Comparison of Corneal Thickness Measurements with Noncontact Specular Microscope and Ultrasonic Pachymeter

ABSTRACT

PURPOSE: To evaluate the corneal thickness values before and after refractive keratoplasty with the Topcon SP-2000 noncontact specular microscope(Topcon American Corp.) and the common standard ultrasonic pachymeter. METHODS: Corneal thickness was measured in 40 eyes of 20 patients, 5 times each before and after keratoplasty, with the Topcon SP-2000P and an ultrasonic pachymeter (DGH 1000, DGH Technology Inc.). The main outcome measures were accuracy and reproducibility as assessed with coefficient of variation (CV) of corneal thickness measurements. RESULTS: The reproducibility of corneal thickness measurements was better with the ultrasonic pachymeter (CV 0.36%) than with the Topcon SP-2000 (CV 1.23%). Mean corneal thickness measured by the Topcon SP-2000P before and after keratoplasty was significantly less than the mean values obtained with the ultrasonic pachymeter (14.4 micro meter; P<.0001, 5.6 micro meter; P<.01; respectively). Compared with theoretical ablated corneal thickness, ablated corneal thickness measured with the Topcon SP-2000P was significantly less (P<.01) than values obtained with the ultrasonic pachymeter (P=0.07) CONCLUSIONS: The noncontact Topcon specular microscope provides measurements of corneal thickness that are significantly less than those of the ultrasonic pachymeter, both before and after keratoplasty. These results indicate that these 2 techniques are not clinically comparable, thereby precluding interchangeable use of their data in planning or assessing keratoplasty.