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C.J. Roberts, A. Mahmoud, D. Castellano; Evaluation of the Performance of the Posterior Edge Tracker of the Orbscan II Corneal Topographer . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2543.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The Orbscan Corneal Topographer (Bausch & Lomb) is widely used in the refractive surgery community since it is the only commercially available system capable of mapping a large portion of the posterior corneal surface. However, the system performance has never been evaluated for accuracy of the posterior edge extraction algorithm. Methods: A retrospective study was performed on 12 eyes of 6 patients where Orbscan II Corneal Topography had been acquired and the slit images saved, both before and one month after LASIK using the Summit Apex Plus excimer laser and the SKBM microkeratome. Slit images were reprocessed using version 3.0 after disabling the Orbscan autoeditor in order to evaluate the performance of the edge tracker. Under these conditions, the Orbscan software displays green lines overlaid on top of the slit images, demonstrating the location of both the anterior and posterior edges, as identified by the edge tracking algorithm. Lastly, for a single case where the Orbscan exam had been acquired and the slit images saved under version 2.0, the same set of slit images were reprocessed with both version 2.0 and version 3.0 of the software. Results: Examination of the slit images demonstrated greater corneal scatter post-operatively, which caused the version 3.0 edge extraction algorithm to identify and begin tracking a low contrast posterior corneal edge outside of the pupil, and then "get lost" as it crossed the pupil margin where contrast in the posterior edge sharply increased. The 3.0 edge tracking algorithm consistently moved into the corneal stroma, seeking a lower contrast edge, only in the post-operative images. For the post-op case of the slit images processed with both version 2.0 and 3.0, there was greater than 40 microns difference in the central elevation of the extracted posterior surface that could be attributed to inaccurate tracking by the algorithm used in version 3.0. Conclusions: The posterior surface edge tracker in version 3.0 of the Orbscan software is not "tuned" to the higher scatter conditions in the post-LASIK cornea, which generates artificially "low" pachymetry values, as well as artificially "high" central elevation in the posterior cornea which might be misinterpreted as ectasia. The edge tracker has adequate performance in pre-operative slit images, as well as in software version 2.0 which is no longer available.
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