Matching Scleral Design with Ocular Shape – Advancements in measurements and manufacturing can improve fitting efficiency and success.
The majority of practitioners fitting scleral contact lenses rely on diagnostic lenses for fitting. Diagnostic lenses primarily allow practitioners to estimate the amount of sagittal depth needed to properly vault the chord diameter of the ocular surface that is being fit. Additionally, the diagnostic lens allows for assessment of haptic alignment. Previous ocular coherence tomography studies have shown that scleral shape is non-rotationally symmetric, which suggest that generally many eyes would be best fit with sclerals that have toric or quadrant specific haptic designs. A recent study by DeNaeyer et al using corneo-scleral topography classified the anterior ocular surface into four shape categories, which included: 5.7% spherical, 28.6% toric, 40.7% asymmetric, and 25% toric with a periodicity different from 180. Most of the eyes in this study had irregular corneas and these results can differ when compared to a regular cornea dataset.
The results suggests that only about 6% of eyes are best fit with a spherical haptic back surface, while almost 30% of patients would be best matched with a toric haptic back surface. The trend industry wide is moving towards fitting scleral lenses with toric haptics. Many labs now have toric diagnostic lenses within their standard fitting sets. In fact, it has been shown that patients often prefer haptic back surface toric designed scleral lenses over spherical designs3. From a lens design perspective, the last two groups of the DeNaeyer study can be combined and the results suggest that 65% of eyes would benefit from a haptic back surface design beyond a toric haptic.
This would include back surface quadrant or multi-meridian haptic designed lenses. The clinical challenge of designing back surface haptics with more than two meridians is deciding on the number of meridians to use and how much they should differentiate. Careful evaluation, photographs, and lab consultation can significantly improve the process. Taking advantage of new technology, such as corneo-scleral topography can immediately improve fitting efficiency and success. These instruments allow the practitioner to not only map the corneal but the conjunctival/scleral surface as well. Assessment of sagittal maps allows the practitioner to immediately know whether the eye is spherical (6%), toric (30%), or asymmetric (65%). Additionally, the micron differential between meridians, whether two (toric) or more (multi-meridian), can be used to design a customized scleral haptic scleral lens.