Effects of corneal cap thickness on corneal morphological and biomechanical properties of rabbits in vivo following SMILE

To investigate the effects of 100 μm and 160 μm corneal cap thicknesses on corneal morphology, endothelial cell density, biomechanics and epithelial thickness in rabbit eyes after SMILE, and to provide experimental reference for clinical surgical parameter selection. Twenty-four New Zealand white rabbits were randomly allocated to a 100-μm corneal cap group (group A) and a 160-μm group (group B), with 12 rabbits in each group and balanced sex distribution (6 males and 6 females in each group). SMILE was performed on the right eye of all rabbits using a uniform VisuMax program with programmed refractive correction of − 4.00 D, an optical zone of 6.50 mm, a cap diameter of 7.60 mm, and an incision at the 120° position with a 45° angle. Corneal morphology (Pentacam), endothelial cell density (specular microscopy), biomechanical properties (Corvis ST), and epithelial thickness (anterior-segment OCT) were measured preoperatively and at 1 week, 1 month and 3 months postoperatively. Between-group comparisons at each time point were performed with independent-samples t-tests; longitudinal trajectories were analyzed with a two-way mixed-design analysis of variance (group as the between-subjects factor; time and the group × time interaction as within-subjects effects). Multiplicity was controlled using the Benjamini–Hochberg false-discovery-rate procedure for secondary outcomes. Baseline central endothelial cell density was significantly higher in group B than in group A (2994.42 ± 237.96 vs 2693.60 ± 345.72 cells/mm 2 ; P = 0.021); baseline anterior and posterior Km and anterior chamber volume also differed between groups ( P < 0.05), reflecting imperfect baseline balance in this small randomized cohort, whereas the remaining baseline indicators were comparable. Both groups demonstrated comparable postoperative corneal flattening, with comparable central endothelial cell density throughout the 3-month follow-up. Early postoperative intergroup differences in corneal asphericity and Corvis ST parameters were observed but were largely nominal after false-discovery-rate adjustment. Group B had lower Def Amp Max and higher SSI at 1 and 3 months (FDR-corrected). The group × time interaction was significant for Def Amp Max, IR, SP-A1, SSI, ARTh, and corneal epithelial thickness, indicating different postoperative remodeling trajectories between the two caps. No significant intergroup differences in corneal morphological parameters were detected at 3 months. In this rabbit SMILE model, both 100-μm and 160-μm caps produced comparable postoperative corneal flattening and no significant reduction in central endothelial cell density over the 3-month follow-up. Cap thickness was associated with different postoperative biomechanical and epithelial remodeling trajectories (significant group × time interactions for Def Amp Max, SSI, and epithelial thickness), without affecting corneal morphological outcomes at the final follow-up.