Vancouver, British Columbia-In the absence of other risk factors, eyes with a post-LASIK initial residual stromal thickness (RST)
Vancouver, British Columbia-In the absence of other risk factors, eyes with a post-LASIK initial residual stromal thickness (RST) <250 µm usually do not develop ectasia, according to Simon P. Holland, MD.
Dr. Holland, clinical associate professor of ophthalmology, The University of British Columbia, and research director, Pacific Laser Eye Centre, Vancouver, and colleagues investigated risk factors for ectasia in a retrospective case-control study. They compared features of a non-ectatic control group of 100 eyes from patients treated at his clinic with a case population of 21 eyes (16 patients) referred to a university-based corneal practice for management of post-LASIK ectasia. The non-ectatic controls were randomly selected from a larger group of 459 non-ectatic eyes that he and his colleagues operated on between 1996 and 2001, which had an initial post-LASIK RST of 250 µm.
Preoperative information was incomplete for many of the referred patients with ectasia, and initial RST was known for only seven of those eyes. However, in five (71%) eyes, initial RST exceeded 250 µm, and mean initial RST in the ectatic eyes was actually significantly higher than in the controls, 256.6 versus 223.1 µm, respectively. Initial RST in the ectatic eyes ranged from 211 to 307 µm, while it was between 174 and 250 µm in the controls without ectasia.
An important risk factor "Based on the findings of this study and other published data, forme fruste keratoconus detected by topographic screening programs is likely one of the most important risk factors for ectasia," Dr. Holland said. "However, our results indicate that LASIK may be safe for the majority of patients when the residual stromal thickness is 250 µm or less. Perhaps 225 µm may be a good cut-off in eyes without other risk factors, but ectasia can also occur when the residual stromal thickness exceeds 250 µm."
Dr. Holland had accumulated a large series of eyes with an RST <250 µm because between 1996 and 1999, he used an RST cut-off of 200 µm as part of his LASIK candidate selection criteria.
"I am not aware of any cases of ectasia developing among eyes operated on during that period," he stated.
All of his center's surgeries were performed using the Automated Corneal Shaper (ACS) microkeratome (Bausch & Lomb) with a plate size of 130, 160, or 180 µm. RST value was estimated for all cases and controls by subtracting the sum of the flap thickness (based on manufacturer-estimated blade depth) plus the ablation depth from the corneal thickness determined with ultrasound pachymetry.
"Use of a derived RST is one of the limitations of this study because the ACS microkeratome may cut a flap that is much thinner than what is stated on the plate. However, to know the actual RST would require use of ultrasound biomicroscopy, high-frequency ultrasound, or even online optical coherence pachymetry," Dr. Holland said.
Preoperative topography was available from 14 of the eyes with ectasia and all 100 controls. Forme fruste keratoconus was present in eight (57%) eyes with ectasia compared with none of the 100 controls (p <0.00001). However, there was no significant difference between the ectasia cases and the non-ectatic controls in the proportion of eyes found on topography to have oblique astigmatism preoperatively, 14% versus 12%, respectively, and only one (17%) of six eyes with ectasia evaluated preoperatively with Orbscan showed a posterior elevation >40 µm.
"Both oblique astigmatism and a posterior Orbscan elevation >40 µm are associated with forme fruste keratoconus. We did not find they were significant risk factors for ectasia, but that may reflect the small number of eyes in our study," Dr. Holland said.