Please confirm that an ethics committee approval has been applied for or granted: Not relevant

Background and Aims Apart from sub-Tenon’s blocks fascial plane blocks do not achieve sufficiently reliable analgesia to be used as stand alone surgical regional anaesthesia. In theory, this is due to insufficient filling of fascial compartments. A misconception of the fluid mechanics relying on fast-injection bulk flow and diffusion instead of accounting for porosity, viscosity and slow creeping flow (viscous fingering) in compartments with extensive 2D but small height extension

Methods Plane (n=5) and spherical (n=5) Hele-shaw cells with glycerol as highly viscous fascial sheath simulation betweeen solid phase and cling film. Single and triple outlet cannula used to breach cling film and inject 5 ml NaCL-solution over 30 seconds.

Results Slow injection induced viscous fingering with central zones of high flow and peripheral zones of fingering progression front slow flow instead of concentric extension of sodium solution. This ‘fractal’ propagation front filled the 20cm x 13 cm compartment to a great extent and gave way to diffusion after cessation of injection. The same applied to spherical models. Fast injection favored bulk flow.

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Abstract EP198 Figure 1

spherical and plane viscous fingering in fascial sheath blocks

Conclusions Viscous fingering may arise in slow-injection fascial block that maintain small height of the compartment. Given fluid mechanic determinants like porosity, viscosity ratio between LA and fascial sheath content and control of turbulence during injection, slow creeping flow inducing viscous fingering may fill fascial sheath compartments to a greater extent. These flow patterns need to be assessed in cadaver and volunteer studies, using MRI or 3 D ultrasound reconstructions and may favor slow injection/catheter techniques in fascial plane blocks.