contact lens primer
Post-PK Contact Lens Care
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
Keratoconus, bullous keratopathy, Fuch's dystrophy and corneal injury are conditions for which penetrating keratoplasty (PK) may be indicated. Spectacles and spherical and toric soft lenses may correct residual refractive error if the post-transplant corneal contour is regular in shape and acceptable vision is achievable. Rigid lenses can correct post-PK residual refractive error or irregular astigmatism resulting from a distorted corneal surface. It has been reported that up to 50 percent of keratoconus patients who undergo PK benefit from post-surgical contact lens wear. If the corneal surface has healed well, contact lens fitting may be performed as soon as three months after the surgery, even with sutures in place with the surgeon's permission.
Fitting Made Difficult
Post-PK corneas present eyecare practitioners with the most challenging contact lens fitting experiences, resulting from their varied corneal topographies. Rigid lens designs with spherical base curves are generally indicated. However, if the refractive astigmatism and corneal toricity are significant and share a similar axis (or 90 degrees apart), a bitoric or base curve toric rigid contact lens may provide the optimal fitting relationship and the best optical correction. When prescribing a toric RGP contact lens, keep in mind that the central (donor) and the peripheral (host) portions of the cornea probably have a different amount or axis of corneal toricity. Topography and fluorescein pattern evaluation will help you determine the optimal design.
If the transplanted cornea is considerably steeper than the host cornea, initially try small diameter spherical or aspheric RGP contact lenses. If the transplant is flatter than the host cornea, consider reverse geometry RGP lenses to minimize excessive central clearance or edge lift and to aid lens centration. If the corneal shape is highly irregular, such as with a tilted graft, a spherical RGP lens with a larger diameter may provide the optimal fit.
Corneal topography is very helpful in understanding the overall contour and localized areas of curvature change of post-PK corneas. Average K values may help you select the initial base curve, but fluorescein pattern interpretation is critical in determining the most appropriate base and peripheral curve radii and optic zone diameter. To enhance lens centration it is often necessary to prescribe lenses with larger overall diameters. It is more difficult to "align" the corneal contour with lenses that have 10.0mm or larger overall diameters (up to 10mm). You may obtain better lens centration at the expense of corneal insult.
Prescribe mid- or high-Dk rigid lens materials to minimize corneal neovascularization.
To monitor the appropriateness of the fitting relationship, observe the fluorescein pattern and corneal staining. An area of harsh touch may cause a corresponding area of mechanical staining in the long-term. Excessive clearance (perhaps evident as an area with bubble formation) may cause localilzed stipple staining or dimple veiling. Adjust the contact lens-to-cornea fitting relationship to minimize corneal staining.
Removing sutures often chan-ges corneal topography, necessitating refitting. For example, the optimal lens design may change from reverse geometry to spherical after suture removal.
Dr. Edrington is a professor and in the contact lens service at the Southern California College of Optometry. E-mail him at firstname.lastname@example.org.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean for clinical affairs at The Ohio State University.