Corneal thickness, curvature, and elevation readings in normal corneas: combined Placido-Scheimpflug system versus combined Placido-scanning-slit system

Purpose: To evaluate agreement in central corneal thickness (CCT), keratometry, and anterior and posterior elevation map measurements in normal corneas between a combined Placido-Scheimpflug system and a combined Placido-scanning-slit elevation topography system.

Setting: Department of Cataract & Refractive Surgery, Rothschild Foundation, Paris, France.

Design: Evaluation of diagnostic test or technology.

Methods: Measurements were performed with a combined Placido-Scheimpflug system (TMS-5) and a combined Placido-scanning-slit system (Orbscan II). Ultrasound (US) pachymetry was used as the reference for CCT measurements. Bland-Altman plots were used to evaluate agreement between instruments.

Results: The mean CCT measurements by US pachymetry, the Placido-Scheimpflug system, and the Placido-scanning-slit system were 556.74 μm ± 42.45 (SD), 543.23 ± 36.73 μm, and 564.45 ± 41.26 μm, respectively. Although the CCT readings were statistically significantly thinner with the Placido-Scheimpflug system than with the other systems, there was high correlation between instruments. Peripheral corneal thickness readings were also thinner with the Placido-Scheimpflug system than with the Placido-scanning-slit system. Keratometry and anterior and posterior best-fit sphere (BFS) measurements were comparable between the 2 optical devices. Anterior and posterior maximum central elevations measured by the 2 instruments were not comparable or strongly correlated. Repeatability after 3 successive measurements was excellent for all parameters except maximum central elevation.

Conclusions: Although highly correlated, with corneal thickness readings were not interchangeable between the 2 optical devices. No statistically significant differences in keratometry or BFS measurements were observed between the 2 devices. There were important discrepancies in the maximum central elevation between the 2 topographers.

Financial disclosure: Drs. Gatinel and Saad are consultants to Technolas Perfect Vision. No other author has a financial or proprietary interest in any material or method mentioned.