Evaluation of two scheimpflug camera-based devices in the analysis of keratoconus

Purpose
This study aimed to evaluate the measurements of the anterior segment of the eye by two Scheimpflug camera-based systems: the Pentacam HR imaging system and the Sirius imaging system in keratoconus patients.
Patients and methods
Keratoconus patients were recruited prospectively. Measurements with the Oculus Pentacam and with the CSO Sirius were performed according to the manufacturer’s instructions. For every eye, the following parameters were analyzed statistically. The anterior ketatometric reading of the flattest meridian K1, anterior K2, anterior mean , anterior , pachymetry at the thinnest location, the highest anterior corneal elevation in the 3-mm pupillary area, the highest posterior corneal elevation in the 3-mm pupillary area, and the best-fit sphere for the anterior corneal surface and posterior corneal surface at the same diameter of analysis. Agreement between Sirius and Pentacam was assessed by calculating 95% limits of agreement and plotting Bland–Altman graphs.
Results
Fifty eyes from individuals (26 men, 24 women) aged 20–38 years were evaluated. The mean difference between the measurements of both machines was −0.54±1.02 D. The mean difference between the measurements of both machines was −1.40±1.53 D. The mean difference between the measurements of both machines was −0.60±2.38 D. The mean avgerage difference between the measurements of both machines was −0.89±1.06 D. The mean thinnest location pachymetry difference between the measurements of both machines was 9.20±14.14 μm. The mean highest anterior elevation difference between the measurements of both machines was 6.86±11.43. The mean highest posterior elevation difference between the measurements of both machines was 20.52±21.32. The mean anterior best-fit sphere difference between the measurements of both machines was 0.14±0.13. The mean posterior best-fit sphere difference between the measurements of both machines was 0.16±0.16. The difference was statistically significant.
Conclusion
All the measurements between both Pentacam and Schwind Sirius showed a significant positive correlation, except for the highest anterior and posterior elevation measurements. Schwind Sirius produces keratometry measurements higher than Pentacam in keratoconus patients. However, corneal thickness, the radius of the best-fit sphere, and the highest anterior and posterior elevation measurements by Pentacam were higher than Schwind Sirius measurements. The differences between the measurements of Sirius and Pentacam were statistically significant. Therefore, it is recommended that the measurements of these devices not be used interchangeably.