Comparison of equivalent keratometric indices on Scheimpflug tomography with Placido-based topography system at different optical zones.

Aim: To compare Holladay equivalent keratometric indices on Scheimpflug tomographer and keratometry on Placido topographer at different optical zones.

Methods: In this observational study, 50 right eyes of 50 patients were recruited. All patients were scanned on both machines. Data on K1, K2, mean keratometry (Km) and meridian of steepest keratometry (MoSK) were collected for central 3 mm and 5 mm optical zones from Holladay Equivalent keratometry report on Scheimpflug tomographer and keratometric data from Placido topographer. Difference (Diff(S-P)) in Km and MoSK were calculated by subtracting corresponding values of Placido from Scheimpflug scans. Concordance was evaluated using paired t tests, the Pearson correlation and Bland-Altman analyses.

Results: Comparing 3 mm vs 5 mm optical zones on each machine, there was no significant difference in Km and MoSK values but Km showed good correlation unlike MoSK. Comparing Scheimpflug versus Placido, Km showed good correlation at all scan sizes, unlike MoSK. 95% limits of agreement between the machines for Km was -1.20D to 1.08D(3 mm) and -1.46D to 1.23D(5 mm); whereas, for MoSK it was -83.05 to 85.82°s(3 mm) and -107.9 to 77.48°s(5 mm) respectively. The Diff(S-P) for Km at 3 mm (-0.45±0.79) was significantly less than that at 5 mm (-0.10±0.70) (p<0.01) whereas Diff(S-P) for MoSK were not significantly different at both optical zones.

Conclusions: Mean keratometry correlated well between Scheimpflug and Placido scans. Axes showed poor correlation between optical zones on same machine and between the machines with same optical zone. Scheimpflug showed flatter keratometry at smaller optical zone. These machines cannot be used interchangeably in refractive surgery.