[Evaluation of refractive prognosis factors in myopic LASIK]

Our objective was to evaluate myopic Lasik refractive results according to preoperative refraction findings.

In 200 eyes, classified into 4 groups according to preoperative sphere and cylinder findings, different postoperative refractive parameters were compared: predictability, visual acuity (VA), security index (SI) and efficacy index (EI).

Grouped by sphere: spherical correction was better than cylindrical correction. Highest negative spheres resulted in undercorrection (p<0.0171). The residual astigmatism was similar and the predictability within +/- 1.0 D was lower for the most myopic groups. Grouped according to cylinder: the higher the cylinder, the more cylinder undercorrection (p<0.05) and more sphere overcorrection was achieved (p=0.0469). Between the lowest and highest cylinder levels, the residual astigmatism difference was of -0.32 x 90 degrees and predictability +/-1.0 D was of 98 and 94% for the sphere and 96 and 88% for the cylinder respectively. Defocus Equivalent <1 D for the lowest and highest myopic groups was 74 vs. 46% for the cylinder and 82 vs. 38% for the sphere respectively. Pre and postoperative VA were lower for higher spheres or cylinders. SI >1.0 was similar to EI (+/-0.9), for preoperative sphere and cylinder groups. Best corrected visual acuity (BCVA) of 20/25 by sphere was 100% vs. 60% and of 20/30 by cylinder was 100% vs. 83%, for lower and higher myopic groups respectively.

Analysis by grouping preoperative refractive errors is helpful for establishing its influence and improving LASIK prognosis and treatment. This procedure is effective and safe for all groups, and its inaccuracy is due more to astigmatism than to spherical error. As astigmatism increases, less spherical and higher cylindrical ablation should be performed.