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Old 03-09-2006, 03:17 AM
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Join Date: Mar 2003
Location: Portland, OR
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How do contact lenses for astigmatism work?

So, how do contact lenses for astigmatism work? I understand the concept for nearsightedmes and farsightednes, but doesn't the corrective lens for astigmatism have to be oriented in a specific way? How do they keep the lens from rotating in your eye? Anybody?
Old 03-09-2006, 03:37 AM
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Toric lenses, to use the technical term, are usually heavier at the bottom than the top, so that they take up the correct alignment through gravity. They also need to be "tailored" more precisely to the patient's eye, so that they're a closer fit and less likely to rotate than normal spherical lenses.
Old 03-09-2006, 04:43 AM
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Quote:
Originally Posted by Tevildo
Toric lenses, to use the technical term, are usually heavier at the bottom than the top, so that they take up the correct alignment through gravity.
Gravity doesn't play a significant role in most modern lenses anymore. The more normal mechanism relies on blinking to keep them oriented. The top and bottom edges are shaped so that the motion of either the top eylid alone or both working together, depending upon the specific lens design, will cause a drag force which the lens rotates into, much like a weather vane always turns to face into the wind.

There's another type which can be used in the case of severe corneal astigmatism, which shapes the inner curvature of the lens so that the lens naturally sits on the cornea in the correct orientation, the way that would happen if you sat a football into an oval indentation.
Old 03-09-2006, 12:20 PM
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Thank you.

(P.S.: Is it considered appropriate on SDMB to thank your answerers?)
Old 03-09-2006, 01:12 PM
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Join Date: Mar 1999
Location: Florida, USA
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Quote:
Originally Posted by Tevildo
They also need to be "tailored" more precisely to the patient's eye, so that they're a closer fit and less likely to rotate than normal spherical lenses.
When I was being fitted for my toric lense, it took 3 or 4 tries with different lenses before one sat right in my eye. I was sure it was a lost cause, and resigned myself to a blurry eye or (ugh) glasses, when finally one worked correctly. That was a fine day.
Old 03-09-2006, 01:25 PM
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My lenses have a little white line on the "bottom" end to help orient them correctly.
I never bother to check, 'cause they end up fine for the reason Q.E.D. stated above.
Old 03-09-2006, 01:29 PM
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Location: Richmond, VA
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Quote:
Originally Posted by garygnu
My lenses have a little white line on the "bottom" end to help orient them correctly.
That's mostly for the eye doctor, not you. The registration marks allow him to see if the lens is properly being rotated into alignment. He'll look to see if it's within a certain number of degrees of the proper orientation, and how much it varies with each blink as part of the fitting process. As Revtim apparently found out, it doesn't always go so smoothly.
Old 03-10-2006, 03:52 AM
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Join Date: Apr 2001
Location: Oklahoma
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So, when you put in a toric lens, will the lens automatically rotate to where it needs to be, or do you have to put it in just right? I also thought the little marks were to help people put them in right, but I just got new contact lenses (acuvue advance for astigmatism), and they have lines on two sides. That threw me. My optometrist didn't mention a special way to put them in, so I just plop 'em in. Occasionally they'll get blurry until I blink a lot, and then my vision will get better.
Old 03-10-2006, 08:39 AM
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Location: Florida, USA
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I just put my toric in, and blink a few times.
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