Article Date: 12/1/2002

CORNEAL RESHAPING
Combining Two Methods of Correcting Myopia
With a thin cornea post-LASIK, a patient successfully wears reverse geometry lenses to correct residual myopia.
By S. Barry Eiden, OD, FAAO, and Amber Dawson, OD

LASIK has matured from an "experimental" procedure into a mainstream third option for vision correction, joining spectacles and contact lenses. Since the initial FDA approval of the excimer laser for PRK in 1995 and the rapid conversion of its use for LASIK soon after, well over three million eyes have been treated in the United States alone. Initially hailed as the procedure that would make glasses and contact lenses obsolete, 2001 signaled the first year that the number of LASIK procedures performed actually declined as compared to the year before. As time has gone by, increasing numbers of cases have come to the forefront which have resulted in limited outcomes and significant complications. Management of LASIK complications and limited outcomes is becoming an ever-growing component of many eye care practices.

Because LASIK is dependent on ablation and removal of corneal stromal tissue, the total amount of vision correction is limited by corneal thickness. Numerous cases present to practitioners in which enhancement treatments are impossible due to inadequate corneal thickness. Until now, options for LASIK undercorrection have been limited to spectacles and conventional contact lenses (both rigid gas permeable and hydrogel).

We present a case of a patient who underwent bilateral LASIK for relatively high myopia. The patient subsequently had an enhancement treatment for undercorrection, yet was still left with a residual myopic refraction. Corneal thickness following enhancement was inadequate for further LASIK treatment. We successfully performed corneal refractive therapy/accelerated orthokeratology to reduce the residual myopia during waking hours.

Case Report

YC, a 51-year-old Asian female, underwent bilateral LASIK for the treatment of myopia in 1998. Pre-surgical refractive error was OD ­7.00 ­1.00 x 60 and OS ­8.00 ­0.75 x 100. Prior to surgery each eye was correctable to 20/20-2 visual acuity. A monovision correction was performed in which the left eye was calculated for near vision correction. Following three months of post-operative care, the refraction stabilized at OD ­2.00 ­0.75 x 50, 20/25-1 and OS ­3.75 ­0.50 x 90, 20/25-1.

Although satisfied with her uncorrected near vision, YC obviously was dissatisfied with the distance vision result of her surgery. The patient underwent an enhancement LASIK procedure OD for distance vision. Over a period of one year following enhancement OD, the residual refraction regressed to OD ­1.75 ­0.75 x 45, 20/25+ (uncorrected visual acuity was 20/200) and OS ­4.50 ­0.25 x 120, 20/25+1.

Due to the patient's habitual close nearpoint working distance, she had no complaints about her near vision, but distance vision was still a significant problem. Unfortunately, the patient's residual central pachymetry of the right eye was 378 microns. Based on an approximation of 12 microns per diopter of laser correction and a flap thickness of 180 microns, there was inadequate stromal thickness to allow for a safe second enhancement OD. She wore spectacles and a daily disposable contact lens OD to manage her residual myopia. As expected, the patient was far less than satisfied with her condition and was very interested in any other technologies that could improve her uncorrected vision in her right eye.

Figure 1. Patient's post-LASIK topography prior to CRT lens treatment.

Corneal Refractive Therapy after LASIK

Based on our experience in treating normal myopic individuals with corneal reshaping (accelerated orthokeratology), the concept of treating YC's residual myopia in this manner was proposed. Prior to attempting CRT with YC, we clearly advised her that this was a non-approved and untested use of corneal reshapingtechnology. We felt that the risks associated with this attempt were quite minimal, and therefore we proceeded to treat YC accordingly.

Conceptually, we felt that the lens design should be based on both YC's pre- and post-operative corneal topographic findings. We calculated the central treatment zone (diameter 6.0mm) based on the post-operative findings (Figure 1) and the intermediate and peripheral curves based on pre-operative findings. The reverse curve was modified based on calculations from both the pre- and post-operative topographic findings. The initial lens design was a four-zone reverse geometry lens made from Boston XO (Bausch & Lomb) material. The initial parameters are indicated in Table 1.

TABLE 1:  Initial Lens Design

MATERIAL: Boston XO Blue
BASE CURVE: 9.70mm
OPTIC ZONE: 6.0mm
TOTAL DIAMETER: 10.0mm
REVERSE CURVE: 7.50mm
REVERSE WIDTH: 0.60mm
ALIGNMENT CURVE: 8.50mm
ALIGNMENT WIDTH: 1.0mm
PERIPHERAL CURVE: 11.50mm
PERIPHERAL WIDTH: 0.40mm
CENTER THICKNESS: 0.21mm
POWER: +0.75D

The lens centered well over the corneal surface and demonstrated a classic fluorescein pattern found with reverse geometry lens designs. At the 24-hour follow up visit, the refraction had reduced to ­1.00 ­0.50 x 40, 20/25+2, with a corresponding improvement in uncorrected acuity to 20/50. The patient was given daily disposable lenses of an appropriate power to use as needed until the next scheduled follow-up visit.

The 72-hour visit revealed no significant change in refractive status or uncorrected acuity. By the one-week follow-up visit, only a subtle improvement had occurred. The refraction OD was ­0.75 ­0.50 x 50, 20/25, and uncorrected acuity was 20/40-. These measurements were taken about 10 hours after removal of the contact lens. There was no evidence of biomicroscopic complications, and YC reported excellent tolerance of the lens.

Based on the limited performance of the initial lens, we proceeded to modify the design by flattening the central treatment zone, enlarging the diameter of the treatment zone and correspondingly modifying the reverse and intermediate zones (Table 2).

One week after dispensing the second lens, the patient returned for follow up. She reported that her uncorrected vision had improved significantly, comfort with the lens and without the lens had remained stable, and at the time of the visit she had been without her lens for 14 hours. Uncorrected visual acuity had improved to 20/25-2, and refraction had correspondingly improved to ­0.25 ­0.25 x 40, 20/25. Biomicroscopy remained normal in the treated eye.

A subsequent follow-up visit was performed about six weeks after dispensing the second lens. Uncorrected visual acuity was 20/25-1 with a manifest refraction of +0.25 ­0.50 x 45, 20/25. The lens had been removed about nine hours prior to the visit.

Figure 2. Post-corneal reshaping corneal map shows no evidence of lens-induced irregularity.

Comparison of corneal topography maps prior and following corneal refractive therapy OD demonstrates a flattening of SimKs of a bit less than 1.00 diopter along the flat meridian and of slightly less than 1.50 diopters along the steep meridian. There was no topographic evidence of lens-induced irregularity (Figure 2). The patient further reported excellent stability of vision throughout the day with only mild fluctuations noted. She continues to wear the lens each evening for an average period of seven to nine hours, and noted that failure to use the lens for more than one night resulted in noticeable reduction of vision at distance by the following afternoon.

Discussion

Corneal reshaping, or accelerated orthokeratology, is an increasingly popular alternative to vision correction. Its primary use is for the temporary correction of normal myopia and astigmatism and the associated control of progressive myopia. Over time we expect to see more creative uses of this technology, such as we have presented in our case. Consider using reverse geometry GP lenses in managing refractive surgery undercorrections in cases of inability to perform enhancements due to inadequate corneal thickness, or cases of small degrees of undercorrection where the risks of enhancement procedures may outweigh the benefits. Consider these lenses also in situations in which patients may not feel comfortable undergoing further surgical procedures following initial refractive surgery.

TABLE 2 Modified and Final Lens Design
MATERIAL: Boston XO Blue
BASE CURVE: 9.85mm
OPTIC ZONE:  6.5mm
TOTAL DIAMETER: 10.5mm
REVERSE CURVE: 7.55mm
REVERSE WIDTH: 0.60mm
ALIGNMENT CURVE: 8.55mm
ALIGNMENT WIDTH: 1.0mm
PERIPHERAL CURVE: 11.50mm
PERIPHERAL WIDTH: 0.40mm
CENTER THICKNESS: 0.21mm
POWER: +1.50D

Calculating lens parameters used for the treatment of refractive surgery undercorrections is a challenge and does not fit into the nomograms currently available for reverse geometry lens fitting. Using both pre- and post-surgical data provides information to assist in the design of lenses for these cases. The central treatment zone should be based on the post-surgical topography, while reverse curve parameters are determined by information gained from pre- and post-surgical topography in terms of treatment effect in the region of the cornea corresponding to the reverse curve section of the lens. Peripheral curves are primarily based on pre-surgical findings. Ultimate determination of lens parameters are made with the evaluation of lenses on the eye in terms of positioning, corneal alignment (fluorescein patterns), lens movement and ultimately clinical performance of reduction of refractive error.

The most appropriate fitting and management techniques for treating residual refractive error following refractive surgery with reverse geometry GP lenses will need to be determined based on a large-scale treatment trial. Currently, the treatment described here is considered an off-label use of this lens design.

Dr. Eiden is president of a private group practice specializing in primary eye care, contact lenses and refractive surgery. He is also an assistant clinical professor at the University of Illinois at Chicago Medical Center in the Department of Ophthalmology, Cornea and Contact Lens Service.

Dr. Dawson specializes in comprehensive family eye care, binocular vision services and the diagnosis and treatment of ocular disease. She also has specific expertise and experience in pediatric eye care.

 


Contact Lens Spectrum, Issue: December 2002