Description of a novel technique for ultrasound‐guided posterior tibial nerve block

Posterior tibial nerve block (PTNB) is an important procedure for anesthetizing the cutaneous innervation of the heel and plantar regions of the foot.1 Emerging from the popliteal fossa as a branch of the sciatic nerve, the tibial nerve (TN) courses down the posterior compartment of the leg and terminates in the tarsal tunnel posterior to the medial malleolus. The segment of the TN that is distal to the proximal aspect of the fibrous arch of the soleus muscle is commonly referred to as the posterior tibial nerve (PTN), although it is merely a continuation and not a distinct branch of the TN. It then divides into its terminal branches: the medial plantar nerve, the lateral plantar nerve, and the medial calcaneal nerve (MCN).2 The primary clinical indications for a PTN block include tarsal tunnel syndrome, Baxter's neuropathy, medial plantar neuropathy (Jogger's foot), postoperative analgesia for forefoot surgery, and regional anesthesia for procedures such as plantar fascia injections, needle fasciotomy or tenotomy, and platelet-rich plasma injections.2, 3 As the precise location of the PTN varies among individuals, especially in patients of large body habitus or with altered anatomy due to prior trauma or surgery, using anatomical landmarks for a palpation-guided injection may be challenging.4, 5 Additionally, although the PTN may trifurcate into its three terminal branches at the tarsal tunnel, the MCN often arises directly from the PTN more proximally. Palpation-guided PTNB based on anatomical landmarks may be distal to the branching of the MCN. Electrical nerve stimulation has been employed to assist with localizing the nerve, however its effectiveness is limited as a motor response is not always elicited.6 More recently, ultrasound has been shown to be a useful instrument to overcome some of these limitations.6, 7 The current protocol for ultrasound-guided PTNB uses a high-frequency linear-array transducer placed in an oblique axial plane on the medial ankle posterior to the medial malleolus and proximal to the tarsal tunnel. This allows the clinician to locate the tibial artery and PTN. The needle is inserted long-axis relative to the transducer (ie, in-plane) on the posteromedial ankle, medial to the Achilles tendon, and lateral to the transducer.1, 2 Redborg et al.8 confirmed that ultrasound-guided PTNB yielded a greater likelihood of successful blockade when compared to a landmark-based PTNB 30 minutes post–intervention. However, in the authors' clinical experience, a limitation of the classic medial approach to PTNB is that needle conspicuity and maneuverability relative to the transducer may be challenging. The sonographic window between the Achilles tendon and the transducer with the medial approach to PTNB is often narrow, requiring the clinician to float the transducer on stacked ultrasound gel to obtain a more parallel relationship between the transducer surface and the needle, potentially decreasing transducer stability. Without ultrasound gel stacking, the needle is often steeply inclined relative to the transducer, and needle conspicuity is compromised. Additionally, the short distance between the skin and PTN in the medial approach may affect needle maneuverability about the nerve. We introduce a novel technique for ultrasound-guided PTNB in which the needle enters the skin from the posterolateral ankle, thus increasing the conspicuity of the needle and improving needle maneuverability and the technical feasibility of the procedure. The patient is placed in the prone position and the PTN is examined using a high-frequency linear-array ultrasound transducer placed on the posteromedial aspect of the distal leg (Figure 1). Practitioners should identify and avoid the Achilles, peroneus longus, and peroneus brevis tendons, as well as the sural nerve, peroneal and tibial arteries, and peroneal and tibial veins, all of which may be near the intended needle path depending on specific site of insertion, patient body habitus, and individual patient anatomical factors. Color or power Doppler should be used to confirm the location of vasculature, but the authors generally recommend turning off Doppler functions prior to needle insertion to avoid motion artifact. Alternatively, some ultrasound machines may have a split-screen function wherein B-mode and Doppler functions can be visualized separately and simultaneously. The needle insertion site will typically be in the interval between the Achilles tendon and the peroneal muscles and/or tendons at the posterolateral leg, at a location corresponding approximately 5–8 cm proximal to the lateral malleolus. Clinicians should identify the MCN exiting from the PTN and ensure that the procedure is performed proximal to this location. The procedural region (both the expected needle insertion site and the area where the transducer will contact the skin) is then prepared in a sterile fashion, and a sterile ultrasound cover kit including sterile ultrasound gel is used. A 25-gauge 5.08-cm needle is inserted and advanced under direct ultrasound visualization, using a posterolateral approach, in a long axis relative to the transducer (ie, in-plane approach), and guided into the space around the PTN (Figure 2). At the typical site of needle insertion, the needle may be advanced through the soleus, peroneus brevis, peroneus longus, and/or flexor hallucis longus muscles on its trajectory toward the PTN (Figure 3). After visualization of the needle tip in the target area, the injectate is dispensed into the space around the PTN. Typically, the authors use 5 mL of 1% lidocaine without epinephrine for this injection. The needle tip is carefully maneuvered in the circumferential area surrounding the PTN, injecting approximately 1 mL of injectate each in regions lateral, medial, deep, and superficial to the nerve until the complete injectate is administered, while exercising caution to not contact the nerve with the needle. A “target sign” of hypoechoic injectate surrounding the relatively hyperechoic PTN is seen following a successful injection. In the authors' clinical experience performing such procedures, the present technique of ultrasound-guided PTNB has been found to be consistently effective for obtaining excellent regional anesthesia when performed just prior to an ultrasound-guided intervention such as a needle plantar fasciotomy with platelet-rich plasma injection. No complications have been noted. To date, an ultrasound-guided posteromedial approach to PTNB has been the methodologic approach most commonly described in the literature for anesthetizing the heel and plantar regions of the foot, principally because the PTN courses posterior to the medial malleolus, with easily palpable landmarks and less tissue to traverse, seemingly making needle placement and maneuverability ideal.2, 3 However, in the authors' clinical experience using the approach described herein, an ultrasound-guided posterolateral PTNB may provide easier needle maneuverability, increased conspicuity of needle placement, and better technical feasibility than the posteromedial approach. In the presently described approach, the needle enters the skin from the posterolateral ankle in such a fashion that the needle is essentially parallel to the transducer surface, which increases needle conspicuity. Additionally, given the increased distance between the skin and PTN in our technique, relative to the traditional posteromedial approach, increased maneuverability of needle trajectory is achieved. Further, there is no need to stack ultrasound gel under the transducer to obtain a parallel relationship between transducer and needle, potentially improving transducer stability. Mehdizade and Adler9 previously observed similar advantages in their novel approach to flexor hallucis longus tendon sheath injection. However, practitioners may need to become accustomed to the greater ultrasound depth setting that is required with the present technique, as well as comfort with inserting the needle more distant from the transducer than is typical for most ultrasound-guided procedures. A potential limitation of this approach is suboptimal visualization of the peroneal artery and sural nerve during the procedure, so practitioners should be careful to scan these structures preprocedurally, note their location, and be attentive to their location following needle insertion. By improving needle conspicuity and maneuverability as well as transducer stability, the posterolateral approach may augment the previously documented benefits of ultrasound-guided PTNB. Future cadaveric investigations may be helpful to confirm the reproducibility and accuracy of this approach. The authors declare no conflicts of interest.