Biomechanical stability after Small Incision Lenticule Extraction (SMILE) using a 110 or 160μm cap thickness: An ex-vivo study on human donor corneas.

Abstract

Purpose : To examine the biomechanical stability after Small Incision Lenticule Extraction (SMILE) using a 110 or 160μm cap thickness.

Methods : A total of 32 human donor corneas were allocated to four groups, by the combination of two cap thicknesses (110 and 160μm) and two spherical corrections (-4D and -8D). Each cornea was mounted on an artificial anterior chamber, with the chamber pressure adjusted by an 8% dextran media column attached.
Corneal tomography was obtained before and after SMILE with a chamber pressure of 15 and 40mmHg (Pentacam HR, Oculus, Wetzlar). Primary outcomes were anterior and posterior sagittal curvature for the 3mm diameter zone (Sag3mm, r) and the Total Corneal Refractive Power (TCRP, 4mm, apex, zone). The average changes after surgery (Δ= postop - preop) and with increased chamber pressure (δ= 40mmHg - 15mmHg) were compared between groups. The precise cap thickness was measured with optical coherence tomography. Mixed ANOVA was used for statistical analysis.

Results : The cap thickness averaged 117±8μm and 117±15μm for the two 110 μm groups, and 161±4μm and 157±5μm for the two 160μm groups, respectively. As seen in Table 1, a 110μm cap caused more anterior flattening and less posterior steepening than a 160μm cap for the -8D groups (p<.002), whereas no differences were seen between the -4D groups (p>.059). Before SMILE, an increased chamber pressure did not significantly change the anterior or posterior radius of curvature in any of the groups (p>.160, Table 2). After SMILE, increased chamber pressure caused a significant anterior steepening in all groups (p<.014). However, the δr_postop for the anterior curvature as well as the δTCRP_postop were comparable when using a 110 and 160μm cap (p<.171).

Conclusions : This ex vivo study demonstrated, that a 160μm cap caused less anterior and more posterior curvature steepening than a 110μm cap, and consequently less myopic correction. A tendency towards greater corneal compliance was seen when using a 110μm cap thickness in SMILE, but the differences were similar for the 110 and 160μm cap thickness.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Table 1: Postop – preop values (Δ). Significant difference between: (a) 110 and 160μm with -8D correction; (b) -4D and -8D with 160μm cap; (c) -4D and -8D with 110μm cap.

 

Table 2: 40mmHg - 15mmHg values (δ) before and after SMILE. (a) Significant different from zero for all comparisons.