[Analysis on corneal deformation and corneal biomechanical changes after small incision lenticule extraction]

Objective: To investigate biomechanical effects of central corneal thickness (CCT), and to explore the possible safe range for the tissue removal and percentage of tissue ablation (PTA) of small incision lenticule extraction (SMILE). Methods: Clinical observational study. One hundred and eighty-seven cases (365 eyes) undergoing SMILE surgery were enrolled. Corneal hysteresis (CH) and corneal resistance factor (CRF) were measured before and after surgery by the ocular response analyzer (ORA). Linear regression of CH, CRF and CCT, and percentage of tissue ablation were calculated. The fifth order polynomial functions provided the closest fit to the data of ΔCH, ΔCRF and the thickness of tissue removal. Results: CH and CRF decreased by 20.7% and 33.0%, respectively, after SMILE, and statistically significant differences were found between preoperative and postoperative values (P<0.05). CH and CRF showed a positive correlation to CCT (r=0.483, P<0.05. r=0.507, P<0.05). The fifth order polynomial functions showed that ΔCH and ΔCRF increased with the increasing tissue removal. The variety rate of ΔCH and ΔCRF was lower after 50 μm to 140 μm of tissue removal, but was higher when the tissue removal was less than 50 μm and more than 140 μm. The variation trend was similar for PTA. Conclusions: Corneal thickness has an effect on the corneal biomechanical properties, which decrease with the increase of tissue removal. It seems that the tissue removal of less than 140 μm and the PTA of less than 25% are safer in SMILE, and further investigations are needed. (Chin J Ophthalmol, 2017, 53:11-17).