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Y. Nawa, K. Masuda, T. Ueda, Y. Hara, H. Uozato; Evaluation of Apparent Corneal Ectasia Displayed With Orbscan After Keratorefractive Surgery . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2754.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Since Wang reported, corneal forward shift after keratorefractive surgery has been reported by many authors. However, we doubt corneal forward shift displayed with the difference map of Orbscan after myopic correction. The posterior surface of the cornea is observed through a lens, composed of the overlying epithelium and stroma. The lens changes its shape and refractive power after surgery. The difference map of Orbscan does not consider the difference of the magnification ratio. We propose a new idea why the cornea apparently shifts forward after myopic LASIK or PRK. Methods: The method to calculate the magnification ratio of the posterior surface of the cornea was described previously (Nawa et al, JCRS 2003). Using the method, we evaluated the relationship between the amount of myopic correction and apparent forward shift. Results: If we assume the preoperative corneal thickness is 600 micron, myopic correction is 10D, and postoperative corneal thickness is 480 micron, the posterior surface of the cornea become apparently smaller by 0.8%. If we assume the radius of curvature of the posterior cornea is 6200 micron, the posterior surface of the cornea become smaller by 50 micron. If it is directly translated to the difference map, the posterior cornea apparently shift forward by 50 micron. Conclusions: Surprisingly, the calculated relationship between the amount of correction and apparent forward shift well corresponds to the past reports. This report strongly suggests that most of the corneal forward shift displayed with Orbscan could be artifact.
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