Mechanistic insights into bupivacaine spread through anisotropic tissue planes and fascial barriers: experimental evidence for interfascial block dynamics

The mechanism by which local anesthetics spread and produce analgesia in interfascial plane blocks remains unclear. Clinical efficacy often exceeds the visible spread of injectate on imaging. This cadaveric study quantified the distribution of bupivacaine within skeletal muscle and across fascial barriers to explore diffusion as a potential contributor to interfascial plane block dynamics.

Fresh and frozen-thawed human soleus muscles were injected with a mixture of bupivacaine and methylene blue, and samples were collected along and perpendicular to fiber orientation. Fresh lower limbs were used to assess transfascial transfer across sequential crural fasciae after injection with bupivacaine, iodinated contrast, and methylene blue. The limbs were imaged with CT and samples were collected from crural compartments separated by fascial layers. Finally, simulated posteromedial quadratus lumborum block was performed in fresh cadavers, with samples harvested from the quadratus lumborum, psoas major, and diaphragm. Bupivacaine concentrations were quantified using high-performance liquid chromatography-mass spectrometry.