Injection inside the paraneural sheath of the sciatic nerve: direct comparison among ultrasound imaging, macroscopic anatomy, and histologic analysis.

BACKGROUND AND OBJECTIVES: There exists little anatomic knowledge regarding the structure and sonographic features of the sheath enveloping the sciatic nerve in the popliteal fossa. We investigated the spread of an injection inside the sheath to (1) determine whether the sheath is a structure distinct from the nerve or part of the epineurium and (2) to develop an ultrasound-guided injection technique.

METHODS: Using gross dissection, ultrasound examination, and histologic study, we characterized the tissue layer surrounding the sciatic nerve in the popliteal fossa of 28 unembalmed cadaver legs.

RESULTS: Grossly, we identified a thin, transparent, and fragile tissue layer surrounding the epineurium. Sonographically, this layer was identified with injectate as a hyperechoic layer detaching from the surface of the sciatic nerve. Histologically, the sheath was seen as a multilayered circular fascia as part of the paraneural tissue. An injection of 10 mL inside the sheath spread 10 to 15 cm closely along the nerve, however, not completely circumferential, compared with 5 to 6 cm if the injection was outside the sheath. Characteristics of the ultrasound-guided injection technique are described.

CONCLUSIONS: There is a distinct tissue layer surrounding the popliteal sciatic nerve as a paraneural sheath that has distinct gross anatomic, histologic, and sonographic features. This sheath may have implications for regional anesthesia involving the popliteal sciatic nerve. We suggest that the ultrasound-guided injection technique described here could be used in future clinical studies investigating the importance of the paraneural sheath.