AIM: To evaluate the effects of different corneal cap thicknesses during small-incision lenticule extraction(SMILE)on corneal densitometry(CD), visual quality, and corneal biomechanics during the first postoperative year, and to explore its safety.

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.

AIM: To compare the changes in corneal epithelial thickness(CET)after small incision lenticule extraction(SMILE)and femtosecond laser-assisted in situ keratomileusis(FS-LASIK).METHODS: A total of 187 patients(187 eyes)who underwent either SMILE or FS-LASIK at Urumqi Aier Eye Hospital between December 2022 and November 2023 were collected. The patients were divided into SMILE group and FS-LASIK group according to surgical methods. The CET of the patients was measured by optical coherence tomography(OCT)system before surgery and at 1 wk, 1, 3, and 6 mo postoperatively.RESULTS: Changes in corneal epithelial thickness(△CET)in the central, paracentral, and mid-peripheral regions were compared at 6 mo postoperatively. The SMILE group was characterized by the most significant thickening in the central area and the least thickening in the mid-peripheral area; while the FS-LASIK group was characterized by the most significant thickening in the paracentral area and the least thickening in the mid-peripheral region. At 1 wk, 1, 3, and 6 mo postoperatively, within the 0-7 mm corneal area, the △CET for both the SMILE and FS-LASIK groups was correlated with the preoperative spherical equivalent.CONCLUSION: Within 6 mo postoperatively, both SMILE and FS-LASIK showed a similar trend in epithelial thickening but with distinct characteristics. The change in corneal epithelial thickness for both procedures was positively correlated with the preoperative diopter.

AIM: To observe the changes of corneal densitometry(CD)and higher-order aberrations after small incision lenticule extraction(SMILE), and to explore their correlations and influencing factors.METHODS: Prospective study. A total of 62 high-degree myopia and compound myopic astigmatism patients(62 eyes)who underwent SMILE in Urumqi Aier Eye Hospital from December 2022 to November 2023 were collected. The CD, root mean square of corneal higher-order aberrations(RMS HOA), spherical aberration, vertical coma, horizontal coma, and corneal epithelial thickness(CET)of the patients were measured before surgery, and at 1 d, 1 wk, 1, 3, and 6 mo after surgery, respectively.RESULTS:There were 4 patients lost to follow-up during the period, all due to failure to attend scheduled reexaminations. The number of complete and valid cases was 58 eyes. The CD in the central and paracentral regions of the anterior, middle, and total layer of the cornea increased significantly on the first day after surgery(all P<0.003). At 6 mo after surgery, there was a slight but statistically significant decrease in the CD of the central and paracentral regions of the anterior and the total anterior layer(all P<0.003). Compared with the preoperative baseline values, the RMS HOA, spherical aberration, and vertical coma significantly increased at 6 mo after surgery(all P<0.003), while the change in horizontal coma was not statistically significant. Compared with the preoperative baseline values, the CET in the central, paracentral, and peripheral regions of the cornea, as well as the total average CET in the 0-7 mm range of the cornea increased at 6 mo after surgery(all P<0.003). At 1 d after surgery, CD of the anterior layer and total layer in the central region of the cornea were positively correlated with the CET in the central region(rs=0.327, rs=0.250, all P<0.05). At 6 mo after the surgery, the CD of the middle layer and posterior layer in the central corneal region were negatively correlated with the preoperative spherical equivalent and the change of RMS HOA(all P<0.05).CONCLUSION:The anterior CD decreases at 6 mo after SMILE for high myopia correction, and there is a certain correlation between the changes in CD and some clinical parameters.