Ultrasound-guided saphenous nerve block at the ankle is utilized as a diagnostic and therapeutic maneuver in the evaluation and treatment of foot and ankle pain thought to be mediated via the distal saphenous nerve. Ultrasound-guided saphenous nerve block at the ankle can also be used for surgical anesthesia for distal lower extremity surgery when combined with tibial and common peroneal nerve block or lumbar plexus block. Ultrasound-guided saphenous nerve block at the ankle with local anesthetic may be used to palliate acute pain emergencies, including postoperative pain, pain secondary to traumatic injuries of the lower extremity including ankle and foot fractures, and cancer pain while waiting for pharmacologic, surgical, and antiblastic methods to become effective.

Ultrasound-guided saphenous nerve block at the ankle can also be used as a diagnostic tool when performing differential neural blockade on an anatomic basis in the evaluation of ankle and foot pain as well as in a prognostic manner to determine the degree of neurologic impairment the patient will suffer when destruction of the saphenous nerve is being considered or when there is a possibility that the nerve may be sacrificed during surgeries in the anatomic region of the saphenous nerve at the ankle. Saphenous nerve block at the ankle with local anesthetic and steroid is occasionally used in the treatment of persistent ankle and foot pain when the pain is thought to be secondary to inflammation or when entrapment of the saphenous nerve at the ankle is suspected. Saphenous nerve block at the ankle with local anesthetic and steroid is also indicated in the palliation of pain and motor dysfunction associated with diabetic neuropathy.

Electrodiagnostic testing should be considered in all patients who suffer from saphenous nerve dysfunction to provide both neuroanatomic and neurophysiologic information regarding nerve function. Magnetic resonance imaging and ultrasound imaging of the popliteal fossa as well as anywhere along the course of the saphenous nerve are also useful in determining the cause of saphenous nerve compromise. Plain radiographs of the ankle should be obtained in all patients who have trauma to the ankle to rule out fractures of the medial ankle, which can damage the saphenous nerve (Fig. 158.1).

The saphenous nerve is the largest sensory branch of the femoral nerve. The saphenous nerve provides sensory innervation to the medial malleolus, the medial calf, and a portion of the medial arch of the foot. The saphenous nerve is derived primarily from the fibers of the L3 and L4 nerve roots. The nerve travels along with the femoral artery through Hunter canal and moves superficially as it approaches the ankle (Fig. 158.2). It passes over the medial malleolus of the tibia, splitting into terminal sensory branches (Figs. 158.3 and 158.4). The saphenous nerve is subject to trauma or compression anywhere along its course. The nerve is frequently traumatized during vein harvest for coronary artery bypass grafting procedures. The saphenous nerve is also subject to compression as it passes over the medial condyle of the femur. Less commonly, the nerve is damaged or compressed distally as it passes beneath the fascia of the dorsum of the foot.

Ultrasound-guided block of the saphenous nerve at the ankle can be carried out by placing the patient in the supine position with the arms resting comfortably along the patient’s chest and the affected lower extremity externally rotated (Fig. 158.5). A total of 4 mL of local anesthetic is drawn up in a 12-mL sterile syringe. If the painful condition being treated is thought to have an inflammatory component, 40 to 80 mg of depot steroid is added to the local anesthetic. A linear high-frequency ultrasound transducer is placed in a transverse plane just above and slightly in front of the medial malleolus (Fig. 158.6). The compressible saphenous vein is seen lying just adjacent to the nerve. Both structures lie just above the anterior border of the medial malleolus (Fig. 158.7). Color Doppler can be utilized to help identify the saphenous artery and vein (Fig. 158.8). When the saphenous vein and adjacent nerve are identified on ultrasound imaging, the skin is prepped with anesthetic solution, and a 1½-inch, 22-gauge needle is advanced from the superior border of the ultrasound transducer and advanced utilizing an in-plane approach with the
trajectory being adjusted under real-time ultrasound guidance until the needle tip is resting in proximity to the saphenous nerve (Fig. 158.9). When the tip of needle is thought to be in satisfactory position, after careful aspiration, a small amount of local anesthetic and steroid is injected under real-time ultrasound guidance to confirm that the needle tip is in proximity to the saphenous nerve, but not within the nerve itself. There should be minimal resistance to injection. After needle tip placement is confirmed, the remainder of the contents of the syringe are slowly injected. The needle is then removed, and a sterile pressure dressing and ice pack are placed at the injection site.