Safety thresholds for removing and preserving corneal tissue in small-incision lenticule extraction: A...

Ranges of corneal tissue removal and residual stroma are the most essential safety consideration for corneal refractive procedures and directly determine the eligibility of candidates who may benefit from surgery. Small-incision lenticule extraction (SMILE), a laser procedure, has been performed for more than 10 years with well-established efficacy and predictability. However, determining the optimal safety limit for corneal tissue removal is still a challenge, perplexing surgeons.

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Six electronic databases (PubMed, Web of Science, Cochrane Library, Embase, China National Knowledge Infrastructure, and Wanfang Data) were searched. Corneal removal and remaining thresholds were mainly quantified by maximum lenticule thickness (MLT), residual stromal thickness (RST), and percentage of tissue alteration (PTA_SMILE, the ratio of MLT to preoperative central corneal thickness (CCT)). Thin corneas (<500 μm) and eyes with extremely high myopia (>-9 D) were regarded as two special conditions for independent analysis.

Twenty-five studies (3247 eyes) were included. Corneal stability post-SMILE declined with increased tissue removal and decreased residual stroma. From a corneal biomechanical perspective, postoperative weakening of corneal biomechanical strength was more pronounced when RST was <280 μm or PTA_SMILE was >28%, as demonstrated by various measurements including deformation amplitude ratio, corneal hysteresis, and corneal resistance factor. Regarding posterior corneal elevation, thresholds of MLT and PTA_SMILE to prevent forward shifts following SMILE were 140-145 μm and 27%-28%, respectively. In meta-analysis, the safety index in thin corneas and extremely high myopia were 1.07 (95% CI: 0.98-1.15) and 1.17 (95% CI: 1.12-1.21), respectively. For thin corneas, the stiffness parameter at first applanation (SP-A1) exhibited less reduction compared to normal corneas after equal tissue removal, indicating biomechanical safety with minimal stiffness alteration.

Until more specific metrics are validated, RST (280 μm) and PTA_SMILE (28%) may serve as benchmarks for establishing safety thresholds for corneal tissue removal and residual stroma in SMILE. Biomechanical assessments, like SP-A1, may be promising indicators, while the preoperative CCT has limited association with postoperative morphological and biomechanical stability.