curl@wjvax.UUCP (Jim Curl) (12/24/85)
I while back I read a few postings about contact lenses and astigmatism.
Someone stated that there are two different types of astigmatism, corneal
and lenticular; the first pertaining to a non-spherical quality of the
cornea and the second referring to a similar feature of the lens.
The orignal poster said that it was impossible for a normal eye exam to
uncover which type one had since they both produced the same effects.
Having just gone in for a set of contact lenses, I questioned the
doctors where I went. In fact, they have a means (which they apparently
used on me) to measure the curvature of the cornea directly and can then
deduce what type of astigmatism you have. In my own case, they found that
I have both corneal *and* lenticular astigmatism. And while corneal
astigmatism tends to fall on the 90 or 180 degree axes while lenticular
can be at any angle, my own case is oddball angles for both eyes and both
corneas and lenses; i.e., each of the four angles is somewhat different.
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.
j curl
san jose, cadr@ski.UUCP (David Robins) (12/31/85)
> 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!