Posterior Abdominal Wall
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.
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 (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).
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).
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.
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).
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).
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).
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).
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).
