To evaluate and compare central corneal thickness (CCT) measurements using Pentacam, Orbscan and US (Ultrasound) pachymetry exclusively in thin cornea with normal topographic pattern.

: 88 eyes of 44 candidates for refractive surgery with thinnest pachymetry on Orbscan of 500µ or thinner, normal topographic pattern , no sign of keratoconus and BCVA of 20/20 were enrolled in this study. Simultaneous Pentacam and US pachymetry was performed by the same examiner for all patients. Main Outcome Measure was thinnest central corneal thickness (CCT). Exclusion criteria were history of ocular surgery, any topographic abnormalities suggesting forme fruste keratoconus or keratectasia and recent contact lens wear.Pachymetric readings by three mentioned devices were recorded and analyzed.

: This study shows that Orbscan can produce more comparable results to US pachymetry in thin corneas if AF (acoustic factor) of 0.94 is taken. The difference between US and Orbscan II ×0.92 lies in the range of -34 to +34µ. The difference between the thinnest point and central readings can amount up to 16µ with Orbscan II ×0.92 and 2µ with Pentacam. The mean differences between modalities were 0.2 µm between Pentacam and US (P =0.727), 30.1 µm between uncorrected Orbscan and US (P <0.001), 10.4 µm between Orbscan II ×0.92 and US (P <0.001) and 0.2 µm between Orbscan II ×0.94 and US (P=0.851).

: It was demonstrated that Pentacam shows better agreement with US in the range of thin corneas in comparison with corrected Orbscan readings. We speculate that a dynamic graded spectrum of AF in reverse proportion to the CCT can constitute a better approach to Orbscan correction.