Accurate biometry, accurate keratometry, use of both an appropriate IOL power calculation formula and personalized A-constant, and the elimination of astigmatism all are essential factors for obtaining a good refractive result following cataract surgery in eyes with a history of keratorefractive surgery.
Excimer laser and incisional refractive surgery procedures performed more than 5 years ago, however, create a multifocal cornea that presents an additional challenge for precise IOL power selection, said Jack T. Holladay, MD, MSEE, at the refractive surgery subspecialty day at the annual meeting of the American Academy of Ophthalmology.
Obtaining an accurate keratometry measurement probably represents the largest source of error in IOL power calculation in eyes with a history of corneal refractive surgery, Dr. Holladay said. The K values generated by two of the standard tools, keratometers and topographers, are based on measurements of the paracentral region of the anterior surface.
In eyes that have undergone corneal refractive surgery, however, the central value cannot be extrapolated accurately from paracentral measurements.
In addition, the IOL formula assumption about the normal physiologic ratio between the anterior and posterior surfaces of the cornea is erroneous in these eyes where only the anterior surface has been altered, according to Dr. Holladay.
Central corneal power
"Both keratometry and topography overestimate the true net corneal power after myopic corneal refractive surgery, although a central corneal power (CCP) value from topography is somewhat better than a keratometry-derived measurement, resulting in an approximate –15% error," he said.
Various mathematical methods have been developed for calculating the true power of the cornea in postLASIK eyes. Among these are the historical method that uses data on the prerefractive surgery K reading and the amount of the refractive change. Although this method still is the gold standard, there is a potential for error if the refractive change is due, in part, to the crystalline lens. The historical method assumes all changes are due to the corneal change, Dr. Holladay said.
Scheimpflug tomography (Pentacam, Oculus; Galilei, Ziemer) has emerged as promising technology for accurate determination of CCP in post-refractive surgery eyes because it directly measures the central anterior and posterior surfaces of the cornea.
In a study evaluating the variability of these measurements within a single eye, Dr. Holladay and colleagues found that tomography-derived CCP was accurate to within about ±0.5 D in postLASIK or postPRK eyes and ±1 D in eyes with previous RK.
"In looking at the distribution of power over the central 4.5-mm zone, we found there was almost 5 D of multifocality in postLASIK eyes, and in the postRK eye, up to 13 D of variability along with two peaks. The problem is that we do not know which peak the patient is looking through and, therefore, we do not know which power to use," Dr. Holladay said (Figure 1).