Scleral Shape Revisited
Contact Lens Case Reports
Scleral Shape Revisited
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
At Pacific University, we have a lens design triage that we use in managing patients who have irregular corneal astigmatism. Clinical experience has taught us that the vast majority of these patients can be effectively managed with one of the many new largediameter (10.5mm to 11.5mm) corneal lens designs. If a patient fails to be a good candidate for that modality, our remaining options are a high-Dk piggyback system or a scleral lens design. It is our mantra that patients must "earn the scleral lens option," therefore, the vast majority of our patients who have irregular astigmatism continue to be fitted with large-diameter corneal or piggyback lenses.
Having said that, a number of individuals have corneal shapes that cannot be effectively managed with more traditional modalities; for them, scleral lenses are often an ideal option.
Scleral Shape and Lens Design
Historically, scleral topography has been a difficult anatomical feature to measure, and its shape has most often been described as a curved surface. Therefore, many of today's scleral lenses continue to incorporate spherical or aspherical radii in the scleral haptic portion of the lens. This feature creates an environment in which the periphery of the lens curves in toward the sclera, often resulting in excessive lens tightening on the scleral surface. Recent studies at Pacific University, (in publication) have shown that in the majority of cases, the sclera is not curved but rather is better described as a straight line or tangent. Surprisingly, this tangency extends from approximately 10.0mm out to a cord of 20mm.
Figure 1. Scleral shape as seen and measured by the Visante (Zeiss) anterior segment OCT camera.
Figure 2. Scleral designs that incorporate curved surfaces (spherical or aspherical peripheral radii) may be prone to scleral tightening.
Figure 3. A 16.5mm tangent angle scleral lens design.
Our studies indicate that between the cord of 10.0mm to 15.0mm, the scleral shape is surprising spherical in the eight principle meridians, suggesting that with smaller scleral lens diameters (16.5mm or less), rotationally symmetric haptic designs often perform well. However, our studies go on to indicate that between the cord of 15mm to 20mm, the scleral shape is significantly more meridionalspecific in which one or more of the quadrants are steeper or flatter compared to the rest. This may be an anatomical byproduct of the proximity of the extra-ocular muscle insertion points. Therefore, with larger-diameter scleral lens designs, more complex toric or non-rotationally, quadrant-specific haptic designs may be indicated. These findings may have significant implications for future scleral contact lens designs. 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.
Contact Lens Spectrum, Issue: September 2010