Comparison of ultrasound pachymetry and partial coherence interferometry in the measurement of central corneal thickness

Purpose: To compare central corneal thickness (CCT) measurements obtained with 3 ultrasound pachymeters and with partial coherence interferometry (PCI) and evaluate the effect of repeated contact by a pachymetry probe on corneal thickness. Setting: Department of Ophthalmology, University of Vienna, Vienna, Austria. Methods: Central corneal thickness was measured in 20 eyes of 20 healthy volunteers with 3 different ultrasound pachymeters (DGH 500, DGH Technology Inc.; SP 2000, Tomey Inc.; Paxis, Biovision Inc.) and with PCI. In each eye, 5 measurements with PCI were followed by 5 measurements with each ultrasound pachymetry device and another 5 measurements with PCI. Results: The mean CCT measured with the DGH 500, SP 2000, and Paxis was 541.0 &mgr;m, 539.2 &mgr;m, and 545.1 &mgr;m, respectively. Although the differences among the 3 ultrasound pachymetry devices were within 6.0 &mgr;m, they were statistically significant (P< .05). The PCI measurements were significantly smaller (P <.05) than the ultrasound measurements: 518.8 &mgr;m before and 517.5 &mgr;m after repeated contact with the ultrasound pachymetry probe (P <.05). The mean precision (standard deviation) was 0.77 &mgr;m for PCI and between 2.40 &mgr;m and 3.58 &mgr;m for ultrasound pachymetry measurements. The correlation coefficients for the intraobserver variability were 0.999 for PCI and between 0.987 and 0.995 for ultrasound pachymetry measurements. Conclusions: Partial coherence interferometry was the more precise method of measuring CCT and had better intraobserver variability. Repeated contact by a pachymetry probe reduced corneal thickness by 1.3 &mgr;m. However, the reason for the smaller measurements with PCI than with ultrasound pachymetry remains unclear.