Sagittal height and scleral shape are the two most important factors that influence the fitting of a scleral contact lens. Until recently, these factors have been difficult to measure, which forced the majority of practitioners to rely on the “art” of diagnostic lens fitting. Previous studies using optical coherence tomography (OCT) have shown that the scleral surface is nonrotationally symmetrical and that it becomes more asymmetric with increasing distance from the limbus (van der Worp, 2010). Recently, the development of corneo-scleral topography has given us the ability to comprehensively study conjunctival scleral shape.
The Scleral Shape Study Group, an international panel of experts on scleral shape, recently published a paper on the qualitative assessment of scleral shape patterns that presents a classification system for scleral shape (DeNaeyer et al, 2017). A retrospective analysis was performed for 140 eyes of prospective scleral lens patients who had undergone measurement with corneo-scleral topography. A circumferential shape plot at a 16mm chord was created with the X-axis as a meridian in degrees and the Y-axis as sagittal height (Figure 1).
Scleral Shape Analysis
Scleral shape patterns were reviewed in all cases and were classified according to recurring characteristics, resulting in four shape categories (Table 1). Group 1 were spherical scleras (5.7% of eyes), defined as having a plot with low amplitude deviations of ≤ 300 microns. Group 2 were regular toric scleras (28.6% of eyes), defined as having a sagittal height plot with a sin2 pattern, periodicity of 180°, and greater than a 300-micron difference between highest to lowest depression. The largest category was the group 3 asymmetric scleras (40.7% of eyes), which had plot asymmetries of > 300 microns. Group 4 (25% of eyes) had a sagittal height plot with a repeating sin2 pattern, but a periodicity different from 180°.
|GROUP||PATTERN DESCRIPTION||N (%)|
|3||Asymmetric High and Low Points||57 (40.7%)|
|4||Periodicity Different from 180°||35 (25%)|
Importantly, group 3 and 4 categories include a total of 65.7% of eyes that have scleral shapes different from commonly designed spherical or toric scleral lens haptic back surfaces. If we decrease the thresholds for all categories by half to 150 microns, groups 3 and 4 increase to a combined 80.7% of eyes evaluated.
This study represents the first classification for scleral shape that includes four categories. Measurement of the sclera allows for more efficient and accurate determination of lens design. The results suggest that a majority of eyes may benefit from custom-back-surface haptics beyond a toric design. CLS
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