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Customized pachymetric guided epithelial debridement for corneal collagen cross linking

George D Kymionis, Vasilios F Diakonis*, Efekan Coskunseven, Mirco Jankov, Sonia H Yoo and Ioannis G Pallikaris

BMC Ophthalmology 2009, 9:10  doi:10.1186/1471-2415-9-10

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Placido based topographies do not accurately reflect the thinnest corneal area in keratoconus and should not be used in customized pachymetric guided epithelial debridement technique for collagen cross linking in thin corneas

Vedat Kaya   (2010-06-25 13:00)  Beyoglu Eye Research and Training Hospital email

We read the paper by Kymionis et al with great interest, in which they report performance of corneal cross linking surgery by leaving the epithelium intact at the area of topographic steepening in two eyes with keratoconus and post-LASIK corneal ectasia, and localized low pachymetry (less than 400 microns).1
In the technique they describe, the authors consider the area of maximum topographic steepening equivalent to the area of thinnest point. (In that paper, Figure 1 displayed the area that was left with intact epithelium, and Figure 2 depicted the corneal steepening at the same location.)
It is well known that the placido based devices construct the axial, tangential and refractive power maps by using the radius of curvature information. In eyes with keratoconus or corneal ectasia, the point of maximum corneal protrusion (apex of the cone) is the highest point in the corneal elevation map, where the slope is virtually zero (similar to the summit of a hill, which is flat). The maximum steepness, i.e., maximum refractive power, and minimum radius of curvature are just below to the apex. In other words, the highest keratometric values in axial topography do not point to the apex of the cone. In order to define the apex of the cone, the point of maximum elevation in elevation topography and the thinnest point in pachymetric map should be used, instead of the area of maximum steepening at the axial map.
On the other hand, it is clearly seen that the corneal area with intact epithelium was located in the peripheral cornea in their study (Figure 1). If in their cases, the area of corneal thinning were peripherally located as shown in Figure 1, diseases that would cause peripheral thinning (like pellucid marginal degeneration and Terrien’s degeneration) should have been considered in the differential diagnosis instead of keratoconus, which is a disease of the central and para-central cornea.2
Sincerely,
Vedat Kaya, M.D.
Beyoglu Eye Research and Training Hospital, Istanbul, Turkey
Canan Asli Utine, MD, MSc
Yeditepe University, Department of Ophthalmology, Istanbul, Turkey

References
1. Kymionis GD, Diakonis VF, Coskunseven E, Jankov M, Yoo SH, Pallikaris IG. Customized pachymetric guided epithelial debridement for corneal collagen cross linking BMC Ophthalmology 2009;9:10 ( available from: http://www.biomedcentral.com/1471-2415/9/10)
2. Krachmer JH, Feder RS, Belin MW. Keratoconus and related noninflammatory corneal thinning disorders. Surv Ophthalmol 1984;28:293-322.

Competing interests

Authors have no competing interest relevant to this study.

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