Samuel Arba Mosquera

Purpose: To examine the influence of eye movement variability on postoperative higher-order aberrations (HOAs), and refractive and astigmatic changes after femto laser in situ keratomileusis (LASIK). Setting: American University of Beirut Medical Center. Design: Retrospective study. Methods: A retrospective analysis was conducted on 94 eyes from 47 patients treated with the SCHWIND AMARIS system. Intraoperative eye-tracking data captured horizontal (X) and vertical (Y), and radial (R) positional shifts, with variability quantified using mean and SD. Correlation analyses examined the impact of movement fluctuations on postoperative corneal HOAs at 3 months, both unscaled and scaled by planned spherical equivalent (SEq) and astigmatism. Analyses were conducted on the full dataset and repeated using 1 eye per patient (right eyes only) after exclusion of high-influence points to eliminate pseudoreplication and assess robustness. Results: Increased intraoperative fixation variability was associated with greater postoperative aberrations, with the strongest and most consistent correlations observed for spherical aberration (SA). Variability in horizontal, vertical, and radial eye positions showed significant associations with induced SA, which were strengthened when positional measures were scaled by planned SEq. Several associations, particularly those involving SA, remained significant after removing high-influence points and were partially preserved in the 1-eye sensitivity analysis. Conclusions: Fixation instability (leading to movement variability) is consistently associated with increased induction of SA, and to a lesser extent, other HOAs after FemtoLASIK. Improvements in eye-tracking technology performance and fixation stability may reduce aberration induction and improve refractive outcomes.

Keratorefractive lenticule extraction (KLEx) relies on dense, spatially uniform femtosecond laser photodisruption to create intrastromal cleavage planes that permit controlled mechanical separation of corneal tissue. While scanning trajectories vary between discrete meander scans and continuous Archimedean spirals, the geometric limits governing coverage efficiency remain unified under local planarity. In this work, we present a mathematical framework describing the asymptotic fill fraction achieved by circular laser spots. By modeling the fundamental 3-bubble dynamics via Boolean triangle tessellation, we derive exact closed-form equations for aligned and staggered spot arrangements. We determine the dependence of the row-to-row phase shift (δ) in the global average fill fraction. We establish analytical boundaries for full coverage (saturation) based on circumradius limits and introduce an invertible master equation to directly calculate required pulse densities for any target fill fraction. This comprehensive model confirms that hexagonal packing is the unique asymptotic optimum and geometrically supports physiological advantages of high-anisotropy scanning strategies.

Femtosecond laser-assisted refractive correction relies on temporally and spatially separated pulses that generate coalescent cavitation bubbles, forming a cleavage plane for tissue separation. Achieving optimal outcomes requires balancing laser-induced stress, mechanical dissection stress, and surface roughness. This work introduces a nonlinear absorption model and a theoretical framework to identify the optimum spatial and temporal distribution of single pulses. The analysis, based on inequalities involving scaling factors for spot size and track distance, defines a bounded solution space. Within this domain, the most favorable setting corresponds to minimum dose with maximum asymmetry, ensuring energy efficiency while enhancing surface smoothness, whereas the least advantageous of the optimum conditions occurs for higher dose and minimum asymmetry (compatible with optimum conditions), both enabling a theoretical bridge-free dissection. Bubble overlap emerges as a key determinant of cutting efficiency and smoothness, and an optimal window for overlap factors is delineated, minimizing treatment dose while preserving corneal quality through smoother stromal cuts.

To develop predictive models for postoperative visual acuity-both near and distance, monocular and binocular-and a composite outcome metric, distance-corrected visual gain (DCVG), in patients undergoing presbyopic corneal refractive surgery. A retrospective analysis was performed on 914 eyes from 457 patients treated with PresbyMAX who completed 6 months of follow-up. Machine learning algorithms, including Random Forest regression and classification models, were applied to predict corrected distance visual acuity (CDVA) and distance-corrected near visual acuity (DCNVA), both measured using the same distance correction. Feature importance was evaluated using Shapley additive explanations (SHAP) and partial dependence plots (PDPs). Binocular models outperformed monocular in predictive accuracy, and CDVA was more predictable than DCNVA. A clinical trade-off was observed whereby improvements of up to three lines in DCNVA could be achieved without significant CDVA loss. The optimal planned addition ranged between +1.50 D and +2.25 D, balancing near and distance vision outcomes. Patients 45 to 56 years of age showed greater potential for near vision gain without compromising distance vision. The DCVG metric effectively quantified overall visual benefits, with only a minority of cases showing a net loss. Predictive modeling combined with the DCVG metric enables personalized surgical planning by identifying key factors influencing outcomes. This approach supports better management of the visual trade-offs inherent in presbyopic refractive surgery. Machine learning-based prediction models and the DCVG metric facilitate individualized clinical decision-making, improving patient counseling and optimizing the balance between near and distance vision in presbyopic treatment.

A Virtual Postoperative Corneal Simulation may assist complex therapeutic laser refractive corneal surgery cases. This study aims to clinically validate the accuracy and precision of the ForeSight predicted corneal geometries compared to actual postoperative corneal measurements. Private practice, Jerry Tan Eye Surgery, Singapore. An observational case series involving 52 eyes of 28 patients, where original planned treatments were previewed by a ForeSight topographic algorithm. Corneal measurements included tomography, topography, epithelial and stromal thickness using a Spectral domain optical coherence tomographer and topographer. Pre-operative measurements were input into the preview algorithm along with the original treatment parameters (including all relevant parameters like the optical zone size, the type of wavefront-driven treatment (Ocular or Topography), refraction, and type of ablation—e.g. Photorefractive Keratectomy (PRK) or Laser assisted in-situ Keratomileusis (LASIK). Postoperative measurements and refractions were compared to the preview. Results show that there is a close correlation between the preview and actual post-operation data (Correlations for keratometry readings with R2 above 0.95, for corneal toricities R2 up to 0.41, for central corneal thickness R2 above 0.97, for ablation depth R2 above 0.95, for spherical equivalent correction R2 above 0.96, for astigmatic corrections R2 above 0.66, for postoperative spherical equivalent refraction R2 below 0.12, while postoperative astigmatic refractions R2 up to 0.59). The post-operative topographies were accurately depicted by the algorithm. The estimated postoperative refraction from the preview was consistent with the trend observed in actual postoperative refraction. Despite its early stage of development, the preview algorithm showed promising accuracy and enormous potential. Future refinements may make complex cases accessible to all surgeons, aiding in the correction of complex corneal refractive surgery cases.This preview algorithm aims to educate surgeons and patients alike in the correction of complex corneal refractive surgery cases.

(1) Background: A decrease in corneal hydration during refractive surgery is observed clinically as well as in laboratory settings, but the associated consequences are not yet fully understood. The purpose of this paper is to analyze the impact of the gain of ablation efficiency due to hydration changes during cornea refractive surgery. (2) Methods: We developed a simulation model to evaluate the influence of hydration changes on the ablation algorithms used in laser refractive surgery. The model simulates different physical effects of an entire surgical process, simulating the shot-by-shot ablation process based on a modeled beam profile. The model considers corneal hydration, as well as environmental humidity, along with the laser beam characteristics and ablative spot properties for evaluating any hydration changes and their effect on laser refractive surgery. (3) Results: Using pulse lists collected from actual treatments, we simulated the gain of efficiency during the ablation process. Ablation efficiency is increased due to dehydration effects during laser treatments. Longer treatments suffer larger dehydration effects and are more prone to overcorrections due to gain of efficiency than shorter treatments. (4) Conclusions: The improper use of a model that overestimates or underestimates the effects derived from the hydration dynamics during treatment may result in suboptimal refractive corrections. This model may contribute to improving emmetropization and the correction of ocular aberrations with improved laser parameters that can compensate for the changes in ablation efficiency due to hydration changes in the cornea.

This study analyzed the impact of corneal epithelial refraction on the correction of hyperopic and mixed astigmatism eyes treated with transepithelial photorefractive keratectomy. From the epithelial refraction provided by the diagnostic device, OCT correlations were evaluated with respect to manifest refraction. The postoperative outcomes showed a mean sphere of −0.03 D and a mean cylinder of −0.33 D, with 93% and 98% having 0.5 D, 1 D, or less spherical equivalent refractive error. The epithelium showed preoperative toricity: at 6 mm, the epithelium showed a compensational effect of ~15% for the refractive astigmatism, whereas at 3 mm, the compensation accounted for ~25% of the refractive astigmatism. No correlation was found between preoperative epithelial refraction and refractive deviation after hyperopic or mixed astigmatic transepithelial photorefractive treatment. This work provides insight into the refractive compensatory impact of the epithelium, suggests how one can benefit from that in transepithelial corrections, and sets a framework for the potential induction of errors in non-transepithelial corrections.

Purpose: To compare the visual outcomes and changes in corneal higher order aberrations (HOAs) after keratorefractive lenticule extraction (KLEx) using conventional symmetric spot/track distance laser spacing versus modified asymmetric spot/track distance laser spacing. Setting: XXX Design: Retrospective comparative case series. Methods: This study included patients who underwent KLEx for myopia treatment. These patients were treated with either conventional symmetric spot/track distance laser spacing or modified asymmetric spot/track distance laser spacing, with both protocols maintaining constant treatment dose and pulse energy. Visual acuity and corneal HOAs were measured preoperatively and 1 month after surgery. Results: A total of 530 eyes of 265 patients were analyzed. The postoperative uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA) showed no significant differences between the symmetric and asymmetric spacing groups. The postoperative total HOA in the symmetric and asymmetric spacing groups were 0.62±0.20 and 0.50±0.15, respectively (PPPP<0.001*). Conclusions: KLEx using modified asymmetric laser spacing yielded postoperative visual outcomes comparable to those achieved with conventional symmetric laser spacing while inducing significantly fewer corneal HOAs in the early postoperative period.