Comparison of corneal thickness measurements using ultrasound and Orbscan slit-scanning topography in normal and post-LASIK eyes

Abstract

PURPOSE: To compare corneal thickness measurements made by ultrasonic and slit-scanning techniques in normal eyes and in eyes after laser in situ keratomileusis (LASIK). SETTING: Corneal Diseases and Excimer Laser Research Unit, University of Dundee, Dundee, Scotland. METHODS: Central corneal thickness (CCT) was measured in 101 eyes of 59 normal subjects and in 30 eyes of 21 post-LASIK patients. Measurements were made with an Orbscan slit-scanning elevation topographer and immediately afterward with an ultrasound pachymeter. RESULTS: The difference in mean CCT between ultrasound (538.0 microm +/- 36.7 [SD]) and Orbscan (566.6 +/- 40.7 microm) pachymetry was statistically significant (P <.001) in the normal eyes; the Orbscan measurement was approximately 28 microm higher than that of the ultrasound pachymeter. The difference in mean CCT between the ultrasound and the slit-scanning techniques was also statistically significant in the post-LASIK eyes (mean values 475.3 +/- 50.3 microm and 461.9 +/- 74.2 microm, respectively; P <.0001). Differences in CCT in individual subjects were much more variable in the post-LASIK eyes than in the normal eyes. The Bland and Altman method for assessing clinical agreement between 2 instruments showed that in 95% of cases, the CCT measurements with both instruments would be within 65 microm in normal eyes and 150 microm in post-LASIK eyes. CONCLUSION: Central corneal thickness measurements were, on average, 28 microm higher with the Orbscan than with the ultrasound pachymeter in normal eyes and 13 microm lower in post-LASIK eyes. The degree of variability within each group indicated that these 2 techniques are not clinically comparable, precluding interchangeable use of their data in planning or assessing corneal surgery.