The Gluteal Region

The buttock looks like a solid slab of muscle, and for most of medicine that is all it is — a place to give an injection. Anatomically it is something much more interesting: it is a corridor. The pelvis is a closed bony bowl, and everything that has to get out of it to reach the perineum or the lower limb must squeeze through one of two gaps in its back wall. The gluteal region is where those structures surface, fan out, and go their separate ways.

That is why this unit is examined so heavily. Ask a student "what is in the buttock?" and you get a shrug. Ask "what passes above piriformis and what passes below it, in order?" and you are asking about the entire outflow of the sacral plexus. TMU asks the second question, and it asks it as a definition and as a drawing.

Under the skin lie the cluneal nerves — superior, medial and inferior — purely sensory branches supplying the skin of the buttock. Clunis is simply Latin for buttock, so the name is doing no more than telling you where they go.

Gluteus maximus is the coarsest, thickest muscle in the body, and it functions as the lid over everything else in this region. Reflect it and the whole neurovascular corridor is exposed at once. Notice where it inserts: only part of it reaches bone at the gluteal tuberosity of the femur — the larger share goes into the iliotibial tract, the same band you met in Unit 1. That shared insertion is why the muscle can hold the knee straight from as far away as the hip.

Its action is worth thinking about rather than memorising. It extends and laterally rotates the thigh, but you barely use it in level walking. You recruit it when you climb stairs, rise from a chair, or run — any moment that demands powerful hip extension against gravity. And when the foot is planted, the muscle works in reverse, pulling the trunk upright over the fixed limb.

The bony and ligamentous framework: sacrotuberous and sacrospinous ligaments convert the sciatic notches into the greater and lesser sciatic foramina.

Gray's Anatomy for Students, 4th Ed., Fig. 6.44, p. 566

Lift gluteus maximus and you find the region is built in two storeys. The superficial group — gluteus medius and gluteus minimus, with tensor fasciae latae in front — are the abductors. The deep group — piriformis, obturator internus, the two gemelli and quadratus femoris — are a row of small lateral rotators running almost horizontally from the pelvis to the greater trochanter.

The important structural point, and the one that makes the rest of this unit make sense: the nerves and vessels run in the plane between those two layers. They are not buried in muscle; they lie in a fascial sheet sandwiched between the superficial and deep groups, which is precisely why reflecting gluteus maximus exposes them all at once.

A. The deep lateral rotators, with piriformis dividing the greater sciatic foramen. B. Why gluteus medius and minimus matter in gait.

Gray's Anatomy for Students, 4th Ed., Fig. 6.45, p. 568

If you learn one structure in this unit, learn this one. Piriformis arises from the front of the sacrum, passes out of the pelvis through the greater sciatic foramen, and inserts on the greater trochanter. In doing so it does something no other muscle does: it fills the middle of the greater sciatic foramen and splits it into two smaller openings — a suprapiriform foramen above it and an infrapiriform foramen below it.

Think of it as a bar placed horizontally across an open doorway. The doorway was one space; now there is a gap above the bar and a gap below it, and every structure that wants through has to commit to one or the other. Learn which structures chose which gap and you have learned the gluteal region.

The gap above piriformis is the simple one. Only one neurovascular bundle uses it, and the three parts of that bundle keep the same order you would expect of any nerve travelling with its vessels.

Nerves of the gluteal region. The superior gluteal nerve emerges above piriformis; everything else emerges below it.

Gray's Anatomy for Students, 4th Ed., Fig. 6.48, p. 571

The gap below piriformis is the crowded one — eight structures in a single row — and it is the drawing TMU sets as homework. Do not try to memorise eight items as a flat list. Read the row as three groups, going from lateral to medial.

Furthest lateral is the traffic heading for the back of the thigh: the sciatic nerve, and immediately beside it the posterior femoral cutaneous nerve. Next comes the bundle that stays in the buttock to supply gluteus maximus: the inferior gluteal nerve, artery and vein. Most medial is the traffic bound for the perineum: the internal pudendal artery and vein and the pudendal nerve. Lateral to medial is therefore thigh → buttock → perineum, which is also the order of how far each has to travel.

The sciatic nerve is the thickest nerve in the body — about as wide as your little finger where it crosses the buttock. It carries fibres from L4 to S3 and it is really two nerves travelling in one sheath: a tibial division and a common fibular (peroneal) division, which usually separate only at the top of the popliteal fossa.

You need its surface projection, because that line is what tells a clinician where not to put a needle. Mark the midpoint between the posterior superior iliac spine and the ischial tuberosity, then mark a point midway between the ischial tuberosity and the greater trochanter. The nerve runs along a curved line joining those points, then descends down the midline of the back of the thigh.

The sciatic nerve emerging below piriformis and descending through the posterior thigh, dividing into tibial and common fibular nerves.

Gray's Anatomy for Students, 4th Ed., Fig. 6.70, p. 597

Both gluteal arteries are branches of the internal iliac artery inside the pelvis, and each simply follows its matching nerve out of the appropriate foramen — the superior gluteal artery above piriformis, the inferior gluteal artery below it. Once you know the foramina, you already know the arteries.

What makes them clinically interesting is that they anastomose freely with the medial and lateral circumflex femoral arteries and with the first perforating branch of the deep femoral artery. This cruciate anastomosis around the hip provides a collateral route if the external iliac or femoral artery is blocked — one of the reasons the lower limb tolerates proximal arterial disease better than you might expect.

Superior and inferior gluteal arteries and their anastomoses with the circumflex femoral vessels.

Gray's Anatomy for Students, 4th Ed., Fig. 6.50, p. 574

This is the one piece of gluteal anatomy every doctor and nurse uses, and it follows directly from everything above. The sciatic nerve leaves below piriformis and runs down the middle of the buttock; the superior gluteal nerve runs forwards above piriformis. Both are therefore in the middle and lower parts of the region. So the safe territory is the part furthest from both.

Divide the buttock into quadrants with a vertical line and a horizontal line. The upper lateral quadrant — specifically its anterior corner — is the safe injection site. A needle placed there enters gluteus medius, anterosuperior to the margin of gluteus maximus, well away from the sciatic nerve and from the other gluteal nerves and vessels.

The quadrant scheme. The anterior corner of the upper lateral quadrant is the safe site.

Gray's Anatomy for Students, 4th Ed., Fig. 6.49, p. 573

The two foramina — the table to draw from memory

Muscles of the gluteal region and their nerves

Obturator externus is grouped with the medial compartment by innervation (obturator nerve) though it acts as a lateral rotator — TMU's slide lists it with the posterior hip group.