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Application for ESRA Abstract Prizes:Background and Aims Thiel-embalmed cadavers are commonly used for teaching/studying ultrasound-guided fascial plane (FB) blocks due to their improved sonoanatomy. Postmortem degradation or removal of hyaluronic acid (HA), a key interstitial matrix viscosity component, may alter local anesthetic (LA) spread in Thiel tissues, challenging this model’s fidelity to replicate in vivo fluid dynamics. Using a Hele-Shaw cell model, we aimed to simulate and compare LA distribution in environments mimicking different viscoelastic properties.Methods We constructed 8×9 cm Hele-Shaw cells using a glass plate covered by cling film simulating FP, containing four different Thiel media: A: Thiel with 1% high-molecular-weight (MW) HA; B: Thiel with 1% low-MW HA; C: Thiel; D: Thiel incubated on a gelatin-pad. 5 mL Mepivacaine 1% was injected over 90s via a pump Endpoints included viscous fingering (VF), propagation speed (cm 2/s), spread area at 45s and qualitative flow behavior (bulk-flow vs VF).Results High-MW HA demonstrated VF patterns, fastest propagation and largest spread with significant difference compared to pure Thiel solution. Low-MW HA presented intermediate spread behavior with less pattern stability. Thiel solution exhibited rapid, turbulent bulk flow without structured propagation, reduced speed and spread compared to high-MW HA. Gelatin conditioning demonstrated fluid behavior consistent with partial viscosity restitution.Abstract P304 Figure 1Demonstrate representative spread patterns across the four groups. VF was most prominent in the high-MW HA (group A) environment, nearly absent in low-MW HA (group B) unmodified thiel solution (group C) and partially restored in gelatin (group D)Abstract P304 Figure 2Propagation speed and covered area differed significantly (pConclusions FP composition and rheological properties substantially influence LA spread. In Thiel cadavers, likely depleted of native HA, fluid dynamics may shift toward disorganized bulk flow. Reconstituting viscoelasticity with HA or collagen analogues restoring near-physiologic conditions. Therefore, injectate distribution in Thiel models should be interpreted and compared to in vivo conditions with caution.