Canada, Australia, and Europe enjoyed several types of extended-wear contact lenses several years before we did. However, there is no Food and Drug Administration in those parts of the world, and because of this lack of strict monitoring control and quality, these lenses have posed a health problem, especially as related to corneal complications.

At first the only type of extended-wear lenses that the FDA approved were therapeutic bandage lenses worn on the cornea and the sclera for use in treating certain eye disease. However, the potential of these lenses for correcting refractive errors was soon realized, and eventually the FDA allowed a few contact lens manufacturers to produce extended- wear corneal lenses. A handful of qualified ophthalmologists and optometrists were then permitted to fit and investigate the extended-wear lenses as visual corrective devices.

They were approved for wear by aphakes since they have a pressing need for an alternative to the often unsatisfactory spectacles and available contact lenses. After five years of rigorous testing by a select group of ophthalmologists (of which I was one) and optometrists on a specially selected group of volunteer patients, the first two brands of extended-wear contact lenses were approved for general cosmetic use: Perma-Lens, fabricated from perfilcon A, a terpolymer, and made by Cooper Vision; and Hydrocurve II, composed of the polymer bufilcon A, and made by Continuous Curve, Inc., and Revlon.

The reaction of the eye to extended contact lens wear is even more important than in daily-wear contact lenses. The lenses must allow plenty of oxygen to reach the cornea because the cornea does not have the same opportunity to recover from prolonged oxygen deprivation as it does with daily-wear contact lenses. In addition, during sleep the eyelids are closed so the ocular physiology is different: there is no available oxygen from the atmosphere and there is an absence of oxygen- filled tears. The semirigid gas-permeable lenses would logically seem to fit the bill for optimum oxygen supply, but studies so far have revealed drawbacks in these lenses during prolonged wear. Except for the pure silicone lenses used by aphakes, the contact lenses that have been FDA approved to date for extended wear are all soft, and made of plastics similar to those from which conventional soft lenses are made, but modified to yield greater oxygen permeability. The rate of oxygen-carbon dioxide transmission through the lens material is determined by two major factors: the water content and the thickness of the lens. The higher the percentage of water contained in the lens material, and the thinner the lens, the more gas is exchanged. A very thin, very highwater-content lens, however, has so far proven to be too fragile to be practical. Thus, a lens manufacturer can take either of two roads to maximize gas transmissibility. It can design a relatively thick lens with a high water content (Perma Lens), or a very thin lens with a lower water content (Hydrocurve II).

Extended-wear lenses are therefore from 55 percent (Hydrocurve) to 75 percent (Perma-Lens) water. Conventional daily-wear soft lenses are on the average about 30 to 40 percent water. The Hydrocurve extended-wear contact lens is as thin as a single strand of hair (0.05 mm).

Oxygen transmissibility is not the only factor in lens wear, however, and thickness and water content not the only variables. The different materials have different properties, and there are different diameters, edge designs, and other design factors to be considered. For instance, the lower the water content of a lens, the higher the tensile strength it has, and the less it tends to accumulate deposits and support the growth of bacteria and fungus. This can mean a longer useful lens life, more comfort, and less frequent replacements. Lower water content yields a slightly more rigid lens, which generally provides better visual acuity. On the other hand the higher the water content, the softer will be the lens and the more comfortable it may feel. But the higher the water content, the more important it is to have adequate tear flow, since the tear flow is essential to keep the lens hydrated. Thinner lenses are more apt to fold and tear and are more difficult to handle. Also, it may be difficult to determine whether they have inverted or not.

Even with more than one brand to choose from, it is impossible to predict whether someone will be able to wear contact lenses for an extended period of time. The cornea and tear composition of each person are different, as is the tendency to form protein deposits and the ability of the eye to adapt to life with less oxygen and a diminished tear pump. It is up to the doctor to weigh the obvious benefits against the possible risks to the health of the eye. Now that extended wear has been FDA approved, any eye practitioner can dispense them, regardless of how much (or how little) experience he has had with them. This is the most promotable form of contact lens wear by far, and the media exposure can sometimes paint too pretty a picture, giving the consumer unrealistic expectations. It is especially important to have a highly experienced ophthalmologist monitor the health of your cornea during extended contact lens wear. True success comes only with strict supervision. Patients must understand the importance of compliance with a strict follow-up schedule of numerous eye examinations, during which subtle changes in the eye may be discovered, indicating that extended wear must be stopped.

Any sign of a problem (red eyes, reduced vision, discharge, or irritation) should be reported to the doctor at once and the lenses should be removed immediately. Reinsertion of the lenses should occur only after your doctor has made certain that your eyes have returned to their normal, healthy state.