• Lower Extremity
• Acute Pain
• Anatomy

To the editor,

Sciatic nerve blocks are commonly performed in the popliteal fossa by injecting local anesthetic at the level of the major bifurcation of the common sciatic nerve into its common peroneal and tibial branches. Injections near this branch point have the advantage that they are relatively shallow and will distribute to both major branches. If single injections via needle or catheter are placed too distal there is a risk of incomplete block because the branches have separated.1 More proximal sciatic blocks, at the infragluteal level, are placed around the combined components of the sciatic nerve within its common paraneural sheath. However, here we report sonographic imaging of a sciatic nerve variation which is divided proximally with independent common peroneal and tibial nerve contributions in this anatomic region.

A 25-year-old man with a history of right anterior cruciate ligament tear presented to our outpatient orthopedic surgery center for reconstruction of the ligament under general anesthesia. Preoperative femoral and sciatic analgesic nerve blocks were proposed, and informed consent was obtained from the patient. Permission was obtained from the patient to present the following information and images. After the femoral nerve block, the patient was turned to the left lateral position. Placement of the ultrasound probe in the infragluteal position unexpectedly revealed two apparent nerve structures coursing separately within the proximal thigh, deep to the biceps femoris muscle (figure 1A). These two distinct structures then joined together, further distally, to form what appeared to be the sciatic nerve proper in the mid-thigh (figure 1B). However, this unified anatomic structure was visible for only a few centimeters proceeding distally, before the ultrasound image revealed the expected, normal division of the SN in the proximal portion of the popliteal fossa (figure 1C). Our interpretation of this ultrasound anatomy was that the TN and CPN had failed to fuse together normally on exiting the sacral plexus, entering the thigh as separate entities, and then coming together distally to form the unified SN, before branching again into the component nerves at the typical level, in the proximal popliteal fossa. This variant anatomy complicated our plan for a single-needle insertion for the sciatic block, at the infragluteal site, since injection around only one of the visible nerves at the proximal level may have led to partial blockade, with the potential for inadequate postoperative analgesia.

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Figure 1

Ultrasound images at multiple levels along the course of the sciatic nerve in the posterior thigh. (A) At infragluteal level, sciatic nerve is apparent as two separate branches. (B) At mid-thigh, the two branches of the sciatic nerve have united into one single peripheral nerve structure. (C) At the proximal popliteal fossa, the sciatic nerve has divided, as expected, into its tibial and common peroneal components.

We altered our needle insertion site to the mid-thigh, where the nerve was visible as a single structure, just proximal to the point at which it divided into the CPN and the TN in the popliteal fossa (figure 1B). The injectate for the analgesic nerve block, consisting of 20 mL of 0.125% bupivacaine, was placed around the nerve with real-time ultrasound guidance and confirmatory nerve stimulation, and the patient’s surgery proceeded without complication. The patient had excellent analgesia in the recovery area, and noted numbness and tingling of the foot, along with weakness of plantar and dorsiflexion. He reported a pain level of 1 on a 1–10 scale. Follow-up phone call revealed good analgesic relief for the duration of block (14 hours), as well a full return of SN function.

Proximal division of the sciatic nerve by the piriformis muscle is relatively common and well-described in the anatomic literature.2 However, this variation has not been described with ultrasound and only recently described with other imaging modalities.3 4 When present in a given individual, identification of the common epineural sheath forming along the nerve course is important for block success. Reference values for nerve size may be helpful for detection of this variation. The cross-sectional area of the common sciatic nerve should be relatively conserved along the nerve path and among individuals.5 We would like to raise awareness of this anatomic variation because, if unrecognized, proximal sciatic nerve block failure rates could be as high as 15% when guided by ultrasound or nerve stimulation.