Clinical Anatomy of the Sacral Plexus

Fig. 12.1

Schematic diagram of the lumbosacral trunk and the sacral plexus

The sacral plexus itself lies anterior to the piriformis and between it and the endopelvic fascia (thus, it is extraperitoneal). Anterior to the plexus are the internal iliac vessels, the ureters, and the sigmoid colon on the left and distal ileal loops on the right. The superior gluteal vessels pass between the lumbosacral trunk (L4, L5) and S1 or between the S1 and S2 roots. The inferior gluteal vessels pass either between the roots of S1 and S2 or between S2 and S3 (the gluteal vessels generally follow the course of the sacral nerves in the frontal plane).

The sciatic nerve exits through the greater sciatic foramen to enter the gluteal region on its way to the posterior compartment of the thigh. The pudendal nerve and internal pudendal vessels course from the greater to the lesser sciatic foramina, crossing the sacrospinous ligament posteriorly near its attachment to the ischial spine. As they transit through the gluteal region on their way into the perineum, they lie superficial to the sacrospinous ligament but deep to the sacrotuberous ligament. Finally, the plexus gives off several direct muscular branches, including nerves to the piriformis, obturator internus, superior and inferior gamelli, and quadratus femoris muscles.

12.2 Direct Muscular Branches of the Sacral Plexus

12.2.1 Nerve to the Piriformis

The nerve to the piriformis usually arises from the posterior branches of S1 and S2 anterior rami (occasionally only S2) and enters the muscle through its anterior surface to supply it.

12.2.2 Nerve to Obturator Internus and Gemellus Superior

Arising from the anterior branches of L5, S1, and S2 anterior rami, this nerve leaves the pelvis through the greater sciatic foramen below the piriformis, supplies a branch to the gemellus superior, crosses the ischial spine, re-enters the pelvis (perineum) through the lesser sciatic foramen, and supplies the obturator internus by piercing it through its pelvic surface.

12.2.3 Nerve to Quadratus Femoris and Gemellus Inferior

This nerve arises from the anterior branches of L4, L5, and S1 anterior rami, exits the pelvis via the greater sciatic foramen below the piriformis, crosses the ischial tuberosity deep to the sciatic nerve, and supplies branches to the gemellus inferior and quadratus femoris as well as an articular branch to the capsule of the hip joint.

12.3 Major Terminal Nerves of the Sacral Plexus

12.3.1 Superior Gluteal Nerve

The superior gluteal nerve is formed from the posterior (dorsal) branches of L4, L5, and S1 anterior rami. The nerve leaves the pelvis via the greater sciatic foramen above the piriformis along with the superior gluteal artery and vein to enter the gluteal region, where it splits into superior and inferior branches. The superior branch travels with the upper ramus of the deep branch of the superior gluteal artery to supply the gluteus medius and sometimes the gluteus minimus. The inferior branch travels with the lower ramus of the deep branch of the superior gluteal artery across the gluteus minimus, supplying it and the gluteus medius and terminating in the tensor fasciae latae, which it also supplies.

Innervation	Motor – gluteus medius and minimus, tensor fasciae latae
	Sensory – none

12.3.2 Inferior Gluteal Nerve

The inferior gluteal nerve arises from the posterior branches of L5, S1, and S2 anterior rami. The nerve leaves the pelvis through the greater sciatic foramen below the piriformis muscle and enters the gluteal region, where it divides into several branches that enter the gluteus maximus through its deep surface to supply it.

Innervation	Motor – gluteus maximus
	Sensory – none

12.3.3 Posterior Femoral Cutaneous Nerve

Arising from the posterior branches of S1 and S2 and anterior branches of S2 and S3 anterior rami, the posterior femoral cutaneous nerve leaves the pelvis via the greater sciatic foramen below the piriformis and courses inferiorly deep to the gluteus maximus with the inferior gluteal artery, lying either medial or posterior to the sciatic nerve. The nerve continues down the posterior aspect of the thigh deep to the fascia lata but superficial to the long head of the biceps femoris. When the nerve reaches the posterior aspect of the knee, it pierces the deep fascia and travels with the small (short) saphenous vein to the mid-calf where its terminal branches communicate with the sural nerve. This is a purely cutaneous (sensory) nerve supplying the skin over the gluteal region, perineum, posterior thigh, and upper leg.

As the nerve crosses the lower border of the gluteus maximus, three or four gluteal branches (also known as the inferior clunial nerves) wind around the muscle’s inferior border to supply the skin over its lower lateral aspect. The perineal branch supplies the skin on the upper medial aspect of the thigh, crosses the ischial tuberosity, pierces the fascia lata, and terminates in cutaneous branches to the skin of the scrotum and labia majora.

Innervation	Motor – none
	Sensory – cutaneous innervation to the gluteal region, perineum, back of thigh, and back of proximal leg

12.3.4 Sciatic Nerve

The sciatic nerve is formed within the pelvis from the fusion of the lumbosacral trunk (L4–L5) with the anterior rami of S1–S3. A flattened oval shape in cross section, the sciatic nerve is the widest, thickest, and largest peripheral nerve in the human body and at least 2 cm wide at its origin in the average adult. The sciatic nerve, although appearing as one nerve, is really two nerves, the tibial and common peroneal (fibular) nerves, enclosed within a common epineural sheath.

The nerve leaves the pelvis through the greater sciatic foramen below the piriformis (generally) and descends just medial to the midpoint of an imaginary line drawn between the greater trochanter of the femur and the ischial tuberosity along the back of the thigh (Fig. 12.2). It is accompanied medially by the posterior femoral cutaneous nerve and the inferior gluteal vessels. Proximally, the nerve lies deep to the gluteus maximus, resting first on the posterior surface of the ischium and then coursing inferiorly posterior to the obturator internus, gamelli, and quadratus femoris muscles to enter the back of the thigh, where it divides. Inferior to the lower border of the gluteus maximus, the nerve is relatively superficial, after which it lies on the posterior aspect of the adductor magnus under the cover of the long head of the biceps femoris muscle (Fig. 12.3).

Fig. 12.2

Initial course of the sciatic nerve

Fig. 12.3

Cross section of the thigh showing position of the sciatic nerve with respect to major muscles, vessels, and femur

The nerve generally divides into the tibial nerve medially and the common peroneal (fibular) nerve laterally near the apex of the popliteal fossa (approximately two thirds of the way down the thigh) (Fig. 12.4). The point where it divides, however, is variable and can be at a more proximal location or anywhere between the piriformis muscle and the popliteal fossa or, occasionally, even more distally. In about 2 % of the population, the nerve pierces the piriformis. Its surface projection on the back of the thigh is along an imaginary line drawn from just medial to the midpoint between the greater trochanter and ischial tuberosity to the superior apex of the popliteal fossa.