Corneal thickness measurement by confocal microscopy, ultrasound, and scanning slit methods

PURPOSE: To measure corneal thickness by using a calibrated confocal microscope and to compare this mead surement to thickness determined by ultrasonic and noncontact scanning slit pachymetry. DESIGN: Comparison of corneal thickness measured by using four instruments in normal subjects. METHODS: Thickness measured by a clinical confocal microscope (Tandem Scanning) was calibrated from mea, surements of polymethylmethacrylate contact lenses with known thickness. Corneal thickness was measured in one eye of 24 normal subjects by using this instrument, two ultrasonic pachymeters (DHG-1000 and Sonogage), and a noncontact optical scanning slit pachymeter (Orbscan II). RESULTS: Mean corneal thickness measured by confocal. microscopy was 516 +/- 30 mum (+/-SD). This was less than the mean thickness measured by both ultrasonic pachymeters, 554 +/- 28 mum by the DGH, and 555 +/- 28 mum by the Sonogage (P <.001). Thickness measured by the Orbscan II pachymeter was 540 +/- 35 mum (P <.001, compared with either confocal or ultrasound) after applying an "acoustic factor" of 0.92, a default correction of the software. CONCLUSION: Corneal thickness measured by calibrated confocal microscopy is approximately 39 mum (7.0%) less than thickness measured by two commonly used ultrasonic pachymeters and approximately 24 mum (4.4%) less than thickness measured by the corrected Orbscan II pachymeter. These differences are important for planning and measuring the effects of refractive and other surgical pro, cedures. The precision of confocal microscopy is limited by corneal motion in an anterior-posterior direction. The difference between instruments suggests that verification of clinical ultrasonic pachymeters should be revisited. (C) 2004 by Elsevier Inc. All rights reserved.