The cornea is not a piece of plastic.

To the Editor: In her recent editorial (Roberts C. The cornea is not a piece of plastic. J Refract Surg 2000;16:407-413), Dr. Roberts points out that biomechanical flattening enhances a myopic excimer laser ablation and works against a hyperopic ablation. Would the circular severing of corneal lamellae in hyperopic ablation not lead to a contraction of the central part of these lamellae with a heaping up of tissue? This effect could compensate for the flattening by the peripheral pull or even exceed it. One might also argue that central islands in myopic ablation seem to contradict an additional central flattening effect. To the Editor: We congratulate Cynthia Roberts for her editorial , " The Cornea Is Not a Piece of Plastic. " Besides high frequency ultrasound, Orbscan topography, and pachymetry data, there is also direct histologi-cal evidence 1 for the paradoxical addition of stromal tissue outside the ablation zone. In a study on his-tologic changes after +10 D hyperopic PRK on rabbit corneas, we found stromal thickening on the order of 20 µm on the shoulder and in the center of the treatment. However, we have questions about the proposed mechanism of flattening of this peripheral swelling. There is no doubt that the swelling of the " sponges " goes in the direction of the least resistance, which is outward. This swelling itself of the peripheral superficial cornea without any change in curvature of the posterior corneal surface, is sufficient to cause central flattening. The possibility proposed by Roberts—that the swelling due to the cross-linking pulls outward and laterally on the center and flattens it both anteriorly and posteriorly (Fig 4 in Roberts editorial)—is not necessary and not supported by posterior corneal surface Orbscan data. Indeed, two Orbscan studies after PRK 2,3 suggest the opposite effect: a central outward bulging of the posterior corneal stroma to steepen both the anterior and posterior corneal curvatures. We think, 1) that the swelling causes flattening of the anterior surface merely by tissue " addition, " and 2) that the central thinning causes a discrete pro-trusion of the posterior surface counteracted and overridden by the first mechanism (and the abla-tion) on the anterior surface, and 3) that in the case of hyperopic correction, the central swelling may enhance the optical effect instead of causing regression. We agree with the conclusion that the first step is to gain a better understanding of the corneal bio-mechanical …