This nerve is formed from the second, third, and fourth lumbar roots. Within the pelvis it passes along the lateral border of the psoas muscle and enters the thigh beneath the Poupart ligament, lateral to the femoral artery. Branches arising within the pelvis supply the iliacus and psoas muscles. Just below the Poupart ligament the nerve splits into anterior and posterior divisions. The former supplies the pectineus and sartorius muscles and carries sensation from the anteromedial surface of the thigh; the posterior division provides the motor innervation to the quadriceps and the cutaneous innervation to the medial side of the leg from the knee to the internal malleolus. The distinction between femoral neuropathy and a third lumbar root lesion is made by detecting weakness of the hip adductor (innervated by the obturator nerve) in the case of the root lesion.
Following injury to the femoral nerve, there is weakness of extension at the knee, wasting of the quadriceps muscle, and failure of fixation of the knee. The knee jerk is abolished. If the nerve is injured proximal to the origin of the branches to the iliacus and psoas muscles, there is additionally weakness of hip flexion.
The most common cause of femoral neuropathy is diabetes. The nerve may also be involved by pelvic tumors. Not uncommon is injury to the nerve during pelvic operations. Usually this is the result of improper placement of retractors, which may compress the nerve directly or indirectly by undue pressure on the psoas muscle. Bleeding into the iliacus muscle or the retroperitoneum, observed in patients receiving anticoagulants and in hemophilia patients, is a relatively common cause of isolated femoral neuropathy (Goodfellow et al). The presenting symptom of iliacus hematoma is pain in the groin spreading to the lumbar region or thigh, in response to which the patient assumes a characteristic posture of flexion and lateral rotation of the hip. A palpable mass in the iliac fossa and the signs of femoral nerve compression (quadriceps weakness and loss of knee jerk) follow in a day or two. Infarction of the nerve may occur in the course of diabetes mellitus and polyarteritis nodosa. Not infrequently acute femoral neuropathy is of indeterminate cause.
Sciatic Nerve (See Also Chap. 10)
This nerve is derived from the fourth and fifth lumbar and first and second sacral roots, for which reason a ruptured disc at any of these levels may simulate sciatic neuropathy (sciatica). The sciatic nerve supplies motor innervation to the hamstring muscles and all the muscles below the knee through its two divisions, the tibial and peroneal nerves (see later); the sciatic nerve conveys sensory impulses from the posterior aspect of the thigh, the posterior and lateral aspects of the leg, and the entire sole. In complete sciatic paralysis, the knee cannot be flexed and all muscles below the knee are paralyzed. Weakness of gluteal muscles and pain in the buttock and posterior thigh point to nerve involvement in the pelvis. Lesions beyond the sciatic notch spare the gluteal muscles but not the hamstrings. Partial compressive lesions are more common and tend to involve peroneal-innervated muscles more than tibial ones, giving the impression of a peroneal palsy.
Rupture of one of the lower lumbar intervertebral discs is the most common cause of sciatica, although it does, of course, not directly involve the sciatic nerve. The associated motor and sensory findings allow localization of the root compression (L4-L5 disc compressing L5 root: pain in posterolateral thigh and leg with numbness over the inner foot and weakness of dorsiflexion of the foot and toes; L5-S1 disc compressing S1 root: pain in posterior thigh and leg, numbness of lateral foot, weakness of foot plantar flexion and loss of ankle jerk), as discussed in Chap. 10.