High-definition ultrasound imaging defines the paraneural sheath and the fascial compartments surrounding the sciatic nerve at the popliteal fossa.

Background And Objectives: The connective tissue layers that surround the sciatic nerve at the popliteal fossa are poorly defined. We present high-definition ultrasound images of the sciatic nerve, which were acquired during ultrasound-guided popliteal sciatic nerve block (SNB), that clearly demonstrate these fascial layers.

Methods: Four patients undergoing hallux valgus surgery received an ultrasound-guided popliteal SNB using a high-definition ultrasound system. In the ultrasound images, the paraneural sheath was identified as a hyperechoeic fascial layer between the outer surface of the sciatic nerve (epineurium) and the epimysium of the surrounding muscles. The paraneural sheath was distinct from the epineurium, better delineated after the local anesthetic injection, and enveloped not only the sciatic nerve but also the common peroneal and tibial nerves separately. In the postblock sonograms, the local anesthetic was compartmentalized into 2 broad areas, that is, external (subepimyseal) and internal (subparaneural) to the paraneural sheath. The popliteal SNB was effective for surgical anesthesia in all 4 patients.

Conclusions: We have demonstrated the paraneural sheath and the fascial compartments, that is, the "subepimyseal perineural compartment" and the "subparaneural compartment" that surround the sciatic nerve and act as conduits for local anesthetic spread during a popliteal SNB.