Myopic Surface Ablation in Patients With Asymmetric Topography
American Journal of Ophthalmology, May 2017
Some individuals are not suitable candidates for refractive surgery because of their risk for iatrogenic corneal ectasia. Brenner et al. determined refractive results and theoretical elastic response of photorefractive keratectomy (PRK) among patients with asymmetric corneal topography and compared the findings with those of matched normal-topography controls. The authors found that in certain cases of asymmetric topography, myopic surface ablation could induce a premature and amplified corneal biomechanical elastic response rather than a long-term progressive pathologic process.
In this retrospective interventional study, patients with asymmetric topog- raphy (30 eyes; superior-inferior dioptric difference [SI index] >1.4 D) were observed for 1 year prior to PRK to monitor corneal stability. Patients in the control group (30 eyes) had normal anterior corneal surface and did not qualify for LASIK based on central corneal thickness. The study groups were matched for age, preoperative spherical equivalent (SE), mean keratometry, and percentage of tissue altered (PTA). All patients underwent myopic surface ablation for astigmatism.
The mean preoperative SI index was 2.06 ± 0.56 D for the group with asymmetric topography and 0.14 ± 0.73 D for the control group. From 3 to 12 months postoperatively, mean keratometric re-steepening was 0.51 ± 0.39 D in patients with asymmetry and 0.19 ± 0.40 D in the control group. The mean PTA of 14.42% generated theoretical elastic modulus reductions of 10.25% in patients with asymmetry and 2.45% in control participants. Three years postoperatively, 90% of all eyes were within ±0.50 D of SE, and the theoretical elastic modulus did not differ significantly between the groups.
Although refractive results were comparable between the study groups, the authors concluded that the asymmetric corneas had increased short-term stress relaxation, greater keratometric re-steepening, and reduced theoretical elastic modulus. In the long term, myopic surface ablation induced a premature and amplified, but not progressive, corneal biomechanical elastic response in some patients with asymmetric corneal topography.
The original article can be found here.