Flap creation technique, ablation depth affect biomechanical results

October 15, 2008

A retrospective study of eyes undergoing wavefront-guided PRK, LASIK with a mechanical microkeratome, or LASIK with a femtosecond laser shows that biomechanical changes are affected by flap creation technique and ablation depth.

Key Points

Los Angeles-Flap creation with a femtosecond laser appears to result in a smaller and more consistent change in corneal biomechanics after LASIK than when a mechanical microkeratome is used, said D. Rex Hamilton, MD, MS, FACS.

His comments were based on a retrospective study analyzing changes in corneal biomechanical properties measured with a proprietary tonometer (Ocular Response Analyzer, Reichert) as well as correlations between the changes and ablation depth in eyes undergoing PRK (n =33), LASIK with a microkeratome (n = 32), or LASIK with a femtosecond laser (n = 32).

"We know that laser refractive surgery induces changes in corneal biomechanics as a result of the effects of both flap creation and tissue ablation. The finding that there is a more consistent and smaller effect on corneal biomechanics using the femtosecond laser for flap creation compared with a mechanical microkeratome probably reflects differences in flap architecture," said Dr. Hamilton, director, Laser Refractive Center and assistant professor, Jules Stein Eye Institute, University of California, Los Angeles.

All patients in the study had been treated for myopic astigmatism by Dr. Hamilton using a wavefront-guided technique (CustomCornea, Alcon Laboratories) with an excimer laser (LADARVision 4000, Alcon). Flaps in the microkeratome LASIK group were created with a proprietary instrument (One Use, Moria). The 60 kHz femtosecond laser (IntraLase, Advanced Medical Optics) was used in the femtosecond laser LASIK group. Eyes for inclusion in the study were selected with the aim to match ablation depth.

"Since ablation depth influences corneal biomechanics, we wanted to make sure we were comparing apples to apples between the groups," Dr. Hamilton explained.

In addition to ablation depth, information was extracted on corneal hysteresis (CH) and corneal resistance factor (CRF) measured before and 3 months after surgery with the tonometer, as well as preoperative central corneal thickness (CCT) and flap thickness determined intraoperatively by subtraction ultrasound. The analyses compared changes in CH and CRF between the groups and correlated the change in CH within each group with depth of ablation using Pearson correlation testing.

Patient age, mean manifest refraction spherical equivalent, and ablation depth all were similar in the three groups, and no difference was seen in flap thickness between the microkeratome LASIK and femtosecond laser LASIK groups. The PRK eyes had a thinner CCT compared with both LASIK groups, however.

"In this retrospective study, the latter finding probably reflects the fact that PRK may be more likely to be chosen as the procedure of choice in eyes with thinner corneas," Dr. Hamilton said.

Statistically significant differences were seen preoperatively in CH and CRF between groups, with both values being significantly lower in the PRK eyes compared with either of the other two cohorts. This difference also is probably related to the thinner corneas in the PRK group, Dr. Hamilton said.

The mean change in CH and CRF were similar across all three groups, although there was a trend for a lesser change in CRF with PRK compared with either of the flap procedures. The correlation analyses suggested a relationship between change in CH and ablation depth in all groups in which the change in CH increased with increasing ablation depth.

"An effect of increasing ablation depth on CH would be expected since the effect of surgery on the biomechanics of the cornea would increase as more tissue is removed," he said.

The correlation between change in CH and ablation depth was strongest and statistically significant in the femtosecond laser group, where the data showed the least scatter. The correlation between CH and ablation depth in the PRK group also was statistically significant, but the correlation was less strong than in the femtosecond laser LASIK group. The data in the microkeratome LASIK group showed the greatest scatter, and no statistically significant relationship was found between change in CH and ablation depth.

"Based on these findings, it appears that the [proprietary tonometer] may be a useful aid in helping clinicians to assess patients for adverse outcomes after LASIK," Dr. Hamilton said. "By allowing an estimation of changes in corneal biomechanics based on ablation depth, clinicians may be able to identify patients at risk for post-LASIK ectasia or for under- or overcorrections."