New York Mets infielder Kazuo Matsui sat out Thursday's game against the Astros after suffering a scratched cornea the night before. Apparently, the injury was caused by one of the "therapeutic contact lenses" that he has been wearing while he sleeps. What's a therapeutic contact lens?
A device that reshapes your cornea. Myopia, or nearsightedness, occurs when two clear structures at the front of the eye—the cornea and the lens—focus incoming light at a point in front of the retina, instead of on its surface. This can happen because the cornea is too steeply curved or because the distance from the cornea to the retina is too great. In either case, the problem can be solved if the cornea is flattened, which is exactly what Matsui's lenses are supposed to do.
LASIK eye surgery relies on a similar principle. Doctors cut open a flap on the front of the cornea and then remove corneal tissue to flatten it. With therapeutic contacts, a rigid lens of the right shape can sit on the surface of the eye and force the cornea into a flatter shape.
While the LASIK surgery permanently changes the shape of the cornea, therapeutic lenses have only a short-term effect. When the lenses come out, the patient can see without a problem—but this clarity diminishes over a day or two as the eyes begin to spring back into their original shape.
The practice of reshaping the cornea with rigid contact lenses is called orthokeratology. It may have originated in ancient China, where the nearsighted are said to have slept with sandbags or stones upon their eyelids. In modern times, the practice was first proposed in the early 1960s. At the time, some eye doctors were already prescribing teenagers the flattest contact lenses available; they hoped to keep the cornea from becoming too curved and halt the development of myopia. (Whether orthokeratology lenses can really do this has been the subject of considerable debate.)
In the early years, orthokeratology lenses could be worn only during the day. (The material used to make contact lenses did not allow enough oxygen to reach the cornea when the eyes were closed.) Doctors prescribed progressively flatter lenses until the cornea had reached its ideal shape. After that, the patient needed to put on a pair of "retainer" lenses every now and again to renew the effect. The procedure was inconvenient, unpredictable, and capable only of correcting relatively mild myopia.
The procedure has since improved. Overnight lenses can be made from new, more permeable materials. Using "overnight orthokeratology," Matsui can wear the lenses while he sleeps and take them out every morning. During the day, he can play baseball without worrying about losing a lens or having a limited field of view.
New manufacturing techniques have also made it possible to design a lens with "reverse geometry," in which the center is flat and the perimeter is curved, the opposite of regular lenses. This shape is supposed to produce better results, more quickly. Since 2002, the FDA has approved a number of these devices for overnight wear.
In general, modern orthokeratology has several advantages: It's cheaper than LASIK surgery, and the procedure is reversible. But doctors still disagree about how well orthokeratology works and what its dangers might be. Its popularity has increased over the past five or six years, but some recent evidence suggests that orthokeratology can lead to eye damage. Children may be at special risk.
Explainer thanks Nathan Efron of the University of Manchester.