20 Pearls for Managing Post-PK Patients With GP Lenses, Part 2
BY EDWARD S. BENNETT, OD, MSED, ROBERT M. GROHE, OD, KEN MALLER, OD, & SCOTT EDMONDS, OD
This is the second in a three-column series pertaining to GP management of a challenging and variable condition: post-penetrating keratoplasty (PK). This article focuses on material selection, lens design, and fitting considerations.
Step 7. Use a high-Dk lens material to minimize vascularization of the graft.
Step 8. Selecting a base curve radius can be challenging, although the initial choice simply represents a starting point with the intention of making changes based upon the fluorescein pattern. Using the simple selection criteria of the specific design you are fitting is a good starting point; often these designs use approximately these criteria:
a. With keratometry, the initial base curve radius equals the average of the flat K and the steep K, or with corneal topography use the Sim K.
b. Use the temporal peripheral keratometry value.
c. With topography, use the cursor value in the temporal quadrant at the 4mm to 5mm area.
Step 9. Do not use central corneal toricity as a primary determinant for selecting the base curve radius as, for the most part, this area of the cornea does not — or should not — support the lens. This is especially important when considering a bitoric lens design in cases of moderate central but less peripheral corneal toricity. Bitoric lens designs are contraindicated for most post-PK patients because of the resulting corneal irregularity, although they can be successful if the topography reveals a mixed (both prolate and oblate) astigmatism or a tilted graft.
Step 10. Design the final base curve radius to avoid extreme bearing or central bubbles. There will probably be several midperipheral micro bubbles, which usually disappear by the first follow-up visit. However, central bubbles are unacceptable and require refitting with a flatter base curve radius.
Step 11. Patients should wear the best initial lens design for two weeks to allow for settling and to yield a more stable and reliable over-refraction. A final over-refraction at dispensing can be misleading and may result in unnecessary exchanges.
Step 12. Even with large-diameter lenses (often 10.3mm to 11.2mm), it is advisable to keep the optical zone diameter relatively small so you can take advantage of using multiple peripheral curves to better align with the corresponding region of the midperipheral cornea. The outer curve can consist of a flat curvature with a narrow (0.2mm) width.
Step 13. Vary the peripheral curve system and the resulting edge clearance as needed. The ability to do this is one of the most important benefits that has resulted from new technological advances in manufacturing. Whether this is accomplished with differing edge curves (i.e., steep, standard, and flat such as with the Rose K [Menicon/Blanchard Contact Lens] system), by tucking the inferior edge inward to reduce excessive edge clearance in this region (i.e., the Steep-Flat system from Lens Dynamics or the Asymmetric Corneal Technology from Blanchard), or even by varying the eccentricity in each peripheral quadrant (as with the QuadraKone from TruForm Optics), multiple options are available to achieve an acceptable peripheral lens-to-cornea fitting relationship. CLS
Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and is executive director of the GP Lens Institute. Dr. Grohe specializes in contact lenses and anterior segment in his suburban Chicago practices while also being associated with the Northwestern University School of Medicine. Dr. Maller works in a private practice in Fort Lauderdale, FL, that focuses solely on contact lenses primarily for irregular corneas and orthokeratology. Dr. Edmonds owns and manages the Edmonds Eye Associates a hospital based multi-location private practices. He is the co-director of the Contact Lens and Low Vision Service at Wills Eye Institute.