Prescribing for Astigmatism

Dealing with the Past: Modern Lenses for RK Patients

prescribing for astigmatism
Dealing with the Past: Modern Lenses for RK Patients

Twenty years ago the pages of the fledgling Contact Lens Spectrum gave frequent attention to the then-new phenomenon of radial keratotomy (RK). The debate over the advisability of the procedure is long quieted down, but the sins of that generation are now visited on the current generation of contact lens practitioners in the thousands of post-RK patients presenting with progressive hyperopia along with both regular and irregular astigmatism. It seems appropriate for this anniversary issue to review briefly the options available today for this group of patients who remind us frequently of the past.

Gas Permeable Lenses

The mainstay for post-RK fitting continues to be the GP lens in one form or another. GP lenses provide neutralization of the corneal irregularities that are common in these patients, and stability for those who suffer from significant fluctuation in refractive error over the course of the day. Achieving centration and avoiding excessive bearing are key challenges.

Practitioners can often use conventional spherical or aspheric lenses effectively. Preoperative readings can help you determine an initial base curve to choose, but these are rarely available. Instead, use the topographical map to determine a mean curvature for the cornea and start with that. If centration is an issue (and it often is because of uneven healing) increasing the diameter is helpful. Don't be surprised if you need a diameter in the 10mm to 11mm range.

When the best-fitting lens ends up excessively steep in the central zone, it may give rise to air bubble formation that degrades the vision, and you may need a reverse geometry design. Usually it's sufficient to make the central zone two to three diopters flatter than the "fitting" curve. Keep in mind that you're not trying to do orthokeratology; the goal is simply to decrease the tear volume in the central zone.

Soft Contact Lenses

As long as there's minimal diurnal variation to the refraction, soft contact lenses can be effective. Because many post-RK patients are hyperopic, one concern has been the high incidence of vascularization with thick, low-Dk lenses.

Fortunately, silicone hydrogel lenses have alleviated this problem, and also bring an additional advantage with some materials of higher modulus, which is effective in correcting some irregular astigmatism. The expansion of toric silicone hydrogels, and the eventual availability of custom designs in these materials, will make this an increasingly appealing choice.

Piggyback and Hybrid Lenses

When patients need a rigid lens for vision, but comfort is unacceptable, the use of a silicone hydrogel lens along with a GP lens can provide post-RK patients with the combination they need. It's also possible to use the piggyback approach to improve centration of a rigid lens by utilizing a plus power silicone hydrogel to effectively steepen the central curvature.

Hybrid lenses also hold some promise for post-RK patients. A post-surgical hybrid design is expected in the near future.

A Vast Improvement

Fortunately for patients and practitioners alike, contact lens technology has improved vastly since the time these patients underwent this ill-conceived surgery. The harm done to many of these patients cannot be undone, but we are now far better equipped to help them deal with the consequences.

Dr. Bergenske, a past chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for over 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. E-mail him at: