Post-LASIK corneal changes may affect enhancement

July 15, 2007

A femtosecond laser (IntraLase, Advanced Medical Optics) and microkeratome (M2, Moria) differ when it comes to changes in the flap and stromal bed that develop between primary LASIK and enhancement procedures. A greater increase in flap thickness at the time of enhancement after myopic LASIK might be the result of epithelial hyperplasia, according to Ronald Krueger, MD, speaking at the annual meeting of the American Society of Cataract and Refractive Surgery.

Key Points

All cases were reviewed retrospectively. Eighty-six eyes underwent a LASIK enhancement procedure after being treated with the microkeratome.

In all cases, ultrasound pachymetry was performed with intraoperative readings using the subtraction method with a 50-mHz probe sonogauge; the average of three measurements was recorded. All procedures were done using the 110-μm head of the microkeratome. Another 123 eyes had undergone treatment with the femtosecond laser (15 and 30 kHz) and underwent the same intraoperative measurements as the group in which the microkeratome was used, Dr. Krueger explained. He is medical director, Department of Refractive Surgery, Cole Eye Institute, The Cleveland Clinic, and he acknowledged his co-authors, medical student Daniel Pierre and Maria Regina Chalita, MD, PhD, who both contributed significantly to the work.

In the myopic eyes treated with the microkeratome, there was a significantly greater flap thickness at the time of the enhancement procedure compared with the thickness obtained at the primary treatment. In the hyperopic eyes, the flap thickness at the time of the enhancement procedure was slightly less than that at the time of the primary procedure. In the eyes with mixed astigmatism, the flap thickness was slightly greater at the enhancement procedure than during the primary procedure, but the difference did not reach statistical significance, Dr. Krueger reported.

When the investigators evaluated the residual stromal beds in myopic eyes treated with the microkeratome, they found that the beds were significantly thinner than the value obtained at the time of the primary surgery.

In the femtosecond laser group, the flaps measured at the enhancement procedure also were thicker than they had been during the primary surgery, but not as thick as the microkeratome group (p < 0.001) (femtosecond laser group, +6.07 μm, p = 0.003; microkeratome group +26.36 μm, p = 0.001).

The residual stromal bed in myopic eyes also was thinner at the time of enhancement in both the microkeratome and femtosecond laser groups, but without statistical significance between groups (femtosecond laser group, –19.45 μm, p < 0.001; microkeratome group, –17.97 μm, p < 0.001, difference, p = 0.63).

"Overall, statistically, the femtosecond laser had less of a thickening effect on the flap at the time of the enhancement procedures compared with the microkeratome. There are potential theories to explain why this happens that may be related to the mechanical tension versus the osmotic hydration components," Dr. Krueger stated. "The fibers in the anterior portion of the cornea have more tensile strength than the lower fibers. Consequently, when the anterior fibers are cut by the flap, they become relaxed, and after laser ablation, the lower fibers take on the load of mechanical tension and undergo compression."