This, the other of the two divisions of the sciatic nerve (it divides in the popliteal fossa), supplies all of the calf muscles—that is, the plantar flexors and invertors of the foot and toes—after which it continues as the posterior tibial nerve. This nerve passes through the tarsal tunnel, an osseofibrous channel that runs along the medial aspect of the calcaneus and is roofed by the flexor retinaculum. The tunnel also contains the tendons of the tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles and the vessels to the foot. The posterior tibial nerve terminates under the flexor retinaculum and divides into medial and lateral plantar nerves (supplying the small muscles of the foot).
Complete interruption of the tibial nerve results in a calcaneovalgus deformity of the foot, which can no longer be plantar-flexed and inverted. There is loss of sensation over the plantar aspect of the foot.
The posterior tibial nerve may be compressed in the tarsal tunnel (an entrapment syndrome as discussed below) by thickening of the tendon sheaths or the adjacent connective tissues or by osteoarthritic changes. Neuromuscular experts have expressed concern that this is one of the over diagnosed entrapment syndromes. Tingling pain and burning over the sole of the foot develop after standing or walking for a long time. Pain in the ankle or foot is added in some cases and the pain may be referred proximally along the sciatic nerve. Pressure over the nerve in the inferior malleolar region produces pain, which radiates to the terminal distributions of the nerve. Usually there is no motor deficit. Relief is obtained by severing the flexor retinaculum.
Reference has been made in several places in the preceding pages to the most frequently encountered entrapment neuropathies. A nerve passing through a tight canal is trapped and subjected to constant movement or pressure, forces not applicable to nerves elsewhere. The epineurium and perineurium become greatly thickened, strangling the nerve, with the additional possibility of demyelination. Function is gradually impaired, sensory more than motor, and the symptoms fluctuate with activity and rest. The most frequently compressed nerves are the median, ulnar, peroneal, tibial, and plantar in approximately that order. It is well to keep in mind the systemic processes that enhance pressure palsies by infiltration of the nerve or surrounding tissues. The main ones are hypothyroidism, amyloid, pregnancy, and hereditary liability to pressure palsies.
Table 43-8 lists the common entrapment neuropathies and the locations of compression. Detailed accounts of these disorders are contained in the monographs of
Dawson and colleagues and of Asbury and Gilliatt.
Complex Regional Pain Syndrome; Causalgia, Reflex Sympathetic Dystrophy (See Chaps. 7 and 10)
One unfortunate result of partial injury to a peripheral nerve is the delayed appearance of severe pain roughly in the distribution of the affected nerve. This complex problem, which consists of burning pain termed causalgia and associated local trophic and autonomic changes that are subsumed under the term reflex sympathetic dystrophy, is discussed further in Chap. 7 in the context of other pain syndromes and in Chap. 10.
Traumatic Interruption of Nerves