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//MUSCLES

MUSCLES

The majority of the muscles of the posterior abdominal wall are functionally part of the lower limb or vertebral column. They provide the surface against which the neurovascular structures of the retroperitoneum lie, and they are supported and separated from the majority of the retroperitoneal structures by fascial layers (Figs 62.4, 62.5; see Fig. 62.14).

  

Fig. 62.4  Bones and deep muscles of the posterior abdominal wall. Left psoas major and the diaphragm have been removed.
(Adapted from Drake, Vogl and Mitchell 2005.)

  

Fig. 62.5  Muscles of the posterior abdominal wall. A, Coronal True FISP MRI. B, Axial T2 weighted MRI. (Provided by Dr Louise Moore Chelsea & Westminster Hospital.)

  

Fig. 62.14  Muscles and nerves of the posterior abdominal wall. The left psoas major has been removed to expose the origins of the lumbar plexus and quadratus lumborum.

Quadratus lumborum

Quadratus lumborum is an irregularly shaped quadrilateral muscle, which is broader at its inferior attachment than superiorly.

Attachments

Quadratus lumborum is attached below by aponeurotic fibres to the iliolumbar ligament and the adjacent portion of the iliac crest for approximately 5 cm. The superior attachment is to the medial half of the lower border of the 12th rib, and by four small tendons to the apices of the transverse processes of the upper four lumbar vertebrae. Sometimes it is also attached to the transverse process or body of the 12th thoracic vertebra. Occasionally, a second layer of this muscle is found in front of the first, attached to the upper borders of the transverse processes of the lower three or four lumbar vertebrae and to the lower margin and the lower part of the anterior surface of the 12th rib.

Relations

Anteriorly are the colon (ascending on the right, descending on the left), kidney, psoas major and minor, and diaphragm. The subcostal, iliohypogastric and ilioinguinal nerves lie on the fascia anterior to the muscle, but are bound down to it by the medial continuation of the transversalis fascia.

Vascular supply

Quadratus lumborum is supplied by branches of the lumbar arteries, the arteria lumbalis ima, the lumbar branch of the iliolumbar artery and branches of the subcostal artery.

Innervation

Quadratus lumborum is innervated by the ventral rami of the 12th thoracic and upper three or four lumbar spinal nerves.

Actions

Quadratus lumborum fixes the last rib, and acts as a muscle of inspiration by helping to stabilize the lower attachments of the diaphragm. It has been suggested that this action might also provide a fixed base for controlled relaxation of the diaphragm in the precise adjustment of expiration needed for speech and singing. With the pelvis fixed, quadratus acts upon the vertebral column, flexing it to the same side. When both muscles contract, they probably help to extend the lumbar part of the vertebral column.

Psoas major

Psoas major has several sites of abdominal attachment. Posteriorly, the attachments are to the anterior surfaces and lower borders of the transverse processes of all the lumbar vertebrae. The muscle arising from these attachments is referred to as the posterior mass. The muscle also has an anterior mass. It consists of two different sets of attachments. The first part is five slips of muscle attached to the bodies of two adjoining vertebrae and their intervertebral disc (from the 12th thoracic vertebra and the thoracolumbar disc to the body of the fifth lumbar vertebra). The second part is a series of tendinous arches extending across the narrow parts of the bodies of the five lumbar vertebrae between these slips. The upper four lumbar intervertebral foramina bear important relations to these attachments of the muscle. The foramina lie anterior to the transverse processes (the posterior attachments) and posterior to the vertebral bodies, discs and tendinous arches (anterior attachments). The roots of the lumbar plexus therefore enter the muscle directly between the two masses and the plexus is lodged within it. The branches then emerge from the borders and surfaces of psoas major.

Psoas minor

Psoas minor is often absent but, when present, lies anterior to psoas major. It arises from the sides of the bodies of the 12th thoracic and first lumbar vertebrae and from the disc between them, and ends in a long, flat tendon attached to the pectineal line and iliopectineal eminence and, laterally, to the iliac fascia.

Erector spinae

The muscles of the erector spinae group (see Ch. 42) do not form part of the posterior abdominal wall itself, but are closely associated with the fascial layers of the posterior wall.

Iliacus

Iliacus is a triangular sheet of muscle that arises from the superior two-thirds of the concavity of the iliac fossa, the inner lip of the iliac crest, the ventral sacroiliac and iliolumbar ligaments and the upper surface of the lateral part of the sacrum. It reaches as far as the anterior superior and anterior inferior iliac spines, and receives a few fibres from the upper part of the capsule of the hip joint. Most of its fibres converge into the lateral side of the strong tendon of psoas major. It lines the posterior wall of the lesser pelvis formed by the ilium.

Posterior abdominal wall hernias

Herniation through the posterior abdominal wall is extremely rare because the fascial layers usually provide an excellent protection against protrusion of the posterior abdominal viscera, which are relatively immobile. However, the posterior free border of external oblique and the inferior free border of latissimus dorsi create an area of potential weakness, referred to as the lumbar triangle. Spontaneous hernias through this tissue are very rare in the absence of previous surgical access such as a nephrectomy.

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