Plantar laceration repair and foreign body removal are common emergency department procedures. Local infiltration of anesthetic to the sole of the foot is notoriously painful and does not anesthetize the deeper structures of the foot. The landmark-based, blind ankle block technique involves anesthetizing individual nerves—the posterior tibial, deep and superficial peroneal, sural and saphenous nerves—and provides anesthesia to the entire foot. Unfortunately, this procedure is both time-consuming and unreliable. Instead, consider a selective, ultrasound-guided blockade of the posterior tibial nerve that provides anesthesia to both the skin of the sole of the foot and internal structures allowing for painless deep exploration when removing foreign bodies. An additional application for this block includes analgesia for calcaneal fractures. Ultrasound-guidance allows the emergency provider direct visualization of the tibial nerve and confirmation of local anesthetic spread around the nerve, resulting in improved success compared to landmark based techniques.
The posterior tibial nerve is a division of the sciatic nerve (L4-S3). The sciatic nerve travels down the posterior aspect of the leg dividing into the tibial nerve and common peroneal nerve above the popliteal fossa. The posterior tibial nerve passes posterior to the medial malleolus, typically just posterior to the tibial artery (Fig. 1A). The tibial nerve continues on to supply the skin sole of the foot and the majority of the internal structures of the foot (Fig. 1B). The ankle joint has multiple innervations and the posterior tibial block will not be sufficient for ankle dislocation or fracture reductions (for complete ankle anesthesia a popliteal sciatic and saphenous blocks are needed). The dorsum of the foot and the extreme postero-lateral portion of the heel are innervated by the sural nerve. The medial aspect of the ankle and foot is innervated by saphenous (via the femoral) nerve.
Procedure: Sterile Preparation. Avoid injection at any skin site with signs of infection. The skin should be prepared with antiseptic solution, and a high-frequency linear (15-6 Mhz) ultrasound probe should be disinfected with quaternary ammonia cleaning wipes prior to the procedure. A sterile probe cover is not necessary, but the probe should be covered with a sterile adhesive dressing (Fig. 2).
Patient Positioning. This block is found to be most comfortable to perform when the patient is supine with the knee flexed, the hip externally rotated, and the ankle supported by blankets. Alternately, the patient can be placed in a lateral decubitus position, with the affected side down exposing the medial aspect of the ankle (Fig. 3).