry readings. The lens produced is then treated as a trial lens and it is modified if a perfect result is not obtained on lens dispensing or first after-care. Experience suggests that this is a very successful technique, with little need for remaking lenses? ° The patient does not receive the negative impression given by wearing an uncomfortable spherical trial lens. However, there are obvious limitations imposed by the less than perfect correlation that exists between keratometry readings and the appropriate BOZR to achieve alignment and the difficulties in accurately estimating the surface powers in very high ametropia.

Use of a spherical trial lens. A trial lens is inserted to give alignment along the flattest meridian. An over- refraction is then performed to establish the power required along that meridian. The power required along the steeper meridian is then calculated by consideration of the residual astigmatism with the spherical lens and the induced astigmatism created by the derived toric back surface. This technique im- proves ~the likelihood that the lens will provide good 'first-fit' success, but the patient receives an unfavour- able impression of RGP lenses if the fit is poor and the

$6


over-refraction may be unstable if there is profuse lacrimation.

Toric trial lenses. This is an excellent method, but requires a relatively large fitting set and more calculation than the empirical approach. It is the method of choice in a contact lens specialist practice, but is unlikely to be used in the average situation.

In this paper, the empirical method will be used to demonstra te the ease with which lenses can be designed.

Fitting of Tor ic Optic Zones by empir ica l m e t h o d A toric back surface will generally become necessary as the corneal astigmatism exceeds 3.00 dioptres or so. No trial lens is used, but a lens is ordered which is designed to fit on alignment with both the steepest and flattest meridians. For BOZDs between 7.00 and 7.40, each radius is fitted 0.05 mm flatter than the keratometry reading. For BOZDs between 7.40 and 8.00, each back optic zone radius is fitted 0.10 nun flatter than the kera tometry reading to give alignment. The total diameter is generally 1.50 to 2.00 mm smaller than the horizontal visible iris diameter (HVID). The ocular prescription is used to calculate the back vertex power along each lens meridian, k is important to realise that in this method the lens will be fitted so as to align along both of the principal corneal meridians and should appear like a spherical lens on a spherical eye. Without more complicated analysis the practitioner will not be aware whether a bi-toric lens is actually required or not.

E x a m p l e 2: Spectacle Refraction: + 2 . 5 0 / - 3.50 x 15 Back vertex distance=10 mm Ocular Refraction: +2 .58 / -3 .57 x 15 Keratometry: 8.10 along 15; 7.40 along 105. HV[D: 11.50 mm

BVP + 3.00 along 8.20

- 0.50 along 7.50

This is all one needs to do to specify the lens. The peripheral curves are derived as for the toric periphery lens described above. Figure 4 shows the same patient as in Figure 3 fitted with a toric back surface lens, which is fitted in alignment in both meridians. There is an even band of mid-peripheral alignment and, were it not for the presence of front surface fluorescein, there would be the same appearance as a spherical lens on a spherical eye.

It is not necessary to know whether the lens is a back surface toric or bi-toric in this simple method. It is the laboratory's responsibility to determine that the lens is manufactured correctly so that when the practitioner measures the back surface radii on the Radiuscope and the BVPs along the two meridians, they conform to current standards. However, for complete understanding of the optical principles involved, the following calculations are given: 1. Calculate astigmatic effect of lens back surface:

470 470 5.25D

8.20 7.50 -

.

.

Determine unmodified BVPs:

+ 3.00 along flat

- 2.25 (+3.00 - 5.25) along steep

Calculate induced astigmatism:

134 134 - - - 1.54D

8.20 7.50 (induced cyclinder is always opposite

corneal cylinder so it is - 1.54 x axis 7.50)

First of all, decide on the back optic zone radii to be ordered. For a TD of 9.60, use a BOZD of 7.50. As described above, fit each meridian 0.1 mm flatter than keratometry to give alignment. Consideration of the powers required along each meridian yields:

- 1.00 ~ - 0.50

+2.50 Fitted 0.1mm flatter along each meridian

+3.00

By virtue of the fact that we are fitting 0.1 mm flatter than the keratometry reading in each meridian, we will produce a tear lens having a power of approximately - 0 . 5 0 D. Therefore, the power that the lens needs to provide is reduced by -0 .50 or increased by +0.50 D.

Final order : C3 8.20 : 7.50/9.30 : 8.60/10.85 : 9.60

7.50 8.30 8.30

Figure 4. Toric BOZRs of 8.20× 7.50 shows even mid- peripheral alignment and good centration. But for the front surface fluorescein, fitting pattern would be as a spherical lens on a spherical eye. Engraved dots were used for demonstration purposes to show stability of fitting.

$7

DAVID M. RUSTON

. Calculate residual astigmatism:

= ocular astigmatism - corneal astigmatism

= 3 . 5 7 - 3.94 = -0.37 (× 7.50)

. Calculate front surface cylinder required to neutra- rise the cylinder produced in step 4 above:

= 1.54 + 0.37) = 1.75D (x 8.20)

6. Quick check on calculation:

Surface powers required :

along flattest + 3.00

along steepest - 0.50(-0.25 + 1.75)

Thus the laboratory will produce a bi-toric lens which will have a cylinder on the front of 1.75 dioptres, which when placed on the focimeter will have back vertex powers of +3.00 and -0.50. Providing that the back surface radii and the back vertex powers check correctly, then the front surface cylinder must be correct.

T i p s f o r a l i g n e d b a c k s u r f a c e t o r i c R G P l e n s e s

• A back surface toric RGP lens over-corrects the corneal astigmatism due to induced astigmatism.

• A front surface cylinder is generally required to correct the induced astigmatism and any residual astigmatism that may be present.

• No prism is required to stabilise the lens, but if it does rotate then vision will only be affected if there is a correction incorporated for residual astigmatism.

• In the majority of cases designing the lens by the empirical method works well.

cylinder is thereby avoided. The procedure is as follows: • Calculate the amount of induced astigmatism that

would arise if the lens back surface were fitted in close alignment along both meridians.

• Then calculate the residual astigmatism. • The steeper BOZR is then flattened in order to

reduce the induced astigmatism to such a level that it neu-tralises or nearly neutralises the residual ast igmatism (i.e. be of equal magnitude and opposite in sign).

This is best explained by way of an example:

E x a m p l e 3 :

Spectacle Refraction: +2 .00 / - 5.25 x 90 Back vertex distance=10 mm Ocular Refraction: +1 .95 / - 5.10 x 90 Keratometry: 8.30 al 90; 7.50 al 180. HVID: 11.50 mm

. First calculate induced astigmation for

8.35: 7.30/9.50: 7.55

134 134 - - 1.70

8.35 7.55

(ocular cylinder is against-the-rule, so induced astigmatism is with-the-rnle)

2. Calculate corneal astigmatism:

337 337 ¢.33 × 90

8.30 7.50

Advantages for aligned back surface toric RGP lenses 3. • Close alignment in both principal meridians ensures

best optimum dynamic fitting and even bearing relationship to cornea.

• Improved centrafion characteristics. • Rotation is unlikely to occur if corneal astigmatism

is significant. 4. • Easy design process with empirical fitting procedure.

The disadvantages of this technique are: • Bi-toric lenses will frequently be manufactured

which will increase costs. • If the front surface cylinder is not carefully worked,

than the wettability may be reduced. 5. • There are fewer modifications possible post-manu-

facture.

Then calculate residual astigmatism:

Residual astigmatism

= ocular astigmatism - corneal astigmatism

= 5.10 - 4.33 = 0.75 D(x90)

The induced and residual astigmatism are of opposite sign but are not equal. Assume patient can tolerate uncorrected astigmatism of 0.50 D. Therefore, we need the induced astigmatism to be no more than 1.25 D.

Try calculating induced astigmatism for

8.30 7.70

Compromise back surface toric lenses This type of lens can only be used when the ocular cylinder is greater than the corneal cylinder. This results in residual astigmatism that is opposite in sign to the induced astigmatism. The idea is to adjust the fitting so that the induced astigmatism negates the residual astigmatism. A front surface

134 134 - - - - -1.26

8.30 7.70

. Thus there is now likely to be a small (0.50 D) amount of astigmatism left uncorrected when the final lens is worn. This will generally be tolerated by the patient.

$8

THE CHALLENGE OF FITFING ASTIGMATIC EYES: RIGID GAS-PERMEABLE TORIC LENSES

Final order :

C3 8.30 : 7.50/9.30 : 8.60/10.85 : 9.60

7.80 8.30 9.30 + 2.00 along 8.30

The advantages of this technique are: • No toric front surface is used, which reduces overall

thickness and cost. In addition, more modifications are possible (see later).

• Arguably, the lens wettability will be better, although this should really only hold true if the cylinder is polished on by hand (see later).

The disadvantages are: • The fitting may be compromised too much, redu-

cing the benefits of a toric back surface. • If the difference in radii is less than 0.5 mm the lens

may rotate, reducing vision. Prism ballast may then be required, negat ing the advantages of the technique.

• More calculations are needed to properly access the relationship between residual and induced astigmatism.

Figure 5 illustrates the possibilities which can arise when the fitting of eyes having significant degrees of corneal astigmatism is considered. As can be seen the compromise or back surface toric form is only appropriate when the degree of ocular astigmatism exceeds corneal astigmatism. This implies that the residual and induced astigmatism will be opposite, but not necessa- rily equal.

• Decide on degree of stabilisation required. A typical amount is 1.5 A base down. Truncation can be added later if necessary. It is wise to ask the laboratory to 'dot' the prism base so that the final axis of stabilisation can be recorded, as for soft toxic lenses.

• Write the final order, remembering to include cylinder and an allowance for rotation. This is typically taken to be 5 ° degrees nasal as with soft torics.

Example 4: Spectacle Refraction: R: - 2 . 0 0 / - 2.00 × 75. Back vertex distance 12 mm Ocular Refraction: R: - 1 . 9 5 / - 1.87 × 75 Keratometry: 8.10 along 75, 8.00 along 165. Insert trial lens of 8.10:7.30/9.20 BVP-3.00 Perform over-refraction: + 1 . 0 0 / - 1.25 × 75 Final order: C3 8.10:7.30/9.20 AEL 0.12 B V P - 2 . 0 0 / - 1.25 × 70 1.5 A base 270 (down). Dot prism base.

Tips for front surface toric RGP lenses • Prism ballast is normally enough to stabilise the

lens. • Truncation can be added later if required. • If a high FS cylinder is required (> 2.00 D) then use

2 A ballast. • In view of greater thickness involved, use high

oxygen permeability materials.

RGP torics to improve visual acuity This implies the use of a front surface toric. This will be necessary when a spherical lens gives a good fitting, but there is significant residual astigmatism. The fitting approach is as follows: • Insert trial lens to fit flattest meridian. • Assess fitting and over-refract in normal manner.

Figure 6 illustrates the use of RGP torics in the correction of astigmatism.

Advantages of front surface torics: • Superior vision than spherical lenses • Generally better vision than soft toric lenses • Verification easier than soft toric lenses.

~IOrTI~ES

.... ] I FIT SI~JakLL

SPI~IERICAT_, OR ASP~i~L[C

. . . . . .

, , . , , , , , ' , [ J , , , ,

FIT XK)RIC PERIPHERY RGP LENS

i -I AS TIG~v~&TIS~vf ]

[ >2 D ]

OCLrLAR C YL ! E Q U A L S 1 C ORN~zXi, CY-L

,

Gv i

• . [ " ,] . . . . . . . . . . .

I ; i . 5o P °V S

~ IYI~ C Y L <

C OR/~Ei~ CYL

B :!omC 1

OCL~_~R CYL }

> l C O R N E A L C Y L

, . . . . , , , , . . , | - .

! I ,

CONSIDER BS 7X)R/C

Figure 5. Flow diagram illustrating the options available when fitting eyes having corneal astigmatism greater than 2.00 dioptres.

$9

DAVID M. RUS]?ON

N O O C U L A R CYL B U T C O R N E A L C Y L P R E S E N T

I , I ,I . . . . . . .

S O F T FS T O R I C S P H E R I C A L R G P

R E S I D U A L J A S T I G M A T I S M

I I

C O R N E A L CYL LESS THAN 2.00D B U T

>OR < ocuL CYL ' 1

[ N g ~ e 6 .

CORNEAl. , C Y L >2-3D BUT > O C U L A R C Y L

,, [ ....

BI-TOmC RGP

Flow diagram illustrating the options available when fitting eyes having residual astigmatism.

Disadvantages of front surface torics: • Greater thickness and weight reduces comfort • Mislocation of prism base reduces vision • Inferior location can increase flare

Problem solving toric l enses There are three types of problems: physiological, fitting and visual.

Physiological problems These are really no different to those which may occur with spherical RGP lenses. In view of the greater thickness involved, it is wise to use materials of higher oxygen permeability.

Fit t ing problems A frequent p rob lem that ar ises is tha t of lens awareness. Generally thinner lenses are more comfor- table, so a reduction in overall bulk by careful selection of the BOZD is helpful. Where the discomfort occurs in a patient wearing a well-fitted front surface toric, then it is best to use a soft toric instead. In view of the greater bulk of these lenses, dropping and inferior positioning can be a problem. Possible solutions are an increase in lens d iameter and overall alignment, coupled with a reduction in overall bulk and a negative carrier edge form. Where the empirical method is used, the quality of the original keratometry readings is important. In the event of poor alignment of the keratometer, unusual topography or poor manufacture, the final fitting may be incorrect and therefore require modification.

Visual problems If the lens is a front surface toric and rotates from its intended position, then the solution is exactly as for soft torics.2t If it is a bi-toric, then, as described earlier, there will be a loss of acuity with rotation if the front surface cylinder is required to eliminate residual as well as induced astigmatism. Stabilisation will be encouraged when the back surface closely aligns with the principal meridians of the cornea.

The quality of manufacture is paramount. A crisp

Table 2. Dimensional and optical tolerances for RGP toric lenses after Hough (1998)

Dimension~Power Tolerance

Toric back optic zone radii: difference between BOZRs < or =0.20 mm _+ 0.05 mm difference between BOZRs >0.20 _+ 0.06 mm

but < or =0.40 mm difference between BOZRs >0.40 _+ 0.07 mm

but < or =0.60 mm difference between BOZRs >0.60 mm _+ 0.09 mm

Back optic zone diameter _+ 0.20 mm Total diameter _+ 0.10 mm Front optic zone diameter _+ 0.20 mm Centre thickness _+ 0.02 mm Back vertex power 0 to +_5.00 D +0.12 D

(in weaker meridian) Back vertex power over _+ 5.00 to + 10.00 D + 0.18 D

(in weaker meridian) Back vertex power over _+ 10.00 to _+ 15.00 D +_ 0.25 D

(in weaker meridian) Back vertex power over + 15.00 to _+ 20.00 D + 0.37 D

(in weaker meridian) Back vertex power over _+ 20.00 D _+ 0.50 D

(in weaker meridian) Prescribed prism _+ 0.25A Cylinder power up to 2.00 D + 0.25 D Cylinder power over 2.00-4.00 D _+0.37 D Cylinder power over 4.00 D _+0.50 D Cylinder axis __ 5 °

endpoint in both meridians should be achieved for power and radius. Providing that the appropriate thicknesses have been specified and the fitting is close alignment, lens flexure should not be a problem. If the front surface cylinder has been produced by hand polishing, then there are potential problems with greasing on the surface secondary to poor wetting. It is wise to select a manufacturer who uses a tofic generator or crimps both surfaces (see later).

When is a trial toric lens necessary? In the event that the cornea is distorted, perhaps following surgery or long-term PMMA lens usage, then

$10

THE CHALLENGE OF FITI'ING ASTIGMATIC EYES: RIGID GAS-PERMEABLE TOPIC LENSES

Table 3. Modifications which can be made following manufactures of different types of toric rigid lenses.

Modification Toric periphery Bi-toric Back surface toric Front surface toric

Reduce total diameter or back optic zone diameter Back vertex power +0.75 to - 1.50 D Back vertex power along flat radius up to - 1.50 D Reduce edge thickness Add front surface cylinder Add spherical back peripheral curves Blend peripheral curves Truncate or increase truncation Polish out scratches

Yes Yes Yes Yes Yes Care! Yes Care! N/A Yes Yes Yes Yes Yes Yes Yes N/A N/A Care! N/A Yes Yes Yes Yes

Care! Care! Care! Care! Yes Care! Care! Care! Yes Care! Care! Care!

'N/A' signifies not applicable and 'Care!' signifies that special care is needed.

a trial lens will be more likely to yield a sound final fitting than reliance on keratometry readings alone. Similarly if the refractive endpoint is imprecise or if the ametropia is particularly high, then a trial lens is advised.

If measurement of the keratometry readings and cylinder axes indicate that an oblique bi-toric is necessary, use of a toric trial lens can often be helpful in ruling out inappropriate cases.

Manufacture There are three different techniques involved in the production of toric RGP contact lenses: • Crimping technique • Toric generating lathes • Hand polishing

The crimping technique is a relatively cheap way of manufacturing a toric surface with relatively tow financial outlays. Providing that the technician using the crimping device is skilled, very good results are attainable. Toric generating lathes are expensive, but require less skill in operation. The final lenses are difficult to polish and the polishing action may alter the surface curvature. Hand polishing is now almost obsolete. It is too reliant on technical expertise and associated with poor wetting of the lens front surface.

Verification BOZRs are checked on the optical microspherometer (e.g. Radiuscope) with the two principal meridians being treated as two distinct spherical lenses. The BVPs are measured in the same manner, the powers along the principal meridians being recorded. Total diameter is measured exactly as for a spherical lens, except that where the lens is truncated two diameters will be recorded, the difference between the two being the amount of truncation. The BOZD is again measured as per a spherical lens, but in the case of a toric periphery lens there will be an oval BOZD and the shorter diameter should be measured. Centre thickness and edge thickness are measured as for spherical lenses.

The British/ISO Standards applicable to RGP lenses have recently been reviewedY 9 Those applicable to toric RGP lenses are shown in Table 2.

Modifications possible following manufacture Table 3 illustrates the possibilities in terms of lens adjustment that can be undertaken post manufacture. It is essential that the laboratory undertaking these adjustments is very familiar with both toric lens manufacture and modification. Perhaps the most important learning from Table 3 is that, with the exception of peripheral toric geometries, the polishing of all forms of toric lenses requires great care and patients should be advised accordingly.

Key Points • Spherical RGP lenses cannot always provide both

optimal fitting and total visual correction. • Toric RGP lenses can be fitted without need for

complex calculations nor fitting sets. • A toric back surface lens is often required to

improve the physical fitting of a spherical RGP lens. • A toric front surface may be required to improve the

visual acuity where residual astigmatism is present. • Toric RGP lenses are thicker than spherical lenses

and therefore a higher Dk material ought to be used than for spherical lenses.

• Toric RGP lenses are less amenable to modification following manufacture.

Acknowledgements The author is indebted to Mr John Meyler for many years of collaboration on this topic and on whose published work much of the above is based; to Dr Trusit Dave for his great help in capturing the colour images reproduced here in Figures 4 and 5 and to Mr Nigel Burnett Hodd for the use of his video slit lamp.

Address for Correspondence DM Ruston, 7 Devonshire Street, London WIN 1FT.

REFERENCES

1 Bennett, A.G. Optics of contact lenses, 5th edn, Association of Dispensing Opticians, London (1985).

$11

DAVID M. RUSTON

2 Douthwaite, W.A. Contact Lens Optics and Lens Design, Butter- worth-Heinemann, Oxford (1995).

3 Pearson, R.M. The clinical performance of hard gas-permeable lenses. Clin. Exp. Optom., 69, 98-102 (1986).

4 Sarver, M.D. Vision with hydrophilic contact lenses, f Am. Optom. Assoc., 43, 316-320 (1972).

5 Wechsler, S. Visual acuity in hard and soft contact lens wearers: a comparison. J. Am. Optom. Assoc., 49, 251-256 (1978).

6 Poggio, E.C., Glynn, P~J. and Schein, O.D. The incidence of ulcerative keratitis among users of daily wear and extended wear soft contact lenses. N. Eng. Med. f , 3 2 1 , 7 7 9 - 7 8 3 (1989).

7 Mathews, T.D., Frazer, D.G., Minassian, D.C., Radford, C.F. and Dart, J.K. Risks of keratitis and patterns of use with disposable contact lenses. Arch. OphthalmoI., 110, 1550-1562 (1992).

8 Guillon, M., Guillon, J-P., Bansal, M. Incidence of ulcers with conventional and disposable daily soft contact lenses . f Br. Contact Lens Assoc., 17, 69-74 (1994).

9 Benjamin, W.J. Risks and incidences of ulcerative keratitis. J. Br. Contact Lens Assoc., 15, 143-147 (1992).

lO Buehler, P.O., Schein, O.D. and Stamler, J.F. The increased risks of ulcerative keratitis among disposable soft lens users. Arch. Ophthal., 110, 1555-1558 (1992).

11 Holden, B.A. and Mertz, G. Critical oxygen levels to avoid corneal oedema for daily and extended wear contact lenses. Invest. Ophthalmol. Vis. Sci., 25, 1161-1167 (1984).

12 Holden, B., Mertz, G., McNally, J. Corneal swelling response to contact lenses worn under extended wear conditions. Invest. Ophthalmol. Vis. Sci., 24, 218-226 (1983)

13 Form, D. and Holden, B. Rigid gas-permeable versus hydrogel contact lenses for extended wear. Am. J. Optom. Physiol. Opt., 65, 536- 544 (1988).

14 Tranoudis, I. and Efron, N. Oxygen permeability of rigid contact lens materials, f Br. Contact Lens Assoc., 18, 49-54 (1995).

15 Fatt, I. and Chaston, J. Measurement of oxygen transmissibility and permeability of hydrogel lenses and materials. Int. Contact Lens Clin., 9, 76-88 (1982).

16 Fat, I. and Ruben, C.M. Oxygen permeability of contact lens materials: a 1993 update. J. Br. Contact Lens Assoc., 17, 11-18 (1994).

17 Eft-on, N. Understanding oxygen: Dk/1, EOP, oedema. J. Br. Contact Lens Assoc., 14, 65-69 (1991).

18 Efton, N. and Ang, J.H.B. Corneal hypoxia and hypercapnia during contact lens wear. Optom. Vis. Sci., 67, 512-521 (1990).

19 Lowther, G.E. Toric RGPs: Should they be used more often? Int. Contact Lens Clin., 17, 260-261 (1990).

2o Meyler, J. and Ruston, D. Toric RGP lenses made simple. Optician, 209 , (5504): 30-35 (1995).

21 Ruston D. Toric RGP fitting: a simplified approach. Br. ]. Optom. and Dispensing, 1,146-150 and 187-196 (1993).

22 Lindsay, R. and Westerhout, D. Toric Contact Lens Fitting. Contact Lenses. t?;d Phillips and Speedwell, 4th edn, Butterworth Heine mann, London (1997).

23 Bennett, A~G. and Rabbetts, R.B. Distribution and ocular dioplrics of ametropia. Clinical Visual Optics, 1st edition, pp 433-444. Butterworth Heinemann, London, (1984).

24 Stone, J. and Collins, C. Flexure of gas permeable lenses on toroidal corneas. Optician, 188, 8 -10 (1982).

25 Jones, L. and Jones, D. Photoflle part eight: 3&9 o'clock staining. Optician, 210 , (5526): 20-22 (1995).

26 Edwards, IC and Hough, T. Q&A: fitting the toric cornea. Optician, 217 , 38-39 (1999).

27 Edwards, BL and Hough, T. Toric periphery rigid lenses. J. Br. Contact Lens Assoc., 17, 103-114 (1997).

28 Veys, J. and Davies, I. Basic Contact Lens Practice: Soft toric contact lens fitting. Optician, 210, (5522): 22-28 (1995).

29 Hough, D.A. Contact lens standards. Contact Lens & Anterior Eye (Suppl) 21, $41-$45 (1998).

MULTIPLE CHOICE QUESTIONS

1. Which of the following is NOT one of the possible complications that may arise following wear of a non-toroidal RGP lens on an eye have corneal astigmatism greater than three dioptres? (a) Reduction in the cylindrical power of the spectacle refraction measured following wear. (b) Corneal staining associated with bearing along the flattest corneal meridian. (c) Induced astigmatism due to an astigmatic tear lens (d) Poor eentration (e) Residual astigmatism due to ocular and corneal astigmatism not being equal

2. Toric Periphery lenses are indicated when: (a) The ocular astigmatism is equal to the corneal astigmatism (b) The peripheral cornea is more astigmatic than the centre (c) There is significant residual astigmatism with a spherical lens (d) Both a and b (e) Both a, b and c.

3. To design a fully aligning RGP toric lens that corrects all the ocular astigmatism, the minimum we need to know is: (a) The ocular refraction, keratometry readings and the corneal size (b) The spectacle refraction, keratometry readings, the corneal size and the amount of residual astigmatism with a spherical lens (c) The spectacle refraction, back vertex distance, keratometry readings, the corneal size, the amount of residual astigmatism with a spherical lens and the specification of the best fitting trial lens (d) The best fitting toric lens specification (e) a and e.

$12


. Induced astigmatism is: (a) The astigmatism that remains when all the corneal astigmatism is corrected (b) Residual astigmatism less ocular astigmatism (c) Ocular astigmatism less corneal astigmatism (d) Always opposite in sign to the corneal astigmatism (e) Left uncorrected in a compensated bi-toric.

5. A compromise fitting back surface toric RGP lens will only be visually successful when: (a) The ocular astigmatism is greater than the corneal Co) The corneal astigmatism is greater than the ocular (c) The residual astigmatism and induced astigmatism are exactly the same (d) There is less than 3 dioptres of corneal cylinder (e) The patients ocular cylinder is roughly equivalent to the spherical component.

. From surface toric, back surface spherical RGP lenses need: (a) Prism ballast to make the lens centre (b) To be made of low Dk material (c) Truncation to enable the lens to orientate correctly (d) A special fitting set to enable orders to be made (e) To be ordered with due compensation for the anticipated rotation in the eye.

S13

the challenge of fitting astigmatic eyes: rigid gas-permeable toric lenses

Documents