A COMMON EPINEURAL SHEATH FOR THE NERVES IN THE POPLITEAL FOSSA AND ITS POSSIBLE IMPLICATIONS FOR SCIATIC-NERVE BLOCK

Sciatic nerve block in the popliteal fossa is associated with a highly variable success rate. Frequently, anesthesia is profound in the dist ribution of both the tibial (TN) and common peroneal nerves (CPN), alt hough the response to nerve stimulation or paresthesia is obtained in the distribution of one division of the nerve. However, anesthesia in the distribution of only one division of the nerve is also a common oc currence under apparently identical clinical circumstances. Looking fo r a possible role of a common epineural sheath in these phenomena, we injected dye into the epineural sheath of the tibial nerve in 10 cadav er legs and observed its spread within the sheath. Injections of 15 mL and 30 mL of the dye resulted in a proximal spread of 147 +/- 34 mm a nd 172 +/- 50 mm, respectively, from the injection point 10 cm below t he popliteal fossa crease. In a majority of the legs, the dye reached the division of the sciatic nerve in the popliteal fossa, bathing both the TN and CPN. Gross inspection and histologic examination of the sc iatic nerve specimens revealed a common epineural sheath enveloping th e TN and CPN. The presence of the common epineural sheath and its char acteristics may have important clinical implications for sciatic nerve blockade in the popliteal fossa.