The tibial nerve is the larger of the two sciatic divisions, being almost twice as thick as the common peroneal (fibular) nerve. It traverses the popliteal fossa lying posterior and slightly lateral to the popliteal vessels.

It then passes between the two heads of the gastrocnemius muscle to the upper edge of the soleus muscle, giving off the medial sural cutaneous nerve which unites with the lateral sural cutaneous nerve (a branch of the common peroneal nerve) to form the sural nerve. The tibial nerve continues distally between the posterior tibial muscle and the soleus muscle and runs together with the posterior tibial artery through the calf musculature, between the medial malleolus and the calcaneus, and to the medial side of the foot joint (Anatomy Fig. 62.3). It divides into its two end branches, the medial and lateral plantar nerves, behind the medial malleolus. These pass under the flexor retinaculum to the sole of the foot and provide it with its sensory innervation.

After separating from the tibial nerve, the common peroneal (fibular) nerve runs along the medial edge of the biceps femoris muscle over the lateral head of the gastrocnemius muscle to the lateral angle of the popliteal fossa. At the neck of the fibula, it passes to the lateral surface of the bone. Before entering the peroneus longus muscle, which originates here, it divides into the superficial peroneal nerve (which is mainly sensory innervation of the foot) and the deep peroneal nerve (mainly motor innervation of the foot).

Up to the point at which it divides, its small branches supply the short head of the biceps femoris muscle, the lateral and posterior parts of the joint capsule, and the tibiofibular joint.

The common peroneal nerve also gives off the lateral sural cutaneous nerve (Anatomy Fig. 62.1). The anterior branch of the lateral sural cutaneous nerve runs subcutaneously on the lateral surface of the lower leg as far as the lateral malleolus and supplies sensory innervation to the posterior and lateral aspects of the lower leg. Its posterior branch unites with the medial sural cutaneous nerve (branch of the tibial nerve) in the middle of the lower leg to form the sural nerve, although it can also occasionally continue as a separate branch as far as the lateral malleolus.

The sciatic nerve is imaged by placing the ultrasound probe in a transverse orientation on the posterior aspect of the patient’s leg, in the popliteal fossa (Sonoanatomy Figs. 62.4 and 62.5). It is recommended that a systematic scan be performed starting at the level of the popliteal skin crease (where structures are most superficial) and moving proximally or cephalad (the traceback approach) [6]. On the lateral aspect of the image, the muscle bulk is made up of biceps femoris, and medially, the muscles of semitendinosus and semimembranosus (Sonoanatomy Fig. 62.6). The popliteal vessels are identified as dark hypoechoic circular structures and can be distinguished by the fact that the artery is pulsatile and the vein is compressible. Color Doppler can also be used for additional confirmation (Sonoanatomy Fig. 62.7). The hyperechoic tibial nerve is located superficial (posterior) to the popliteal vessels and is most commonly lateral to the vein. As the probe is moved proximally, the smaller common peroneal (fibular) nerve will be seen to move from the lateral aspect of the thigh toward the midline to join the tibial nerve to form the sciatic nerve within a common (paraneural) sheath (Sonoanatomy Fig. 62.8).