Orthokeratology Decentration: Topography Alignment, Tear Profile Symmetry and Induced HOAs

PURPOSE: To assess how topographic rotational misalignment alters orthokeratology (OK) treatment position, symmetry and visual references. A simulated lens-cornea model that predicts treatment position is introduced. A further aim investigated decentration-induced change in higher-order aberrations, root mean square (ΔHOA RMS). METHODS: Retrospective analysis of topography from 26 myopic OK wearers was conducted on visual and geometric aligned scans. Treatment centres were obtained from tangential maps using ellipse fitting, while chord mu was measured from visual scans. MATLAB modelling related Placido misalignment with treatment position and used lens-cornea height data to generate tear film profiles. An optimisation protocol then identified lens shift and tilt that minimised clearance asymmetry. Lastly, Zernike fits across various pupil sizes, enabled stepwise ΔHOA RMS calculation for decentrations up to 1.0 mm. RESULTS: Rotational misalignment was strongly associated with a change in treatment position (r = -0.93, p < 0.001) and geometric modelling closely matched measured shifts (r = 0.98, p < 0.001). Corneal symmetry indices did not differ between visual and geometric scans and chord mu remained stable after treatment (mean change -0.006 mm, p = 0.67). The tear film model predicted horizontal decentration well (r = 0.77, p < 0.001; mean absolute error = 0.13 mm; bias = -0.02 mm, limits of agreement -0.36 to 0.31 mm), whereas vertical prediction was weak (r = 0.23, p = 0.27). Treatment showed a temporal bias on visual scans that diminished after geometric recentring. Simulated ΔHOA RMS increased with decentration, for a baseline value of 0.10 µm and a 4.0 mm pupil, ~0.40 mm decentration would double RMS to ~0.20 µm. CONCLUSIONS: Visual scan alignment provides stable references for OK centration without distorting treatment appearance. Integrating topography with lens parameters through tear film symmetry modelling may help predict decentration, guiding lens design to optimise visual outcomes.