Accuracy of the Pentacam and IOL Master to Calculate the Effective Corneal Power After Corneal Refractive Surgery

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 Pentacam (Comprehensive Eye Scanner) is a non-invasive, diagnostic system created to take photographs of the anterior segment of the eye by a rotating Scheimpflug camera measurement. This rotating process supplies pictures in three dimensions. The center of the cornea is measured very precisely because of this rotational imaging process. The measurement process lasts less than two seconds and minute eye movements are captured and corrected simultaneously. By measuring 25,000 true elevation points, precise representation, repeatability and analysis are guaranteed. It provides a topographic analysis of the corneal front and back surfaces that is based on the true elevation measurement from limbus to limbus. Both corneal surfaces can be selected for analysis in axial (sagittal), tangential or elevation representation modes.

The Pentacam using tomography calculates a virtual model of the anterior segment of the eye. It is possible to move, zoom and rotate it to detect e.g. iris defects, cornea incisions e.g. RK or size, location and shape of opacifications in the crystalline lens. The slicing function in the three dimensions offers a detailed view of the different layers of the eye. It includes the Holladay report that was developed to improve the calculation of IOLs for patients which have undergone corneal refractive surgery.

The IOL Master is a non-contact optical coherence biometry that makes possible the exact measurement of visual axis length. Its accuracy is not affected by high ametropia, pupil size or state of accommodation. This non-contact technology makes it easy on the patient: no local anesthesia, water bath, or contact probe. For IOL power calculation after previous refractive corneal surgery, one can derive the effective corneal power using either the clinical history method, or the rigid contact lens over-refraction method. It therefore permits selection of a suitable lens after myopic LASIK/PRK without requiring refractive pre-LASIK data or additional contact lens over-refraction.

The purpose of this study is to determine the accuracy of the Pentacam and IOL Master 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.