Tarsal tunnel syndrome can be described as a constellation of signs and symptoms caused by entrapment or compression of the tibial nerve or any of its branches within the tarsal tunnel, the region beneath the flexor retinaculum on the medial aspect of the ankle (Fig. 94.1). The tibial nerve branches that may be involved deep to the tarsal tunnel include the medial plantar nerve, lateral plantar nerve, Baxter nerve (also known as the first branch of the lateral plantar nerve or inferior calcaneal nerve), and medial calcaneal nerve [1]. Anatomically, the tarsal tunnel is a fibro-osseous structure that begins just posterior to the medial malleolus; the roof is the flexor retinaculum (also called the laciniate ligament), and the floor is formed by the tendons of the posterior tibialis, flexor digitorum longus, and flexor hallucis longus muscles. The tibial nerve usually divides into three branches at the level of the ankle: the medial plantar nerve, the lateral plantar nerve, and the medial calcaneal nerve. However, Baxter nerve (i.e., first branch of the lateral plantar nerve) usually branches from the lateral plantar nerve (but can branch off the tibial nerve) just distal to the origin of the medial calcaneal nerve at the level of the tarsal tunnel [2,3]. Baxter nerve then traverses laterally across the anterior aspect of the heel and terminates with motor branches to the abductor digiti quinti (or minimi) pedis muscle [2]. It is likely that tarsal tunnel syndrome occurs infrequently compared with other well-known focal entrapment neuropathies, such as carpal tunnel syndrome, ulnar neuropathy at the elbow, and peroneal (fibular) neuropathy at the knee. In fact, in a retrospective review of isolated tibial neuropathies in the foot, the incidence of Baxter neuropathy (17%) was much greater than that of tarsal tunnel syndrome (5%) [4].

f94-01-9781455775774 — FIGURE 94.1 Medial aspect of the right foot. The tibial nerve traverses through the tarsal tunnel and then branches into the medial calcaneal nerve, medial plantar and lateral plantar nerves, and first branch of the lateral plantar nerve (i.e., Baxter nerve). Note that the medial calcaneal nerve branches may pierce the flexor retinaculum as they course toward the medial plantar aspect of the heel.

There are generally considered to be five basic categories that account for the etiology of tarsal tunnel syndrome: trauma and post-traumatic changes, mass or space-occupying lesions causing compression, systemic diseases, biomechanical causes related to joint structure or deformity, and idiopathic causes. In addition, the underlying pathophysiologic mechanism of tarsal tunnel syndrome remains elusive; a portion of the literature supports the process of demyelination, whereas other sources implicate axonal degeneration as the primary process [5,6]. It is thought that the tibial nerve may be entrapped proximally within the tarsal tunnel, or one of its branches (e.g., the medial plantar nerve) may be entrapped distally in its own calcaneal chamber [1]. Entrapment of the first branch of the lateral plantar nerve (i.e., Baxter nerve) has also been described as a cause of heel pain [7–10]. Therefore in a case of clinically suspected tarsal tunnel syndrome, the tibial nerve and its major terminal branches (including the medial plantar nerve, lateral plantar nerve, and Baxter nerve) should be thoroughly evaluated [4].

In the current literature, there is no mention of an age or gender preference in patients with tarsal tunnel syndrome. One possible explanation for this is the relatively low incidence and various causes of tarsal tunnel syndrome.