Accuracy of the Galilei Analyzer to Calculate the Effective Corneal Power After Corneal Refractive Surgery

• Accuracy of the Galilei to calculate the effective corneal power (keratometry, corneal curvature) in patients who have undergone both corneal refractive surgery and lens extraction with intraocular lens implantation. [ Time Frame: Three Months ] [ Designated as safety issue: No ]
  

The accuracy of IOL calculation is important for the visual outcome of patients undergoing cataract extraction and IOL implantation. Different formulas such as Holladay I, HofferQ, SRKT have been used with excellent results. All formulas use the corneal power among other factors to calculate the IOL power. Corneal refractive surgery i.e. radial keratotomy (RK), photorefractive keratectomy (PRK), and Laser in situ keratomileusis (LASIK) changes the corneal power; therefore, it is difficult to measure the true corneal power after surgery by any form of direct measurement, such as keratometry, or corneal topography. Keratometry and topography assume a normal relationship between the anterior and posterior corneal curvatures, and measure the anterior corneal radius. RK for myopia flattens both the anterior corneal radius and the posterior corneal radius while PRK and LASIK for myopia flattens the anterior corneal radius but leaves the posterior corneal radius mostly unchanged.

Standard keratometry measures an intermediate area and extrapolates the central power based on some very broad assumptions. For this reason, keratometry, autokeratometry and simulated keratometry by topography will typically over-estimate central corneal power following keratorefractive surgery for myopia. This inaccuracy leads to an inability to meet the patients' rising expectations and with the increasing popularity of refractive surgery, calculating intraocular lens (IOL) power after refractive surgery is becoming increasingly important.

Different methods to calculate the effective corneal power (keratometry) after refractive surgery have been described (historical data, effective refractive power, modified Maloney method, etc), however, intraocular lens power calculations in eyes with previous refractive surgery remains difficult because of the inaccuracy of keratometry power measurements.

The Galilei dual Scheimpflug analyzer is a non-invasive, diagnostic system that combines dual rotating Scheimpflug camera measurements and Placido optical system for corneal topography and 3D analysis of the anterior eye segment. It provides pachymetry as well as elevation and curvature mapping of the cornea. Additionally, the Ray tracing system delivers a more accurate total corneal power and anterior chamber depth. The total corneal power that the Galilei provides is an alternative that seems to be more accurate to calculate the IOL power in patients who have undergone keratorefractive surgery prior to cataract extraction and IOL implantation.

The purpose of this study is to determine the accuracy of the Galilei to calculate the effective corneal power (keratometry, corneal curvature) in patients who have undergone both corneal refractive surgery and lens extraction with intraocular lens implantation.