Corneal thickness measurements with the Topcon SP-2000P specular microscope and an ultrasound pachymeter

Objective: To compare the reproducibility of measurements obtained with a n ew pachymetry instrument, the Topcon specular microscope (Topcon SP-2000P; Topcon America Corp, Paramus, NJ), with those obtained by ultrasound pachym etry. Methods: Corneal thickness was measured in 40 eves of 40 patients 3 times e ach with the Topcon 5P-2000P and an ultrasound pachymeter (DGH 500; DGH Tec hnology inc, Exton, Pa) by 2 separate in investigators. Comparisons include d average thickness as measured by each instrument, average thickness for e ach instrument as measured by each investigator, and differences in thickne ss due to corneal abnormalities. Results: Mean corneal thickness measured by the Topcon instrument was signi ficantly less (32 mu m; P<.001) than the mean value obtained with the ultra sound pachymeter. Similarly, mean values obtained with the 2 instruments by the 2 investigators were significantly different (P<.001 and .008 for inve stigators 1 and 2, respectively), with the Topcon value less than the ultra sound value in both cases, Both instruments detected abnormalities in corne al thickness equally well. However, the measurements obtained with the Topc on instrument by the 2 investigators were more consistent (no significant d ifference [P=.32]) than those obtained with the ultrasound unit (difference was significant [P=.02]). Conclusions: The new noncontact Topcon specular microscope provides measure ments of corneal thickness that are somewhat less than those of ultrasound pachymetry, but that seem to be more consistent from one operator to anothe r, possibly as a result of the elimination of observer bias induced by prob e placement required by the ultrasound unit. This consistency may be import ant in the comparison of measurements by different operators over time in p atients being followed up after refractive surgery or other therapeutic int erventions.