A review of rigid lens design

Contact Lens and Anterior Eye (2000) 23, 106-111 © 2000 British Contact Lens Association

www.nature.com/clae

Guest Editorial A REVIEW OF RIGID LENS DESIGN

Keith Edwards*

Introduction

W ithin the United Kingdom, one might be forgiven for believing that the future of rigid gas perme-

able (RGP) lenses may be limited. The call for all lenses to be used for single patient use ~,2 has apparently led to a reduction in the prescribing of rigid lenses within the UK from 14% of new fits in 19993 to 6% in 2000. 4

Nevertheless, rigid gas permeable lenses offer significant benefits for many patients and there is continued application of resources by manufacturers to develop new polymers for these lenses to improve designs for a range of normal and compromised eyes.

Fitting Criteria Due to the inflexible nature of rigid lenses, it is necessary to use either multicurve spherical or flattening aspheric surfaces to achieve the desired fitting objectives. These objectives are generally considered to be near alignment over the back optic zone diameter (BOZD) of the lens with increasing clearance towards the lens periphery. The flattening periphery should result in edge clearance between the lens and cornea sufficient to promote free movement of the lens over the peripheral cornea in secondary positions of gaze and to permit lens removal with the lids.

As knowledge of the average corneal topography has increased, it has led to refined corneal models being developed that simplify the corneal shape to a flattening ellipsoid specified by apical radius (ro) and the rate of flattening expressed either as p-value or eccentricity (e- value). Recent corneal models have shown that the average cornea is flatter in the horizontal meridian than the steeper meridian 5,~ with no significant differences between right and left eyes of the same patients but with some difference between males and females 6, males having flatter eyes.

Having established the dimensions of the average cornea, lens designs can be developed using compu- terised models and simulated fluorescein patterns. System lenses may employ central and peripheral specifications that give constant apical clearance and constant axial edge clearance across a wide range of

*BSc FCOptom DCLP FAAO, British Contact Lens Association Gold Medallist, 1999.

corneal radii provided that the topography conforms to the model being used. Figure 1 shows a simulation of a lens on the eye with the clearances identified and a tear layer profile shown. Tear layer profiles identify the clearance between the lens and the cornea at each point across the lens back surface for any combination of lens geometry and corneal model.

Changes in Peripheral Designs When polymethylmethacrylate (PMMA) was the only lens material available, peripheral clearance had to maximise the tears exchange under the lens, since this was the sole source of oxygen for the cornea. As materials of increasing oxygen permeability were developed, decreased clearance was acceptable since the function of tears exchange was limited to debris removal, with oxygen transmitted through the lens being adequate for corneal physiology. In comparison to early PMMA lens designs, modern RGP lenses often employ larger back optic zone and total diameters and produce less axial edge clearance.

With PMMA lens designs, a typical specification would often utilise set increments of peripheral flattening from the back optic zone radius (BOZR) such as:

BOZR : 7.00/BOZR + 0.50ram :

8.00/10.00 : 8.50/12.25 : 9.007

Distance from centre iD ITIm

Figure 1. Representation of rigid lens on the eye showing tear layerprofile and the apical tear layer thickness (ATLT) and axial edge clearance (AEC).

A review of rigid lens design K Edwards

These lenses showed variations in lens fitting on the eye and both Stone 8 and Rabbetts 9 have published specifications for lenses that have constant axial edge lift (CAEL). Such lenses have a constant difference in primary sag between the BOZR and the peripheral curves at the total diameter for all values of BOZR. While this limits the variation in fluorescein fit at the extremes of the radius range it still does not eliminate it completely. However lenses designed with constant apical clearance (CAC) and constant axial edge clearance (CAEC) could do so.

Table l a shows the specifications of a standard multicurve design, a design with constant axial edge lift and a design with constant apical and axial edge clearance. Table lb shows the edge lifts, apical and edge clearances produced at the extremes and mid- range of lens BOZRs. Standard multicurves show variation in edge lift as well as apical and edge clearances across the range of BOZRs. CAEL lenses show reduced variation in edge clearance, while lenses designed for constant apical and edge clearances show some limited variation in axial edge lift. To obtain constant apical tear layer thickness, variations in BOZD are necessary across the range of BOZRs if a standard increment of 0.05 mm in BOZR is to be maintained. To give CAEL or CAEC, flatter peripheral curves are required for flatter BOZRs.

Aspherie Lenses An alternative to the flattening effect of multi-curve back surfaces is the aspheric back surface geometry.

In its simplest form, a single flattening elliptical curve will more closely follow the corneal contour. It is not possible to have a rate of flattening that is sufficient to create the desired axial edge clearance without disrupt- ing the central fit. As a result, elliptical aspherics incorporate a peripheral spherical curve to produce the desired edge clearance.

In a further development bi-aspheric lens designs have been introduced. Th ese incorporate a fixed elliptical central curve with a flatter peripheral asphere that is often a flatter ellipse or a hyperbola. Even with these designs, a spherical edge curve is often added to refine the edge clearance.

Polynomial designs use an aspheric geometry where the rate of flattening increases with the distance from the lens centre. For each millimetre increase k o m the lens centre the e-value is increased by a specified amount. This produces increasing flattening towards the lens edge, although spherical peripheral curves are still used to enhance the edge clearance.

One theoretical benefit of aspheric lenses is the increased area of alignment between the lens back surface and the eye. This can be quantified using the

107

Table 1. Specification and fitting data for sample rigid lens designs

(a) Lens specification

BOZR BOZD BPR1 BPZD1 BPR2 BPZD2 BPR3 TD

Standard multicurve 1 7.10 2 7.60 3 8.10

Constant axial edge lift 4 7.10 5 7.60 6 8.10

7.00 7.60 8.00 10.00 8.50 12.25 9.00 7.00 8.10 8.00 10.00 8.50 12.25 9.00 7.00 8.60 8.00 10.00 8.50 12.25 9.00

7.00 7.65 7.00 8.29 7.00 8.93

Constant apical clearance and constant axial edge 7 7.10 7.00 7.50 8 7.60 7.40 8.05 9 8.10 7.80 8.70

8.00 8.08 8.50 8.28 9.00 8.00 8.86 8.50 9.15 9.00 8.00 9.67 8.50 10.08 9.00

clearance 8.00 8.15 8.50 9.00 9.00 8.00 9.00 8.50 10.00 9.00 8.00 9.85 8.50 11.25 9.00

(b) Data from lenses shown in (a) (corneal model ro=BOZR, e=0.45)

Axial edge li)? Apical clearance Axial edge clearance Lens BOZR mm (ttm) (ttm) (#m)

1 7.10 0.193 (193) 14 162 2 7.60 0.151 (151) 11 128 3 7.80 0.119 (119) 9 101 4 7.10 0.100 (100) 14 69 5 7.60 0.100 (100) 11 77 6 8.10 0.100 (100) 9 82 7 7.10 0.111 (111) 14 80 8 7.60 0.100 (100) 14 80 9 8.10 0.093 (93) 14 80

Contact Lens and Anterior Eye

A review of rigid lens design I< Edwards

.108 method of establishing the 'percentage area aligned '1° and shows significant improvements in central alignment for aspheric designs over multicurve spherical types} ~ It has also been believed that aspheric lenses may give less peripheral clearance in corneal astigmatism although this is not necessarily the case. 11

Empirical Fitting Given the concerns about multi-patient use of lenses, there is a move towards empirical fitting of RGP contact lenses. With the advanced lens designs based on corneal modelling, system lenses have been developed that should work on a wide range of patients and may be fitted from keratometry data and spectacle refraction corrected for vertex distance. Even with toric lenses fitted in this way, first time success rates in excess of 80% can be achieved (Atkins - personal communica- tion).

In a study of a new system lens design (Edwards and Jones (1991), Bausch and Lomb - in preparation) 51 patients were fitted with a RGP lens according to the manufacturers recommendations. After 6 months 92% of patients continued to wear lenses for more than 8 h per day. All enrolled patients had responded to an adver- tisement calling for study subjects and had signed informed consent forms prior to commencement.

Figure 2 shows the contact lens history of those enrolled. Of the 51 patients entered into the study, two moved from the area before completion and were lost to follow-up. Analysis was completed on the remaining 49 patients. Figure 3 shows the reasons for failure. The two patients (4%) who experienced poor comfort, were both previous soft lens wearers. Nevertheless 85% of soft lens wearers were able to tolerate RGP lens wear. The two patients (4%) who exited for visual reasons, had lenticular astigmatism but the protocol did not permit their exclusion. If the cases of predictable residual astigmatism are ignored then the success rate was 96%. This suggests success rates comparable to soft lens fitting.

Complex Lens Designs Rigid lens designs lend themselves to fitting in cases where corneal topography is more challenging or where the intention is to induce a change in refractive status. For post-refractive surgery and some post-graft cases as well as orthokeratology fitting, so-called reverse geometry lenses can be employed. The essential difference

between conventional lenses and lenses for this type of fitting is that the first peripheral curve (or curves) is (are) steeper than the central curve. In the case of post- surgery fitting this is an attempt to match the new corneal topography while in orthokeratology, it is to create a tears reservoir that can assist in changing the corneal curvatureY

In an attempt to improve the fitting of rigid lenses on post-photorefractive keratectomy (PRK) patients, Ed- wards et aU reported on a unique design that employs a steepening aspheric curve outside the BOZD with a spherical reverse-bevel creating the peripheral clearance. Such a design can be utilised to obviate the apical fitting achieved with conventional RGP designs and results in a fluorescein pattern similar to the keratoco- nus three-point-touch but with alignment over the central corneal area. These designs can also be applied to flat grafts and LASIK cases.

Figure 4 shows the back surface geometry, identify- ing the spherical central zone, the steepening elliptical peripheral zone and the spherical edge bevel. Figure 5 shows how the edge lift is negative for the aspheric curve and positive for the bevel cure, representing steepening and flattening curves relative to the BOZR. However because the peripheral part of the cornea is steeper than the central part this is not reflected in the axial edge clearances. Figure 6 shows the unusual tear layer profile for this type of lens with central alignment, pooling around the BOZD as the lens periphery steepens towards the steeper mid-peripheral cornea and then peripheral clearance in keeping with conventional lenses.

P o o r v i s i o n P o o r c o m f o r t

4 % (2 subjects) 4 % (2 subjects)

S u c c e s s

9 2 %

Figure 3. Success and failure rates for study subjects with reasons for exit.

Previous soft lens wearsrs 25% New to contact lens wear 60%

previous rigid lens wearers 25%

Figure 2. Previous lens wearing history of subjects enrolling in RGP lens study.

__ 4- - - Spherical optic zone - - --~

d# @@~mmmmmmmm~mNNN~k ,

Reverse ('ski') Steepening aspheric bevel peripheral zone

Figure 4. Geometry of reverse geometry aspheric lens showing zones of the back surface design


A review of rigid lens design K E d w a r d s

In a study on the initial lens fitting of 10 post-PRK patients 1~ lens comfort was reported to be good and centration generally good (Figures 7 and 8). All lenses were fitted from the analysis of pre- and post-procedure topographical maps. Of the 10 patients assessed, two regressed to a point where conventional geometries were appropriate. All patients had a previous history of

O~

Spherical

Conic

Ski bevel

- - Tear layer

~ l l l l - - Axial edge lift

Figure 5. Back surface of the lens showing the change in axial edge lift in different zones. The expected tear layer is also shown.

I d m .

I

J

. 3 - ~ . ~ a ~ • ~ ~ ~

Figure 6. Tear layer profile of reverse geometry lens on post-PRK eye.

contact lens wear, three being soft lens wearers and one a soft lens wearer with a history of RGP lens failure. A further five were RGP lens wearers and one was a failed RGP lens wearer.

Regression of the myopia with a resultant change in central corneal radius was a major problem in three cases of high myopia (> 10.00 D) when new lenses with different BOZRs had to be ordered. In one case of corneal astigmatism, a smaller total diameter was required to improve centration by decreasing lid attachment. Overall 1.6 lenses per eye were required to obtain a satisfactory fit. Visual results showed best corrected visual acuity (BCVA) with lenses which was as good as pre-procedure BCVA even where post- procedure results showed a drop in spectacle BCVA. Figures 9 - 1 1 summarise changes in refraction, keratometry and visual performance resulting from the PRK and subsequent lens fitting. This lens design continues to be used for post refractive surgery cases as well as grafting. More recently, Leach 14 has reported on a similar design developed in the USA.

Material Stability One final factor that has exercised the minds of those fitting RGP lenses is the stability of the parameters once the lens has been manufactured and then soaked for a period of time. The issue was first raised by Gordon 1~ for PMMA lenses and subsequently for CAB by Stone 1~ and PearsonY Many practitioners considered that silicone acrylate and fluor-silicone acrylate materials might be worse.

In a study reported at a British Contact Lens Association Conference, Edwards et a118 showed the hydration cycle for a series of RGP materials and PMMA in four back vertex powers. Lenses of powers -5.00, -10.00, +5.00 and +10.00 D were made from four materials (PMMA, RXD Equalens and Quantum 1). Ten lenses in each material and in each power were assessed and the BOZR measured at baseline and then

109

Lens Comfort

No

Good Fair Poor

Figure 7. Rating of comfort for PRK lens trial subjects.

NO Of re

Lens Centration

Tempora l Cent ra l N a s t l

Figure 8. Practitioner assessment of lens centration of PRK lenses.


A r e v i e w o f rigid lens design K Edwards

110 after hydration for 1, 24 and 48 h, 4 days, 1 and 2 weeks and 1 month.

The results for PMMA closely followed those published by Gordon and in general the RGP materials in minus vertex powers followed a similar pattern, with the BOZR flattening during hydration. In plus powers, RGP lenses tend to steepen with hydration while PMMA flattens slightly. The differences in BOZR change between PMMA Equalens and RXD were not significantly different for any power. For Quantum 1 there was statistically significantly less parameter change than for the other materials (P< 0.001). The results are shown in Figures 12 -15 . Similarly good results were subsequently found for Quantum 2.19

The results of these and similar studies suggest that, at worst, RGP materials are no less stable than PMMA

Rx (D)

Refractive Data - BVS M e a n C h a n g e • - 4 . 4 9 D _+3.38

I N P r e - P R K I ~ P o s t - P R K I

which has been considered to be the gold standard. In some cases the materials are more stable than PMMA

Best Corrected Visual Acuity

[ ] P r e - P R K [ ] P o s t - P R K I I C L - R x V / A

1.4

1.2.

1 .

0.8.

0.0

0 . 4 .

0 . 2

0 - P1 P2 P3 P4 P5 P6 P7 P8

Patient

Figure 11. Best corrected visual acuity for pre-PRK, post-PRK spectacles and post-PRK contact lenses for eight subjects.

Pl P2 P3 P4 P5 P6 P7 P8

Patient

Figure 9. Pre- and post-PRK refractive data (BVS is Best Vision Sphere) for eight subjects.

Keratometry Data M e a n C h a n g e in r a d i u s = 0 . 7 6 m m + 0 . 1 0

[ ] P r e - P R K [ ] P o s t - P R K

K ( ram) 9.50 -

9.00 -

8,50 -

5 .00 -

7 .50 -

7 .00 -

6,50

oi ii!i!ii! iiiiiiii iiiiiiii iiiiiiii: :::::::: i!!iiii!

:::::::: iiiiiiii i:i:i:?

P1 P2

Figure subjects.

P3 P4 P6

Patient

1 i

P6 P7 P8

10. Pre- and post-PRK keratometry data for eight

RGP Lens Parameter Stability B V P : - 1 0 . 0 0

I 1 L"-~- RXD 1 E q u a l e n s - - - k - - .PMMA - - > ( - Q u a n t u m j

Change in radius ( m m ) 0. t

-0.05 - - - ~- - - E . . . ~

- 0 . 1

O 1 24 48 96 165 336 672

Measurement point in hours (log scale)

Figure 12. Variation of BOZR with period of hydration for four RGP materials with BVP - 10.00 D.

RGP Lens Parameter Stability B V P : - 5 . 0 0

I .-'-~- RXD - -B- - -Equa leas ---Jk--PMMA - ~< -Quan tum 1 Change in radius ( ram) 0.1

0,05

0

-0.05

-0.1 0 1 24 48 96 t 6 8 336 672


Figure 13. Variation of BOZR with period of hydration for four RGP materials with BVP - 5 . 0 0 D.


A review of rigid lens design K Edwards

RGP Lens Parameter Stability BVP: +10.00

Change in radius [--*- RXD -4l---Equalens --~¢--PMMA - ~<- Quantum ] (rnm) 0.t

0 . 0 5

0

- 0 . 0 5

-0.1 0 1 24 48 9fl t 68 336 672


Figure 14. Variation of BOZR with period of hydration for four RGP materials with BVP+ IO.O0 D.

RGP Lens Parameter Stability BVP: +5.00

I-.- RXD -- I I - -Equalens - -~ - *PMMA - - ~ - Q u a n t u m I Change in r a d i u s

(ram) o.1

0 . 0 5

0 -

- 0 . 0 5

-0.4 0 1 2 4 4 8 96 t6:g 336 672

M e a s u r e m e n t point in hours (log scale)

Figure 15. Variation o/ BOZR with period o/hydration for four RGP materials with BVP+5.00 D.

showing little tendency for parameters to change during hydration.

S u n l r n a r y

While RGP lenses are seen as under threat by new provisions to control cross-infection within some coun- tries, there is still a clinical benefit to these lenses for a wide range of patients.

Practitioner control of parameters and a wide choice of designs and materials all give the contact lens practitioner more control of the fitting characteristics than is often evident with soft lenses.

Complex designs can be made in materials with very high oxygen performance without loss of material stability. System lenses can often be successfully fit on an empirical basis, obviating the need for trial or diagnostic lenses.

Acknowledgements I would like to acknowledge the assistance of my many co-workers who contributed to a number of the studies

referred to in this study. In particular, Randolph Layman provided the original opportunity to carry out some of the clinical work while Debbie Jones did much of the clinical assessment. The late Jonathan Kersley kindly permitted access to his patients and offered suggestions for fitting post-refractive surgery patients. Without the continued enthusiasm and technical/computing skills of Tony Hough, little of the work reported here would have been completed. I am indebted to Tony Hough for the production of Figures 4 and 5.

Address for Correspondence Mr K Edwards, 5 Vicarage Road, Kingfield, Woking, Surrey GU22 9BP, UK email: Keith [email protected]

REFERENCES

1 Anon. Use of trial contact lenses on multiple patients, in UK Medical Device Agency Advice Notice. MDA AN1999(02) (1999). Anon. Single patient use of contact lenses: implications for clinical practice, in UK Medical Device Agency Advice Notice. MDA AN1999 (03) (1999).

3 Efron, N. and Morgan, P. Trends in UK contact lens prescribing 1999. Eurolens Research, Manchester, UK. (1999).

4 E/ton, N. and Morgan, P. Trends in UK contact lens prescribing 2000. Eurolens Research, Manchester, UK (2000).

5 Guillon, M., Lydon, D. and Wilson, C. Corneal topography: a clinical model. Ophthal. Physiol. Opt., 6, 47-56 (1986).

6 Douthwaite, W., Hough, T., Edwards, K. and Notay, H. The EyeSys videokeratoscopic assessment of apical radius and p-value in the normal human cornea. Ophthal. Physiol. Opt., 19, 467-474 (1999).

7 Phillips, A. Rigid gas permeable corneal lens fitting. In: Phillips A. and Speedwell L., (eds). Contact Lenses, 4th edn, Oxford: Butter- worth-Heinemann, pp 313-357 (1997).

8 Stone, J. Corneal lenses with constant axial edge lift. Ophthal. Optician, 15, 818-824 (1975).

9 Rabbetts, R. Large corneal lenses with constant axial edge lift. Ophthal. Optician, 16, 236-239 (1976).

l0 Young, G. The effect of rigid lens design on fluorescein fit. Contact Lens & Anterior Eye, 21, 41-46 (1998).

11 Edwards, K. Contact lens problem-solving: aspheric RGP lenses. Optician, 219(5733), 28-32 (2000).

1~ Mounfford, J. Orthokeratology. In: Phillips A. and Speedwell L., (eds). Contact Lenses, 4th edn, Oxford: Butterworth-Heinemann. pp. 653-692 (1997).

13 Edwards, K.H., Hough, D.A. and Kersley, H.J. Designing rigid lenses for the post-PRK eye. Optom. Vis. ScL, 72 (12s), 13 (1995).

14 Leach, N.E., Bergmanson, J.P.G., Jackson, J.A. and Tran, A. A new reverse aspheric RGP contact lens design for post refractive surgery patients. Poster abstract, Optom. Vis. Sci., 76, 12S, 236, (1999).

15 Gordon, S. Contact lens hydration: a study of the wetting-drying cycle. Optom. Weekly, 56, 55-62 (1965).

16 Stone, J. Changes in curvature of cellulose acetate butyrate lenses during hydration and dehydration. J. Br. Contact Lens Assoc., 1, 22-35 (1978).

17 Pearson, R.M. Dimensional stability of lathe-cut CAB lenses. J. Am. Optom. Assoc., 49, 927-929 (1978).

18 Edwards, K.H., Jones, D.A. and Frazer-Bates, D.F. Parameter stability of RGP Materials. Annual Clinical Conference of the British Contact Lens Association, Glasgow (1990).

19 Edwards, K.H. and Jones, D.A. Parameter stability of high Dk RGP lenses. Optom. Vis. Sci., 6 (12s), 150-151 (1992).

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A review of rigid lens design