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Correction Factor in Orbscan II in the Assessment of Corneal Pachymetry

Purpose: Comparison of corneal pachymetry assessment with ultrasound and Orbscan II using acoustic factor and subtraction methods. Methods: Ultrasound and Orbscan pachymetry for all patients with LASIK performed between July 2002 and May 2003 were retrospectively analyzed. Comparison between the 2 preoperative measurements was analyzed. Results: Using a custom acoustic factor of 0.93, there was no significant difference between the overall ultrasound and Orbscan pachymetry (P = 0.696). However, there was underestimation in thick corneas and overestimation in thin corneas. Using the subtraction method as the correcting technique, the magnitude of over- and underestimation was reduced. Conclusion: The current correction method by means of acoustic factor may result in error in assessing extremes of corneal thickness. Alternative approaches such as the subtraction method can be considered to achieve more accurate results.