Predicting Flap Thickness and Residual Stromal Bed in LASIK Enhancements

Abstract

Abstract: : Purpose:Evaluate the flap thickness and residual stromal bed measured at the time of a LASIK enhancement procedure and compare them with the estimated mathematical calculation based on measurements performed at the primary procedure. Methods:Eighty six eyes that underwent LASIK enhancement were subdivided according to the primary LASIK treatment into: myopia (59 eyes), hyperopia (20 eyes) and mixed astigmatism (7 eyes). Preoperative and intraoperative central pachymetry was performed using a 50Hz probe (Sonogage). All primary procedures were performed with the Moria M2 microkeratome, 110µm head. Measurements of the flap and the residual stromal bed thickness at the primary procedure were compared to the measurements acquired at the time of enhancement. Results:In the myopia ablation group, the flap was significantly thicker at the enhancement measurement (p<0.0001), with mean thickness of 115µm±4.0SE at the primary procedure and 142µm±4.4SE at the enhancement. For hyperopia and mixed astigmatism ablations there was no difference in flap thickness between the primary procedure and the enhancement (p=0.94 and p=0.20, respectively). When comparing residual stromal bed postablation at the primary procedure with residual stromal bed preablation at the enhancement, in the myopia ablation group the stromal bed at the enhancement was thinner than expected from calculations from the primary procedure (347±5.7SE and 364±5.4SE, p<0.0001). The hyperopia and mixed astigmatism groups revealed that the residual stromal bed at the enhancement was thicker than expected by the calculation from the primary procedure (366±9.7 and 411±8.8 for hyperopia (p<0.0001); 413±17.1 and 433±20.9 for mixed astigmatism (p=0.045). Conclusions:Residual stromal bed was thinner than expected after ablations for myopia and thicker after ablations for hyperopia and mixed astigmatism. Different etiologies, such as amount of tissue removed with the laser, corneal biomechanical effects and/or corneal wound healing may have an important role in these clinical findings that should be taken into account when planning a LASIK enhancement.

Keywords: refractive surgery: LASIK • cornea: clinical science • wound healing