contact lens case reports
Fitting Semi-Scleral Lenses on Irregular Corneas
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Semi-scleral GP lens designs have been a tremendous addition for managing patients who have irregular and/or asymmetric corneal astigmatism following corneal injury, surgery or disease. These lenses can also serve as therapeutic lenses in ocular surface anomalies such as chemical burns, ocular pemphigoid and Stevens-Johnson syndrome.
Correcting Corneal Plateauing
Our patient is a 59-year-old male who underwent bilateral penetrating keratoplasty in the 1970s for keratoconus. Post-operatively he experienced significant corneal plateauing (a flat midcentral topography with a steep midperipheral topography). Since the late ’70s, all attempts with contact lenses have failed and the patient has relied on spectacle lenses despite suboptimal visual acuity.
His simulated keratometric readings were OD 37.00 @ 160/43.25 @ 075, OS 39.00 @ 005/42.50 @ 095 with significant corneal steepening in the midperiphery. We elected to fit a semi-scleral lens, which begins with selecting the appropriate lens diameter based on the overall size of the fissures and globe. Lens diameters can range from 13.0mm to 18.2mm, and we determined that the patient's anatomy could support an 18.2mm semi-scleral lens.
To determine the base curve radius on highly irregular or asymmetric corneas, select a diagnostic lens in the middle of the set. The radii of the diagnostic lenses range from 38.00D to 52.00D, so the mean diagnostic lens is 45.00D (7.50mm). Next instill fluorescein onto the concave side of the lens and apply it to the cornea. The resulting fluorescein pattern will be too steep, too flat or optimum (Figure 1). The optimum base curve should provide 25-to-35 microns of apical clearance and increased limbal clearance (as evidenced by a brighter ring of fluorescein at the limbus). Unlike rigid corneal lenses, a properly fitted semi-scleral lens will not move with the blink.
Figure 1. Steep, flat and optimum fluorescein pattern.
Finally, the scleral lens radius should align with the lens edge just above the scleral epithelium. If the scleral radius is too steep, increase the lens edge clearance by 50 microns. If the scleral radius is excessive, decrease the lens edge clearance by 50 microns. Determine lens power by a refraction over the best-fitting diagnostic lens.
Figure 2. Corneal topographies and ultimate fluorescein patterns.
Our patient's final contact lens prescription was OD 49.00D (6.90mm) base curve, 18.2mm diameter, –C13.00D and OS 50.00D (7.65mm) base curve, 18.2mm diameter, –C15.25D. Visual acuity was 20/20 OU and wearing time was 14 to 16 hours a day. CLS
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Paragon Vision Sciences. Mark André is an associate professor of optometry at Pacific University. He is also a consultant for CooperVision.