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Under optimal asymmetric spacing, overlaps occur on the millisecond scale, well beyond the bubble lifetime(< 10 ls), which minimizes destructive pulse – bubble interactions. These considerations support the view that optimal surgical parameters represent a“ sweet spot” balancing laser stress, mechanical stress, and surface smoothness. Our theoretical findings align with prior reports linking dissection quality, occurrence of OBLs, and visual recovery to laser settings [ 24, 36, 37 ]. Specifically, surface smoothness improves with lower pulse energy, tighter track distances, and increased asymmetry( especially above a 2:1 ratio). Laser stress decreases with reduced energy, larger spacing, and higher asymmetry by limiting bubble overlap. Mechanical stress, in turn, is minimized within the asymmetry range of ~ 1.15 – 3.46, where energy distribution balances cutting efficiency with tissue integrity.
There are other considerations which may benefit from the proposed strategy. For example, larger heating effects can cause a problem. For instance, separating the pulses with some form of a regionalized Sobol sampling can reduce cumulative heat load in one area, while still having asymmetry. At this regard, both using( close to) minimum dose as well as using asymmetric spacings with Spot Distance > Track Distance, both are independent simple measures to reduce heating. Actually, both together act synergistically by reducing the total energy deposition while non overlapping consecutive pulses; and still ensuring( facilitating) tissue bridge-free dissections.
Early visual recovery appears to depend more on minimizing laser and mechanical stress than on surface roughness alone. While smoother corneal interfaces predictably enhance vision, they also accelerate epithelial remodeling by providing a better baseline. Thus, reducing energy deposition and bubble interference may be more critical for POD1 outcomes than further minimizing residual roughness, though both factors contribute. In practice, low pulse energy with spot distances slightly larger than bubble size( avoiding spatial / temporal overlap along the pathway) and tight track distances creates the most favorable balance.
There are potential refinements to this work. On the one hand, we calculated the number of pulses based on the area of a flat disc. This may be refined to account for the true cutting surface resembling something between a spherical cap and a parabolic dish( enlarging the treatment area with respect to the disc). Further, one may account for the compression and deformation of the tissue under the contact element during the laser process [ 42 ]. But both these refinements do not change the presented findings, since the treatment area cancels out in the calculation of dose( Eqs.( 1)–( 3)).
We have used the photodisruption model in this work, and not the more recently introduced plasma model [ 9 ], which is less commonly applied so far. Earlier work based on this model proposes that the maximum surface smoothness is achieved at an infinite overlap( F = 1, infinite dose = maximum smoothness), with F = 1.41 as the smallest factor permitting bridge-free dissection, and F = 1.19asareasonable compromise. In that work, we assumed symmetric conditions( F1 = F2). In asymmetric configurations, however, F1 and F2 can be selected independently, constrained by F1 F2 1.41 as a practical compromise between F1 F2 = 2 for a bridge-free tissue dissection, and F1 F2 = 1.27(= 4 / p) for the minimum dose. Furthermore, an asymmetry ratio F2 / F1 > 2 is shown to effectively reduce roughness. This yields, for example, F1 0.73( Spot Distance 1.4 Bubble Diameter) andF2 1.92( Track Distance 0.52 Bubble Diameter), corresponding to the proper optimum solution determined in this study( dose( F1 F2) of 1.41 and asymmetry ratio( F2 / F1) of 2.62). More broadly, we propose a spot spacing range of 100 – 165 % of bubble diameter, with track distances adjusted accordingly, as the geometric – temporal“ sweet spot” for femtosecond laser refractive procedures. The presented findings may provide a foundation for future system designs to incorporate and expand upon these concepts.
5 Conclusion
The bubble overlap denoted by the scaling factors F1 and F2 plays a pivotal role in determining cutting smoothness and efficiency. Extending our previous works, an optimum window for the overlap factors has been defined, which optimizes overall dose per treatment and minimizes corneal surface roughness. Collectively, the clinical and biomechanical data confirm that spot spacings above 5 lm, when used in conjunction with lower pulse energies and asymmetric spacing, provide a robust foundation for safe, efficient, and highquality corneal refractive surgery. A general consensus has emerged among femtosecond laser platform users that the“ sweet-spot” energy density for currently available commercial systems lies in the range of 400 – 500 mJ / cm 2. Aproper optimum to achieve this energy density can vary slightly between clinical settings but increasingly shows a trend towards increased spot and track distance asymmetry, reduced pulse energy leading to a reduced dose, suggesting that the currently recognized“ sweet-spot” energy density will further reduce as the technology evolves. The alignment of empirical results with theoretical models justifies the growing clinical trend toward dose-preserving, low-energy, asymmetric ablation paradigms in modern lenticule extraction.
Funding This research did not receive any specific funding.
Conflicts of interest
The authors are employees of SCHWIND eye-tech solutions GmbH, Germany. The authors declare that there are no financial interests, commercial affiliations, or other potential conflicts of interest that could have influenced the objectivity of this research or the writing of this paper.
Data availability statement This article has no associated data generated.
Author contribution statement
Conceptualization, SAM and SV; Methodology, SAM and SV; Software, N / A.; Validation, N / A; Formal Analysis, SAM and