The sciatic nerve is commonly injured by fractures of the pelvis or femur, fracture/dislocation of the hip, gunshot wounds of the buttock and thigh, and the injection of toxic substances into the lower gluteal region. Total hip arthroplasty is another cause. Tumors of the pelvis (sarcomas, lipomas) or gluteal region may compress the nerve. Sitting for a long period with legs flexed and abducted (lotus position) under the influence of narcotics or barbiturates or lying flat on a hard surface in a sustained stupor may severely injure one or both sciatic nerves or branches thereof. The nerve may be involved by neurofibromas and infections and by ischemic necrosis in diabetes mellitus and polyarteritis nodosa. Cryptogenic forms of sciatica occur and in a referral practice are more frequent than those of identifiable cause. Partial lesions of the sciatic nerve occasionally result in causalgia (see further on).
The common Morton neuroma, typically between the third and fourth metatarsals, causes interdigital or intermetatarsal pain and can be detected by MRI. It is subject to surgical section. Also mentioned here is a distressingly painful compression of the plantar branches of the sciatic nerve.
Just above the popliteal fossa the sciatic nerve divides into the tibial nerve (medial, or internal, popliteal nerve) and the common peroneal nerve (lateral, or external, popliteal nerve). The latter swings around the head of the fibula to the anterior aspect of the leg, giving off the superficial peroneal nerve that provides musculocutaneous branches (to the peroneal muscles) and to the deep peroneal nerve (formerly called anterior tibial nerve). Branches of the latter supply the dorsiflexors of the foot and toes (anterior tibialis, extensor digitorum longus and brevis, and extensor hallucis longus muscles) and carry sensory fibers from the dorsum of the foot and lateral aspect of the lower half of the leg. There was weakness of dorsiflexion of the foot (foot-drop) in all of the 116 cases of common peroneal neuropathy reported by Katirji and Wilbourn, and numbness of the dorsum of the foot was present in most cases. Weakness of eversion of the foot is usually demonstrable; because inversion is a function of the L5 root and the tibial nerve, it is spared in peroneal palsy, thereby allowing a distinction between foot-drop at the two sites. (Foot eversion should be tested with the ankle passively dorsiflexed.) Pain is variable.
Pressure during an operation or sleep or from tight plaster casts, obstetric stirrups, habitual and prolonged crossing of the legs while seated, and tight knee boots are the most frequent causes of injury to the common peroneal nerve. The point of compression of the nerve is where it passes over the head of the fibula. Emaciation in patients with cancer or AIDS increases the incidence of these types of compressive injuries. The nerve may also be affected in diabetic neuropathy and injured by fractures of the upper end of the fibula. A Baker cyst, which consists of inflamed synovium extending into the retropopliteal space, may compress the nerve, and it may be damaged by muscle swelling or small hematomas behind the knee in asthenic athletes. The prognosis is generally good in cases of partial paralysis.