Experience with silicone— PMMA material

Experience with Silicone PMMA material R. N. Pennington

R. Neil Pennington commenced contact lens practice in 1953 and is Foundation Member New Zealand Contact Lens Society (1958) and a Past President.

over well blended multicurve back surfaces in respect to corneal respiration. The advent of gas permeable hard materials though long overdue held considerable promise for improved management of patients both new and in the longer term. The question was would these materials of rather marginal oxygen permeability come up to our expectations'? The paper by Stone (1978) on lathe manufactured CAB lenses indicated a disturbing dimensional instability of this material and others questioned its surface resistance to wear and tear and chemical purity. The report by Sarver et al (1977) on refitting some PMMA wearers with silafocon A (Polycon) was very favourable to this particular gas permeable material and it was decided to use this material in our practice.

The availability of gas permeable flexible and rigid materials while widening the scope of contact lens practice has also brought about a dramatic improvement in the management of those patients who present adaptation problems. For twenty years the fitting of hard corneal lenses has been bedevilled by corneal oedema and its associated effects. Since first fitting the microlens in 1956 I have tried all the permutations including mini diameters and lenses which resemble a vegetable strainer in an attempt to achieve a balance between acceptable vision and minimum tissue disturbance. In many cases small diameters less than 8.5mm present location problems resulting in flare, increased lid insult which encourages excessive tearing, are more prone to loss and are less easy to handle. When they require a tighter fitting to reduce the visual impairment oedema of some degree is almost inevitable. Fenestrations have been a time honoured device for the relief of oedema but they are not without their drawbacks, particularly when introduced to longer term wearers. Over 50% experience intolerable visual problems from disruption of the tear film after blinking. Surveys reported in the literature over this period indicated at least a 3(F~ failure rate for PMMA wearers after six months and this figure would be close to our experience in New Zealand. The contour of the ocular lens surface has been the subject of intensive investigation with claims for superiority of one design over the other. Claim for superiority for example of aspheric over multi-curve back surface is tenable in my view only in respect to the ease of prescribing aspherics and the precision reproduction of any parameter; they have no inherent advantage

Polycon assessment The initial assessment of Polycon was made on established wearers who had been experiencing visual and comfort problems, often of long standing, but who declined soft lenses or were advised against them and who refused to return to spectacles. Commencing late 1977 PMMA wearers were routinely refitted with Polycon lenses of similar parameters except for overall diameter which averaged 0.30mm larger. Over a nine month period 43 patients were refitted with results that exceeded expectation. Of the three patients who failed, one struggled to two hours with Polycon compared to half an hour with PMMA after six months, the other wearing double truncated front surface cylinder for eight years was refitted for chronic bulbar conjunctival congestion but Polycon lenses in identical parameters caused immediate scratchiness, the redness increased and after three

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days were intolerable. Repolishing surfaces and Corneal Oedema edges had no effect whatever. The 40 patients (93%) We continually meet patients, however, who have the who responded favourably experienced almost classical symptoms associated with corneal oedema without exception a dramatic reduction of their without any of the cardinal signs on the slit lamp. particular problem. Some of these wearers were in this category. A pattern of delayed relative intolerance to The means at our disposal for demonstrating PMMA has been observed repeatedly over the past corneal hypoxia in daily practice are limited to the 12.years or so and has presented a major challenge in rather gross degrees and apart from the use of a general practice. The author has labelled this loss of ~pachometer, whose value is questioned in daily tolerance "tissue exhaustion" for want of a better routine, there does not appear to be any instru• term. The sample group analysed here were typical, mentation available or in the pipeline to help us. One they had enjoyed comfortable clear vision up to at can reasonably assume that hypoxia and, to as yet least six years more often eight - ten years before one undetermined extent, raised corneal temperature, or more problems developed. The mean wearing existed in those patients who symptoms were relieved • period in this group was 10.8 years at the time of by the classical modifications of diameter and optic refitting a n d about eight years when symptoms zone reductions, increased lift of secondary radii and appeared. The main reasons for refitting were fenestrations. In this Polycon group these traditional subjectively modifications were unnecessary and as noted above -persistent blurring of vision the overall diameters were in fact often increased. By -discomfort indoors, particulary in warm or air taking advantage of the anticipated corneal oxyconditioned atmospheres, further aggravated genation a larger diameter was used to restrict the with close work and objectively excessive mobility seen with smaller lenses this in turn -corneal tissue disturbance reducing lid margin insult and lessening the entry of -conjunctival congestion foreign bodies, it provided better pupil coverage with - - excessive debris on the front surface of the lens flare reduction appreciated in night driving and lens In 18 (42%) of these patients, oedema of mainly adhesion increased noticeably. On the last point the central circular clouding (C.C.C.) type had been many wearers complained their lenses were now observed later in the day, and in 6 (14%) wearers, 3 difficult to flick out. and 9 staining of marked degree. Greasy lenses were a The author's fitting philosophy has been and still is persistent problem for 16 (37%) wearers. Two with Polycon material minimal apical clearance or wearers were being treated for recurring Keratitis by alignment with a lens diameter sufficient to give good their ophthalmologist, and one for "conjunctivitis'" pupil coverage and location. Could one fit steeper by her G.P. The remaining wearers experienced now and still meet the criteria for a successful fitting'? discomfort at varying stages of their wearing time and The oxygen permeability given by Fatt and Morris symptoms were typical of those expected in corneal (1978) for Polycon in the absence of blinking is below hypoxia. All those with discomfort reported dramatic the minimum level for corneal respiration as esimprovement in terms such as noted in Table A. tablished by Poise and Mandell (1970), but would the The oedema in most wearers when refitted with oxygen supply be sufficient in the presence of Polycon lenses was either eliminated or was only blinking? One patient wearing very steeply fitted faintly visible. All patients with 3 & 9 staining showed aspheric small diameter lenses was refitted with a marked reduction or complete absence after being Polycon in identical parameters except the diameter refitted. The three Keratitis cases have been free of was increased from 8.2 to 8.6mm for pupil coverage. any recurrence for a year now. There was marked central corneal clouding (CCC) and smoky vision in later evening which he had noticed for past year or so. With Polycon refitting the oedema was just visible but smoky vision didn't Design Polycon lenses have been prescribed in all the forms reappear suggesting that some oxygen delivery through the lens took place. The impression gained we commonly have fitted with PMMA material. Bitoric, front surface toric, Keratoconus and prism from this wearer and others similarly refitted exballast in aspheric (conocoid) and multicurve back perimentally was that tissue tolerance was borderline surfaces are routinely manufactured in Polycon and so little latitude existed for stress situations. With material. The ratio of aspheric to multicurve fitted Polycon the least disturbance of corneal tissue and was 4:6 and choice of a particular back surface was curvature was achieved with alignment or minimal based on physical and optical considerations. apical clearance fitting. Kress (1978) reported Comparisons made with some patients wearing an similarly on Polycon. aspheric in one eye and multicurve in the other eye indicated no preference for one back surface form L e n s d e p o s i t s over the other. Blurring of vision from lipids, or mucous on the front

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lens surface is a perennial problem for some wearers and can defy all attempts at amelioration. Since cholesterol was identified in the tears we have had patients adjust their diet but rarely with any benefit. Some are convinced that incorrect blinking is the major cause of greasy lenses but who can claim to train wearers to blink correctly all their wearing hours'? If gas permeable material gave no other improvement than providing a front lens surface which stays free of visually ruinous debris it would still constitute a real breakthrough. Why it performs so much better than PMMA, which has a slightly better wetting angle for instance, is a question for our researchers.

eye had steepened at one week, while the range and mean curvature changes were also identical. Then as adaptation increased some corneas in one or both meridians flattened quite noticeably whence a pattern emerged of a tendency for Polycon wearers after one month to have less flattening than PMMA wearers. This steeper tendency was sustained at three months with the Polycon eyes. As overall flattening of corneal curvature with PMMA has been the general rule after six months it is likely that these Polycon wearers will follow a similar pattern but settle to curvatures nearer the original. Now, can we expect this new generation of gas permeable wearers to suffer less corneal distortion in the long term? The frequency with which long term PMMA patients acquire with the rule astigmatism of often disturbing proportions is a fact of life and one over which we have had scant control but if hypoxia is the major factor in this tissue distortion as Mackie (1978) believes it is, then a major souce of concern may well be removed.

Corneal radii change It seemed worthwhile with Polycon wearers to determine the changes in corneal curvature, the induced refractive changes and the correlation, if any, between these curvature and refractive alterations. A comparison was made of corneal radii changes for PMMA and Polycon first time wearers at intervals up to three months. Results are given in Table I for 36 PMMA and 24 Polycon eyes. The majority had with the rule astigmatism and the flattest meridian in every

Spectacle blur Spectacle blur with Polycon generally followed the pattern with PMMA but recorded a surprisingly greater myopic shift with the gas permeable material up to three months wear. This sample Fig. 1 comprised 24 PMMA and 18 Polycon eyes. About 20%

TABLE 1 Comparison of corneal radii changes induced by micro corneal lenses of differing material. No. eyes PMMA 36 No. eyes Polycon 24 F L A T T E S T R A D I U S 1 Week

4 WeeEs

12 Weeks

*FO.I2-SO.~9 SO.03-SO. 17

FO.09-SO.14 FO. 10-SO.09

Range of Change PMMA Polycon

"SO.02-0.13 SO.112-0.13

Mean radius change PMMA 1 Polycon

SO.06 SO .06

SO.07 FO.08 S0.09 FO.09

SO.07 FO.05 SO .I17 FO.05

% Steeper/Flatter PMMA Polycon

l(~d/11)0/-

S T E E P E S T R A D I U S l Week

78/22 85/15

52/48 62/38

4 Weeks

12 Weeks

Polycon eyes showed an exaggerated increase of myopia from the first week to the third month of wear. For both materials our records show little change in the amount of induced sphere right through to the end of the three months. Our records of PMMA, from six months onward indicate a general overall reduction of the original induced spherical change but an increased cylinder as noted above. Why should a small group of gas permeable wearers selected at random demonstratemore overall corneal steepening and induced myopia than a comparable non-gas permeable group? If the generally accepted modes for these corneal changes, namely hypoxia and mechanical moulding as demonstrated by Carney (1975), are correct these results are rather surprising. Lens diameter admit-

Range of Change PMMA Polycon

FO 11 -SO.13 FO.25-SO.117

FO.15-SO.(17 FO.25-SO.07

FO.17-SO.12 FO.25-80.02

PMMA

FO.05 SO.07

FO.08 SO.06

FO.06 SO.05

Polycon

FO.07 SO,02

FO. 11 SO.O8

FOAM SO.08

84/16 92/8

81/19 85/15

68/32 80/20

Mean radius change

%Steeper/Flatter PMMA Polycon

* F denotes flatter radius mm. * S den(~tes steeper radius ram.

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16"

tedly was increased on an average by 0.30mm and it occurs that a possible mechanical factor could operate here in so far as the reduction of lens motion across the cornea may limit the opportunity of the flatter mid-peripheral lens zones to bear on the steeper central corneal area as must occur with smaller diameters.

j. 12-

J"

J

Correlation corneal and refacting changes The analysis of 24 PMMA and 18 Polycon eyes after 3 5 7 9 11 13 15 days three months is given in Figure 2. Approximately " 60% of Polycon eyes had a correlation with the Fig. 3 Adaptation period for new wearers. corresponding corneal meridian dioptric change at the _+ 0.25 D level whereas the I'MMA eyes recorded wearers now have the advantage of more rapid build a high of 80% correlation at the same level. The up so that at the end of the first week 12 hour wear is maxtmum variance for both materials was + / - 0.75 usual against eight hour for PMMA wearers. Slow adaptation to sunglare and other bright light D. Why PMMA wearers in this sample displayed a sources which has driven many PMMA .wearers into closer relationship is not immediately obvious. sunglasses has not been anywhere like the same problem for Polycon wearers. In fact it is unusual to have complaints of photophobia from new wearers. Those patients refitted with clear Polycon and who so had worn tinted lenses previously experienced no more discomfort with glare than before, their ao + complaint now is finding the lenses. i

Dimensional stability of lens Reports to date confirm that all gas permeable hard materials are prone to some instability of curvature after manufacture and after lenses are worn. Initially the laboratory forwarded Polycon lenses to us in dry containers but odd behaviour of some lenses received at this time caused us to do some comparisons of BCOR in dry and wet states and it was found that changes took place after 24 hours hydration. These changes were clinically significant in some lenses and certainly more movement was found than with PMMA of comparable parameters. As a result of this we now receive Polycon lenses in soaking solution (Soaclens) and they are checked in the laboratory after 24 hours soaking before despatch. A small proportion of BCOR we received in wet state had flattened slightly since despatch, 0.02 - 0.03 on average, but some returned to the radius recorded by the laboratory on the mailing container after the patient had worn the lenses. A series of lenses were checked for base radius movement over an extended period and the BCOR during wear analysis Table 2 indicated a highly acceptable clinical level of lens

20 ~

°i/

025

050

075

a,e~re

Adaptation period Although Boyd (1967) and Feldman (1971) amongst others have advocated all day wear beginning the first day, the majority of practitioners prefer a slower wearing-in period, perhaps we are too cautious. However it is generally felt that a steady increase of wearing time reduces the chance of adverse reactions which apart from other considerations could affect the wearer's confidence in the lens and in the practitioner. Gas permeable Polycon presented the pleasant prospect of reducing the tedium and frustrations of those initial weeks of adaptation. To establish a safe wearing-in schedule a number of new wearers were monitored at intervals over the first fortnight when tissue reactions and the patients' reactions were recorded. The potential of all day wear after the first week was assessed for this randomly selected group. All but one of the nine wearers achieved a comfortable 12 hour wearing time after one week and most elected to continue wearing their lenses for the rest of the evening. The adaptation period we have developed for each material is reasonably conservative, to allow for the slower patient, Figure 3, but even so our Polycon

TABLE 2 POLYCON BCOR DRY WET S T O R E D IN P R E S E R V E D S A L I N E SOLUTION (NORMOL) BCOR

BCORWET

7.30/7.25 7.32/7.27 7.29 7.28 7.29

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TIME LAPSE BETWEEN CHECKS M I N I M U M 24 H O U R S

CT

TYPE

PowER

.13

*cc

-325

No secondary radii

DAYCHECKED l 7 10 14 15

7.04

.11

cc

-1050

7.05/7.10 7.05/7.07 7.06/7.02 No secondary radii

7.06/7.05 7.08 7.50/7.45

.15

cc

-450

7.48/-7.43 7.49/7.46 7.47/7.45 7.46

Finished lens

7.39

.18 7.40 7.40

*mc

-425

N o secondary radii

8.35

.13 8.36 8.37 8.37/8.36

mc

-400

N o secondary radii

7.41

-15 7.41 7.40/7.39 7.40/7.39

mc

.11 7.55 7.55/7.52 7.55/7.51

cc

25 26 1 8 11 15 16 1 4 5 1 2 6 7

-550

Finished lens

7.51

material stability. Very high minus, up to 17.50 D have been very stable after wear and are included in this table. Lees (1978) reported his findings on BCOR and warpage with Polycon lathe cut lenses before and after wear and noted rather more movement than we found; of possible significance is that his lenses were despatched by the manufacturer in the dry state. To complement our monitoring Mr. Shennan, head of the hard lens division of Hirst Contact Lens Limited ran a study on a series of Polycon lenses with aspheric and multicurve back surfaces for a minimum hydration of 48 hours and his results are given in Table 3. A tendency for aspheric to be less stable than multicurve was noted by Shennan and he attributed this to the thinner periphery of the former design. Gas permeable lenses are proving quite difficult to manufacture to the tolerances the practioner has been accustomed to in the past and the increased demands on our technicians must be acknowledged by clinicians. It was suggested by an overseas researcher visiting our laboratory that instability could be due to imperfect distribution of the silicone through the material. Hirst laboratory then asked Auckland University to analyse Polycon. Their report in brief

1 7 1l 21

-750

2 6 7

Finished lens

1 2 6 7

7.48

7.51

.12 cc -600 No secondary radii

1 2

8.39

8.39

.23 cc -200 N o secondary radii

1 2

.16 mc -350 N o secondary radii

1 2

was:

7.49 7.47

The silicone content is in the order of 10% Silicone is evenly distributed over the surface and does not clump into areas of relatively greater silicone content After annealing at 50°C for 21/2 hours there was no polymer migration into the material and the silicone remained uniformally distributed.

* ce denotes conocoid * mc denotes multicurve

TABLE 3 Years

Worn Design

Reason for Refitting

9

cc

Cleaninglenses 4 - 5 daily

PMMA

1

Mrs. D . A .

Design cc

POLYCON Power - 4.50 - 4.00

Patient's Diam Comments 9.0

Extremely comfortable never need to clean

Clinical

Observations Lenses free of previous heavy debris

lenses 2

Mrs. P . S .

15

mc

Loss comfort recent

mc

3

Mr. W . K .

3

mc

5

Mr. K . W .

Ms H . B .

12

9

mc

"mc

N o 3 - 9 staining no

Now really comfortable

Noccc

9.2

me

- 3.25 - 3.25

9.0

Spare pair, no problems

mc

- 8.75 - 6.75

10.2 Like stepping out 0f gumboots into running shoes

/vlarked epithelial necrosis

mc

- 5.50 - 3.00

8.9

Don't feel lenses now aching gone, no blurring

No epithelial staining clean lenses

mc

- 7.50 - 8.75

9.0

Vision always clear, lenses feel much lighter

Clean lenses no Keratitis 9 mths

M o d e r a t e ccc, never

achieved comfortable level 4

Perfect comfort, no cornparison with old lenses

- 7.50 - 7.25

years, 3 - 9 staining

mucous build up, aching

epithelial staining

Objectively same as PMMA

back of eyes 6

Mrs. S . F .

15

cc

Greasy lenses, treated

for recurring Keratitis

never had such comfort

4 yrs 7

8

Mrs. M . B .

Mrs. R . M .

9

4

cc

?

Persistent blurred vision in warm room, cleaning often

cc

Burning dry eyes after starting pill, not worn lenses 5 yrs

cc

+ 3.25 + 3.75.

9.2

- 6.00 - 6.00

9.2

35

Vision stays clear now all day

Become uncomfortable after 4 - 5 hrs, glare bothers me

Oily film now

absent

No oedema, mild conjunc• tival congestion, same symptoms with ultra thin soft lenses

PMMA Y ear s 9

Mr. P . C .

Worn Design

Reason For Refitting

2

cc

S m o k y vision later evening m a r k e d ccc (very steep fitting)

POLYCON Power

Patient's Diam Comments

Clinical Observations

cc

- 5.25 - 5.t~)

8.6

V i s i o n always clear, eyes not sore n o w w h e n lenses r e m o v e d

Faint ccc (same p a r a m e t e r s P M M A except d i a m e t e r increased)

Design

111

Miss D. K.

14

mc

Blurring office lenses coated excessive oil film

mc

- 13.511 - 16.511

8.9

D o n ' t feel lenses now blurring less but still a nuisance

Slight reduction a m o u n t lens oiling

11

Mrs. F . O .

9

cc

G r e a s y lenses2 yrs nasal b u l b a r injection

cc

- 2.00 - 2.50

9.0

N o blurring or irritation lenses feel m u c h finer

Clean lenses b u l b a r injection reduced

12

Miss J . A .

16

mc

Mild chronic central o e d e m a , greasy lenses 3 yrs

cc

10.50 - 10.75

8.8

N o vision blurring, lenses feel lighter, no stinging o r b u r n i n g office

No oedema clean lenses

13

Mr. K. M c K I 2

mc

Slight ccc mild lens greasing

mc

- 12.00 - 12.00

9.2

Eyes feel really comfortable vision always

N o ccc clean lenses

14

Mrs. J . S .

17

mc

M e d i u m ccc, G P treating recurring "conjunctivitis"

mc

4.75 - 4.25

8.8

S t o p p e d using drops, lot Faint ccc m o r e comfortable, cat nap no conjunctival 2 - 3 hrs now (never with dongestion PMMA)

15

Miss R. C,

8

mc

Medium ccc, ophthalmologist treating recurring Keratitis 2 yrs

cc

7.75 - 7.11t)

8.2

Eyes never horrible dry feeling, vision n e v e r blurry

N o ecc, n o Keratitis for over 1 yr n o w

16

Mrs. B . B .

21)

cc

M a r k e d central epithelial necrosis

cc

- 6.110 - 5.75

9.2

Eyes m o r e comfortable than can ever r e m e m b e r

N o corneal staining

17

Mrs. J . H .

15

mc

Slight ccc, o p h t h a l m o l o g i s t treating recurring Keratitis 4 yrs

mc

8.50 - 17.50

9.2

Much more comfortable and no blurring particulady w a r m office

C o r n e a s free Keratitis 11 mths, no ccc

18

Mrs. P . M .

6

cc

M e d i u m ccc, overwear s y n d r o m e if wearing time not controlled

cc

+ 4.lXI + 2.1~)

8.3

Easier to wear no b u r n i n g e n d of day

Faint ccc n o over wear p r o b l e m s for 1 yr

Conclusion

Carney L. G., Am. J. Upturn. and Physiol. optics, 52 (7): 445 - 454, 1975. Boyd H. H. Contact Lens Symposium in Munich - Feldating, S. Karger, New York 1967. Feldman et al, Contact Lens Med. Bul., 2, 8 - 10, 1971. Lees M. E., The Ophthal. Opt., Vol. 18, No. 22, pp. 816-819, 1978. Kress J. A., The Ophthal. Opt., Vol. 18, No. 11, pp. 413 - 414, 1978.

The corneal lens in this material has resulted in the least disturbance of corneal physiology and associated tissues and provided dramatic relief for visual problems caused by excessive lens deposits. Its dimensional stability has been clinically acceptable over a wide range of minus powers and its surface resistance to abrasion approaches that of PMMA with reasonable handling. It meets most of the criteria for the successful fitting of corneal lenses and is now our material of choice.

References Sarver M.D., Poise K.A., and Harris M.G., AM. J. Optom. 54, No. 4, pp. 195 - 20, April 1977. Fatt I. and Morris J. A. Eye I. Vol. 5 No. 1, pp. 10 1i, 1978 Poise K. A. and Mandell R. B. Arch. Ophthalmol.; 84 (10): 505 : 508, 1970.

Address for further correspondence: Kelvin Chambers, 16 The Terrace, Wellington 1, New Zealand.

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