Changes in Corneal Volume at Different Areas and Its Correlation with Corneal Biomechanics after SMILE and FS-LASIK Surgery

Purpose. To investigate the variations of corneal volume (CV) after small incision lenticule extraction (SMILE) and femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) and analyze the influences of biomechanical properties on the changes of refraction and CV. Methods. Ninety-seven eyes of 97 patients undergoing SMILE and FS-LASIK were included in this retrospective study. CV was measured with Scheimpflug-based corneal topography at preoperatively and at day 1, week 1, and months 1 and 3 postoperatively. CV measured within 5 mm diameter was defined as central region volume (CV5) and between 5 mm and 10 mm diameter was defined as peripheral region volume (CV5-10). An Ocular Response Analyzer (ORA) was used to assess corneal biomechanical properties including corneal hysteresis (CH) and corneal resistant factor (CRF). The reduction of study parameters (o) were calculated by subtracting the preoperative value at various time points from the postoperative values. Results. CV had significant reduction after the SMILE and FS-LASIK procedure (P<0.05). CV5 increased significantly from postoperative day 1 to month 3 (P<0.001) in SMILE, while both CV5 and CV5-10 increased significantly in FS-LASIK (P<0.001). The increase in CV5 after SMILE was 0.11 +/- 0.16 mm(3)?which was significantly different from FS-LASIK (0.20 +/- 0.13 mm(3), P=0.004). In the SMILE group, oCV5 correlated with oCRF (r = 0.498, P<0.001) and oCH (r = 0.374, P=0.007). In the FS-LASIK group, oCV5 and oCRF had a significant correlation (r = 0.363, P=0.012), but not with oCH. Conclusions. Dynamic changes in corneal volume were found after SMILE and FS-LASIK surgery. The central region significantly increased after SMILE, while both central and peripheral regions increased following FS-LASIK in the early postoperative period. SMILE was associated with less change in biomechanical properties per unit of reduction in CV compared with FS-LASIK.