Corneal pachymetry in normal and keratoconic eyes: Orbscan II versus ultrasound

Purpose: To compare corneal thickness measurements using Orbscan II (OII) and ultrasonic (US) pachymetry in normal and in keratoconic eyes. Setting: Eye Department, Heartlands and Solihull NHS Trust, Birmingham, United Kingdom. Methods: Central corneal thickness (CCT) was measured by means of Oil and US pachymetry in 1 eye of 72 normal subjects and 36 keratoconus patients. The apical corneal thickness (ACT) in keratoconus patients was also evaluated using each method. The mean of the difference, standard deviation (SD), and 95% limits of agreement (LoA = mean +/- 2 SD), with and without applying the default linear correction factor (LCF), were determined for each sample. The Student t test was used to identify significant differences between methods, and the correlation between methods was determined using the Pearson bivariate correlation. Bland-Altman analysis was performed to confirm that the results of the 2 instruments were clinically comparable. Results: In normal eyes, the mean difference (+/- 95% LoA) in CCT was 1.04 mum +/- 68.52 (SD) (P>.05; r = 0.71) when the LCF was used and 46.73 +/- 75.40 mum (P = .0001; r = 0.71) without the LCF In keratoconus patients, the mean difference ( 95% LoA) in CCT between methods was 42.46 +/- 66.56 mum (P<.0001: r = 0.85) with the LCF, and 2.51 +/- 73.00 mu m (P>.05: r = 0.85) without the LCF. The mean difference (+/- 95% LoA) in ACT for this group was 49.24 +/- 60.88 mum (P<.0001: r = 0.89) with the LCF and 12.71 +/- 68.14 mu m (P = .0077; r = 0.89) when the LCF was not used. Conclusions: This study suggests that Oil and US pachymetry provide similar readings for CCT in normal subjects when an LCF is used. In keratoconus patients, Oil provides a valid clinical tool for the noninvasive assessment of CCT when the LCF is not applied.