Soft lenses - shallower or steeper BC to prevent decentering?

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Wooly - 03 Jul 2005 15:53 GMT

I wore hard and/or RGPs for years until I moved to Austin. Allergies
and ineffective treatment of same drove me back to spectacles for more
than 10 years. Last week's exam netted me several sets of Biomedics
soft lenses to try. The left lens is a toric and seems to fit
beautifully, tho the power the doc recommended I start with is too
strong.

The right lens decenters badly with a BC of 8.6 I know that a
too-tight lens will tend to peel off with a blink and a too-loose lens
will slide around, but I can't figure out if a bigger number denotes a
shallower or a steeper BC.

I'd be happy to Google if I knew what to look for - my efforts thus
far have produced a lot of spammy pages telling me to ask my eyecare
professional for a contact lens fitting, and about contact lens care
and all manner of other non-relevant stuff.

+++++++++++++

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This practice has cut my spam by more than 95%.
Of course, I did have to abandon a perfectly good email account...

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p.clarkii@gmail.com - 03 Jul 2005 23:48 GMT

a smaller base curve number means steeper. a bigger number means
flatter.
conventional wisdow is that steepening the base curve (smaller number)
would allow the lens to center better, but that isn't necessarily so
with thin piable disposable lenses. its really just trial and error.

if you are wearing biomedics disposables, the available base curves are
8.6 and 8.9

you may need to try a different brand of lens for your right eye. the
toric lens in your left eye is somewhat larger (diameter) and thus may
be stabilized better due to a larger surface area in contact with your
cornea/tear film.

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Wooly - 04 Jul 2005 19:37 GMT

>a smaller base curve number means steeper. a bigger number means
>flatter.
[quoted text clipped - 4 lines]
>if you are wearing biomedics disposables, the available base curves are
>8.6 and 8.9

I guess the logic seems backwards to me - I'd expect a too-tight lens
to decenter on its own and a too-loose lens to decenter on blink. I
do have a lens check appointment this week so I'll see what else they
can try on my right eye.

+++++++++++++

Reply to the list as I do not publish an email address to USENET.
This practice has cut my spam by more than 95%.
Of course, I did have to abandon a perfectly good email account...

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Mike Tyner - 04 Jul 2005 19:54 GMT

> I guess the logic seems backwards to me - I'd expect a too-tight lens
> to decenter on its own and a too-loose lens to decenter on blink.

Too-tight lenses usually center very well, and they're often very
comfortable for the first couple of hours. They often "pooch" in the center,
a bag of tears that wrinkles or varies in thickness on blink, yielding
variable focus.

A loose lens usually does "bob" too much on blinking, but sometimes when
they're too flat, they just stick like cellophane, high up and temporal.

But don't get locked into a simple flat-and-loose continuum. Corneas are
seldom spherical, but rather be oblate or prolate. Contacts can be spherical
or aspheric, oblate or prolate.

The bottom line is there's no measurement that guarantees the fit. You gotta
try several in real life before saying one is the "best."

-MT

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