Future challenges to aberration-free ablative procedures.

ne of the exciting new research areas in laser refractive surgery is the development of sophisticated devices to measure the wave-front of the optical system of the entire eye, the various implementations of which are described in this proceedings. The ability to objectively measure performance of the eye, as opposed to simple corneal shape, is critical to " customizing " ablation algorithms and generating an overall improvement of visual outcomes after refractive surgery. Anterior corneal surface topography cannot take into account contributions of optically important structures inside the eye, such as the posterior corneal surface and the crystalline lens. If a laser were programmed strictly with anterior topography data, the correction would be at best incomplete, and at worst simply wrong. Therefore, wavefront analysis is clearly important, particularly if the ultimate goal is to correct higher order aberrations along with the sphere and cylinder. However, the question must be asked whether wavefront analysis alone is sufficient to fully predict visual outcomes. Will it replace corneal topography in the quest for the perfect " aberration-free " guided procedure? Or, on the other hand, is there a piece of the puzzle still missing? If so, can corneal topography complement the wavefront measurements to help complete the picture of corneal response? To answer these questions, the underlying theory of laser refractive surgery can be examined. Munnerlyn and colleagues 1 described what is referred to here as a " shape-subtraction " model of refractive surgery, based on geometric equations for altering the surface curvature with the cornea mod-eled as sphere. In a " shape-subtraction " model of a myopic procedure, the desired sphere of lesser curvature is superimposed on the original sphere of greater curvature, with the apex displaced by an amount determined by the chosen ablation diameter. If the intervening tissue is removed or " subtracted " from the original sphere by a laser, the desired sphere of lesser curvature is the result. In other words, the cornea is analogous to a piece of plastic that can be sculpted to a new shape, without taking into account how the cornea might respond to a change in its structure imposed by an ablative procedure. The Munnerlyn formulas have been empirically modified since they were first implemented, based on statistical analyses of large numbers of treated patients. Therefore, current ablation algorithms are optimized to the mean population response. This approach has produced a majority of …