Measuring corneal thickness with the ConfoScan 4 and z-ring adapter

PURPOSE. To assess the precision and accuracy of the z-ring adapter with the ConfoScan 4 confocal microscope for measuring corneal thickness. METHODS. Thirty healthy corneas of 15 volunteers were scanned twice with a ConfoScan 4 confocal microscope equipped with a z-ring adapter (Nidek, Inc., Fremont, CA) and with a Tandem Scanning confocal microscope (Tandem Scanning Corporation, Reston, VA). Corneal thickness was determined from the position of the focal plane at the epithelial and endothelial surfaces. Distances measured by both instruments were calibrated from scans of 15 polymethylmethacrylate contact lenses with known thicknesses between 400 and 650 μm. Corneal thickness was also measured by ultrasonic pachymetry (DGH Technologies, Inc., Exton, PA). RESULTS. Corneal thickness measured with the ConfoScan 4 (mean, 529 ± 35 μm) was not significantly different from thickness measured with the Tandem Scanning confocal microscope (mean, 531 ± 30 μm) (P=0.30). The mean difference between the first and second scans was 1.1 ± 20 μm and -2.6 ± 17 μm with the ConfoScan 4 and Tandem Scanning microscopes, respectively. Both confocal microscopes indicated thinner corneas than ultrasonic pachymetry did (567 ± 35 μm) (P<0.0001). CONCLUSIONS. Mean corneal thickness measured with the z-ring adapter with the ConfoScan 4 agrees with mean corneal thickness measured with the Tandem Scanning confocal microscope when both instruments are correctly calibrated. Ultrasonic pachymetry indicated a mean corneal thickness that was at least 35 μm greater than was indicated by either confocal microscope.