Predicting Changes in Corneal Asphericity After Hyperopic LASIK

Recently, the first experimental work on corneal asphericity after hyperopic laser in situ keratomileusis was published.1 The authors show that corneal asphericity (Q) becomes more negative after surgery, especially when greater degrees of refractive correction are applied. This trend is contrary to that with myopia, given that Q augments (becomes more positive), as demonstrated by Holladay and Janes.2 Anera et al.3 analyzed the changes in corneal asphericity for myopia and demonstrated that the main factor explaining the changes is the form of the ablation provided by the paraxial formula of Munnerlyn used in practical surgery. If the Munnerlyn formula is applied, the p-factor (p = 1 + Q) after surgery, p′, is given in the function of the presurgery data as3,4 where p′ is the postsurgery p-factor, p is the presurgery p-factor, and R and R′ are the radii of the anterior cornea before and after surgery, respectively. Given that R′ > R in myopia, we would expect, as the experimental results show, an increase in the corneal asphericity after surgery.3 The same equation (1) would justify the contrary trend in the case of hyperopia and would partly explain the data of Chen and coauthors1 as this equation is valued for individual data, which we do not have. We can make a theoretical estimation: If we apply equation 1, knowing that 1/R′ = 1/R + 0.375 D (with D being the number of diopters to correct), a presurgery average eye with R = 7.7 mm and Q = −0.26 would diminish its Q after surgery to Q = −0.34 and Q = −0.42 for D = +2 and D = +4, respectively. Although this equation cannot explain the entire change in corneal asphericity (since factors such as the biomechanics3 of the cornea can also influence corneal shape), it does explain the general trend detected experimentally in hyperopia. As (R′/R)3 <1 for hyperopia correction, this trend could be expected in prolate and oblate presurgical eyes. Jose R. Jiménez PhD Rosario G. Anera PhD Luis Jiménez del Barco PhD Enrique Hita PhD aGranada, Spain