Shape Change With Contact Lenses In Situ
BY JOSEPH T. BARR, OD, MS,
It doesn't just roll off your tongue does it? Does orthokeratology? You could call it "OK." Does corneal refractive treatment? Then you could call it "CRT." Does overnight orthokeratology make sense? You could call it "ONOK" or "OO." What about accelerated overnight orthokeratology ? You could call it "AOOK." Do any of them roll off your tongue like LASIK? Today's orthokeratology and overnight orthokeratology (not yet FDA approved in the United States) are not your grandfather's ortho-k. It's not a process using a sequence of lenses to flatten the cornea over many months to reduce myopic dependence on glasses and contact lenses. The idea today is to get it right with one pair of lenses. Some feel the old name is associated with a not-so-popular, cult-like niche. So maybe it needs a new name.
At least one manufacturer likes the term corneal refractive therapy. It happens to have a branded product with that name. But can we really get everyone to change the words they use in a field that is already full with nonstandardized jargon? Time, success and marketing will tell.
Modern (overnight) orthokeratology has the goal of leaving the cornea shaped much like it underwent PRK or LASIK. The lens back surface doesn't gradually flatten like 99.99 percent of the other lenses. The base curve is flatter than the flat central corneal meridian by about 0.50 diopters more than the spherical component of the refractive error. At least we can agree on the term base curve. These lenses have been designed with three, four and five posterior curves, areas or zones. Some of the zones are like typical spherical curves, some are aspheric, some are sigmoidal and some are sections of cones, i.e. flat, but angled.
Then it gets tricky. What do we call the second curve, area or zone on the back surface? A number of terms have been proposed. Patent and financial interests are involved here, folks, so this is not simple. We could call it the reverse zone or the return zone or the steepening zone or the secondary zone (which it is, by the way). Calling it a curve is not always exactly right. ANSI Standards would dictate the words "intermediate zone" or "secondary curve" for this area that adds sagittal depth to the posterior surface.
The next area or zone is designed to align the cornea and keep the lens centered. It is actually a tertiary zone. The last zone is to get a little edge lift, or it may just be the edge curve.
Whatever you call the curves or zones on these lenses, two things are for sure. They are not your grandfather's ortho-k lenses, and we should stick to ANSI standard terms to describe them: base curve, secondary or intermediate curve or zone, tertiary (alignment) curve or zone, peripheral curve or zone. ANSI standards do not describe a reverse or return secondary curve area.
But is it orthokeratology, corneal refractive treatment or what?
Contact Lens Spectrum, Issue: October 2001