Gas permeable perspectives

Journal of the B. C L .A . Vol. 8. No, 1, pp. 27 - 29, 1985 Printed in Great Britain

©1985 by the British Contacl Lens Assoc.

GAS PERMEABLE

PERSPECTIVES*

RICHARD M . H I L L t

(Received 30 October 1984, in revised form 3 December 1984) Abslract -- Over the last 20 years, increasing emphasis (and clinical dependence) has been placed on materialproperties as a means of meeting daily and extended wear contact lens requirements. Oxygen transmissivity remains high on that list of properties. Here, a brief overview of certain expectations and limitations in relation to the oxygen performances of hard and soft materials is given.

GAS PERMEABLE PERSPECTIVES Among contact lens properties, few have had a greater range of impact than oxygen transmissivity. Since its recognition some two decades ago as a significant clinical parameter, it has attracted and held the attention of virtually every sector associated with contact lenses, from laboratory to practitioner, from patient to government -even to the lawyers who defend patent boundaries.

quite abruptly; and secondly, a decrease in oxygen below the normal level in air (21%) even down to just 16 % is readily detectable by the cornea. That marked change in the cornea's oxygen responses as it passes below the 1.5% oxygen mark, corresponds very closely with the original Poise and Mandell slit-lamp detection threshold for oedema (2), and suggests that we should permit no lens design, through poor transmissivityor a faulty pump to manoeuvre the cornea below that level. In terms of response time (within 90 seconds) and intensity (an uptake increase of 3 to 5X normal), this corneal reaction then gives fair warning of short term stress, and speaks clearly to the "minimum" oxygen aspect of our question. That the cornea can "notice" a decrease in oxygen level of just 5% (and very likely even smaller decrements), offers a clue to the "optimum" oxygen part of our question. Although the possible consequences of such minor stress over the long term (e.g. the 50+ years of corneal life beyond fitting at age 18), have not been seriously considered, perhaps they should be in light of the finite odds for future infection, dystrophy, diabetes, or the occurence of trauma (natural or associated with surgery). It is a fundamental responsibility of the practitioner to assure, with best materials and designs, the most favourable status of a patient's cornea over that long term, as well as the short.

MINIMUM VERSUS OPTIMUM Curiously however, the most fundamental question associated with oxygen transmissivity ("What does the cornea really need?") remains only generally answered, and even then heavily qualified by a wide array of contingencies and conditions. Perhaps greater confidence (or at least conviction) might be gained from a look at the extremes of the matter, that is by asking how little oxygen is the cornea prepared to tolerate (i.e. the short term perspective), and how much would the cornea like, if any amount were available (a question with possible long term implications)? Recent investigations of Benjamin provide clues at both of these extremes (1). His approach was to allow human corneas to stabilize to each of a series of oxygen levels maintained over them by goggles, then on their return to air he would use a probe against the eye to measure the oxygen wants of the cornea (increased if deprived vs. little or no change from normal if not deprived). His major observations were two: first once below 1.5 % oxygen the comea's demand mounts

HARD VERSUS SOFT A major return to translimbal, semi-scleral design would have been judged by most practitioners of the late 1950's and early 1960's as extremely unlikely, yet within the decade following a substantial number of fitters were doing just

*Excerpts from Lectures presented at the Nissel Contact Lens Conferences on Gas Permeability at Edinburgh, June 18, and London, June 20, 1984. tProfessor, College of Optometry, Ohio State University, Columbia, Ohio, U.S.A. 27

28

Richard M. Hill

that in soft lens form. Similarly, a significant shift of prescriptions from soft back to hard lens forms was anticipated by very few just five years ago -yet today, even the possibility of extended wear is no longer an exclusive soft lens preserve. Oxygen permeability has had a role in both of those design shifts, but we now appear to be nearing a practical plateau among the hydrophilic forms. Figure 1 gives the oxygen performance expected of a 79% water lens under optimum conditions of pH and hydration (3). The thinner end of the thickness spectrum shown is already very difficult to approach with any practicality (the best case being the centre of a very high minus prescription), and most certainly will require a substantial advance in plastics technology to achieve still thinner matrices in the future. There are as well a number of factors and conditions which can work against (depress) the oxygen curve of such high water content materials, for example, water loss into a dry work or living environment, and in some cases due to a lowere~d pH following prolonged eye closure.

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Maximum performance still depends too on lens design. Even with the best permeability a hydrophilic material can provide, the need to clear waste products, both chemical and solid, from the post-lens tear-pool remains important for best comfort and health. In this regard thinness, expecially extreme thinness, can exert a unique and not always favourable influence on hydrophific lens movement. Up to a point, thinness might be deliberately employed to stabilize a lens, if for some reason base curve or diameter options were limited, but

too much stability can lead to debris entrapment. However, proneness to local drying or wrinkling together with handling and fragility problems, have served as practical guards against superstabilizing, or the "cling" effect (i.e. akin to the behaviour of a refrigerator plastic film wrap against a container). But as further materials appear which may be both stronger and thinner, continuing watchfulness for the "too stable" lens remains in order. 0,7/,20

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2. The effects of various flatness-thickness lens design combination on corneal swelling found for Lens Brand A (a thicker, lower water, less supple example) and Lens Brand B (a thinner, higher water, more supple example). The first number over each point is the amount flatter (mm) the lens iS r compared to the cornea's curvature; the second number is the lens thickness (mm) corresponding to where the cornea's thickness was measured. Modified from Ref. 4.

Figure 2 illustrates such movement control in relation to two types of lens: A, of low water content, and thicker; versus B, of higher water content and thinner, on cornea swelling (oedema). While the six human corneas fitted with the less supple type A lens could still benefit from some flattening and thinning as shown in the Figure, lens type B was already too supple and thin, and continued under those strategies to stabilize and cling sufficiently to result in added, rather than reduced, thickening of those corneas (4). Certainly, the advantages of initial comfort and superior oxygen passage gave hydrophilic lenses an impressive market lead over their initial decades. Rigid lens transmissivity however, following a sluggish beginning is now advancing quite rapidly as well, matching and in the laboratory even exceeding, many of the best hydrophilic performances. The Table provides a sampling of EOP (equivalent oxygen percentage (5)) performances associated with hard lens materials spanning some two decades. Those

Gas Permeable Perspectives

with the higher values are (as economic logic insists) the more recent to appear, some not yet being approved in the U.S.A. (6). The highest oxygen performances represented even now seem to be approaching physiologically practical extended wear levels.

1. Benjamin, W. J. and Hill, R. M.: Human cornea: Oxygen

2. 3.

EXPANDING THE PATIENT POOL But as those two major lens classes, "soft" and "hard", continue toward their fullest potentials, neither is likely to eliminate the other. Instead, through such choices we should be finding ourselves in an ever improving position to meet the unique needs of those millions of potential wearers waiting at the practical wearing perimeter. And additions to our current patient pool represent of course the very best kind of growth for all parties involved. Table OXYGEN PERFORMANCES OF H A R D CONTACT LENSES OF DIFFERENT MATERIALS AND COMBINATIONS BUT ALL OF THE SAME THICKNESS* Example Material # Class 1 PMMA (standard) 2 PMMA (modified) 3 Silicone- Combination 4 CAB (one type) 5 Silicone-Combination 6 Silicone-Combination 7 Silicone-Combination 8 Alkyl-Styrene 9 Silicone-Combination 10 Flurocarbon (one type)

EOP**

(%) 0 0.6 1.1 2.2 4.5 5.1 5.1 7.7 9.4 15.6

*A limiting thickness of 0.07mm was used here throughout for comparative purposes, and to provide a bench mark for the best performance likely to be realized at, for example, the center of a very high minus prescription. **Equivalent Oxygen Percentage: A value of 21% would be equivalent to no lens impedient, i.e. as if the cornea were exposed directly to air. These values were measured at a representative altitude of 230 meters above sea level.

29

REFERENCES

4. 5.

6.

uptake immediately following graded deprivation. Graefe's Archives of Clinical & Experimental Ophthalmology. Submitted 1984. Poise, K. and Mandell, R. B.: Critical oxygen tension at the corneal surface. Archives of Ophthalmology 84: 505508 1970. Flynn, W. J. and Hill, R. M.: Have we hit the high water mark? Contact lens Forum (in press 1984). Paugh, J. R. and Hill, R. M.: Is flatter and thinner always better? Journal of the American Optometric Association 53:303-304 1982. Hill, R. M.: Oxygen permeable contact lenses: How convinced is the cornea? in Curiosities of the Contact Lens, by R. M. Hill, Professional Press inc., Chicago 1981, p. 99. Flynn, W. J. and Hill, R. M.: Hard gas permeables: An oxygen update. Contact Lens Forum (submitted 1984).

Richard M. Hill, College of Optometry, The Ohio State University, 338 West 10th Avenue, Columbus, OHIO 43210-1240, U.S.A.