Elastic Modulus and Silicone Hydrogel Contact Lens Fitting

What is meant by the modulus of a material? In fact, there are several different “moduli” that can be measured, but Young’s modulus, also called the elastic modulus and denoted by the letter E, is the one that is usually quoted in connection with contact lenses. Basically, if a material sample is deformed by a given force, Young’s modulus is the ratio of that force to the change in length produced. Putting this more precisely:

So, the stiffer the material, the higher will be its Young’s modulus because there is a greater resistance to deformation. The actual value obtained depends critically on the measuring conditions. Hence it can be tricky to compare values from different sources unless the experimental details are known. With this proviso, some representative values for currently available silicone hydrogel lenses are shown in Table 1.

It is also worth remembering that, like oxygen permeability, modulus is a material parameter and so the effective stiffness of a particular contact lens will also be influenced by its specific geometry. Thus, a lens with a low modulus may still be relatively stiff if it has a thick and chunky design.

Table 1

How does modulus influence contact lens fit?

The purpose of optimizing fit is to distribute the weight of the lens as evenly as possible across the eye’s surface and minimize mechanical interaction with ocular tissue while positioning the optics appropriately to correct refractive error. Should a lens not conform properly to the eye’s shape, local pressure variations will occur that, depending on their magnitude and distribution, can have significant clinical consequences. Very broadly, the magnitude of these pressure variations increases with increasing modulus (G. Brent, 2005, personal communication), although the lens design also has a substantial influence on the final outcome.

An example of such a situation is edge fluting. This occurs when a relatively stiff lens is poorly matched in shape to the ocular surface. Pressure variations in the post-lens region, combined with elastic forces in the lens, cause the lens edge to lift from the eye. As can be seen from the example shown in Figure 1, gross cases are easy to spot at the biomicroscope. More subtle presentations can be missed unless high molecular weight fluorescein is used to aid detection by highlighting the rippling of the post-lens tear film (arrowed in Figure 1b). Fluting invariably results in poor comfort as well as ocular surface damage and is essential to eliminate.

Figure 1: Edge fluting viewed with A) white light and B) high molecular weight fluorescein. Subtle edge fluting is indicated by the white arrow at the 7 o’ clock position.

In the process of achieving a good physical fit, the traditional view has been that the higher the modulus of the material, the less forgiving will be the lens in conforming to the eye. Consequently, achieving a satisfactory result demands more control over the lens’s shape parameters as modulus increases. Perhaps the extreme example would be rigid gas permeable (RGP) materials, which have moduli up to about 10³ MPa and require base curve increments of 0.1 mm or even 0.05 mm to satisfy the needs of fitters.

One might expect, therefore, that the number of available fitting increments for soft contact lenses, while being fewer than for RGPs, would still be positively correlated with material modulus. A glance at Table 1 suggests that this is not quite the case, at least for current silicone hydrogels. Irrespective of modulus, none are offered with multiple diameters. Only two lens types have more than one base curve and while one of these is indeed the highest modulus material available, the other is the lowest!

Although this does appear slightly anomalous, one must presume that silicone hydrogel manufacturers are comfortable that acceptable fits can be achieved on the majority of eyes with the parameters they provide, and there is no doubt that this is mostly true. It almost goes without saying, however, that the variation in eye shape among the general population will guarantee circumstances in practice where the fit is not good enough. Unless the practitioner intervenes in such instances, the interaction between lens and eye can be clinically problematic. When faced with the absence of a suitable alternative fitting increment, there is little alternative but to switch between lens types. For the conscientious practitioner this can be irksome if the attributes of the preferred variety are, in all other respects, ideal.

Obviously manufacturers have vested interests in encouraging practitioners to remain within their respective stables and perhaps offering them more fitting flexibility might be one means to that end. Of course the realities of modern production methods and the pressures of the market place do limit the scope for flexibility in this regard, but that would surely respond to a sufficiently loud clamour for change from practitioners in the trenches. On the other hand, their continued silence will be equally telling about the concerns that are of primary importance in modern contact lens practice.