Influence of rigid lens decentration and rotation on visual image quality in normal and keratoconic eyes

Purpose To investigate whether the movement of a rigid sphero-cylindrical contact lens has a greater impact on the visual image quality in highly aberrated eyes than in normal eyes. Methods For 20 normal and 20 keratoconic SyntEyes, a previously determined best sphero-cylindrical rigid lens was permitted to shift by up to +/- 1 mm from the line of sight and rotate up to +/- 15 degrees. Each of the 52,111 lens locations sampled was ray-traced to determine the influence on the wavefront aberration. In turn, the logarithm of visual Strehl ratio (log(10)[VSX]) was calculated for each aberration structure and was used to estimate the associated changes in logMAR visual acuity. Finally, contour surfaces of two-letter change in visual acuity were plotted in three-dimensional misalignment space, consisting of decentrations in the x and y directions and rotation, and volumes within these surfaces were calculated. Results The variations in image quality within the misalignment space were unique to each eye. A two-letter loss was generally reached with smaller misalignments in keratoconic eyes (10.5 +/- 4.7 degrees of rotation or 0.27 +/- 0.13 mm of shift) than in normal eyes (13.4 +/- 1.8 degrees and 0.39 +/- 0.15 mm, respectively) due to larger cylindrical errors. For keratoconic eyes, on average, 14.4 +/- 14.9% of misalignment space saw VSX values above the lower normal VSX threshold, well below the values of normal eyes of 48.5 +/- 18.5%. In some eyes, a specific combination of lens shift and lens rotation away from the line of sight leads to a simulated improvement in visual image quality. Conclusion Variations in visual image quality due to the misalignment of rigid sphero-cylindrical contact lens corrections are larger for keratoconic eyes than for normal eyes. In some cases, a specific misalignment may improve visual image quality, which could be considered in the design of the next generation of rigid contact lenses.