Comparison of Javal-Schiotz keratometer, Orbscan Ilz and Pentacam topographers in evaluating anterior corneal topography

Background: To consider inter-instrument variability in keratometry measurements across the ACS and to explore instrument protocols for determining ACS keratometric analogues (KAs). Methods: Mean keratometry/KAs of the right eye were recorded using Javal-Schiotz keratometer (J-S), Orbscan Ilz and Pentacam from 124 subjects (78 females; mean +/- SD age: 24.71 +/- 6.61 years). Mean radii of curvature were obtained for 1-mm wide annular zones extending up to 6 mm (horizontally) and 4 mm (vertically) from the apex for Orbscan and Pentacam. Zonal mean radius of curvature was calculated by averaging keratometry values for all measured points within the zone. Results: KA (mean +/- SD): Horizontal: Orbscan (7.80 +/- 0.31 mm) and J-S (7.82 +/- 0.29 mm) were not significantly different (p = 0.072). Pentacam (7.86 +/- 0.29 mm) was significantly flatter than J-S (p < 0.001) and Orbscan (p < 0.001). Vertical: Orbscan (7.64 +/- 0.31 mm) was significantly steeper than J-S (7.67 +/- 0.29 mm, p < 0.005) and Pentacam (7.70 +/- 0.29 mm, p < 0.001). Pentacam was significantly flatter than J-S (p < 0.001) and significant flatter than Orbscan across the entire profile (1-4 mm zones horizontal and vertical, p < 0.001). LoAs (CI): J-S/Orbscan: +/- 0.75 mm (0.05-0.18); J-S/Pentacam: +/- 0.72 mm (0.01-0.12); Pentacam/Orbscan: +/- 0.16 mm (0.04-0.08). There was a +0.03 mm positive bias for Orbscan compared to J-S, +0.06 mm positive bias for Orbscan compared to Pentacam and -0.03 mm negative bias for Pentacam compared to J-S. Conclusions: Algorithms used by Orbscan and Pentacam to solve the peripheral paraxial ray problem produce significantly different KAs. Instrument-specific KAs cannot be used inter-changeably between instruments. Differences in KA between instruments are not significant for ocular surgery, but may influence rigid contact lens fitting. Pentacam measures flatter than Orbscan and J-S.

Automated summary

This study considers inter-instrument variability in keratometry measurements and explores instrument protocols for determining ACS keratometric analogues (KAs). Results show significant differences in KAs calculated by Orbscan and Pentacam, indicating that instrument-specific KAs cannot be used interchangeably. These differences have no significant impact on ocular surgery but may affect rigid contact lens fitting.