Long-term Corneal Biomechanical Properties of Thin Corneas After Small Incision Lenticule Extraction Surgery: A Prospective Controlled Study.

Purpose: To investigate long-term corneal biomechanical changes in thin corneas after small incision lenticule extraction (SMILE).

Methods: Patients with indications for SMILE were enrolled in this study between November 2017 and March 2018. Patients were matched for age, spherical diopter, cylinder, spherical equivalent (SE), and lenticule thickness (LT), and then categorized into the thin cornea group (preoperative thinnest central corneal thickness [CCT] of 500 µm or less, 32 eyes) or normal cornea group (CCT of greater than 500 µm, 32 eyes). Corneal biomechanical properties were measured using the Corvis ST system. Data were collected at 1 day, 3 weeks, 3 months, and 3 years postoperatively.

Results: At 3 years postoperatively, the safety indexes were 1.06 ± 0.12 and 1.09 ± 0.12 ( = .23) in the thin cornea and normal groups, respectively; the respective effective indexes were 0.89 ± 0.23 and 0.98 ± 0.18 ( = .12). Recovery of overall corneal stiffness was observed in both groups. Comparative analysis of biomechanical parameters revealed that the change between preoperative and 3-year postoperative values was smaller in the thin cornea than in the normal group, without statistical significance. Correlation analysis showed that SE, LT, and CCT were the main parameters affecting changes in corneal biomechanical properties in the normal group. No significant correlations were found between the stress-strain index, Corvis biomechanical index for laser vision correction, and preoperative CCT or age.

Conclusions: With rigorous preoperative screening and appropriate surgical design, thin corneas are biomechanically stable in the long term after SMILE. Moreover, postoperative biomechanical strength increases over time. .