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Refractive surgery and supervision: physical and biological constraints to an exciting perspective

Recent technological advancements have opened new perspectives to photorefractive surgery: the target is going to broaden from the simple correction of defocusing errors in ametropic eyes to the compensation of high order optical aberrations on an individual basis, even in emmetropic subjects. In this way, optimization of visual performance may result in `supervision', namely a visual acuity equal to or better than logMAR equals -0.3 (Snellen: 20/10). Assuming that all monochromatic aberrations can be completely nulled through corneal sculpting, several factors remain to limit the ultimately achievable visual acuity: diffraction, photoreceptor spacing, internal noise, chromatic aberration. All these factors have been included in a numerical model based on first principles and best fitting of experimental data, providing estimates on visual acuity and contrast sensitivity. In an aberration-free human eye, the maximum achievable visual acuity is logMAR equals -0.24 (Snellen: 20/11.5) at 2.7 mm pupil size. Diffraction turns out to be the limiting factor up to 2 mm pupil size, chromatic aberration from 2 to 5 mm, and internal noise dominates above 5 mm. The maximum visual acuity gain attainable with a `super-vision' procedure appears thus restricted to only 2 lines above the `mean' logMAR equals 0 level (20/20). Chromatic aberration requires thorough consideration in all photo-refractive procedures.