Introduction:

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.

Anatomy:

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).

Survey Scan: The ultrasound system should be placed contralateral to the affected extremity, allowing the clinician to view the screen and the site of injection with an unobstructed line-of-sight. A high-frequency linear transducer (15-6 MHz) is to be placed just proximal and posterior to the medial malleolus in a transverse orientation (Fig. 4). The tibial nerve will be seen as a hyperechoic structure adjacent to the pulsatile artery. For the novice sonographer, we recommend first locating the medial malleolus, then the tibial artery, and finally the adjacent nerve. Color Doppler can be used to confirm the location of the artery if there is any doubt (Fig. 5). (Interestingly, we have found significant variation in the location of the tibial nerve relative to the artery and it is not uncommon to find the nerve anterior to the artery.) Carefully slide the probe cepahalad (along the nerve), rotating and tilting the transducer as needed to achieve optimal visualization of the nerve in short-axis (Fig. 6).

Needle insertion and injection: This block can be performed using either an in-plane (parallel to the transducer axis) or out-of-plane (perpendicular to the transducer axis) approach. The goal is to position the needle tip just adjacent to the tibial nerve without passing the needle into the nerve itself or puncturing the tibial artery. The tibial nerve is normally quite superficial and a short (1.25 inch), small gauge (21-27) needle is typically sufficient. We recommend the non-expert sonographer used lidocaine with epinephrine as the local anesthetic rather than bupivicaine. Intravascular injection of bupivicaine can be dangerous, and is not recommended until the provider is very comfortable with needle tip visualization. When using the out-of-plane approach, center the probe directly over the target nerve and enter the skin just proximal to the probe, with the needle directed steeply (60-90 degrees relative to the skin (Fig. 7). Watch closely for deformation of the tissue as the needle advances to confirm optimal trajectory. Small back-and-forth movements with the needle can sometimes aid in finding the needle tip if it cannot be directly visualized. Always, take care to avoid the adjacent tibial artery. The in-plane approach allows the entire needle (shaft and tip) to be visualized and can be performed with a medial-to-lateral or lateral-to-medial approach (Fig 8). When you are satisfied that the needle tip is positioned just adjacent to the nerve, slowly inject 5-10 mL after a negative aspiration. The local anesthetic should be observed to spread in real-time around the nerve creating a “donut sign”(Fig. 9). Subtle repositioning of the needle tip, even simply rotating the needle bevel can facilitate optimal circumferential spread of the local anesthetic around the nerve.

Summary:

Bedside ultrasound can be a used to identify the posterior tibial nerve and target local anesthetic injection for nerve blockade in patients undergoing painful procedures on the sole of the foot such as laceration repair or foreign body removal.

Common Questions:

I have done this block blind for years. Why do I need ultrasound? Several studies have shown that the blind techniques for tibial nerve blockade are associated with very high failure rates. This may be due in part to variations in the anatomic location of the tibial nerve relative to the artery. When a block is placed under direct visualization, one can confirm a “donut sign” of circumferential spread of local anesthetic and be confident in a successful block.

How long do I have to wait for anesthesia after the block is placed? How long does it last? The block should start working within 10 minutes, and achieve maximal anesthesia within 15-20 minutes. A block using lidocaine will typically last for at least 90 minutes, sometimes longer. Using a larger volume of a longer acting local anesthetic such as bupivacaine can result in a dense block that lasts for several hours or longer.

Dr. Herring and Dr. Miss are Ultrasound Fellows at Highland General Hospital. Dr. Nagdev is Director of Emergency Ultrasound at Highland General Hospital.

[References can be found in the online version at www.acepnews.com.]