Corneal Power Measurement With the Dual Scheimpflug-Placido Topographer After Myopic Excimer Laser Surgery

PURPOSE: To evaluate the ability of the Sirius Scheimpflug-Placido Topographer (Costruzione Strumenti Oftalmici, Florence, Italy) in predicting corneal power in eyes with previous myopic corneal refractive surgery. METHODS: The following corneal refractive power measurements were obtained with Sirius in 38 eyes with prior myopic excimer laser surgery: simulated keratometry (SimK), true net power based on the Gaussian optical formula (TNP), and the mean pupil power (MPP) calculated by ray tracing within the diameter of the entrance pupil range from 2.5 to 6 mm. These values, along with the corresponding keratometric value, which was based on the equation that converted the MPP within 4.5-mm entrance pupil diameter to a keratometric reading in 75 myopic eyes, were compared using the clinical history method. RESULTS: SimK significantly overestimated corneal power by 0.67 +/- 0.52 diopters (D), whereas TNP significantly underestimated corneal power by -1.05 +/- 0.48 D. The only measurements that showed no statistically significant difference by the clinical history method were the 5 mm MPP, 5.5 mm MPP, and K-MPP4.5POST, and the 95% limits of agreement (LOA) were -1.10 to 0.62, -0.91 to 0.89, and -0.67 to 1.05 D, respectively. CONCLUSIONS: The Sirius Scheimpflug-Placido Topographer is an alternative method of measuring corneal power in eyes with previous corneal refractive surgery for myopia when preoperative data are not available. The best options seem to be the 5 mm MPP, 5.5 mm MPP, and the keratometric reading calculated from the regression formula.