Comparison of residual stromal bed and flap thickness in primary and repeat laser in situ keratomileusis in myopic patients.

Purpose: To compare the change in residual stromal thickness and flap thickness between primary laser in situ keratomileusis (LASIK) and repeat LASIK in myopic patients.

Setting: Melbourne Excimer Laser Group, East Melbourne, Australia.

Methods: This retrospective nonrandomized comparative trial comprised 46 eyes of 34 patients who had repeat LASIK. The thickness of the residual stromal bed was calculated by subtracting the calculated stromal ablation from pachymetry of the stromal bed after cutting the flap in primary treatment and directly measuring during retreatment. The thickness of the LASIK flap in primary and repeat LASIK was calculated by subtracting the central pachymetry of the stromal bed after creating the flap from pachymetry before cutting and lifting the flap, respectively. The main outcome measures were comparison of the residual stromal bed and flap thickness between the primary treatment and the retreatment.

Results: The mean thickness of the calculated residual stromal bed after primary treatment was 329.8 microm +/- 40.8 (SD), and the mean measured residual stromal bed at retreatment was 317.3 +/- 42.8 microm. The mean difference in residual stromal bed thickness was 12.5 +/- 13.0 microm (P<.001). Sixteen eyes (34.7%) had a decrease in bed thickness between 11 microm and 20 microm. The mean flap thickness during primary LASIK and repeat LASIK was 145.2 +/- 17.1 microm and 169 +/- 18.3 microm, respectively. The mean interval between primary treatment and retreatment was 7.4 +/- 4.1 months. The mean change in flap thickness was 23.8 +/- 15.2 microm (P<.001). Fifteen eyes (32%) had an increase in flap thickness between 11 microm and 20 microm. There was a negative correlation between refractive error before primary treatment and the difference in flap thickness. No correlation was found between the difference in flap thickness and the interval between the primary treatment and the repeat treatment.

Conclusions: Intraoperative pachymetry of the stromal bed during retreatment is strongly recommended as the residual stromal bed and flap thickness changes between primary and retreatment. There is a tendency for the measured stromal bed at retreatment to be thinner than the calculated stromal bed and for the flap to be thicker than previously measured.