Corneal power measurements using scheimpflug imaging in eyes with prior corneal refractive surgery

To evaluate the accuracy of central corneal power measurements by Scheimpflug imaging (Pentacam) for eyes that had corneal refractive surgery.

This study comprised two groups: a pilot group of 100 eyes with prior hyperopic or myopic LASIK that did not have cataract surgery, and a test group of 41 eyes with prior radial keratotomy (RK) and cataract surgery. In the pilot group, Pentacam images and refraction were taken preoperatively and 3 months after LASIK. The historical method was used to compute the theoretical postoperative keratometry (K) -reading and then compared to the measured equivalent K-reading (EKR) from the Pentacam. The EKR is the same value measured by standard keratometry or topography on the front surface, adjusted for the effect of the back surface power difference from normal. In the test group of RK eyes, the postoperative refraction and EKR were measured 3 months after cataract surgery. The Holladay IOL Consultant Program was used to back-calculate the theoretical K-reading. The EKR measurements were then compared to the back-calculated corneal power.

The optimal zone sample size was determined to be 4.5 mm for the pilot group. The mean prediction error for this group was -0.06+/-0.56 diopters (D) (range: -1.63 to +/-1.34 D). Using the 4.5-mm zone determined in the pilot group, the EKR value for the test group of 41 RK eyes had a mean prediction error of -0.04+/-0.94 D (range: -1.84 to +/-2.27 D).

When historical refractive data are not available, Scheimpflug imaging with the Pentacam provides an alternative method of measuring the central corneal power in eyes that previously received corneal refractive surgery.