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Posterior Abdominal Wall Anatomy


Posterior Abdominal Wall Anatomy
Posterior to the abdominal cavity lie the lumbar spine, the psoas, iliacus and quadratus lumborum muscles, and associated fasciae and nerves (Fig. 4.99). The posterior abdominal wall extends inferiorly to the pelvic brim and superiorly to the attachment of the diaphragm (p. 203), while laterally it merges with the anterolateral abdominal wall. The lumbar spine and the postvertebral muscles (erector spinae) are considered in Chapter 8.



Muscles
Psoas major
This long fusiform muscle (Fig. 4.99) attaches to the sides of the last thoracic and all five lumbar vertebral bodies, to the intervening discs and to the fronts of the lumbar transverse processes. At the side of each lumbar vertebral body, psoas major attaches to a fascial tunnel conveying a lumbar artery and vein (Fig. 4.103).
The muscle inclines downwards, passing behind the inguinal ligament to enter the anterior compartment of the thigh (Fig. 4.100). The psoas major tendon, which also receives most of the fibres of iliacus, passes in front of the hip joint capsule, from which it is separated by a bursa (Fig. 6.68), and attaches to the lesser trochanter of the femur.
Psoas major is innervated by the anterior rami of the upper lumbar nerves and its principal actions are flexion and medial rotation of the hip joint. In addition, the muscle flexes the lumbar spine both anteriorly and laterally. Within the substance of psoas major the anterior rami of the lumbar nerves form the lumbar plexus, whose branches emerge from the lateral, anterior and medial surfaces of the muscle.
Psoas minor
When present, this small muscle lies on the anterior surface of psoas major and gives way to a long narrow tendon (Fig. 4.99), which attaches to the iliopubic eminence of the hip bone.
Iliacus
This fan-shaped muscle attaches to the upper portion of the abdominal surface of the ilium and adjacent part of the sacrum (Fig. 4.100). Most of its fibres attach to the tendon of psoas major although some reach the femur below the lesser trochanter. The muscle is innervated by the femoral nerve (Fig. 4.103) and assists psoas major in flexing the hip joint.
Quadratus lumborum
Quadratus lumborum (Fig. 4.99) is anchored inferiorly to the iliolumbar ligament and adjacent part of the iliac crest. Superiorly, it reaches the medial part of the lower border of the twelfth rib. There are intermediate attachments to the transverse processes of the upper four lumbar vertebrae. This muscle is innervated by the subcostal nerve and anterior rami of the upper three lumbar nerves and is a lateral flexor of the lumbar spine. When the diaphragm contracts during inspiration, quadratus lumborum stabilizes the twelfth rib.

Fasciae
The psoas and iliac fasciae form a continuous layer covering the anterior surfaces of their respective muscles (Fig. 4.99). The psoas fascia fuses superiorly with the diaphragmatic fascia and laterally and inferiorly with the transversalis fascia. Fascial thickenings over the upper parts of psoas major and quadratus lumborum form the medial and lateral arcuate ligaments (lumbocostal arches), which provide attachment for the diaphragm (Fig. 4.103). Fascial layers covering the anterior and posterior surfaces of quadratus lumborum fuse to form the thoracolumbar fascia which gives attachment to transversus abdominis. The psoas fascia can direct the spread of tuberculosis from the lumbar vertebrae into the anterior triangle of the thigh, producing a psoas abscess.

Nerves
On each side of the midline the sympathetic trunk enters the abdomen behind the medial arcuate ligament of the diaphragm and descends on the medial border of psoas major (Fig. 4.97). The anterior rami of the subcostal and lumbar nerves emerge through their respective intervertebral foramina and enter the substance of psoas major. All spinal nerves within psoas receive grey rami communicantes from the sympathetic trunk, but only the last thoracic and upper two lumbar nerves supply white rami to the trunk.
Subcostal (T12) nerve
This nerve follows the lower border of the twelfth rib and enters the abdomen behind the lateral arcuate ligament of the diaphragm (Fig. 4.103). It crosses the anterior surface of quadratus lumborum and continues on the deep surface of transversus abdominis. The nerve pierces transversus to enter the neurovascular plane of the abdominal wall, and its subsequent course is similar to that of the lower intercostal nerves (p. 145).
Lumbar plexus
Within psoas major the anterior rami of the upper four lumbar nerves form the lumbar plexus, whose branches are distributed to the lower part of the abdominal wall, the lower limb and the sacral plexus in the pelvic cavity (Figs 4.101 & 4.104).
First lumbar nerve
Although a few first lumbar (L1) fibres contribute to the genitofemoral nerve (Fig. 4.101), most form a nerve that emerges from the lateral border of psoas major and crosses the anterior surface of quadratus lumborum (Fig. 4.103).
After a variable distance, this nerve divides into iliohypogastric and ilioinguinal branches, which continue into the anterolateral abdominal wall. Their paths and distribution are considered on p. 145.
Genitofemoral (L1, L2) nerve
This nerve emerges through the anterior surface of psoas major on which it descends, dividing into two branches (Fig. 4.101). The genital branch enters the deep inguinal ring and innervates the cremaster muscle, while the femoral branch passes behind the inguinal ligament to reach the subcutaneous tissue covering the femoral triangle (Fig. 6.9).
Lateral cutaneous (L2, L3) nerve of thigh
Emerging from the lateral side of psoas major, this nerve inclines across the anterior surface of iliacus (Fig. 4.102). It passes behind or through the inguinal ligament close to the anterior superior iliac spine (a site of possible entrapment) and supplies skin on the anterolateral aspect of the thigh (Figs 6.9 & 6.21).
Femoral (L2, L3, L4) nerve
This large nerve appears at the lateral border of psoas major and descends in the gutter between this muscle and iliacus (Fig. 4.102). It innervates iliacus (Fig. 4.103), passes behind the inguinal ligament lateral to the femoral artery and enters the anterior compartment of the thigh (Figs 6.8 & 6.18, p. 262).
Obturator (L2, L3, L4) nerve
Descending vertically within psoas major, this nerve emerges from the medial border of the muscle near the pelvic brim (Figs 4.102 & 4.103). It passes behind the common iliac vessels and runs downwards and forwards on the lateral pelvic wall, as far as the obturator canal (p. 237). Within the canal it divides into anterior and posterior branches, which enter the medial compartment of the thigh (p. 266).
Lumbosacral trunk
Fibres from the fourth lumbar anterior ramus join those of the fifth to form the lumbosacral trunk (Fig. 4.103). The trunk emerges from the medial side of psoas major and crosses the sacroiliac joint to enter the pelvis, where it contributes to the formation of the sacral plexus (p. 237).

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