In the reply1 to a letter about the article on corneal thickness measured by Orscan (Bausch & Lomb),2 Miyata could not satisfactorily explain why the corneal thickness was underestimated after myopic LASIK. In the article, Miyata et al. speculated that the reasons were changes in the refractive index, stromal haze, changes in the anterior contour of the cornea, and inappropriate reconstruction algorithms that might occur after LASIK. We propose another explanation of the underestimation of corneal thickness measured by Orbscan after myopic LASIK.
The posterior surface of the cornea is observed through a lens composed of the overlying epithelium and stroma. The lens changes its shape and thickness after myopic LASIK. Then, the posterior surface of the cornea observed through the lens becomes relatively smaller than that observed before the surgery.
We have described a method to calculate the change in the magnification ratio of the posterior cornea after myopic LASIK under the paraxial assumption.3 If the preoperative and postoperative corneal thicknesses are 600 μm and 480 μm, respectively, the corneal refractive index is 1.376, and the amount of myopic correction is 10.0 diopters, the image of the posterior cornea will become smaller by approximately 0.8%.4 If the posterior radius of curvature of the cornea is 6.2 mm, ie, 6200 μm, it will become smaller by 50 μm. Details of the calculation algorithm of the Orbscan have not been released by the manufacturer. If the apparent change in the posterior cornea is directly translated to the apparent decrease in the corneal thickness, the cornea will become thinner by 50 μm. Figure 1 shows the relationship between the amount of myopic correction and the actual and hypothetically calculated apparent corneal thickness. It looks very similar to Figure 3 in the article.2
The change in the magnification ratio of the posterior cornea could be related to the apparent forward shift of the posterior cornea displayed with Orbscan after myopic correction.4 I hope this hypothesis will help explain many of the discrepancies in the data obtained by Orbscan after keratorefractive surgery.
Shinji Maruoka MD
Yoshiaki Nawa MD
Kozo Masuda ORT
Tetsuo Ueda MD
Yoshiaki Hara MD
Hiroshi Uozato PhD
1. Miyata K. Reply to letter by ACK Cheng. J Cataract Refract Surg 2004; 30:2251-2252
2. Miyata K, Tokunaga T, Nakahara M, et al. Residual bed thickness and corneal forward shift after laser in situ keratomileusis. J Cataract Refract Surg 2004; 30:1067-1072
3. Nawa Y, Ueda T, Masuda K, et al. Evaluation of the corneal endothelium after hyperopic laser in situ keratomileusis. J Cataract Refract Surg 2003; 29:1543-1545
4. Nawa Y, Masuda K, Ueda T, et al. Evaluation of apparent ectasia of the posterior surface of the cornea after keratorefractive surgery. J Cataract Refract Surg, in press.