Influencing factors of corneal biomechanics following small incision lenticule extraction

Background The visual quality and corneal biomechanical change are two major sides for evaluating the outcomes of cornea refractive surgery.It was determined that small incision lenticule extraction (SMILE) can improve the visual outcomes, but its long-term effects on corneal biomechanical are not known clearly. Objective This study was to investigate the change of corneal biomechanics and influence factors following SMILE. Methods A serial cases-observional study was performed.One hundred eyes of 58 patients with a mean spherical diopter (-5.00±1.55)D and cylindrical diopter (-0.98±0.87)D were included in Tianjin Eye Hospital from August 2011 to July 2013 under the informed consent.Corneal biomechanical parameters, including corneal resistance factor (CRF) and cornea hysteresis (CH) were measured using the ocular response analyzer (ORA) in preoperation and postoperative 1, 3, 6 and 12 months, respectively.The patients were grouped based on different lenticule thickness ratio (LTR), preoperative CRF values and residual stromal thickness (RST) separately, and the correlations of LTR, preoperative CRF and RST with ΔCRF and ΔCH were assessed.In addition, the associations between ΔCRF or ΔCH and related preoperation parameters were evaluated. Results The CRF and CH values at postoperative 1 month were (7.12±1.20)mmHg and (7.90±0.97)mmHg, which were significantly lower than preoperative (10.17±1.46)mmHg and (10.00±1.16)mmHg, respectively (t=15.552, P=0.000; t=13.411, P=0.000). The CRF values at postoperative 3, 6 and 12 months were (7.06±0.90), (6.98±1.11) and (6.87±1.07)mmHg, and those of CH were (8.12±0.84), (8.12±0.97) and (8.14±0.86) mmHg, and no significant differences were found in CRF and CH between the adjacent time points (CRF: P=0.848, 0.992, 0.270; CH: P=0.370, 0.791, 0.777). Positive correlations were seen between the ΔCRF or ΔCH and LT/preoperative central corneal thickness (CCT) (LTR), preoperative CRF or preoperative CH values, respectively (LTR: r=0.468, P=0.000; r=0.299, P=0.004; preoperative CRF: r=0.696, P=0.000; r=0.590, P=0.000; preoperative CH: r=0.576, P=0.000; r=0.690, P=0.000). ΔCRF and ΔCH were negatively correlated with preoperative spherical equivalent diopter (r=-0.496, P=0.000; r=-0.292, P=0.010), the sum of preoperative spherical diopter and cylindrical diopter (r=-0.484, P =0.000; r=-0.293, P=0.005) or RST/preoperative CCT (r=-0.362, P=0.000; r=-0.243, P=0.019) and were positively correlated with lenticule (r=0.495, P=0.000; r=0.325, P=0.002). No significant association was found between ΔCRF or ΔCH and age, preoperative CCT and preoperative mean keratometry (all at P>0.05). Conclusions Corneal biomechanical strength is decreased at the early stage after SMILE.However, biomechanical strength gradually enhances 1 month after surgery and tends to stability.The large RST/preoperative CCT can improve postoperative CRF and CH.The another main factor affecting the corneal biomechanics after SMILE is corrected-diopter. Key words: Corneal surgery, laser; Refractive surgical procedures/methods; Corneal stroma/surgery; Biomechanical phenomena; Small incision lenticule extraction; Influencing factors