Comparison of Keratometric Changes After Myopic Ablation: Ray Tracing Versus Simulated Keratometry

Abstract

Ray tracing methods underestimate corneal power as opposed to the overestimation in corneal power after refractive surgery, directly related to the level of myopia, observed in Placido-based systems. They have the potential to overcome the errors in calculations based on anterior curvature alone. However, ray tracing methods need to be validated and optimized before it can be used routinely in IOL calculation.

Assessing the corneal power change after refractive surgery using Scheimpflug imaging

“…Assessing corneal power in patients who have previously undergone keratorefractive surgery has been the subject of a significant amount of research over the past several years . Keratometry and computerised videokeratography fail to provide correct measurements of the corneal power after corneal surgery.…”

“…A few other studies have compared the manifest refractive change with corneal power change by ray tracing using the Galilei and the Sirius devices. Mello et al .…”

“…Furthermore, keratometers and topographers have a central scotoma. The inherit limitations explain why these instruments give inaccurate measurements after corneal surgery . Many formulas have been suggested through the years to circumvent these issues, with The Clinical History Method being one of the most recognised calculations .…”

“…Recent studies suggest that devices employing Scheimpflug technology, such as the Sirius (Costruzione Strumenti Oftalmici, Florence, Italy), the Galilei (Ziemer Ophthalmic Systems AG, Port, Switzerland), and the Pentacam HR (Oculus Optikgeräte GmbH, Wetzlar, Germany), may be used to accurately measure the effective refractive change for surgically treated corneas. The Pentacam HR (Oculus Optikgeräte GmbH) device uses a rotating Scheimpflug camera and presents several methods for calculating corneal power including Gaussian optics and ray tracing (http://www.pentacam.com/sites/calc_corneal_power.php#ptop).…”

Evaluation of the Pentacam Ray Tracing Method for the Measurement of Central Corneal Power After Myopic Photorefractive Keratectomy

“…As they suggested, the intentional central steepening combined with a paracentral flattening in an aspheric ablation algorithm can cause paracentral overcorrection and may cause the inaccuracy of TCRP4. Recently, another study by Mello et al 12 reported that the ray tracing corneal power of the Galilei overestimated the corneal refractive change after wavefront optimized laser in situ keratomileusis using Allegretto Wave Eye-Q.…”