Central and peripheral corneal thickness measurement with Orbscan II and topographical ultrasound pachymetry

To compare thickness measurements of the central 6.0 mm of the cornea obtained with the Orbscan(R) II topography system and topographical ultrasound pachymetry.

School of Optometry, University of Santiago de Compostela, Galicia, Spain.

In 24 right eyes, pachymetric measurements were taken at the center and 1.2 mm and 3.0 mm on the superior and inferior hemimeridians. A 1-sample t test was applied to assess the significance of the relationship between Orbscan II and ultrasound methods. The relationship between the 2 was assessed by analyzing regression and plotting the differences against the mean corneal thickness. Orbscan II data were analyzed in 3 ways: (1) without the application of an acoustic equivalent correction factor; (2) with a correction factor of 0.92, as recommended by the manufacturer; (3) with correction using the equations derived in this study. The data were systematically compared with those of ultrasound pachymetry.

Before the correction factor was applied, the Orbscan II overestimated the corneal thickness at all locations, with the mean difference (48.15 microm +/- 33.74 [SD]) significantly different from zero (P <.001). Differences increased toward the periphery, and the reliability of Orbscan II readings seemed to decrease with thicker corneas. After the acoustic equivalent was applied, the differences were significantly less; however, this effect did not seem clinically significant as large differences remained. When specific corrective equations were applied for each corneal location, the level of agreement between Orbscan II and ultrasound pachymetry improved substantially; the mean (-0.11 +/- 15.22 microm) was not statistically different from zero (P >.05).

The acoustic equivalent correction factor proposed by the manufacturer to obtain corneal thickness measurements with the Orbscan II compared to those from ultrasound pachymetry was not valid for all corneal topography positions. Orbscan II measurements agreed better with those of ultrasound pachymetry when equations for the central and each peripheral location across the topography were applied.