Breaking the 4-hour rule: Multiscale computational modelling redefines post-injection restrictions for botulinum neurotoxin A.

This study used a validated, literature anchored multiscale modelling platform (AesthetiSIM) to investigate whether common post injection behaviours influence the early distribution of botulinum toxin A (BoNT A). A cohort of 10,000 paired digital twins was simulated, with each virtual patient evaluated under a rest condition and a matching behaviour scenario. Behaviours included repeated bending, light exercise, superficial rubbing, mild heat exposure, and a combined conservative case incorporating all perturbations. Across all single behaviours, changes in effective spread radius at 4 h were small. Median ΔR values were 0.08 mm for bending, 0.11 mm for exercise, 0.14 mm for rubbing, and 0.09 mm for heat, while the combined scenario produced a median change of 0.22 mm. Upper tail values remained limited, with Q ΔR ranging from 0.36 to 0.52 mm for individual behaviours and 0.69 mm for the combined condition. The probability of exceeding a 1 mm margin was low: ≤1.2 % for single behaviours and 2.1 % for the combined scenario. Behaviour related changes in off target binding were similarly minimal. Median ΔF values ranged from 0.004 to 0.008 for individual behaviours and 0.012 for the combined scenario, with Q values remaining below 0.043. Exceedance of the 0.05 margin ranged from 1.3 % to 4.4 %. Global sensitivity analysis showed that most variation in spread and binding was driven by intrinsic tissue and injection factors such as diffusivity (Sobol S1 0.28 for R95), internalization rate (S1 0.19 for R), and injection depth (S1 0.14), not by behaviour induced interstitial flows. These findings indicate that within the first 4 h after injection, the distribution of BoNT A is shaped primarily by injection geometry and rapid target binding and is only weakly affected by everyday movements or mild environmental exposures. Although these results provide quantitative support for reconsidering universal 4-h restrictions, clinical trials are still required to validate these observations in real patients.