Cannula Gauge and Tissue Penetration Forces in the Temporal Region: A Biomechanical Basis for Injection Safety

Blunt-tip cannulas are widely used for facial filler injections, yet quantitative data on tissue penetration force (TPF) across temporal planes remain limited. We quantified the effects of cannula gauge, tissue plane, and advancement direction on TPF in the temporal region to provide a biomechanical basis for injection safety.

In 21 fresh Korean cadavers, we performed 3,664 insertions with 21G-27G cannulas at two sites-temporal crest (TC) and zygomatic arch (ZA)-advancing vertically and horizontally at a controlled, uniform speed. Forces were recorded continuously with a handheld digital force gauge. Anatomical layers were identified (skin, superficial temporal fascia [STF], deep temporal fascia [DTF]), and measurements were mapped to two procedural planes: L2 (subdermal; superficial to STF) and L4 (inter-fascial; between STF and DTF). Data were analyzed using linear mixed-effects models to account for repeated measurements within cadavers, and p values from pairwise comparisons were adjusted using the Benjamini-Hochberg false discovery rate (FDR) procedure (α = 0.05).

TPF increased with larger cannula diameter and with depth-i.e., skin ≤ STF < DTF (FDR-adjusted p < 0.05)-and was higher in horizontal than vertical advancement (mixed-effects model p = 0.0068). A cannula diameter of 21G produced the greatest resistance, whereas that of 27G produced the lowest. Increasing age was associated with lower TPF (p < 0.05). No sex or side of the body differences was detected.

Temporal TPF shows systematic dependencies on gauge, plane, and direction, delineating mechanical conditions that may precede vascular penetration events. Although vascular penetration force (VPF) was not measured, the findings support a "TPF-first" strategy of pre-dissection/hydrodissection to create a low-resistance track, followed by low-pressure, retrograde micro-aliquots to limit instantaneous force peaks and preserve a favorable safety margin by maintaining TPF less than VPF.

Quantitative TPF profiles can assist in selecting cannula gauge/plane and in structuring low-pressure, incremental, retrograde injections in the temporal region to mitigate vascular injury.

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