Chapter 14: The gluteal region
The skin of the buttock is supplied by a number of small nerves derived ultimately from the region of the twelfth thoracic nerve to the third sacral nerve (T12 to S3). A bursa is found lateral to the greater trochanter, and the strong gluteal aponeurosis over the gluteus medius muscle continues inferiorward as the iliotibial tract of the fascia lata. The superolateral quadrant of the buttock is relatively free of nerves and vessels and is frequently used for intramuscular injections in order to avoid the sciatic nerve and other important structures. An alternative site is over the gluteus medius in a triangular area bounded by the anterior superior iliac spine, the tubercle of the iliac crest, and the greater trochanter (fig. 14-1B).
Muscles of gluteal region (table 14-1)
The glutei maximus, medius, and minimus, from superficial to deep, form the bulk of the buttock (fig. 14-1). They are supplied by the gluteal nerves and vessels, which reach them through the greater sciatic foramen (Figs. 14-2 and 14-3). The gluteus maximus, a large muscle with numerous attachments, is a powerful extensor of the thigh or of the trunk lower limbs are in a fixed position. Surprisingly, however, it is not important posturally, is relaxed when one is standing, and is little used in walking. It is employed in running, climbing, and rising from a sitting or stooped position. It also controls flexion at the hip upon sitting down (paradoxical action). The glutei medius and minimus abduct the thigh at the hip joint and rotate it medially.
During walking, the glutei medius and minimus of the grounded limb abduct the pelvis, i.e., tilt it so that the swinging limb can clear the ground (see fig. 18-2). Paralysis results in a characteristic lurching or waddling gait in which the hip on the weak side deviates laterally when that foot is in contact with the gound. A series of small lateral rotators of the thigh is found largely under cover of the gluteus maximus. One of them, the piriformis, emerges from the greater sciatic foramen, and the sciatic nerve appears at its inferior border (figs. 14-1, 14-2 and 14-3).
Several important nerves from the sacral plexus either supply or traverse the gluteal region. The superior gluteal nerve (L4,5, S1) (figs. 14-2 and 14-3) passes posteriorward through the greater sciatic foramen immediately superior to the piriformis muscle. It passes anterolaterally between the gluteus medius and minimus and supplies both of those muscles as well as the tensor fasciae latae. The inferior gluteal nerve (L5, S1,2) (figs. 14-2 and 14-3) traverses the greater sciatic foramen just inferior to the piriformis muscle and supplies the gluteus maximus. The pudendal nerve is merely passing through the gluteal region, making a brief appearance at the infrior aspect of the piriformis, only to re-enter the pelvis via the lesser sciatic foramen (fig. 14-3). The posterior femoral cutaneous nerve (S1-3) (figs. 14-2 and 14-3) passes posteriorward through the greater sciatic foramen just inferior to the piriformis, descends deep to the gluteus maximus and continues to the middle of the posterior thigh, where it pierces the fascia, and will ultimately reach the calf. The largest nerve in the body, the sciatic nerve, consists of two parts, tibial and fibular, which are initially bound together and then separate at a variable level into two nerves. A branch of the sacral plexus, the sciatic nerve (L4-S3) (figs. 14-1, 14-2, 14-3 and 15-8) traverses the greater sciatic foramen, below the piriformis. The fibular component, however, may pierce the piriformis (or even emerge superior to that muscle), in which case it remains separate. The sciatic nerve descends under cover of the gluteus maximus and then along the middle of the posterior thigh. In terms of surface anatomy, the sciatic nerve leaves the pelvis a little more than a third of the way along a line from the posterior superior iliac spine to the ischial tuberosity (fig. 14-4). It then descends about halfway between the ischial tuberosity and the greater trochanter. The words sciatic and ischium are related.
The gluteal arteries (see figs. 13-5 and 14-2) are variable, but they arise, directly or indirectly, from the internal iliac artery. The superior gluteal artery, the largest branch of the internal iliac artery, accompanies the corresponding nerve and, under cover of the gluteus maximus, divides into branches that supply the gluteal muscles. The inferior gluteal artery, another branch of the internal iliac artery, accompanies the corresponding nerve, and, under cover of the gluteus maximus, lies medial to the sciatic nerve, to which it gives a companion artery. The gluteal veins accompany the arteries and drain into the internal iliac vein. They can return the blood from the lower limb even when the femoral vein is ligated.
The exit of the superior gluteal nerve and artery from the pelvis is indicated by the superior point of trisection of a line from the posterior superior iliac spine to the upper end of the greater trochanter (fig. 14-4). The exit of the inferior gluteal nerve and artery is indicated by the inferior point of trisection of a line from the posterior superior iliac spine to the ischial tuberosity.
14-1 Where is an intramuscular injection usually given?
14-2 What is the role of the glutei medius and minimus in walking?
14-3 From which spinal segments is the sciatic nerve derived?
14-4 Which lateral rotator of the thigh has a variable relationship to the sciatic nerve?
Figure 14-1 The gluteal region, posterior aspect. A, The deep relations of the gluteus maximus. The sciatic nerve usually emerges inferior to the piriformis and lies on a succession of lateral rotators. B, Intramuscular injection. An intragluteal injection may be made safely in the area between the second and third fingers.
Figure 14-2 The arrangement of the structures emerging from the greater sciatic foramen. The foramen gives exit to the pirformis, to seven nerves (sciatic, posterior femoral cutaneous, superior gluteal, inferior gluteal, pudendal, nerve to the obturator internus, and nerve to the quadratus femoris), and to three groups of vessels (internal pudendal, superior gluteal, and inferior gluteal). The nerve to the quadratus femoris and the veins that accompany the arteries are not shown.
Figure 14-3 Oblique vertical section (see inset) to show relations in the gluteal region. Note that (A) a nerve and artery leave the greater sciatic notch superior to the piriformis, (B) seven nerves (the nerve to the quadratus femoris is not shown) and two arteries emerge inferior to the piriformis (see the list in the legend of fig. 14-2), and (C) two nerves and an artery regain the pelvis through the lesser sciatic foramen. The sciatic nerve lies successively on the ischium, gemelli and obturator internus, and quadratus femoris. (Modified from Castaing and Soutoul)
Figure 14-4 The surface anatomy of the femoral artery, gluteal nerves, and sciatic nerve.