Comparison of the posterior corneal elevation and biomechanics after SMILE and LASEK for myopia: a short- and long-term observation

Abstract

Purpose This study compares the posterior corneal elevation and corneal biomechanics after small incision lenticule extraction (SMILE) and laser-assisted subepithelial keratomileusis (LASEK) for myopia correction in a short-and long-term observation. Methods This prospective study included 32 patients in the SMILE group and 32 patients in the LASEK group. Corneal posterior central elevation (PCE), posterior mean elevation (PME), corneal back power (Kb), and anterior chamber depth (ACD) were evaluated with Pentacam, and intraocular pressure (IOP), corneal hysteresis (CH), and corneal resistance factor (CRF) were evaluated with the ORA at pre-operation and 3 months and 3 years post-operation. Results Three months post-operatively, CH, CRF, and IOP decreased significantly and central posterior surface shifted backward in both groups (p < 0.05). CH was lower in the LASEK group (p = 0.03) and change of CH and CRF per unit corneal tissue removed (ΔCH/ablation depth (AD) and ΔCRF/AD) was lower in SMILE than in LASEK (p = 0.01, 0.03). Three years postoperatively, the PME shifted more posteriorly in LASEK (p = 0.04), but was stable in SMILE (p = 0.06). Kb flattened and ACD was shallower in both groups (compared to preoperative data, p < 0.001). CH in the LASEK group increased and is comparable to that in the SMILE group at 3 years post-operative. Conclusion Both SMILE and LASEK can change the posterior surface and corneal biomechanics. SMILE may have less influence on corneal biomechanics than LASEK at an early stage post-operative in terms of per unit corneal tissue removed, but the effect became comparable in a long-term observation.

Corneal biomechanical properties after SMILE versus FLEX, LASIK, LASEK, or PRK: a systematic review and meta-analysis

“…Seventeen studies reported a cap thickness between 100 to 120 μ m [25, 30, 39–41, 45, 47–53, 55, 56, 58, 59]. Only one study reported a 90 μ m thickness cap [36].…”

“…Only one study reported a 90 μ m thickness cap [36]. The cap diameter was between 7.2 to 8 mm in 16 studies, [25, 30, 36, 38, 41, 45–51, 53, 58–60] and the diameter of the optical zone was between 6 to 7 mm in 19 studies [25, 30, 36, 38–41, 45, 47–49, 51–56, 58, 59]. Twelve studies were performed with an energy between 115 to 190 nJ [30, 36, 38, 40, 41, 45, 47, 49–51, 53, 54, 58].…”

“…Two of the four studies which involved LASEK as the comparator reported an 8.5 mm flap diameter in 2 studies [50, 53] and optical zone was 6.25 to 6.75 mm in 1 study [58] with and the energy for ablation of 150 nJ in all 3 studies [50, 53, 58].…”

Short-term changes in the anterior segment and retina after small incision lenticule extraction

“…Results from previous studies suggested that anterior segment changes would last for a long time, while posterior segment changes are only observed for a short time and then gets resolved. Prior studies have reported that PCE and ACD decreased after surgery and even several years later [ 6 , 7 ], and this change was more significant in younger patients [ 12 ]. It has also been reported that changes in elevation correlated with residual bed thickness [ 13 ].…”

“…Cornea biomechanics decrease after both LASIK and SMILE [4,5]. Posterior corneal elevation (PCE) and anterior chamber depth (ACD) are also reduced [6,7], but few studies have investigated changes in the retina.…”