orthokeratology today
Troubleshooting Astigmatic Orthokeratology Fits
BY JOHN MOUNTFORD, DIP. APP. SC, FAAO, FCLSA
Practitioners often ask me to review
difficult orthokeratology cases that haven't gone according to plan. In other
words, they haven't achieved the refractive target or there's unwanted residual
astigmatism. In the huge majority of cases, the simple truth is that the
practitioners have forgotten the basic rules that govern the success or failure
of ortho-k.
Adjust Your Expectations
The most common mistake is that they haven't
properly reconciled the pre-fit refractive astigmatism and the corneal
astigmatism. The FDA has approved most of the corneal reshaping lenses for
reducing up to 6.00D of myopia and 2.00D of astigmatism, but these are the
exceptions rather than the rule.
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Figure 1.
Mildly astigmatic cornea fit with a spherical lens.
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Figure 2.
Same eye with a toric lens, showing improved ortho-k
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Published studies show either no reduction of
astigmatism or, at best, a 50-percent reduction of corneal astigmatism over the
central 2.00mm chord. Therefore, if a patient has 2.00D of pre-fit astigmatism,
the best possible outcome is a residual 1.00DC.
So, let's have a quiz: What's the likely outcome
for the following four patients?
1. The refraction is �2.50/�1.50 @ 180 with K
readings of 44.00D sphere.
2. Same Rx as above, but the K readings are
44.00D @ 180 and 45.50 @ 90.
3. Same Rx as above, but the K readings are
44.00D @ 180 and 44.50 @ 90.
4. The refraction is �2.50/�1.00 @ 180. The K
readings are 44.00 D @ 180 and 46.00 D @ 90.
I'd really like to see some answers to these
questions, which I may include in a later column. Send them to
besty@bigpond.net.au. The most
important hint I can give is that you should always compare the refractive to
the corneal cylinder and take into account the likely effects of induced
lenticular astigmatism.
Review the Topography
The second most common problem is that the
practitioner has poorly understood the pre-fit corneal map. The most common
cause of chronic lens lateral decentration is a markedly flatter nasal cornea
compared to the temporal cornea, and superior decentration resulting in a
difference of greater than 50μm of corneal sag between the flat and steep
meridians.
Figure 1 shows a mildly astigmatic cornea fit
with a spherical lens. The difference in sag was 58μm between the flat and steep
meridians. Note the small treatment zone, and the K readings showed an increase
of the corneal cylinder from 1.20D to 1.609D.
Figure 2 shows the same
eye fit with a "toric" lens where the difference in sag between the steep and
flat meridians was 50μm. Note three main improvements: Increased treatment zone
diameter, better centration and decreased lateral decentration, all resulting in
an increased prescription change.
Dr. Mountford is an optometrist
in private practice specializing in advanced contact lenses for keratoconus,
post refractive surgery and pediatric aphakia. He is a visiting contact lens lecturer
to QUT and UNSW, Australia.
Contact Lens Spectrum, Issue: September 2006