readers' forum
Fitting Gas Permeable Lenses: A New Paradigm
BY PERRY ROSENTHAL, MD

While looking back at my 42 years of fitting hard contact lenses. I was struck by how little their design methodology and fitting objectives have changed. PMMA lenses were principally designed to maximize the efficiency of tear pump, which was  their sole means of supplying oxygen to the corneal surface. When gas permeable (GP) polymers replaced PMMA, the philosophy of propelling lenses over the cornea with each blink survived virtually intact. Is this fitting concept valid today?

Friction generated during lens movement is a major cause of GP intolerance. In contrast, scleral lenses form a cushion of lubricating fluid between the lens and the corneal surface, which virtually eliminates corneal friction.

GP Design Fundamentals

Fitting GP lenses has been more an art than a science, challenging to teach and frustrating to learn. Moreover, traditional GP fitting objectives do not address corneal friction. If we want to approach or exceed the wearing comfort and fitting simplicity of soft lenses, we need to revisit the fundamentals of GP lens design.

To minimize corneal friction, we should limit blink-induced lens movement and the area of corneal contact. Logically, this means large diameter lenses that vault most of the corneal surface and are supported by the less sensitive corneal periphery. Large, well-centered lenses that have limited movement ­ does this ring a bell?

Think soft lenses. Because they are largely insensitive to lid anatomy and do not affect the blink reflex, their optimal diameter is a simple function of corneal size. Moreover, if we design GP lenses to vault most of the corneal surface, we eliminate the need for complex corneal topography to dictate lens design requirements.

Until now our dependence on base curve radius to control the fitting relationship of GP lenses has limited our design options. If we remove that and introduce mathematical design tools for manipulating the lens vault, we could apply soft lens design and fitting principles to GP lenses.

Introducing a New Paradigm

Imagine a zone between the base curve and peripheral zone that, like an accordion, you can elongate or shorten to elevate or lower the central zone of the lens. Suppose you can precisely control the vault in this manner with a mouse click. With the base curve largely irrelevant as a fitting parameter, the principle fitting parameters now become diameter (based on corneal size) and vault.

How do you choose the vault? Unlike base curves, this value has a forgiving range of acceptability ­ it merely needs to clear the central cornea. Furthermore, lenses with inadequate vault display fluoroscein patterns that are simple to identify.

The minimal movement of large diameter lenses promotes accumulation of glue-like mucin in the lens-cornea interface. The last obstacle to overcome is to prevent lens adhesion by providing sufficient tear flow under the lens while avoiding air bubbles. We can achieve this by using the same principles for corneal GP lenses that were successfully incorporated into the fluid-ventilated Boston Scleral Lens.

Advanced mathematical design tools, high oxygen permeable polymers and new generation CNC lathe technology have opened the door to a new generation of comfort-driven GP contact lens designs for a broad array of applications. These simpler designs that require only a few base curves will make GP lenses, like soft lenses, accessible to practitioners at all levels of skill and experience.

Dr. Rosenthal is assistant clinical professor of ophthalmology at Harvard Medical School, he is also the founder of Polymer Technology Corporation, founder and president of the non-profit Boston Foundation For Sight and a consultant to Bausch & Lomb.