GP insights
Baffled
by Bitorics? Part 2
BY
JOHN MARK JACKSON, OD, MS, FAAO
My last column discussed bitoric GP lenses and
how they are easier to fit than many clinicians believe. While I still maintain
that this is true, things don't always go according to plan and simple
rules don't always lead to simple solutions.
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Figure
1. Corneal toricity confined to the central portion of the cornea.
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All Rules Have Exceptions
In the first article, I discussed the generally accepted rule
that you should consider fitting a bitoric contact lens when the corneal astigmatism
is more than 2.00DC and definitely fit one when it's more than 3.00DC. In most cases,
a bitoric lens will better match the corneal shape and provide a much better fit.
But all rules have exceptions, and I recently had just such a
situation. I usually order bitoric lenses empirically based on refraction and
keratometry readings rather than performing a diagnostic fit. This is what I did
for the patient I saw recently who had a spectacle Rx of –2.00 –4.00
x180 and Ks of 42.00/46.00 @ 090. With 4.00D of with-the-rule corneal cylinder,
I had no doubt that I needed a bitoric contact lens.
I
ordered the lens empirically as usual. When we tried the lens on, however, the fit
looked terrible. It didn't provide the typical bitoric fluorescein pattern of even
alignment through the optic zone. Instead, the lens decentered and was too tight
vertically.
Topography Lights the Path
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Figure 2. A more common limbus-to-limbus corneal
toricity. |
At this point I did what I probably should have done in the first
place: Corneal topography. The patient's topography map showed that although he
had a large amount of corneal astigmatism, it was fairly confined to the central
cornea (similar to what appears in Figure 1) rather than stretching limbus-to-limbus
(Figure 2). Because the keratometer measures only the central 3mm of the cornea,
it's unable to detect this difference between the central and peripheral cornea.
In this case, because the peripheral cornea is mostly spherical,
a spherical lens is probably the better choice. The spherical base curve will usually
bridge over the central toric cornea and align in the spherical peripheral cornea,
providing an acceptable overall fit. However, when the corneal toricity stretches
from limbus-to-limbus, as is more often the case, a bitoric contact lens will provide
better stability by matching the differing corneal elevations.
Lesson Learned
Most of the time, fitting a bitoric contact lens is a very straightforward
process and you can obtain good results with just the keratometry values. Although
you don't always need the corneal topographer, it can help troubleshoot a wayward
fit or help to pick the proper initial design, as it would have in this case. The
corneal topographer is a remarkable instrument...but it helps only if you use it.
Dr. Jackson is an assistant
professor at Southern College of Optometry where he works in the Advanced Contact
Lens Service, teaches courses in contact lenses and performs clinical research.
Contact Lens Spectrum, Issue: December 2006