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From: dr@ski.UUCP (David Robins)
Newsgroups: net.med
Subject: Re: Contact lenses and astigmatism...
Message-ID: <191@ski.UUCP>
Date: Tue, 6-Aug-85 14:05:27 EDT
Article-I.D.: ski.191
Posted: Tue Aug  6 14:05:27 1985
Date-Received: Sat, 10-Aug-85 22:57:45 EDT
References: <3420@fritz.UUCP>
Organization: Smith-Kettlewell Institute, S.F., CA.
Lines: 59

> Hello.  I hope to glean some information from informed persons on the
> topic of contact lenses.  First, I have a mild astigmatism
-------------->
> from the incorrect curvature axis.   This  space is  filled with tear
> solution such that the  combination of  hard lens,  tear, and natural
> cornea  together  form  a  `new'  cornea  with  the  correct  optical
> properties and curvatures in both axis.   Thus any  `floating' of the
> hard lenses won't  greatly affect  vision quality, unlike soft lenses
> (that correct for astigmatism).  
> 
> I would appreciate if any informed netters out there could tell me if
> the  above information is  pretty much correct, or if the optometrist
> was just passing gas.  Thanks in advance.

The optometrist gave you a very thorough explanation, and your
understanding of it is correct.

Soft contact lenses for astigmatism do need to stay oriented to the
corneal axes in order to compensate for the astigmatism.  This is
accomplished by either ballast weighting (thickening) the bottom of
the lens, so gravity pulls this edge down, or by truncating the edge,
so contact with the lower lid  keeps it in position.

Trouble is, at each blink, the lids move sideways slightly, thereby
torting the lens.  After the blink, the lens realigns itself, but this
takes a short time (less than 1 second, usually). However, if there is
a significant astigmatism, or if one is acutely aware of one's vision,
the time when the lens is not aligned may cause blurred vision, and
can thus be annoying.

These problems are largely eliminated by rigid contact lenses.  Hard
contact lenses (non-permeable) and gas-permeable contact lenses are
spherical on the front and back surfaces.  As long as there is not too
much astigmatism (less than about 2.50 or 3.00 diopters of keratometer-
reading curvature), the spherical surface sits on the toric surface of
the astigmatic cornea reasonably well, thereby forming a new,
spherical front optical surface.  This eliminates the optical
aberrations.  In high astigmatism cases, the posterior curve of the
contact lens can be made toric, instead of spherical, to get a better
fit.  However, then the anterior surface must be toric also.
Manufacturing such lenses is an art, and fitting them is not easy.

Gas-permeable CL's are not as rigid as hard CL's, and
they can bend a bit if there is much corneal toricity.  However, on
the whole, gas-permable CL's are healthier for the cornea.  Hard CL's,
with no oxygen transmission, have sometimes caused permanent corneal
warping.  
Any rigid CL will put pressure on the toric cornea, causing a temporary 
curvature change.  This may be noticeable after taking the lenses off
and putting on one's glasses.  The usual CL should not take more than
about 1/2 hour to bounce back.  Corneal moulding takes longer to undo,
even up to several weeks in severe cases.
-- 
====================================================================
David Robins, M.D. 
Smith-Kettlewell Institute of Visual Sciences
2232 Webster St; San Francisco CA 94115
415/561-1705
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