Does SMILE cap thickness affect postoperative corneal properties?

This prospective study compared biomechanical changes and posterior corneal elevation after small incision lenticule extraction (SMILE) using 2 different cap thicknesses.

Study design

A total of 58 consecutive patients underwent SMILE; 1 eye received a depth of 110 μm while the other eye to a depth of 130 μm. A Scheimpflug camera was used to assess posterior corneal elevation (PCE) along 3 optical zones (apex and 2- and 4-mm diameters). A Hartmann-Shack aberrometer was used to measure wavefront aberrations. Measurements were taken preoperatively and at 1 and 3 months after surgery. Dynamic Scheimpflug imaging was used to evaluate the biomechanics preoperatively and at 1 day, 1 month and 3 months postoperatively.

Outcomes

No significant difference was found in either vision correction or corneal biomechanics between the 2 groups. In both groups, PCE significantly flattened at the apex and at the 2-mm annulus at 1-month postop, which was especially prominent in the 110-μm group. Three months postoperatively, the 110-μm group was still flattened significantly (P=0.002) whereas the displacement at the apex in the 130-μm cap group had resolved (P=0.074).

There was no significant difference in wavefront aberrations between the groups after surgery, except in vertical coma (P<0.001).

Limitations

The total number of patients was small (58). Follow-up was only 3 months, limiting long-term conclusions from these results. The displacements for the 110-μm cap group and 130-μm cap group were −1.73 and −1.22, respectively, at 1-month postop and −1.02 and −0.81, respectively, at 3 months postop. The displacements decreased in both groups significantly, especially in the 110-μm cap group, and perhaps with more time, the intergroup differences may disappear.

Due of the different arrangement of collagen fibers in the anterior cornea and greater difficulty when separating the lenticule in the 110-μm cap group, there may have been more corneal swelling which may have lead to the greater posterior corneal displacement observed within this group.

Clinical significance

The differences in corneal biomechanics between the 110-μm cap group and 130-μm cap group were small; however, a thinner superficial lenticule might cause displacement of the PCE to be more persistent. This flattening of the posterior surface increases the radius of the curvature of the posterior surface of the cornea and decreases the total corneal refractive power, resulting in a hyperopic shift. This phenomenon might be related to the swelling of the cornea after surgery and the redistribution of the corneal tension, because central ablation may reduce cohesive tensile strength.