The Cornea is Not a Piece of Plastic

he future of laser refractive surgery is exciting with the potential for ever-improved postoper-ative visual performance. In the past, the operative goal has been 20/20 uncorrected visual acuity with zero residual refractive error. Criteria for a successful procedure are no longer 20/40 or better with ±1.00 diopter (D) of residual refractive error, because neither surgeons nor patients are satisfied with such gross measurements of visual performance. Hence, considerable research effort is being devoted to develop customized procedures for each patient. The new goal is 20/10 uncorrected visual acuity with aberration-free postoperative vision. How can these lofty goals be accomplished? First, lasers have been improved with the development of scanning, small spot systems, as opposed to broad-beam systems. Scanning systems have brought customized procedures into the realm of feasibility. Second, more comprehensive and sophisticated input data can be used to guide the laser based on individual patient measurements, as opposed to the simple refractive sphere and cylinder. Two types of approaches are currently being pursued—wavefront-guided and topography-guided procedures. Early results are promising, yet neither approach has demonstrated consistently superior results to non-guided procedures in controlled, scientific studies. Is a piece of the perfect vision puzzle still missing? Is there an additional, complimentary approach to customization that has yet to emerge? Are we leaping into the future ahead of our understanding of how these procedures may be implemented to be optimally successful? Munnerlyn and colleagues 1 gave us an elegant analysis of how corneal surface shape may be altered to correct both myopia and hyperopia. His geometric approach can be thought of as a " shape-subtraction " model, where tissue of an appropriate profile is simply " subtracted " using an excimer laser to produce the desired surface curvature. Essentially, the cornea is treated like a piece of plastic to be sculpted into the ideal surface shape. The Munnerlyn approach, combined with empirical experience, has been relatively successful in correcting spherical and cylindrical errors for the majority of patients treated to date. However, only 50% to 85% achieve 20/20, despite greater than 90% of patients being " satisfied. " In addition, significant postoperative optical aberrations are produced using conventional algorithms 2-4 , encouraging the development of aberration-reducing ablation profiles. To improve the percentage of patients who achieve postoperative visual acuity of 20/20 or better , as well as to minimize optical aberrations, it is important to examine critically our current conceptual model …