Effect of corneal thickness on the agreement between ultrasound and Orbscan II pachymetry

To study the agreement between ultrasound (US) and Orbscan II (Bausch & Lomb) measurements with different ranges of central thickness in normal corneas.

Noor Ophthalmology Research Center, Tehran, Iran.

The central corneal thickness was measured first by the Orbscan II and then with a US pachymeter in 177 right eyes. Data were compared in 3 thickness groups: less than 500 microm, 500 to 600 microm, and more than 600 microm. In all cases, uncorrected Orbscan II data were used. For each range, a correction factor was calculated through regression analysis, which was then used in a new set of analyses. To study the agreement between the 2 methods, the 95% limits of agreement (LoA) and intraclass correlations coefficients (ICC) were determined.

The mean interdevice difference was 2.7 +/- 16.9 (SD) in the less than 500 microm group (P = .365), 21.3 +/- 3.1 in the 500 to 600 microm group (P = .0001), and 27.2 +/- 20.9 in the more than 600 microm group (P = .0001). No clinically acceptable LoA were found in any group. The application of regression equations to the Orbscan II readings improved the agreement in the less than 500 microm group better than the other 2 groups. The ICC was 0.76, 0.61, and 0.43 in the less than 500 microm, 500 to 600 microm, and more than 600 microm groups, respectively.

The Orbscan corneal thickness readings had relatively lower validity than US measurements. The 2 devices should not be used interchangeably, especially when the validity of the measurement is vital. In patient screening for corneal surgery, it is advisable to recheck corneal thickness with a US pachymeter.