Assessment of Visual Quality Changes as a Result of Spectacle Misalignment

Background: A substantial proportion of presbyopic patients experience visual symptoms or fail to adapt to progressive addition lenses (PALs), and lens tilt caused by suboptimal frame fitting has been suggested as one of the possible contributing factors. The aim of this theoretical study was to quantify the sensitivity of retinal image quality, wavefront aberrations and model-predicted monocular visual acuity to controlled horizontal and vertical misalignment (tilt) of single-vision (SV) spectacle lenses across a range of spherical refractive powers and viewing directions. No clinical, comfort or adaptation outcome was measured. Methods: A tolerance analysis of spectacle lens mispositioning was performed by numerical simulation in Zemax OpticStudio using the Liou–Brennan schematic eye, augmented with a 0.11 mm posterior retinal shift to account for higher-order aberrations of the average healthy eye. Twelve HOYA single-vision spectacle lenses (made from CR-39 1.50 and Eyas 1.60 materials, refractive powers ±2.00, ±4.00 and ±6.00 D) were tilted around the vertical axis (lens front face angle, LFFA) up to 5° and around the horizontal axis (pantoscopic angle, PA) up to 12.5°, in 2.5° steps, the latter range covering the whole span of pantoscopic angles encountered in practice, both for axial fixation and for an eccentric 20° nasal viewing direction at infinite object distance. Wavefront aberrations (Zernike astigmatism,C22, C2−2) and visual acuity (V, in logMAR units) were computed via a previously validated polychromatic point-spread-function-based algorithm. Results: For the central (on-axis) viewing direction, none of the tested spectacle lenses were sensitive to tilt. For 20° oblique gaze, however, astigmatism became disturbing for negative lenses at or below −6.00 D and for positive lenses already around +5.00 D. Visual acuity simulations confirmed that lenses of ±4.00 D and stronger lost on average 0.03 logMAR (≈one third of an ETDRS line) at 5° of tilt, while ±2.00 D lenses were unaffected. Extending pantoscopic tilt to the upper physiological limit increased the effect in a monotonic manner: at PA = 12.5°, the oblique astigmatism of +6.00 D lenses reached 0.69–0.73λ (≈0.95–1.00 D of cylinder), and their predicted acuity deteriorated by 0.09–0.12 logMAR, i.e., about one ETDRS line, whereas +4.00 D lenses lost at most 0.03 logMAR, and ±2.00 D lenses remained unaffected throughout. Conclusions: In this optical model, the effect of SV lens tilt on predicted visual acuity depended on lens power, tilt axis and viewing direction: tilt was optically inconsequential for axial fixation and for low powers, whereas at 20° eccentric fixation, lenses of ±4.00 D and above showed a small model-predicted degradation that became clinically relevant in magnitude only for +6.00 D lenses at pantoscopic angles approaching 12.5°. These model-derived results describe the optical tolerance of SV lenses only; they were not obtained with PAL designs and do not quantify clinical visual performance, adaptation or wearing comfort.