A comparison of two pachymetric systems: slit-scanning and ultrasonic.

Purpose: This article compares measurements of corneal thickness obtained with the Orbscan Topography System with those obtained with the ultrasonic pachymeter, when used to measure central-comeal thickness in normal subjects.

Methods: Slit-scan topography (Orbscan II system, version 3.0, Bausch & Lomb, Rochester, NY) and ultrasonic pachymetry (Ophthasonic A-Scan/ Pachometer III, Accutome, Malvern, PA) were used to measure central-corneal thickness in 92 right corneas of 92 healthy adult subjects. A correlation analysis was used to evaluate the relationship between both instruments. Measurements were also compared by plotting the difference between the methods against the average. The hypothesis of zero bias was examined by a paired t-test. The 95% limits of agreement also were calculated.

Results: The mean corneal thickness was 559.9 microm +/-3.73 SEM with the Orbscan system and 551.2 microm +/- 4.57 SEM with the ultrasonic pachymeter, values that were significantly different (paired t-test, P = 0.000). The coefficient of determination was 0.860 (y = -65.78 + 1.135 x; P = 0.000). The mean difference between the measurements from both devices was found to be significantly different from zero (mean: 8.74 microm; paired t-test; P = 0.000), with Orbscan being slightly higher than the Ophthasonic A-Scan.

Conclusions: The Orbscan system measurements of central corneal thickness were greater than ultrasonic pachymeter measurements, a difference that was statistically significantly.