POST-LASIK ORTHO-K
Fitting
Post-LASIK Progressive Myopes with Accelerated Ortho-k
Reverse-curve
lenses and overnight orthokeratology offer an excellent alternative to additional
surgery.
By Bruce T. Williams, OD
After
having successfully fit many orthokeratology patients, and having tentatively avoided
post-LASIK and other post-refractive surgery patients who requested that I help
improve their declining distance vision, I decided to research the literature to
see if I could find any significant contraindications to post-refractive surgery
contact lens corneal reshaping. I found three studies that provided some insight.
Alharbi
and Swarbrick (2003) indicated that the reduction in myopia with orthokeratology
results from epithelial thinning and increased stromal thickening in the midperiphery.
The consequent change in corneal sagittal height and curvature is the primary factor
underlying the ortho-k refractive effect. Reverse geometry lenses appear to cause
a migration of the epithelial cells to the lower-pressure area under the reverse
curve.
Wang et al (2003) from the University
of Waterloo in Waterloo, Ontario, Canada used optical coherence tomography to measure
thickness changes in the epithelium as well as total corneal thickness. Their findings
indicated that, "Immediately after removal of the test lens, the central epithelium
was 5.1 percent ± 4.5 percent thinner than baseline, and all other locations
(P < 0.005 post hoc tests) and the epithelium in the midperiphery showed significant
thickening (1.9 percent on the temporal side and 2.4 percent on the nasal side,
both P < 0.006 compared with the baseline). There were no significant changes
in epithelial thickness with the control lens during the study period (post hoc
tests: P > 0.05)."
Sridharan and Swarbrick (2003) concluded
that the ortho-k response results from the redistribution of the corneal epithelium.
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Figure 1. Difference map OD of Patient FS following
overnight ortho-k treatment.
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Getting Started
Because of the relatively small (only about 5
percent) changes in the tissues involved, I felt it safe to perform orthokeratology
on post-refractive surgery patients. Careful monitoring to grade changes in the
cap interface and position, as well as microfolds, Bowman's cracks or any evidence
of cap dehiscence, is a critical part of follow up for these patients.
Fitting reverse curve lenses is generally
straight forward. The base curve is a mathematical calculation that's based on either
the spherical correction or the spherical equivalent and the flat curve or mean
curve on the cornea. Some reverse curve lenses are fit using the e-value or slope
of the cornea as measured from the center to the periphery. Others use a four-curve
design that contains a base curve, a return curve, a landing zone with a plano or
flat flap, and an engineered edge to provide proper fluid control. I've used several
designs and have found that they all have advantages and disadvantages for specific
corneas. Very steep corneas respond better to thicker, traditional lathe-cut and
polished four-curve designs while "average" curvatures respond well to a thinner,
"harmonic surface" design. Very flat corneas remain challenging and, depending on
the amount of myopia, you can attempt fitting them in stages using a very thin design
and a larger diameter than that of lenses used on an average cornea.
Fitting Post-refractive Surgery Patients
My intention wasn't to re-invent reverse-curve
lenses to accommodate post-refractive surgery corneas, but to use the current technology.
Getting the proper fluorescein pattern with
a reverse-curve, accelerated ortho-k lens was a challenge when fitting post-LASIK
patients because of the relatively flat optical zone with respect to the steeper
midperipheral cornea. After trying several lens designs, I was able to achieve
the best patterns with the flexibility of the adjustable return zone depths (RZD)
and landing zone angles (LZA) of Paragon CRT lenses.
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Figure 2. Difference map OS of Patient FS following
overnight ortho-k treatment. |
I initially fit four post-refractive
surgery patients with Paragon CRT. This is currently an off-label use of these lenses.
Because the myopia was mild in all of these cases, ranging from –0.75D to
–1.75D, I felt that the procedure would be safe because it involved a minimal
amount of tissue movement. The following reports describe three of these four very
successful post-refractive surgery cases — two LASIK and one PRK —
achieved with Paragon CRT.
Case #1 Patient FS is
a 36-year-old male who underwent LASIK surgery in 1997 with a pre-op refraction
of OD –7.75 –0.75 x 070 and OS –7.75 –0.75 x 090. His refraction
one-day post-op was OD plano –0.50 x 005 and OS plano –0.75 x 010.
By 1998, his refraction had slipped
to OD –0.50 –0.50 x 010 and OS –0.25 –0.75 x 180.
By 2001 his refraction was OD –1.25
–0.50 x 010 and OS –0.25 –0.50 x 175.
In the summer of 2002, we found that
his refraction had progressed to OD –1.25 –0.75 x 010 and OS –1.00
–0.50 x 175.
Finally, by the summer of 2003 he measured
OD –1.75 –0.25 x 010 and OS –1.00 –0.50 x 170, and he was
wearing eyeglasses full time. BCVA was 20/20 at all visits.
I began the procedure in July 2003.
I determined a base curve of 8.9mm OD based on FS's manifest refraction sphere (MRS)
of –1.75D and flat K of 40.37D. I determined a base curve of 8.8mm OS based
on his MRS of –1.00D and flat K of 40.00D. I achieved the best fluorescein
patterns with RZDs and LZAs of 500μ and –34 degrees respectively.
We saw FS the next morning while he
was still wearing the lenses. Although the refraction had improved to OD +0.25 –1.00
x 010 and OS plano –0.50 x 150, the fluorescein pattern had sufficiently changed
to indicate that there was excessive edge lift. I didn't want to lose the central
bearing, so I replaced these initial lenses with a second set that had the same
base curves, but I reduced the RZDs to 475μ and increased the LZAs to –35
degrees.
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Figure 3.
Difference map OD of Patient DH following overnight
ortho-k treatment. |
This
lens provided the best fit for the left eye, but I still had centering problems
with the right lens. I increased the LZA OD to –37 degrees. I wanted
to achieve a final correction of OD +0.25 DS and OS plano. FS's visual acuity is
20/20 OU, and he wears the lenses every other night and is happy with the results.
Figures 1 and 2 show the difference maps OD and OS following FS's ortho-k treatment.
Case
#2 Patient DH is a 31-year-old male who'd undergone LASIK approximately
three years prior. He came to our practice for punctal plugs because his surgeon
had suggested that his poor vision resulted from dry eyes. His uncorrected visual
acuities were 20/80 OD and 20/60 OS and his refraction measured –1.25D OD
and –1.00D OS.
We trial fit and dispensed Paragon
CRT lenses with parameters of 8.7mm base curve, 475μ RZD and LZA of 34
degrees OD and 8.6mm base curve, 475μ RZD and LZA of 34 degrees OS. His subsequent
refraction was plano OD and +0.25D OS. Figures 3 and 4 show the subsequent difference
maps.
Case #3 Patient SR is
a 37-year-old male who'd undergone PRK four year prior. He reported that his vision
was fuzzy like a wet windshield. His refraction was –1.25D OD and –0.75D
OS. I noted some residual haze at the level of the pupil margins and the mires
were slightly distorted.
I fit SR with Paragon CRT lenses
with parameters of 8.6mm base curve, 500μ RZD and LZA of 33 degrees OD and
8.5mm base curve, 500μ RZD and LZA of 33 degrees OS. His refraction is now
plano OU. It's important to note that his vision is much crisper because of reduced
surface irregularities from the procedure, which not only flattens the cornea but
smoothes the surface as well.
A Great Alternative
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Difference map OS of Patient DH following
overnight ortho-k treatment. |
After monitoring my four patients for several
months and seeing no change in the appearance of their refractive surgery aftermath,
I've concluded that the ortho-k procedure is safe and effective for such patients.
They're happy to have an alternative to surgical enhancement or re-treatment. Don't
be afraid to ask lens manufacturers to accommodate curvatures and diameters to achieve
the optimum results for these patients. I find manufacturers very helpful and anxious
to know about my patients' progress.
I feel that the opportunity to actively participate
in controlling myopia in post-refractive surgery patients is a responsibility of
the eyecare profession whether we use conventional or orthokeratology lenses. Overnight
ortho-k lenses offer patients an alternative to the current options of additional
surgery, daytime spectacles or contact lenses. Corneal refractive therapy/ortho-k
is another tool to help us provide our patients with optimal visual quality and
convenience.
Dr. Williams is a member of the American Optometric
Association, the Orthokeratology Academy of America, the British Contact Lens Association
and the British Ortho-K Society. He practices in Seattle, WA, in a contact lens-focused
general practice that specializes in orthokeratology. He has also given several
presentations on the subject of orthokeratology.
The author has no financial or proprietary
interest in any of the products mentioned in this article.
For references article, visit
www.clspectrum.com/references.asp and click on document #123.
Contact Lens Spectrum, Issue: February 2006