Impact of ablation ratio on 5-year postoperative posterior corneal stability after refractive surgery: SMILE and FS-LASIK

Abstract

Background To investigate the impact of the ablation ratio on 5-year postoperative posterior corneal stability in myopic eyes after small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) surgery. Methods A prospective, nonrandomized, cohort study: 80 eyes of 43 patients underwent SMILE surgery and 63 eyes of 32 patients underwent FS-LASIK surgery at the EYE & ENT Hospital, Fudan University. Ablation ratio was defined as lenticule thickness (SMILE cases) or ablation depth (FS-LASIK cases) divided by central corneal thickness (CCT). Posterior corneal elevation changes were recorded as posterior central elevation (PCE), posterior corneal surface at thinnest point (PTE) and posterior corneal mean elevation (PME). Patients were followed up at 6-month and 5-year interval to investigate the impact of the ablation ratio on posterior corneal elevation after SMILE and FS-LASIK surgery. Results PCE dropped at the 6-month follow-up for both SMILE (decreased by −1.11 ± 2.93 μm, P < 0.05) and FS-LASIK groups (decreased by −0.46 ± 3.72 μm, P < 0.05). PTE also dropped in SMILE (reduced by −2.04 ± 3.02 μm, P < 0.05) and FS-LASIK group (reduced by −1.28 ± 4.21 μm, P < 0.05) at the 6-month follow-up. Stable PCE (elevation change: SMILE −0.28 ± 4.03 μm; FS-LASIK 0.79 ± 4.13 μm, P > 0.05) and PTE (elevation change: SMILE −0.08 ± 4.28 μm; FS-LASIK 1.42 ± 3.85 μm, P > 0.05) for both groups were recorded at the 5-year follow-up compared to the 6-month visit. Ablation ratio was strongly correlated with 5-year postoperative PCE (β = 2.68 ± 1.05, P < 0.01) and PTE (β = 2.35 ± 1.17, P < 0.05). Cut-off value for 5-year postoperative raised PCE and PTE was 27.3 and 27.1%, respectively. Conclusions Ablation ratio was strongly correlated with postoperative posterior corneal elevation in a 5-year follow-up in both SMILE and FS-LASIK groups. PCE and PTE underwent slight backward displacement 6-month postoperatively and remain stable at the 5-year follow-up. Threshold of the ablation ratio for resisting forward displacement of posterior corneal surface was 27.3 and 27.1% for SMILE and FS-LASIK groups, respectively.

Comparison of 6.0 mm versus 6.5 mm Optical Zone on Visual Outcomes after LASIK

“…Despite larger optical zones improving visual acuity and reducing HOAs, larger optical zones require more tissue removal to achieve the same refractive power, leading to a smaller residual stromal bed [26] and limited potential for future enhancements. For every attempted diopter of myopic correction, an additional 3−4 microns of tissue is ablated with the 6.5 mm optic zone compared to the 6.0 mm optic zone, which may potentially increase the risk of ectasia [27] and forward displacement of the posterior cornea [28]. On the other hand, our data reflected a significant difference in rates of enhancement one year after LASIK, with the 6.0 mm group having higher rates of enhancement.…”

Corneal morphological changes after small incision lenticule extraction for myopic anisometropia

“…Changes in PCE after SMILE have been reported that PCE tended to decrease as compared with that preoperatively, and this decline remained stable from 1 week to a 5 years follow-up period ( 27 , 28 ). In the present study, changes in PCE in the higher myopia eyes were consistent with the previous studies in the higher and lower myopia eyes.…”

Predictive factors of posterior corneal shift after small incision lenticule extraction: a 5‐year follow‐up study

“…A PTA value of ≥40% was significantly associated with the development of corneal ectasia (Santhiago et al 2014). A modified PTA formula was proposed for SMILE (Moshirfar et al 2017), but due to its low practicability, the PAD, also named as MCCT index or ablation ratio, has been used in most studies for the evaluation of SMILE outcomes (Wu et al 2014;Li et al 2020;Liu et al 2021). The PSBT was demonstrated to affect corneal stability after laser ablation, and a minimum value of 60% was usually accepted by surgeons for LASIK (Wang et al 2017).…”

“…Pairwise comparisons of the area under the ROC curve (AUC) were performed using DeLong's test. With a target power of 80% and significance level of 5%, we determined a 0.33-lm difference in PCE for a sample size of 161 eyes (Li et al 2020). Percentage ablation depth (PAD) was defined as the ratio of maximum lenticule thickness (MLT) to preoperative central corneal thickness (CCT).…”