J. Eur. Opt. Society-Rapid Publ. 22, 24( 2026) 253
as lower pulse energies( enabled by the asymmetric spacings).
Theoretical models and experimental findings suggest that surface smoothness improves with well-managed bubble overlap, but this does not need to be necessarily achieved from redundant overlap along a single path. Instead, track overlap can be leveraged more efficiently, enabling a more balanced energy distribution. Amann and Arba Mosquera [ 24 ] explored the effects of pulse energy, spacings, and total delivered treatment energy independently using a computer modelling, reporting clear benefits in reducing pulse energy( for the same total treatment fluence); but also confirming that the asymmetric spacings were more beneficial in one than in the other direction( from a model perspective), in which consecutive pulses are placed further apart( even by distances beyond the bubble diameter, fully avoiding bubble overlap of consecutive pulses); with consecutive laps( or lines in a meander arrangement) placed tighter to maintain the total number of pulses and treatment fluence, as well as complete the overlap of the tracks for a successful separation. Further, this work allowed the inference of upper and lower levels for the required asymmetry.
We visually compare the asymmetric pattern( with spot distance < track distance) derived from the optimization approach used in this work, with a reverse asymmetric configuration in which the spot distance exceeds the track distance. Although both patterns can be designed to achieve the same nominal dose( F1 F2 = 1.41), their behavior in the tissue may differ. The reverse asymmetry( F2 / F1 = 0.58) appears to produce more persistent tissue bridges and less favorable stromal smoothness compared to asymmetric pattern with F2 / F1 = 1.73( Fig. 6). One possible explanation for this could be that increasing the spot distance reduces beneficial intra-path overlap while shifting most of the overlap to the inter-track domain, thereby altering the spatial – temporal balance of cavitation events. While reverse asymmetry might offer practical advantages, such as potentially increasing treatment speed, its influence on bubble dynamics and tissue separation may be less optimal than that of the asymmetric spacing prescribed in this work. These observations highlight that the manner in which asymmetry is achieved may be an important parameter for cutting efficiency and interface quality.
The theoretical findings are clinically supported in a large cohort analysis by Darzi et al. [ 29 ] who explored reasonable ranges of pulse energies and spacings( from 75 nJ to 130 nJ, with symmetric and asymmetric spacings in a wide range, and treatment fluences) confirming clear trends that lower pulse energies, asymmetric spacings, and lower treatment fluences provided better outcomes. Their results suggested that maintaining a consistent dose( ~ 12 μm 2 area assigned to a low energy pulse) achieved through asymmetric spacing such as 6.0 μm · 2.0 μm or 8.0 μm · 1.5 μm, is optimal for cutting efficiency and tissue smoothness. The requirement for overlap could be satisfied more effectively through optimized spatial and temporal distribution of pulses, rather than enforcing spot spacing to remain below bubble diameter. The integration of clinical data and predictive modeling supports the notion that decoupling spatial parameters, while controlling overall dose, enhances both tissue response and visual outcomes, providing strong empirical validation for the refined cavitation-based models proposed in earlier theoretical work [ 24, 26 ].
Sobutas and Arba Mosquera [ 30 ] explored the effects of pulse energy on the treatment depth using a computer modelling with clear benefits in reducing pulse energy( for the same total treatment fluence).
Verma and Arba Mosquera [ 9 ] introduced the plasma model and could mathematically demonstrate that for energies closer the threshold, the cavitation bubble remains constrained within the plasma volume; and only from a determined suprathreshold pulse energy are cavitation bubbles dominating the process.
Ryu et al. [ 31 ] compared using a different platform( two different models from Carl Zeiss Meditec, VisuMax 500 and VisuMax 800) the close-to-threshold plasma KLEx( 85 nJ) and to a conventional low energy regime for the same platform( 100 nJ). Both types of treatments were performed with the same treatment fluence( 625 mJ / cm 2). Authors reported both clinically and statistically significant differences between the two regimes. The study found that Plasma- KLEx with asymmetric spacings( Spot Distance > Track Distance), minimized microcavitation and created a lenticule with near-pure plasma, showing more favorable outcomes than Conventional-KLEx, yielding better early postoperative visual acuities and reduced induction of corneal HOAs. This study confirms the previous findings by Pradhan et. al. [ 28 ] using a different platform.
Park et al. [ 32 ] explored for a different platform the asymmetric spacings compared to a conventional symmetric spacings, both for a low energy regime( 100 nJ) and same treatment fluence( 625 mJ / cm 2) and could confirm both clinically and statistically advantages for the asymmetric spacings, suggesting its universal validity.
The model presented in this work is based on several idealized assumptions, such as constant bubble size and homogeneous tissue, which inherently introduce certain limitations. Most of those limitations have been addressed in previous studies [ 9, 24, 26 ]. Notably, these works demonstrated that above the optima, the variability of the bubble effects is smaller than the energy fluctuations.
The product of the overlap factors( F1 F2) determines the effective dose, with higher values corresponding to easier tissue dissection, whereas the ratio of the overlap factors( F2 / F1) governs the degree of asymmetry, with larger ratios associated with reduced residual roughness. An additional constraint can be imposed such that F2 / F1 F1 F2, which implies F1 1 and thereby prevents spot overlap along the scan pathway independent of dose considerations. Under this framework, achieving a higher dose at the same pulse energy corresponds to an increased product( F1 F2), which is most effective when accompanied by greater asymmetry( see Table 4). This interplay is balanced by reducing track distances, enabling simultaneous control of dissection efficiency( dose) and surface smoothness( asymmetry). While Table 4 illustrates these relationships for a specific threshold and pulse energy, Table 5 extends the analysis across multiple threshold energies and corresponding single-pulse energies.