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From: gordon@cae780.UUCP (Brian Gordon)
Newsgroups: net.med,net.consumers
Subject: Re: Hard lens advise request
Message-ID: <1623@cae780.UUCP>
Date: Wed, 27-Nov-85 15:12:04 EST
Article-I.D.: cae780.1623
Posted: Wed Nov 27 15:12:04 1985
Date-Received: Fri, 29-Nov-85 21:52:14 EST
References: <697@leadsv.UUCP> <557@ttidcb.UUCP>
Reply-To: gordon@cae780.UUCP (Brian Gordon)
Organization: Tektronix, Inc. (CAE Systems Division), Sunnyvale, CA
Lines: 55
Xref: watmath net.med:2862 net.consumers:3495

>>	I have gone for years without considering contacts because they
>>could not be used for astigmatism.  Now that has changed, but I have heard
>>that the sort of contacts that you must wear must keep a specific 
>>orientation; i.e. you cannot but them in just any way.  
>
>Yes, the lenses are thicker (hence heavier) on the bottom.  This tends
>to keep them oriented properly.  This mechanism is good for correcting up
>to about 2.5 diopters of astigmatism.  

Let's see how far an amateur can insert his foot into his mouth.  (I have no
medical background, but great personal interest in recent developments in
contact lenses.)

"Typical" astigmatism can be thought of as a systematic irregularity of the
surface of the cornea.  Eyeglasses "correct" for it by bending (i.e.
distorting) the light as it passes through, so that the redistortion by the
cornea brings it back to its original path.  This involves a cylindrical
correction along an appropriate axis in the lens.  Soft contacts use the
same technique, but have the additional problem of keeping the lens properly
oriented so that the axis of correction is in the right place.

One technique is to "weight" the bottom edge, allowing gravity to help keep
the orientation.  An alternate method is to thin the top and bottom, so that
the eyelids do the work (by migrating the lens so that the thin parts are
hit by the lids, leaving the thicker center band where you want it).  There
are presumably different limits to the degree of astigmatism that can be
corrected while still allowing either type of positioning to be used.

Hard lenses use a completely different technique.  As long as the source of
the astigmatism is the eye surface (this covers a high percentage of cases),
the hard lens, in effect, replaces your cornea -- your effective cornea is
the cornea/tear-layer/contact-lens package, and its surface is not distorted.
Thus, the astigmatic correction is "free" -- not even part of the prescription.
This is obviously an oversimplification of details, but, I believe, essentially
accurate.

The newest "ultra permeable" hard lenses seem to have a limited geographical 
distribution so far, as I have received several requests asking for more 
specific pointers for their eye-care professionals to follow-up.  There are at 
least three vendors, and the brand names are all of the form "xxxxx 39", the 
"39" apparently describing the material.  The ones I have are "Ultraperm 39", 
by Alcon.  They are more permeable than the present extended wear SOFT lenses, 
which makes them about twice as permeable as previous gas permeable hard lenses.
They are also significantly thinner than some older lenses, making adaption
much easier -- but they are still hard lenses, not as easy as soft ones.

If you couldn't tolerate hard lenses a couple of years ago, times may be
different now ...

FROM:   Brian G. Gordon, CAE Systems Division of Tektronix, Inc.
UUCP:   tektronix!teklds!cae780!gordon
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