No existing scientific evidence validates the presumption that thinner preoperative corneal thickness independently increases the risk of post-LASIK ectasia, as long as intraoperative pachymetry is performed to guard against a thicker-than-expected flap. In fact, thin corneas in eyes with normal topography appear to be biomechanically strong. Routine preoperative topographic screening should be performed to detect abnormalities that suggest an increased risk for post-LASIK ectasia.
Most but not all post-LASIK ectasia cases had abnormal findings present on the preoperative topography, said Dr. Trattler, director of cornea, Center for Excellence in Eye Care, Miami. A careful evaluation of the topography is, therefore, a critical part of the preoperative screening process, he added.
Another important step to consider is intraoperative pachymetry, Dr. Trattler said. One of the challenges in evaluating risk factors in eyes that have developed post-LASIK ectasia, he added, is that the true depth of the flap and residual stromal bed often is unknown. Because some metal microkeratomes can cut deeper than expected, a thick flap in an eye with a preoperatively thin cornea can thin the cornea critically, leading to ectasia, Dr. Trattler said.
Evidence exists that thin corneas, when paired with normal topography, can be biomechanically strong and similar in behavior to thicker corneas, he said. Dr. Trattler cited four series from the scientific literature indicating that thin CCT is not an independent risk factor for post-LASIK ectasia in eyes with normal topography.1-4 Binder, Caster, Pallikaris, and Kremer have reported on their LASIK results in patients with normal preoperative topographies and thin corneas. Their results were very positive, with no cases of post-LASIK ectasia, Dr. Trattler said.
One of these series, a 2007 study by Binder, evaluated 9,700 eyes in a computer database, selecting those that had LASIK for myopic refractive errors and whose characteristics included CCT of 500 μm or less and a preoperative normal topography.1
Four hundred eyes were identified, with preoperative corneal thickness ranging from 402 to 500 μm. None of the eyes with these characteristics developed postoperative ectasia over a mean follow-up period of more than 2 years. Alternatively, three eyes with abnormal preoperative topography and normal preoperative thickness went on to develop ectasia.
Data from Dr. Trattler's personal database of 71 post-LASIK ectasia patients also supports his contention that thin corneas are not an independent risk factor. Preoperative CCT data were available on 94 eyes, 10 of which had a thickness of less than 500 μm. All of these eyes except two eyes from one patient, however, had obvious topographic abnormalities that independently increased their risk of post-LASIK ectasia, most commonly FFKC, keratoconus, or pellucid.
The lone patient with bilateral thin corneas had high myopia. Because intraoperative pachymetry was not performed, a deeper-than-expected flap combined with the deep ablation easily could have thinned the residual stromal bed to a critical level, he said.
Dr. Trattler also collaborated with J. Bradley Randleman, MD, and colleagues at the Emory Eye Center in Atlanta to review an unpublished database of ectasia cases.5 Most of these procedures were performed before 2003, all were done with mechanical microkeratomes, and intraoperative pachymetry data were unavailable for most of them.
"The average thickness of the corneas of the ectasia cases was thinner than that of controls, but since we don't have intraoperative pachymetries, we don't know whether some of these cases represented thicker-than-expected flaps," Dr. Trattler said.
He also said that thin corneas still can be biomechanically strong. The corneal collagen crosslinking procedure strengthens corneas, which become thinner. Although African Americans, on average, have thinner corneas than Caucasians, they are not at higher risk for keratoconus or FFKC, conditions associated with corneas of reduced biomechanical strength, Dr. Trattler said.
With available evidence, which does not include any published studies supporting an increased risk of post-LASIK ectasia for thin corneas with normal topography in which intraoperative pachymetry was performed to guard against thicker-than-expected flaps, a reasonable assumption seems to be that thinner corneas are not necessarily biomechanically weaker, he concluded.