Four main nerves of the lumbosacral plexus provide sensory and motor innervation to the lower extremity: the femoral, lateral femoral cutaneous, obturator, and the sciatic nerve.
The anatomy of the lumbar plexus is described in detail in Chapter 8 (Fig. 3–1). The terminal nerves of the lumbosacral plexus relevant for innervating the lower extremity include the lateral cutaneous nerve of the thigh, the femoral nerve, the obturator nerve, and the sciatic nerve. The lateral cutaneous nerve of the thigh and the femoral nerve leave the lumbar plexus along the posterolateral border of the psoas major muscle; the obturator nerve emerges from the medial border of the psoas muscle at the pelvic brim and crosses in front of the sacroiliac joint. 1 The sacral plexus provides sensorimotor innervation to the posterior thigh, most of the lower extremity, the entire foot, and parts of the pelvis. It is formed by the union of the anterior primary rami of the spinal nerves of L4, L5, S1, S2, S3, and S4 (lumbosacral plexus, Fig. 3–2). The sacral plexus lies deep within the pelvis between the piriformis muscle posteriorly and the pelvis fascia anteriorly (Fig. 3–3). The sigmoid colon, ureter, internal iliac artery, and vein lie anterior to it. The superior gluteal artery and vein lies between the lumbosacral trunk and the first sacral nerve, and the inferior gluteal artery and vein lie between the second and third sacral nerves. The nerves forming the sacral plexus converge as they descend towards the lower part of the greater sciatic foramen and unite within the pelvis to form the sciatic nerve (Fig. 3–4). The sciatic nerve is the largest (thickest) nerve of the body and exits the pelvis through the greater sciatic foramen, between the piriformis and the superior gemellus muscles (Fig. 3–5), to enter the “subgluteal space” between the greater trochanter and ischial tuberosity (Figs. 3–6 and 3–7). 2,3 Sciatic nerve and piriformis muscle anomaly are seen in 16.2% (95% CI: 10.7–23.5%) of individuals. 4 The entire sciatic nerve or one of its components (tibial or common peroneal) may rarely exit the pelvis by passing through or above the superior border of the piriformis muscle. 4 The sciatic nerve, after it emerges from the pelvis, descends along the back of the thigh, lying deep to the semitendinosus and biceps femoris muscles, to about its lower third (Figs. 3–8 and 3–9), where it bifurcates into its two branches: the tibial and common peroneal (fibular) nerves. This bifurcation may take place at any point between its origin at the sacral plexus and the lower third of the thigh or at a variable distance from the popliteal crease. 5 The tibial and common peroneal nerves may also arise separately from the sacral plexus.
Anatomical illustration showing the formation of the lumbosacral plexus.