Comparison of central corneal thickness measurement by scanning slit topography, infrared, and ultrasound pachymetry in normal and post-LASIK eyes.

Objectives: To compare central corneal thickness (CCT) measurements by scanning slit topography (SST), infrared pachymetry (IRP), and ultrasound pachymetry (USP), and their agreement in normal and post-laser in situ keratomileusis (LASIK) eyes.

Methods: Sixty normal and 35 post-LASIK subjects were recruited. Only one eye from each subject was analyzed. Non-contact pachymetry was performed first, and the order for SST (Orbscan IIz) and IRP (Tonoref III) was randomized for each patient, to be followed by contact USP (Echoscan US-4000). Pearson's correlation, paired t test, and Bland-Altman plots were used to investigate association, difference, and agreement among different instruments respectively.

Results: The measurements obtained with the instruments were highly correlated. Compared to CCT determined by USP (CCT), CCT determined by SST (after correction with acoustic factor) (CCT) was thicker by 7 µm in normal eyes (P < 0.001). There was no significant difference between CCT and CCT (P = 0.128), but a thickness-dependent deviation in post-LASIK eyes (P = 0.003). The CCT determined by IRP (CCT) was thicker than CCT in normal (P < 0.001) and post-LASIK eyes (P < 0.001) and demonstrated proportional overestimation with thinner corneas, with less predictable ultrasonic equivalent corneal thickness in normal eyes. Conversely, CCT significantly underestimated CCT compared to CCT and showed increasing underestimation with thinner corneas in both normal and post-LASIK eyes (both P < 0.001).

Conclusion: Central corneal thickness determined by SST, IRP and USP were not interchangeable or interconvertible, probably attributed to difference in methodologies. Compensation with algorithms may improve agreements amongst instruments.