Methods: In this retrospective study, a total of 92 eyes of 92 patients (53 normal eyes, 39 keratoconus eyes) were included. Five consecutive measurements were taken with optical low-coherence refl ectometry (OLCR), Scheimpfl ug-Placido Topographer, spectral domain optical coherence tomography (SD-OCT), and ultrasound pachymetry (USP).

Results: In normal eyes, mean OLCR, Scheimpfl ug-Placido Topographer, SD-OCT, and USP CCT measurements were 520.76±4.31 ?m, 519.96±4.33 ?m, 522.40±4.20 ?m, and 528.89±4.11 ?m respectively. There was a statistically signifi cant difference in the CCT measurements between the OLCR and USP and the Scheimpfl ug-Placido Topographer and USP (p=0.003, p=0.01 respectively). All four modalities of CCT measurements correlated closely with each other, with Pearson correlation coeffi cients ranging from 0.957 to 0.983. In keratoconic eyes, average OLCR, Scheimpfl ug-Placido Topographer, SD-OCT, and USP CCT measurements were 449.93±5.77 ?m, 455.48±5.79 ?m, 457.21±5.61 ?m, and 465.87±5.49 ?m respectively. In keratoconic eyes, OLCR measurements were thinner than those of USP by a mean of 15.93 ?m, of SD-OCT by an average of 7.27 ?m (p<0.001, and p=0.007 respectively). All 4 modalities of CCT measurements correlated closely with each other, with Pearson correlation coeffi cients ranging from 0.872 to 0.932.

Conclusion: Scheimpflug-Placido Topographer and SD-OCT measurements showed good agreement with ultrasound pachymetry in normal and keratoconic eyes. However, OLCR measurements were thinner than the other three devices in keratoconic eyes. Scheimpfl ug-Placido Topographer, USP, SD-OCT, and OLCR should not be used interchangeably in keratoconic eyes. Keywords : Central corneal thickness, optical low-coherence refl ectometry, pachymetry, scheimpfl ug-placido topographer, spectral domain optical coherence tomography