METHODS: A prospective randomized contralateral controlled design was employed. Thin cap(110-120 μm)and thick cap(130-140 μm)SMILE procedures were performed on the two eyes of the same myopic patients undergoing surgery, respectively. Postoperative evaluations at 1 wk, 1, 3, 6 mo, and 1 y included CD, uncorrected distance visual acuity(UDVA), corrected distance visual acuity(CDVA), contrast sensitivity, wavefront aberrations, and corneal biomechanics. A linear mixed-effects model was used for statistical analysis.

RESULTS:A total of 60 patients(120 eyes, 60 eyes in the thin-cap group and 60 eyes in the thick-cap group)completed the surgery successfully and attended scheduled follow-up visits. The mean age was 26.3±3.8 y, including 34 males(57%)and 26 females(43%).The intended cap thickness was 115.24±3.10 μm in the thin-cap group and 135.28±3.45 μm in the thick-cap group. There were significant intergroup differences in lenticule thickness and ablation depth(P<0.001), whereas the residual stromal bed(RSB)thickness was comparable between groups(P=0.301). Central 0-6 mm CD significantly decreased over time(P<0.001), and the thin-cap group had overall higher densitometry values than the thick-cap group(P<0.001), with significant differences persisting from 1 wk to 1 y postoperatively(Bonferroni-corrected P<0.05). At 1 y, significant CD differences were most pronounced in the anterior and central layers(both P<0.05), with no significant differences in posterior layers. The thick-cap group showed a significant improvement in UDVA(P=0.002)and better contrast sensitivity(corrected P<0.05), while the thin-cap group had higher values of total root mean square of higher-order aberrations(HOA-RMS), spherical aberration, and coma(all P<0.05). No statistical differences were observed between groups in corneal biomechanical parameters(DA, CBI, SSI)(all P >0.05), and the incidence of safety events was also comparable between the two groups(P≥0.5).

CONCLUSION: Given an equivalent RSB thickness, thicker corneal caps can reduce anterior-layer CD, improve contrast sensitivity and higher-order aberration outcomes, without obvious increases in biomechanical risks or safety concerns.Therefore, it warrants consideration as a preferred intraoperative strategy for SMILE.