Assessing the corneal power change after refractive surgery using Scheimpflug imaging

Abstract

Purpose: To investigate whether the Pentacam HR could accurately predict the surgically induced refractive change in patients operated with small-incision lenticule extraction (SMILE) for myopia or femto-second laser in situ keratomileusis (FS-LASIK) for myopia or hyperopic astigmatism.

Methods: Data from three groups consisting of (1) 410 myopic eyes of 410 patients operated with SMILE, (2) 111 myopic eyes of 111 patients operated with FS-LASIK, and (3) 40 eyes of 40 patients with hyperopic astigmatism operated with FS-LASIK were retrospectively analysed. The change in manifest refraction due to surgery was compared with the objectively measured change in corneal power by the Pentacam HR in three different ways: Sagittal Power (calculated as for placido topographers), True Net Power (calculated by a Gaussian optics formula), and Total Corneal Refractive Power (calculated by ray tracing). Multiple linear regression analysis was performed to investigate which parameters influenced the Pentacam HR's prediction of the change in subjective refraction due to surgery.

Results: The Total Corneal Refractive Power Apex, Zone calculation in a diameter of 4.0 mm effectively predicted the surgically induced refractive change for all three patient groups. The spherical equivalent was predicted with an error of 0.08 ± 0.41 D for the SMILE eyes, 0.05 ± 0.61 D for the myopic eyes operated with FS-LASIK, and -0.15 ± 0.49 D for the hyperopic astigmatic eyes treated with FS-LASIK. Regression showed that preoperative refractive error had a significant impact on the prediction error of the Pentacam HR.

Conclusions: Ray tracing calculations based on Scheimpflug imaging accurately assessed the change in manifest refraction due to corneal laser surgery.

Keywords: cornea/physiopathology; corneal surgery; corneal topography; laser; ocular/physiology; refraction.