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Large-diameter RGP lenses help restore vision in LASIK patients, study shows
Rigid gas permeable lenses may help patients with optical distortions after LASIK recover visual acuity equal to preop levels.
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MAASTRICHT, Netherlands — Rigid contact lenses with large diameters and carefully selected posterior curvatures can improve vision in LASIK patients with decentered ablations, according to a study here.
Decentration of the ablated area in LASIK may occur early in the experience of beginning LASIK surgeons, according to Fred A.G.J. Eggink, OD, FAAO, of the Academic Hospital of Maastricht. It can cause monocular diplopia and a nocturnal halo phenomenon due to multifocality of the corneal surface overlying the entrance pupil.
Most decentered LASIK patients do not have notable distortion of visual acuity during the daytime when their constricted pupil is in the center of their ablated zone. However, when the pupil opens in dim light, the visual zone can extend beyond the ablated zone. In these conditions, the cornea becomes multifocal, and loss of best corrected visual acuity (BCVA) occurs.
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Not straightforward
Fitting decentered LASIK patients with rigid gas-permeable (RGP) contact lenses to correct this problem is “not straightforward,” Dr. Eggink said. Contact lenses with standard geometry are not useful in these patients because of excessive movement and inadequate centration. Problems occur when trying to achieve equal pressure distribution on the cornea, Dr. Eggink said.
Dr. Eggink said that in his experience, using central keratometry as a guide to the posterior optic zone radius of an RGP lens after LASIK will consistently result in a “too-flat fit” that will cause lift-off of the lens. Larger lens diameters allow physicians to achieve better centration of the lenses and “excellent stability on the cornea,” he said.
Dr. Eggink recommended that physicians use RGP contact lenses “with large diameters of 11 mm or larger to avoid instability and decentration of the lens. Use the fluorescein pattern to make adjustments in lens fittings.”
Dr. Eggink uses large-diameter RGP lenses with a posterior optic zone radius that matches the patient’s corneal curvature at a diameter 0.2 mm peripheral to the refractive ablation zone.
“RGP contact lenses offer advantages over soft contact lenses: the RGP lenses correct astigmatism and most other cornea-related optical aberrations, vault over the centrally flattened cornea and provide better and stable visual acuity and improved visual function,” he said.
Successful fit
In this study, Dr. Eggink and colleagues fitted contact lenses in eight eyes of seven LASIK patients; six eyes had a decentered or overcorrected ablation, one eye had an irregular corneal surface and one eye had increased myopia as a result of a retinal detachment. All eyes also had a BCVA loss of at least two Snellen lines.
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In virgin corneas, most contact lens fitting methods use keratometry values in combination with the fluorescein pattern for selection of the posterior radius of curvature. However, after refractive surgery, central keratometry values may not be representative of the entire corneal surface.
“In fact, the radii within the central 3.2-mm optical zone of a cornea after refractive surgery often show no correlation with the radii in more peripheral areas of the same cornea,” Dr. Eggink said.
Using corneal topography can give physicians a better insight into the true shape of the cornea, Dr. Eggink said, which is more helpful than central keratometry. He gave an example from one of his patients.
“When we look at the corneal topography, we see three zones — the decentered deep blue optical zone, the small transition zone and the green/red non-treated areas. When we put the cursor 0.2 mm outside the optical zone into the transition zone, the radius on that spot is 8.4 mm. This radius will determine the back optic zone radius of the trial lens,” he said.
If the lens is too steep, Dr. Eggink said, air bubbles will form under the lens center and disturb the visual acuity. As a result, he designed a lens with an optical zone diameter of 8.5 mm and a total diameter of 12 mm.
Improved sight after fittings
In the eight eyes in this study, mean BCVA before the contact lens fitting was +0.3 LogMAR (Snellen equivalent 20/40). This “improved significantly” (P<.01) to +0.08 LogMAR (Snellen 20/25), Dr. Eggink said.
As long as 6 months after the initial contact lens fittings, no haze was noted and all interfaces were without edema. Dr. Eggink added that the excessive myopic lens power (mean contact lens power was –6.44 D) was due to a “significant” positive tear film noted at the site of the ablation. None of the patients complained of lens dislodgment or of poor centration.
Although topography-guided LASIK is promising and may alleviate the decentration problem, Dr. Eggink suggests using contact lenses until the efficacy of topography-guided LASIK treatments can be firmly established.
Not appropriate for hyperopes
Dr. Eggink and colleagues have had difficulties in fitting contact lenses in hyperopic patients after LASIK.
“In these patients, the center of the cornea is still untreated, and around the corneal peak there is an ablated area that forms a gully drain in which tears accumulate, making it likely that the corneal apex is deprived of sufficient tears for normal contact lens wear,” he said.
For Your Information:Reference:
- Fred A.G.J. Eggink, OD, FAAO, can be reached at the Academic Hospital Maastricht, Department of Ophthalmology, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands; (31) 43-387-5346; fax: (31) 43-387-5343; e-mail: fredeggink@hotmail.com.
- Eggink FAGJ, Beekhuis WH, et al. Rigid gas-permeable contact lens fitting in LASIK patients for the correction of multifocal corneas. Graefe’s Arch Clin Exp Ophthalmol. 2001;239:361-366.