Although soft lenses can work well for many patients who need correction after undergoing laser-assisted in situ keratomileusis (LASIK) or radial keratotomy (RK), rigid gas permeable lenses (RGPs) are an excellent alternative:

1.      RGPs are available in many highly oxygen permeable plastics.

2.      RGPs can be custom designed to accommodate unusual corneal contours.

3.      The stability and rigidity of RGPs produce optimum visual performance.

4.      Efficient tear exchange removes tear debris and corneal byproducts.

5.      Formation of a posterior lens tear pool helps to neutralize corneal irregularities and maximize visual clarity.

Appropriate design can address problems often associated with RGPs, such as discomfort and centering problems.

Fitting standard rgp designs

Standard, spherical RGPs work well for many of these patients. The main rule is to steepen the base curve if the fit is too flat (causing excessive peripheral lift-off) and flatten the base curve if the lens is too steep (indicated by excessive peripheral impingement and central pooling).

I begin by selecting a high oxygen flouorosilicone acrylate material in a base curve that's close to the flat K of the preoperative keratometry measurement. (This may be 3D to 6D steeper than the postoperative K.)

The lens should align over the mid-periphery and vault the central optical zone. If an air bubble results, I try flatter base curves in 0.5D steps until the bubble disappears.

I also:

�         refract over a diagnostic lens to determine power

�         choose large diameter lenses (9.5 to 11 mm) to aid centration

�         choose lenses with large optical zones (around 8 mm)

�         allow 2 mm for transition curves (the intermediate curve should be 1 mm flatter than the central base curve)

�         use a 0.4 mm wide, 12.25 mm radius peripheral curve to provide plenty of edge lift and facilitate tear flow under the lens.

I evaluate lens movement and centration using fluorescein and adjust the base curve and lens diameter accordingly.

Nonstandard RGP alternatives

When a standard RGP can't meet a patient's needs, try these options:

�         Reverse geometry lenses. These incorporate secondary curves that are 3 to 6 diopters steeper than their central curves, helping to maintain alignment over a plateau-shaped cornea. They often create greater stability and centration, while preventing excessive tear pooling.

�         The Lexington RK Splint. This lens was designed for use with corneal transplant patients, but can work well for post-RK patients with minor modifications. It features a central base curve and four peripheral curves. (The latter are designed to simulate the aspheric shape of a normal peripheral cornea.)

�         The NRK lens. This lens, designed for plateau-shaped corneas, offers four different base curves, large diameters in the
10-mm range, and large optic zones (about 8 mm). It can be produced using any material, in sophisticated lens designs such as prism ballast toric, back toric and bifocal. Fitting sets are available.

�         Menicon plateau lenses. These feature a central base curve and three pe-ripheral curves. Fitting sets are available.

�         Macrolens designs. These come in diameters of 14 and 15 mm; they're made of high DK/L materials. A laser hole facilitates tear exchange and oxygen transmission. They're stable on the eye.

I've used this lens on post-graft and keratoconus patients, and it's well-suited for post-RK patients. Overall, patients report that this lens is comfortable, and vision is usually excellent.

Part 3: Soft lens options.

Dr. Koffler is director of the Kentucky Center for Vision in Lexington, Ky., and associate clinical professor of ophthalmology at the University of Kentucky Medical Center. He currently serves on the board of directors of the Contact Lens Association of Ophthalmologists, and is past president of the Kentucky Academy of Eye Physicians and Surgeons.