Corneal densitometry changes after SMILE Xtra versus SMILE for moderate to high myopia

To investigate the changes in corneal densitometry (CD) after small incision lenticule extraction combined with accelerated corneal cross-linking (SMILE Xtra) for correcting moderate to high myopia and to analyze their correlation.

In this non-randomized retrospective comparative study, participants who underwent surgery were enrolled in the standalone SMILE group and SMILE Xtra group, respectively. Preoperative and postoperative uncorrected visual acuity, spherical equivalent (SE), and CD values were recorded at 1, 3, 6, and 12mo. Measurements were taken in the central (0-2 mm), paracentral (2-6 mm), and peripheral (6-10 mm) zones, as well as in the anterior 120 µm, central, and posterior 60 µm layers of the cornea. A generalized linear mixed model (GLMM) was used to compare CD values between groups and at different time points, and the correlation between postoperative CD values and residual bed thickness (RBT) was analyzed.

Totally 31 participants (62 eyes, mean age 20.32±2.76y) were in the standalone SMILE group and 28 participants (53 eyes, mean age 20.45±2.08y) in SMILE Xtra group. At 12mo postoperatively, there were no statistically significant differences in uncorrected visual acuity or SE between the two groups (P>0.05). Preoperative CD values in all regions showed no significant differences between groups (all P>0.05). In the central (0-2 mm) and paracentral (2-6 mm) zones, and the anterior, central, and total stromal layers, group, time, and interaction effects were all significant (all P<0.001). At 1, 3, and 6mo postoperatively, CD values in the SMILE Xtra group were significantly higher than those in the SMILE group within the 0-2 mm and 2-6 mm diameter ranges, as well as in the anterior, central, and total layers (all P<0.05). At 12mo postoperatively, CD values in the anterior layer remained significantly higher in the SMILE Xtra group compared to the SMILE group (P=0.007), whereas no statistically significant differences were observed in other diameter ranges or stromal layers (all P>0.05). Correlation analysis revealed that at 12mo postoperatively, CD values in all regions of both groups showed a significant positive correlation with RBT (all P<0.05), except for the anterior layer in the SMILE group, which showed no significant correlation with RBT (r=0.114, P=0.380).

SMILE Xtra induces transient early increase in CD, primarily in the anterior and central layers. Although values stabilize or decline by 12mo postoperatively, the clinical significance of these long-term patterns requires cautious interpretation, considering the potential influence of post-surgical structural changes. Further research is necessary to fully elucidate the underlying contributions to these densitometric changes.