Scan Geometry Orientation Influences Early Visual Acuity After Low-Energy Keratorefractive Lenticule Extraction: A Randomized Contralateral-Eye Study

Purpose: To determine whether the orientation of asymmetric spot-track scan geometry, independent of pulse energy and total laser fluence, influences early visual and optical outcomes after low-energy keratorefractive lenticule extraction (KLEx). Methods: Forty-one consecutive patients (82 eyes) undergoing bilateral myopic KLEx (SCHWIND ATOS, SmartSight; SCHWIND eye-tech-solutions GmbH) were enrolled in a randomized contralateral-eye comparison: one eye to conventional asymmetric geometry (spot 6 µm, track 2.8 µm) and the fellow eye to reverse-asymmetric geometry (spot 2.8 µm, track 6 µm). Pulse energy was 75 nJ in all 82 eyes; total laser fluence was therefore identical between geometries. Examiners were masked. Outcomes were modeled with linear mixed-effects regression (random patient intercept; fixed effects of arm, log time, preoperative spherical equivalent, and preoperative corneal wavefront higher order aberration for corneal outcomes) at postoperative day 1, week 1, month 1, and month 3. Outcomes included uncorrected distance visual acuity (UDVA), corneal and ocular wavefront higher order aberrations, and refractive predictability. Results: Reverse-asymmetric geometry was associated with a small but statistically significant UDVA difference across the postoperative course: mixed-effects arm effect +0.023 logarithm of the minimum angle of resolution (95% confidence interval +0.007 to +0.038, P = .004). Corneal and ocular wavefront higher order aberrations, spherical aberration, and refractive predictability were equivalent between arms. Δ corneal coma root mean square showed a directional increase with reverse geometry (β = +0.034 µm, P = .092), consistent with the UDVA effect. Programmed laser-on time was halved with reverse-asymmetric geometry (18 vs 37 seconds, a fixed 19-second reduction by construction). No eye lost a line of corrected distance visual acuity at any visit through postoperative month 3. Intraoperative opaque bubble layer was Grade 1 in all eyes. Conclusions: Reversing the asymmetric spot-track orientation, with pulse energy and total fluence held constant, was associated with a small, statistically significant disadvantage in early UDVA of uncertain clinical significance, while preserving safety, refractive predictability, and wavefront quality. These findings suggest that scanning geometry orientation may be an independent design parameter in low-energy KLEx, potentially related to bubble-field geometry and lamellar crack propagation anisotropy.