Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site ski.UUCP Path: utzoo!watmath!clyde!burl!ulysses!allegra!mit-eddie!think!harvard!seismo!lll-crg!well!ptsfa!ski!dr From: dr@ski.UUCP (David Robins) Newsgroups: net.med Subject: Re: Astigmatism Message-ID: <219@ski.UUCP> Date: Mon, 30-Dec-85 16:21:44 EST Article-I.D.: ski.219 Posted: Mon Dec 30 16:21:44 1985 Date-Received: Wed, 1-Jan-86 02:40:42 EST References: <635@wjvax.wjvax.UUCP> Organization: Smith-Kettlewell Institute, S.F., CA. Lines: 40 > I have both corneal *and* lenticular astigmatism. And while corneal > > The result of all this? I can't use hard lenses to correct my astigmatism > since a large component of the astigmatism is lenticular. But since my > overall astigmatism is not that acute, I just bought a pair of non-astigmatic > contacts to correct my nearsightedness. I beg to differ with your fitter. Rigid lenses *CAN* correct corneal *AND* lenticular astigmatism. But, it isn't easy, and comfort may suffer. (Gas-permeable lenses would be the recommended material these days). How this is done: 1) The coneal astigmatism is corrected as always, by covering the astigmatic cornea with a spherical back-surface lens. This is given prism ballast, to keep it oriented correctly on the cornea. The lenticular astigmatism correction is then ground-in on the front surface of the lens (similar to the way eyeglasses are made to correct astigmatism). This is called a front-toric lens. 2) If there is more than about 2.50 or 3.00 dioptics of corneal astigmatism, a spherical lens may rock (see-saw) on the cornea. This is minimized by using small lenses. If a good fit cannot be made with the spherical back-surface lens, then a bi-toric lens can be use. The back of the lens is ground with and "astigmatic" surface to match the corneal curvature, allowing some tear space. This "locks" the lens orientation to the cornea, so prism ballast is not needed. Then, the remaining uncorrected corneal astigmatism is ground on the front surface (The toric back surface does not bridge over the cornea, and thus does not correct the corneal astigmatism). The lenticular astigmatism correction is also ground in to the front surface. Admittedly, the fitting and manufacturing costs of this type of lens is rather expensive, but it can be done if needed. -- ==================================================================== David Robins, M.D. Smith-Kettlewell Institute of Visual Sciences 2232 Webster St; San Francisco CA 94115 415/561-1705 {ihnp4,qantel,dual}!ptsfa!ski!dr The opinions expressed herein do not reflect the opinion of the Institute!