The psoas major is a long fusiform muscle stretching from the sides of lumbar vertebrae to the lesser trochanter of the femur. It’s enclosed in a fascial sheath named psoas sheath.
The muscle originates from 14 fleshy skids which are as follows:
- 5 chemises from intervertebral discs between T12 L5 vertebrae and adjoining margins of the bodies of these vertebrae.
- 5 slides from anterior surfaces and lower edges of the transverse process of 5 lumbar vertebrae.
- 4 skids from tendinous arches bridging the constricted sides of the bodies of lumbar vertebrae. The lumbar vessels pass deep to these arches.
From the site of origin, the muscle descends along the pelvic brim and enters the thigh behind the inguinal ligament. Below the ligament, the tendon creates on the lateral side of the muscle, enters in front of the hip joint, and enters the anterior surface of the tip of the lesser trochanter of the femur. A synovial bursa that might communicate together with the cavity of the hip joint normally separates this tendon from the joint capsule.
The Nerve Supply is by direct branches from ventral rami of L2, L3, L4 spinal nerves.
All these are as follows:
- Acting from above, it’s the main flexor of the thigh in the hip joint.
- Acting from below, it bends the trunk on the thigh, as in lifting the trunk from recumbent to sitting position.
The psoas major is the key muscle of the posterior abdominal wall, because its relationships supply a rational notion about the layout of structures in this region:
Lumbar plexus creates inside the substance of psoas. The plexus can be shown only by ripping the muscle, for it will not break up the muscle into planes.
5 nerves appear from underneath the lateral border of the psoas major from above downward; these are as follows:
- Subcostal nerve.
- Iliohypogastric nerve.
- Ilioinguinal nerve.
- Lateral cutaneous nerve of thigh.
- Femoral nerve.
The upper 4 nerves come above the iliac crest and runs downward and laterally across the quadratuslumborum muscle. The final nerve (femoral nerve) comes below the iliac crest and runs down in the groove between the psoas and iliacus muscles.
1 nerve (genitofemoral nerve) runs downward on the very front of the psoas major and occasionally may be mistaken for the tendon of psoas minor muscle.
3 significant structures being located on the medial side of the psoas major. From medial to lateral side these are: (a) lumbosacral trunk, (b) iliolumbar artery, and (c) obturator nerve.
Lumbosacral Triangle of Marcille
It’s a triangular separation on every side of the body of L5 vertebra with all the apex directed upward. It’s bounded medially by the body of L5 vertebra, laterally by the medial border of the psoas major, and inferiorly (base) by the ala of the sacrum. The apex is composed by the junction of psoas and the body of L5 vertebra. The floor (posterior wall) is composed by the transverse process of L5 vertebra and iliolumbar ligament.
It includes 4 structures. From medial to lateral sides, these are:
- Sympathetic trunk.
- Lumbosacral trunk.
- Iliolumbar artery.
- Obturator nerve.
The psoas major muscle is enclosed in a fascial sheath named (psoas sheath) created by the psoas fascia. The attachmentsof psoas fascia are as follows:
- Above: It’s thickened to create medial arcuate ligament, which goes from the body of L1 vertebra to the tip of its transverse process.
- Laterally: It combines with all the anterior layer of the thoracolumbar fascia.
- Medially: It’s connected to the bodies and intervening intervertebral discs of lumbar vertebrae and presents 4 tendinous arches.
- Below: It fuses with the arcuate line of the pelvis and the fascia covering the iliacus muscle (iliac fascia).
The psoas fascia creates a long tubular sheath (osseofibrous tunnel) termed psoas sheath going from the upper lumbar region of the posterior abdominal wall to the groin. Tubercular infection of vertebrae of the thoraco-lumbar region causes destruction of their bodies resulting in the formation of an abscess. The pus can not propagate anteriorly as a result of anterior longitudinal ligament. For that reason, it propagates laterally into the psoas sheath creating psoas abscess. The pus can also goes into the psoas sheath from the posterior mediastinum via a gap deep to medial arcuate ligament. Pus may subsequently propagate downward along the psoas muscle, below the inguinal ligament into the femoral triangle where it creates a soft swelling.