The influence of centre thickness on miniscleral lens flexure

Purpose To examine the influence of centre thickness upon miniscleral lens flexure and the association between the magnitude of in-vivo lens flexure and scleral toricity. Methods In-vivo lens flexure was measured using a videokeratoscope in 9 healthy young participants (25 ± 4 years) with normal corneae fitted with ICD 16.5 miniscleral lenses (hexafocon B material) with centre thicknesses of 150, 250, and 350 μm. Scleral toricity was determined from sagittal height data over a 15 mm chord obtained from a corneo-scleral topographer. Results On average, lens flexure increased with decreasing centre thickness, but remained <0.50 D (mean increase <0.25 D, p = 0.63). Scleral toricity was positively correlated with in-vivo flexure for the 150 μm (r = 0.77, p = 0.02) and 250 μm (r = 0.72, p = 0.03) lenses. Using a group mean split, eyes with >200 μm scleral toricity exhibited greater in-vivo flexure than eyes with <200 μm (0.40 D more, averaged across all lenses, p = 0.02), and this effect was greatest for the 150 μm lens (0.61 D more, p = 0.04). Conclusions Decreasing the centre thickness from 350 μm to 150 μm resulted in <0.25 D increase in lens flexure for a high Dk and low modulus material. Scleral toricity >200 μm was associated with more in-vivo lens flexure. When intentionally reducing scleral lens centre thickness to enhance oxygen transmissibility, customised back surface designs may be required to minimise in-vivo flexure in eyes with >200 μm scleral toricity at a 15 mm chord.