Flap handling may provide answers to causes of higher-order aberrations

September 20, 2004

Paris

Paris—What causes the increase of higher-order aberrations after LASIK? posed Scott MacRae, MD, of Rochester, NY, to an audience at the European Society of Cataract and Refractive Surgeons meeting.

Dr. MacRae set out to understand the individual effects of the microkeratome cut, mechanical lifting of the cut flap, and laser ablation on the patient's lower- and higher-order aberration pattern after LASIK.

Dr. MacRae presented some findings from an ongoing, 4-year prospective study that he and colleagues at the University of Rochester have studied in relation to this problem.

The study examined outcomes after the corneal lamellar resection, lifting of the flap, and LASIK ablation. Eleven eyes underwent lifting of the flap (flap lift group). In 7 eyes, the flap was cut but not lifted (no flap lift group). Aberrometry, topography, visual acuity, and refraction tests were performed preoperatively and at 20 minutes, 1 day, 1 week, 1 month and 2 months post-flap creation.

After 2 months, massive series of measurements were made. The flap was lifted and laser ablation was performed in the flap-only eye and LASIK was performed in the contralateral control eye. Follow-up occurred post-ablation at 1 day, 1 week, 1 month, and 3 months.

The study showed that the increase in higher-order aberrations was 0% in the no flap lift group versus 30% for the lift group. Furthermore, a small but significant increase in spherical aberration was observed in the flap lift group. Following laser ablation, increase in spherical aberration was observed in both groups proportionate to the amount of sphere corrected.

In the no-flap-lift group, Dr. MacRae said the researchers lifted the flap and found something very interesting.

"Surprisingly, there was no systematic change in coma; there was no systematic change in cylinder, and no systematic change in spherical," he pointed out. "There was a slight tendency toward a hyperopic shift of about 0.2 D."

Dr. MacRae questioned whether flap handling caused higher-order aberrations and whether flap displacement caused a reduction in higher-order aberrations. He believed that it actually does. He urged surgeons to minimize the hydration of the flap after the cut, then reposition the flap carefully, stretching the flap to minimize the gap, and optimize the flap alignment so it is symmetrically aligned. He said these points are quite critical.

In summary, Dr. MacRae said that about 40% to 67% of the increase in positive aspherical aberration is mostly related to the actual ablation. Cutting and not lifting the flap surprisingly does not increase higher-order aberrations.

"This really raises some basic questions of whether we can minimize higher-order aberrations with minimal displacement of flap post-positioning -- and if it would make any difference at all whether you were cutting with a blade or cutting the flap with a laser," Dr. MacRae said. "It may be more important that you are very meticulous how you reposition the flap."