Accuracy of Orbscan optical pachymetry in corneas with haze

PURPOSE: To compare corneal thickness measurements with the Orbscan instrument (Orbtek, Inc.) and an ultrasonic pachymeter (Alcon Surgical) in corneas with haze. SETTING: Department of Ophthalmology and Otolaryngology, University of Bari, Bari, Italy. METHODS: Six eyes of 6 patients having phototherapeutic keratectomy for treatment of grade 2+ haze were assessed preoperatively and postoperatively by Orbscan optical pachymetry and ultrasonic pachymetry. Contralateral transparent untreated corneas of the same patients were used as controls. Two Orbscan and 5 ultrasound pachymetry measurements were performed at each visit and the mean values recorded. The accuracy and repeatability of the measurements were evaluated. RESULTS: Preoperatively, the median corneal thickness in the 6 eyes with haze was 512.7 microm by ultrasonic pachymetry and 375.0 microm by optical pachymetry. The mean difference of 143.1 microm was statistically significant (Wilcoxon signed rank test, P= .0277). Postoperatively, the median pachymetry was 439.0 microm and 433.0 microm, respectively; the mean difference was 10.7 microm (P= .0277). In the control group, the median ultrasonic pachymetry was 530.9 microm and the median optical pachymetry, 529.0 microm; the mean difference was 4.7 microm (P= .1730). Precision was good in all groups; the standard deviation was less than 6.0 microm. CONCLUSIONS: The Orbscan system showed a decreased accuracy in measuring corneal thickness when clinically significant haze was present. The repeatability of the measures was good, suggesting that the cause of error lies in the optical acquisition process.