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Background and Aims Reliability and distribution of Fascial Sheath Blocks remain wanting. While hyaluronic acid-the main receiving molecule in FSB is actively researched, fluid dynamics of FSB remain unknown. We propose a functional comparison of FSB to Hele-Shaw-cells and the influence of viscous fingering (VF) -ie creeping flow in addition to LA-diffusion and turbulent bulk flow. We developed a schematic model of FSB relying on hyaluronic acid in an elastic Hele-Shaw-cell to study mepivacaine distribution.Methods 2 ml of colored 1% hyaluronic acid serum ( molecular weight 1.5–1.8 kD) was placed in a thin layer on 6 glass plates and covered with a 10x12 cm Tegaderm. Average compartment size of hyaluron depot was 70.5 cm2. 5 ml mepivacaine was injected with a 70 mm UPC 22 G needle at 5 ml/90s. The surface of the entire artificial compartment was calculated and its filling by Mepivacain studied quantitatively (% of surface) and qualitatively (viscous fingering). Hight h after injection was measured by ultrasound.Results 6/6 models showed extensive distribution of LA in viscous fingering patterns (see figures 1–3). Viscous fingering advanced in 2 circles: one initiating inner (6/6) and a secondary, wider outer circle (4/6). Compartments were filled to 64% (45.6 cm2) mean with regards to inner circle and 80% (58.6 cm2) with regards to outer circles after injecting 5 ml mepivacaine 1%/90 sec. VF patterns showed long, slim digitations issueing from needle tip and along shaft . Displacing the needle tip outside the first circle, immediately caused emergence of a wider outer circle. The hight of the LA-comparment after injecting 5 ml on a mean surface of 70 cm2 was 3.2 mm.Abstract EP203 Figure 1Viscous fingering of mepivacaine 1% injected into 1% hyaluronic acid serum over 90 sec by injection pump. Mark outer segments of secondary VF-propagation in the corners of the outlined field. Squares are 4 cm2Abstract EP203 Figure 2Injection initiates backward flow along the needle-shaft, followed by viscous fingering from needle-tip and shaftAbstract EP203 Figure 3Injection continues into round shape VF, slowly filling the major part of the compartmentConclusions Viscous fingering is a dominant fluid dynamic pattern of LA apart from turbulent bulk flow. This simple model reproducibly shows VF and allows studying FSB fluid dynamics. Future research needs to explore added collagene network porosity!