prescribing for astigmatism
Using Larger Diameter GPs to Fit Irregular Corneas
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO
Within the last two years, I've been prescribing large overall diameter (OAD) GP contact lenses to manage many of my patients who have irregular corneas. Patients who have pellucid marginal degeneration as well as post-surgical patients (including post-penetrating keratoplasty (PK), post-radial keratotomy (RK) and post-LASIK patients) may particularly benefit from large diameter GP lenses.
Several GP laboratories offer large OAD GP lenses. Lens Dynamics, Inc. makes the Intra-Limbal lens (11.2mm OAD), Innovations in Sight makes the Jupiter lens (15.0mm OAD) and C&H Contact Lens, Inc. makes the Macrolens Einstein (15.0mm OAD).
Why Increase GP OAD?
The emergence of high-Dk GP materials has helped to hasten the growth of larger OAD GP lens fits because the greater oxygen transmissibility reduces the need for lens movement and tear exchange to provide oxygen. The primary benefit of large OAD lenses is enhanced centration. Another occasional benefit is improved comfort resulting from decreased lid-lens interaction.
Large OAD Design Options
In addition to spherical design GPs, bitoric large OAD lenses can optimize the lens-to-cornea fitting relationship and correct regular residual astigmatism. You can prescribe prism to stabilize lens rotation on a front-surface toric design or to center a superior positioning lens.
You can employ a reverse geometry peripheral curve system to decrease edge lift and to enhance lens centration when fitting a post-surgical cornea that has an oblate shape.
Post-PK Fitting Tips
Prescribe small or regular diameter GP lenses for post-PK patients who have steep central corneal curvatures relative to the host cornea. If the donor cornea is flatter than the host, then consider a reverse geometry design in which the "secondary" curve portion of the lens is steeper than the base curve.
For grafts that are highly irregular in shape, consider a large OAD GP lens design. Also, if lens centration isn't adequate, then fit a large OAD lens.
Post-RK Fitting Tips
For post-RK patients, first try fitting a standard sphere (or aspheric) GP lens design. If the central fitting relationship is ideal but peripheral clearance seems excessive, then prescribe a reverse geometry lens design. If the best-fit, standard-diameter reverse geometry design doesn't adequately center, then consider a large OAD lens with a reverse geometry peripheral curve system.
I've observed several post-RK patients who have superior areas of steep curvature that result in "high-riding" or lid-attached fitting relationships. Prescribe prism to center these superior positioning lenses.
Large OAD for Keratoconus?
In my opinion, large OAD GP contact lenses are contraindicated for correcting keratoconus. The exception is for a keratoconus patient who has an inferiorly displaced corneal apex.
In general, keratoconus patients require a relatively flat peripheral curve to obtain adequate tear exchange. The larger the overall diameter, the more challenging it becomes to achieve sufficient tear exchange for most keratoconus patients.
Keep the Goal in Mind
In deciding whether to use a large OAD GP lens, remember that your goal when fitting an irregular corneal surface is to minimize localized areas of excessive clearance and/or harsh corneal bearing.
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at firstname.lastname@example.org.