The Posterior Abdominal Wall: The Floor Nobody Sees

The bony frame

Before the muscles, the scaffolding: five vertebrae, one rib, two crests and a promontory. The core of the posterior abdominal wall is the LUMBAR SPINE — five vertebrae with the largest bodies in the column, kidney-shaped when viewed from above, built to carry the weight of everything above them. Their bodies project FORWARDS into the abdomen in the midline, so the wall is not a flat plane at all: it is a central ridge with a deep gutter, the paravertebral gutter, on either side, and the kidneys and the great vessels are tucked into and against those gutters. Backwards project the spinous and articular processes; sideways project the TRANSVERSE PROCESSES, which are the attachment posts for muscle and fascia and the landmark the ureter crosses on its way down. The lumbar curve is a forward convexity (lordosis) that deepens the gutters further. Above, the wall is closed off by the TWELFTH RIB, short, floating, and much more obliquely set than students expect — it slopes downwards and laterally, and its tip may lie almost as low as the iliac crest in some people or barely emerge from the erector spinae in others. Below and laterally, the ILIAC CRESTS flare out to give the wall its lower boundary at the level of L4. Behind and below in the midline, the body of L5 tips forwards onto the sacrum at the SACRAL PROMONTORY — the sharp shelf that marks the entrance to the pelvis, the point at which the wall stops belonging to the abdomen and begins belonging to the pelvis, and the landmark against which the aorta has already divided.

Psoas major: the muscle that leaves the abdomen

It begins on the spine and ends on the femur — and nothing else in the body does that. PSOAS MAJOR arises from the bodies and intervertebral discs of T12 to L5 and from the front of all five lumbar transverse processes, its vertebral attachments arranged as a series of tendinous arches that bridge the waists of the vertebral bodies and leave a tunnel on each side for the lumbar arteries and veins. From that long origin the muscle forms a thick fleshy belly that runs downwards and laterally along the side of the vertebral bodies, fills the gutter, crosses the pelvic brim in front of the sacroiliac joint and the hip joint capsule, passes UNDER the inguinal ligament through the muscular compartment of the lacuna musculorum, and converges on a strong rounded tendon that inserts into the LESSER TROCHANTER of the femur. Its action follows directly from that geometry: it is the most powerful FLEXOR OF THE HIP, and when the femur is fixed it flexes the trunk on the thigh instead — the muscle that lifts your knee towards your chest, and the muscle that sits you up from lying flat. It also contributes weak lateral rotation of the thigh and, acting on one side, lateral flexion of the lumbar spine; acting together, the two psoas muscles are important postural stabilisers of the lumbar column. Its nerve supply is unusual and worth memorising: the ANTERIOR RAMI of L1, L2 and L3 supply it DIRECTLY, without ever forming a named nerve — the muscle is innervated by the roots that pass through it. PSOAS MINOR, present in only about half of people and absent as often as not, is a slender muscle in front of psoas major arising from the bodies of T12 and L1 and the disc between them, ending not on bone of the limb but on the pecten pubis and the iliopubic eminence through a long flat tendon; it is a weak flexor of the trunk and is supplied by L1.

Iliacus, iliopsoas, and quadratus lumborum

ILIACUS is a broad fan filling the ILIAC FOSSA, the smooth concave inner surface of the iliac blade, and taking additional fibres from the ala of the sacrum and from the anterior sacroiliac and iliolumbar ligaments. Its fibres converge downwards onto the lateral side of the psoas tendon, and most reach the lesser trochanter with it; a few insert directly into the shaft of the femur just below. Because the two muscles share an insertion and a common action they are usually described as a single functional unit, the ILIOPSOAS — the chief flexor of the hip — but their nerve supplies are different, and that difference matters: iliacus is supplied by the FEMORAL NERVE (L2–L3), whereas psoas major is supplied by the anterior rami directly. QUADRATUS LUMBORUM is a flat quadrilateral sheet lateral and posterior to psoas, filling the space between the iliac crest and the twelfth rib. It arises from the iliolumbar ligament and the adjacent posterior part of the iliac crest, and inserts above into the medial half of the lower border of the TWELFTH RIB and, by four small tendons, into the tips of the transverse processes of L1 to L4. Acting on one side it LATERALLY FLEXES the trunk; acting on both sides it helps extend the lumbar spine and, most importantly, it FIXES AND DEPRESSES THE TWELFTH RIB during inspiration, holding it steady so that the costal fibres of the diaphragm have something solid to pull against. It is supplied by the subcostal nerve (T12) and the anterior rami of L1 to L4.

The roof: the back of the diaphragm

The wall does not stop at the twelfth rib; it is roofed by the posterior part of the diaphragm, and the join between the two is one of the most elegant pieces of design in the trunk. The diaphragm's lumbar part arises through two tendinous pillars, the CRURA, which grip the front of the upper lumbar vertebral bodies: the right crus is longer and stronger, from the bodies and discs of L1 to L3, and the left crus from L1 to L2. Between them the MEDIAN ARCUATE LIGAMENT arches across the midline in front of the vertebral column, and beneath that arch the aorta escapes at T12 without ever being squeezed. Lateral to the crura the diaphragm meets a problem: two muscles are already occupying the ground it would otherwise attach to. Its answer is to vault over them. The MEDIAL ARCUATE LIGAMENT — a thickened upper edge of the psoas fascia — arches over psoas major from the body of L1 or L2 to the transverse process of L1; the LATERAL ARCUATE LIGAMENT — a thickened upper edge of the quadratus lumborum fascia — arches over quadratus lumborum from the transverse process of L1 to the twelfth rib. Each ligament is nothing more than a fascial bridge, and each exists because a muscle got there first. Structures slip under these arches as under a doorway: the sympathetic trunk under the medial arcuate ligament, and the subcostal nerve and vessels under the lateral one.

One continuous envelope of fascia

The fascia is not a wrapping. It is the reason things travel where they do. Deep to the peritoneum and the extraperitoneal fat, the whole inner surface of the abdomen is lined by a single continuous membrane, the TRANSVERSALIS FASCIA, which changes its name as it passes over each new muscle without ever changing its nature. Over psoas major it becomes the PSOAS FASCIA (the psoas sheath), attached medially to the lumbar vertebral bodies and the pelvic brim, thickened above as the medial arcuate ligament, and continued downwards into the thigh as the fascia iliaca after fusing with the fascia over iliacus. Over quadratus lumborum it becomes the QUADRATUS LUMBORUM FASCIA, which is in fact the anterior layer of the thoracolumbar fascia and is thickened above as the lateral arcuate ligament. Behind all of this lies the THORACOLUMBAR FASCIA proper, a tough three-layered structure in the lumbar region: a posterior layer behind erector spinae, a middle layer between erector spinae and quadratus lumborum, and the anterior layer in front of quadratus lumborum. The three layers fuse at a lateral raphe near the tip of the twelfth rib, and from that fused edge arise transversus abdominis and internal oblique — which is how the wall behind is mechanically continuous with the anterior abdominal wall, and why a strong trunk is a single cylinder rather than a front and a back. Two consequences follow. First, the fascial planes are potential spaces: fluid entering one of them is guided along it rather than dispersed. Second, the psoas sheath, uniquely, is a plane that leaves the abdomen entirely — the anatomical basis of everything clinical that follows.

Everything that lies on the wall

Strip away the peritoneum and the gut, and the retroperitoneum is laid out like a map. In the midline, slightly to the LEFT, runs the abdominal aorta, entering behind the median arcuate ligament at T12 and ending by dividing into the common iliac arteries at L4; along its course it gives the coeliac trunk at T12, the superior mesenteric artery at L1, the renal arteries at L1–L2 and the inferior mesenteric artery at L3, plus four pairs of lumbar arteries that run backwards around the vertebral bodies deep to psoas. To its RIGHT lies the INFERIOR VENA CAVA, formed by the union of the common iliac veins at L5 and ascending to pierce the central tendon of the diaphragm at T8. On either side, in the paravertebral gutters, sit the KIDNEYS, extending from about T12 to L3 with the right one lower because the liver sits above it, each capped by an ADRENAL (SUPRARENAL) GLAND; from each hilum the URETER descends almost vertically on the psoas fascia, crossing the tips of the lumbar transverse processes and then the pelvic brim — the whole course covered in kidneys and ureters. Between the aorta and the right crus at the level of L1–L2 lies the CISTERNA CHYLI, the sac-like origin of the thoracic duct that receives the intestinal and lumbar lymph trunks. On the anterolateral surfaces of the vertebral bodies, medial to psoas, run the two SYMPATHETIC TRUNKS with their lumbar ganglia. And within the substance of psoas major itself lies the lumbar plexus, whose branches emerge in a fixed and testable pattern: iliohypogastric and ilioinguinal (L1) from the upper lateral border, crossing quadratus lumborum; the genitofemoral (L1–L2) piercing the ANTERIOR surface of the muscle; the lateral cutaneous nerve of the thigh (L2–L3) from the lateral border crossing iliacus to the anterior superior iliac spine; the FEMORAL nerve (L2–L4) from the lateral border, hidden in the groove between psoas and iliacus; the OBTURATOR nerve (L2–L4) from the MEDIAL border, passing into the pelvis; and the lumbosacral trunk (L4–L5) medially, on its way to join the sacral plexus.

The psoas sign, and why pus travels

Two classic clinical phenomena come straight out of this anatomy. The first is the PSOAS SIGN. The commonest position of the appendix is RETROCAECAL, and a retrocaecal appendix frequently lies directly on the psoas fascia. When it becomes inflamed it irritates the muscle beneath it, so the patient instinctively keeps the hip slightly flexed to keep psoas short and quiet. The test is to stretch it: with the patient lying on the left side, the examiner slowly EXTENDS the right hip — or asks the patient to flex the hip against resistance — and a sharp deepening of the right iliac fossa pain is a positive psoas sign, discussed alongside the other localising signs in the large intestine and appendix. The second is the PSOAS ABSCESS. Because the psoas sheath is a closed fascial corridor running from the twelfth thoracic vertebra to the lesser trochanter, pus that enters it anywhere along its length is not contained — it is conducted. Classically this was tuberculosis of the lumbar spine (Pott's disease), which erodes forwards into the sheath and produces a "cold abscess": a fluctuant, painless, non-inflamed swelling that appears in the groin BELOW the inguinal ligament or even in the upper thigh, sometimes months later, in a patient whose only other symptoms are fever, night sweats and weight loss. Today the commoner causes are secondary: Crohn's disease with a fistula burrowing backwards from the terminal ileum, diverticulitis, a perforated appendix, vertebral osteomyelitis, or primary haematogenous seeding with Staphylococcus aureus. The presentation is remarkably consistent — fever, back or flank pain, a limp, and a patient who lies with the hip flexed and resists extension, because the abscess is inside the muscle that flexes it.

A wall that hides things

The retroperitoneal space in front of this wall is loose, fatty and enormously distensible, and it is separated from the peritoneal cavity by a membrane that does not transmit signs well. Together those two facts make it one of the most dangerous hiding places in the body. A leaking abdominal aortic aneurysm, a fractured pelvis, a torn kidney or a spontaneous psoas bleed can pour LITRES of blood into this space with very little to show for it: the abdomen is not distended, guarding is unimpressive, and the parietal peritoneum in front is untouched so there is no early peritonism. The patient's first honest signal may be tachycardia, a falling haemoglobin and flank pain. This is also why the everyday complaints of this wall are so easy to dismiss. Prolonged sitting holds iliopsoas in a shortened position for hours a day; over months the muscle adapts, resists full hip extension, and tilts the pelvis forwards, which exaggerates the lumbar lordosis and loads the facet joints — the low-grade, nagging lumbar ache of a desk-bound life, treated far more effectively by restoring hip extension than by anything applied to the back itself. The lesson is the same at both extremes of severity: symptoms attributed to the hip, the thigh, the back or the abdomen may all be arising from a single sheet of muscle that the examining hand never reaches.