[Two millimeter micro incision lenticule extraction surgery with minimal invasion: a preliminary clinical report]

To report the clinical results of micro incision lenticule extraction (MILE) to correct myopia or myopia with astigmatism, and enlighten its viability and mechanism.

Prospective case-control study. All patients enrolled were treated by the VisuMax femtosecond laser system to correct refractive errors. Sixty-one patients underwent MILE with an incision of 2 mm. Another 53 patients underwent small incision lenticule extraction (SMILE) with a 3-5 mm incision as the control group. All patients took measurements of uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), refractive power and intraocular pressure, slit-lamp microscopy, and corneal topography preoperatively and at 1 day, 1 week, 1 month, 6 months postoperatively. The visual acuity was presented as median. To compare the visual acuity and astigmatism between the MILE group and the SMILE group, as well as before surgery and at each time point after surgery, non-parametric tests were applied in this study. The repeated measures analysis of variance was used to compare the differences between these two surgeries and between pre- and post-operation in diopters and morphological parameters. The paired-sample t test was used to compare the diopters and morphological parameters at each time point postoperatively and preoperatively. The independent-sample t test was applied to compare the basic characteristics preoperatively and the diopters, morphological parameters at each time point between these two groups.

Forty-one eyes (24 patients) in the MILE group and fifty-one eyes (29 patients) in the SMILE group had complete follow-up data. Before surgery and at 1 day, 1 week, 1 month, 6 months after surgery, the spherical diopters in the MILE group were (-5.09 ± 1.04), (0.12 ± 0.32), (0.11 ± 0.29), (0.02 ± 0.33) and (0.02 ± 0.23) D; the cylinder diopters were (-0.90 ± 0.83), (-0.25 ± 0.27), (-0.23 ± 0.30), (-0.20 ± 0.25) and (-0.16 ± 0.21) D. In the SMILE group, the spherical diopters were (-5.37 ± 1.26), (-0.04 ± 0.49), (0.12 ± 0.38), (0.10 ± 0.34) and (0.02 ± 0.33) D; the cylinder diopters were (-0.76 ± 0.65), (-0.22 ± 0.26), (-0.25 ± 0.30), (-0.26 ± 0.29) and (-0.21 ± 0.28) D. No significant difference was found between the two groups (F = 1.042, 0.941, 0.018; P = 0.310, 0.335, 0.894). In the vector analysis of astigmatism, at 1 month and 6 months after surgery, the values on Y axis were -0.06 ± 0.11 and -0.04 ± 0.10 in the MILE group, smaller than -0.14 ± 0.18 and -0.11 ± 0.16 in the SMILE group (Z = -2.076, -2.149; P = 0.038, 0.032). All full-correction patients had UCVA of 20/20 or better, and no BCVA decreased after 6 month follow-up postoperatively. Refractive stability was achieved within 1 month postoperatively, and less volatility appeared in the MILE group than the SMILE group. Before surgery and at 1 day, 1 week, 1 month, 6 months after surgery, no significant difference in the index of surface variance (ISV) and the index of vertical asymmetry (IVA) was found between the MILE group and the SMILE group (F = 0.902, 0.744; P = 0.345, 0.391), whereas the values of ISV and IVA were smaller in the MILE group than the SMILE group at each time point after surgery, and the D-value between the two groups increased with time. Six cases (9 eyes) had an opaque bubble layer while femtosecond laser passed in the procedure, 2 eyes had a small tear at the incision edge, 1 eye had local diffuse inflammatory exudates at 1 day postoperatively, and no eye had transient light-sensitivity syndrome, decrease of corneal transparency or infection.

MILE surgery (1.5-2.0 mm) is safe, predictable, effective and stable to treat refractive errors. It can reduce astigmatism values on oblique axis and maintain the integrality and stability of the structure of the cornea.