Regional corneal topographic responses in overnight orthokeratology and their influence on treatment zone decentration

摘要

The purpose of this thesis was to investigate regional corneal topographic responses in overnight orthokeratology (OK) and their influence on treatment zone (TZ) decentration. In all studies reported in this thesis corneal topography was assessed using the Medmont E300 corneal topographer. In the initial study, eyelid photographs of healthy volunteers were captured using the Nikon D5000 digital SLR camera. Eyelid parameters were determined from the photographs using i-Metrics software. The performance of BE spherical OK lenses on minimally and moderately toric corneas was evaluated in retrospective and prospective studies. When evaluating the performance of toric OK lenses on moderately toric corneas on a small number of participants the DreamLite TRZ toric periphery OK lens design was used. In general, corneal asymmetry was found to be significant between horizontal sectors but not in vertical sectors in minimally toric corneas. Several corneal sectors showed a difference between East Asian and non-East Asian eyes with clinically significant differences apparent only in the superior corneal sector. Upper eyelid curvature in non-East Asian eyes showed a stronger positive association with corneal spherical equivalent power than in East Asian eyes. During spherical OK lens wear on minimally toric corneas, a tendency of TZ decentration to move away from the most prolate corneal region was found. In the same cohort, it was shown that TZ decentration caused non-uniform corneal curvature changes. Significant central temporal flattening and para-central steepening were consistent with temporal TZ decentration. When the TZ decentration during spherical OK was compared between minimally and moderately toric corneas, decentration was significantly greater in moderately toric corneas. Alternative methods of fitting OK lenses with slightly deeper sag heights did not show any benefit in controlling TZ decentration in moderately toric corneas. Toric OK lens fitting on moderately toric corneas showed greater TZ decentration compared to spherical OK lenses, but the TZ margins were well defined giving good visual outcomes. In conclusion, normal corneal shape asymmetry in minimally toric corneas and magnitude of corneal toricity were shown to influence TZ decentration. Toric OK lenses may provide improved visual outcomes in moderately toric corneas.