Peripheral nerve microanatomy: new insights into possible mechanisms for block success.

Postmortem histology and in vivo, animal-based ultra-high-definition microultrasound demonstrate a complex array of non-communicating adipose tissue compartments enclosed by fascia. Classic nerve block mechanisms and histology do not consider this tissue. Injected local anesthetic agents can occupy any of these adipose compartments, which may explain the significant differences in outcomes such as success rates, onset time, block density, duration of nerve block, and secondary continuous block failure. Furthermore, these adipose tissue compartments may influence injection pressures, making conclusions about needle tip location unreliable. This educational review will explain the neural anatomy associated with these fatty compartments in detail and suggest how they may affect block outcomes.