Corneal Magnification

Abstract

I read with interest the article by Nishimura et al.1Nishimura R. Negishi K. Saiki M. et al.No forward shifting of posterior corneal surface in eyes undergoing LASIK.Ophthalmology. 2007; 114: 1104-1110Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar They measured anterior chamber (AC) volume, AC depth (ACD), corneal thickness, central corneal true net power, and posterior corneal tangential curvature using a rotating Scheimpflug camera (Pentacam, Oculus, Wetzlar, Germany) before and after myopic LASIK. They compared the result with that measured by Orbscan (Bausch & Lomb, Rochester, NY). The result showed that the posterior corneal curvature, peripheral corneal thickness, ACDs, and AC volume were consistent, indicating that neither forward shifting of the central posterior corneal surface (bulging) nor backward shifting of the peripheral posterior corneal surface due to corneal swelling after ablation occurred after LASIK. In contrast, the result measured by Orbscan showed a forward shift of the posterior surface of the cornea by an average of 29.0 μm. Recently, Hashemi and Mehravaran also reported a similar result.2Hashemi H. Mehravaran S. Corneal changes after laser refractive surgery for myopia: comparison of Orbscan II and Pentacam findings.J Cataract Refract Surg. 2007; 33: 841-847Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar We previously reported that images of the posterior surface of the cornea could be affected by the overlying corneal lens composed of the anterior surface of the cornea and stroma.3Nawa 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. 2005; 31: 571-573Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar After myopic LASIK, the posterior surface of the cornea apparently becomes thinner, resulting in apparent ectasia of the posterior surface of the cornea displayed in the Orbscan difference map. The method of calculation is shown in the article.3Nawa 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. 2005; 31: 571-573Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar Referring to the data in the Nishimura et al study,1Nishimura R. Negishi K. Saiki M. et al.No forward shifting of posterior corneal surface in eyes undergoing LASIK.Ophthalmology. 2007; 114: 1104-1110Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar I assumed that the amount of myopic correction was 6 diopters (D); preoperative corneal thickness, 0.55 mm; postoperative corneal thickness, 0.48 mm; preoperative anterior corneal power, 46 D; postoperative anterior corneal power, 40 D; and preoperative and postoperative posterior corneal radius of curvature, 6800 μm, and ignored the 20-μm pixel error. The calculated apparent ectasia of the posterior cornea was 30.6 μm, which was quite close to the 29.0 μm of forward shift displayed with Orbscan in the Nishimura et al article. Both the Pentacam and Orbscan are slit-scanning topographers and may have common flaws. It is interesting to note that the ACs of post-LASIK eyes measured by Pentacam appear slightly shallower than those of preoperative eyes,1Nishimura R. Negishi K. Saiki M. et al.No forward shifting of posterior corneal surface in eyes undergoing LASIK.Ophthalmology. 2007; 114: 1104-1110Abstract Full Text Full Text PDF PubMed Scopus (47) Google Scholar which had been reported as a paradox in the article by researchers using Orbscan.4Cairns G. Ormonde S.E. Gray T. et al.Assessing the accuracy of Orbscan II post-LASIK: apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures.Clin Experiment Ophthalmol. 2005; 33: 147-152Crossref PubMed Scopus (53) Google Scholar I suppose this is due to the fact that the images of the anterior surface of the lens also have been affected by the changes of the magnification effect of post-LASIK corneas. Although the authors believed that the Pentacam was more accurate than Orbscan, I hope the manufacturers will disclose why these discrepancies of corneal ectasia occur between the Pentacam and Orbscan. Author replyOphthalmologyVol. 115Issue 3PreviewWe thank Dr Nawa for his interest in our article. We appreciate his suggestion that the magnification effect by the cornea should be taken into account when evaluating the shape of the posterior corneal surface, and we completely agree with him. We were not sure whether the Pentacam (Oculus, Wetzlar, Germany) and Orbscan (Bausch & Lomb Technolas GmbH, Feldkirchen, Germany) corrected such a magnification effect in the machine when we wrote the article. According to his suggestion, we asked Oculus Germany and Bausch & Lomb Technolas about the point. Full-Text PDF