Evaluation and comparison of early outcome and corneal biomechanical changes between SMILE-corneal collagen cross linking and SMILE for myopic eyes

Objective

To evaluate early corneal biomechanical change, safety and effectiveness after small-incision lenticule extraction combined with ultraviolet A corneal collagen cross-linking (SMILE-CXL) for myopic eyes and compare with SMILE.

Methods

A non-randomized controlled clinical study was performed.Forty-four myopic eyes of 25 patients were included in Hankou Aier Eye Hospital from December 2017 to July 2018.SMILE-CXL or SMILE was carried out for 22 eyes of 12 patients or 22 eyes of 13 patients, respectively, based on the normal posterior surface manifestation of Belin by Pentacam tomography or not.The posterior surface manifestation of Belin was normal in the SMILE group, and the posterior surface of Belin was yellow or red with Corvis biomechanical index and tomographic biomechanical index<0.3 (except keratoconus) in the SMILE-CXL group.The refractive diopter of the eyes was detected before and 6 months after surgery, including spherical diopter, cylindrical diopter and spherical equivalent (SE) with a comprehensive refractometer.The corneal biomechanical parameters of the eyes were detected before and 6 months after surgery with a Scheimpflug-based dynamic tonometry (Corvis ST). The safety index, a ratio of postoperative mean best corrected visual acuity (BCVA) to preoperative mean BCVA, and efficacy index, a ratio of postoperative uncorrected visual acuity (UCVA) to preoperative BCVA, were compared between SMILE-CXL group and SMILE group.BCVA and UCVA were examined using standard chart and converted to logarithm of the minimal angle of resolution (LogMAR) units.This protocol complied with the Declaration of Helsinki and was approved by an Ethics Committee of Hankou Aier Eye Hospital (No.WHS2017052701). Written informed consent was obtained from each patient prior to entering into the cohort.

Results

The postoperative UCVA of the eyes in both groups was more than 1.0.There were no significant differences in safety index, efficacy index and SE change between the SMILE-CXL group and SMILE group (all at P≥0.05). At 6 months after surgery, the values of second applanation time (A2T), time from the start until the highest concavity (HC-Time) and DA ratio 2 mm were significantly increased in comparison with before operation in both SMILE-CXL group and SMILE group (all at P<0.05). The changes of A2T, HC-Time and DA ratio 2 mm in the SMILE-CXL group were significantly lowered than those in the SMILE group at the sixth month after surgery (P=0.001, 0.001, 0.036). The deformation amplitude, maximum corneal velocity during the first applanation (Vin), distance between both non-deformed peaks and integrated radius during the maximum depression were significantly increased, and the central curvature radius at highest concavity, cord length of first applanation, cord length of second applanation, Ambrósio’s relational thickness horizontal, stiffness parameter applanation 1 and biomechanical intraocular pressure during the maximum depression were significantly lower at the sixth month after surgery than those before surgery (all at P<0.05). There were no significant differences in the parameters mentioned above between the two groups (all at P<0.05).

Conclusions

Compared with SMILE alone, SMILE-CXL shows a comparable safety and efficacy, and better corneal biomechanical properties.

Biometry; Small-incision lenticule extraction; Collagen cross-linking, corneal; Biomechanical properties, corneal; Scheimpflug-based dynamic tonometry (Corvis ST)