Chicago?In-depth studies of corneal healing after wavefront-guided PRK are providing an understanding of the postoperative course of changes in refractive errors and wavefront aberrations, said Keith Holliday, PhD, at the refractive surgery subspecialty day meeting sponsored by the International Society of Refractive Surgery of the American Academy of Ophthalmology.
Dr. Holliday, director of laser research, Advanced Medical Optics (AMO), Santa Clara, CA, presented observations from evaluation of two eyes that had undergone wavefront-guided myopic PRK using the CustomVue platform (AMO/VISX). Characterization of corneal changes was performed using corneal topography (Humphrey Atlas Corneal Topography System, Carl Zeiss Meditec) complemented by corneal thickness measurements (Haag-Streit Pachymeter). MatLab software was used to analyze the data by calculating the differences in corneal shape when measured between any 2 postoperative days as well as by comparison with the target post-treatment shape. The latter was derived from simulations of the treatment applied to the preoperative corneal shape.
In evaluating the day-to-day healing, comparisons between postoperative days 5 and 8 showed thickness changed faster horizontally than vertically, and that phenomenon was determined to account for early induced astigmatism. Later, between days 14 and 47, the pattern of thickening reversed, and that was accompanied by a reduction in astigmatism.
"The persistence of coma may be an effect of pupil centroid shift between the preoperative evaluation at the aberrometer and treatment in the illuminated operating room. However, that issue is addressed in the new iris registration upgrade for the laser," Dr. Holliday said.
Explaining the wound healing changes that would account for the observed wavefront changes, Dr. Holliday observed that in the first few days after surgery, re-epithelialization occurs from the periphery. As the epithelium is thicker in the peripheral cornea, the central cornea is relatively flatter than intended, and there is an initial defocus error.
Over the next few days, the epithelium becomes more spherically uniform and, as a result, mean surface power gets closer to target.
"By postoperative day 8, which was 4 days after bandage contact lens removal, the defocus error reached its intended value," he said.
At the same time, however, the epithelium is thickening more horizontally in the nasal and peripheral areas than vertically, and that gives rise to astigmatism.
"We speculate this phenomenon may be due to differential densities of stem cell activity or the influence of eyelid pressure," Dr. Holliday said.
Over the next several months, as remodeling continues and the corneal surface smoothens, astigmatism resolves and higher-order aberrations also gradually decrease.