Effect of optical zone diameter on corneal biomechanical properties after small incision lenticule extraction

Abstract

Objective: To investigate the effect of optical zone diameter on corneal biomechanical properties after small incision lenticule extraction (SMILE). Methods: In this prospective case-control study, 62 eyes of 42 patients with myopia and myopic astigmatism that had undergone SMILE procedure were divided into 2 groups according to the optical zone (OZ) diameter: group A, 31 eyes of 19 patients, OZ= 6.5 mm; group B, 31eyes of 23 patients, OZ=6.0 mm. These patients were examined for corneal hysteresis (CH) and corneal resistance factor (CRF) with the Ocular Response Analyzer (ORA) preoperatively and at 1 week, 1 month and 3 months postoperatively; at the same time, the central corneal thickness-corrected indices of DifCH and DifCRF were calculated. Pearson correlation was applied to analyze the correlation between CH and CRF and central corneal thickness. Independent-samples t test was utilized to compare CH and CRF between the two groups. Repeated ANOVA was used to compare the difference of corneal biomechanical properties at different follow-up timepoints. Results: The preoperative mean CRF and CH values were (10.13±1.19) mmHg and (9.65±1.12) mmHg in group A and (10.34±0.89) mmHg and (10.04± 1.05) mmHg in group B, respectively, and no significant differences were found in CRF and CH between the two groups (CRF: t=-0.807, P=0.423. CH: t=-1.405, P=0.165). The mean CRF values in group A at postoperative 1 week and 3 months were (6.62 ± 1.09) mmHg and (6.83 ± 1.07) mmHg, respectively, which were significantly lower than the relevant mean CRF values in group B (t=-2.703, P=0.009. t=-3.733, P= 0.001). Meanwhile, the mean CH values at 1 week, 1 month and 3 months postoperatively in group A were (7.31±1.06) mmHg, (7.37±0.96) mmHg and (7.82±0.97) mmHg, respectively, which were significantly lower than those in group B (t=-2.415, P=0.019. t=-2.113, P=0.039. t=-2.67, P=0.01). Moreover, the DifCRF and DifCH values in group A were significantly lower than those in group B at 3 months postoperatively (t=-3.409, P=0.001. t=-2.064, P=0.044). The CRF and CH values in both groups showed a significant reduction at 1 week, 1 month and 3 months postoperatively (P<0.05). Conclusions: The size of optical zone has some effects on corneal biomechanical properties after SMILE. The smaller optical zone diameter has a relatively smaller effect. (Chin J Ophthalmol, 2017, 53:182-187).