Psoas major muscle
The psoas major muscle (often referred to as the psoas muscle) is one of the muscles of the posterior abdominal wall and lies not in the retroperitoneum but posterior to it, in the iliopsoas compartment.
On this page:
Summary
- origin: vertebral bodies, intervertebral discs and transverse processes of T12 to L5
- insertion: lesser trochanter of the femur
- innervation: branches of the L1-L3 roots of the lumbar plexus
- arterial supply: lumbar arteries, iliolumbar artery, deep circumflex iliac artery, external iliac artery, femoral artery
- action: lateral flexion of the trunk; stabilizer and flexor of the hip
Terminology
The correct terminology is psoas major muscle (as opposed to just psoas muscle) to differentiate it from the psoas minor muscle.
Gross anatomy
The psoas muscle lies in the gutter between the bodies and transverse process of the lumbar vertebra. Its vertebral attachment is to T12-L4 discs, the adjacent vertebral bodies and to fibrous arches that span the concavities of the sides of the upper four vertebral bodies (superficial part). Thus there is one continuous attachment from the lower border of T12 to the upper border of L5. It also attaches to the medial ends of the transverse process of L1 to L5 (deep part). It fuses with the iliacus muscle to form the iliopsoas muscle at the level of L5-S2 and passes inferiorly, deep to the inguinal ligament, to insert into the lesser trochanter of the femur 1-3.
The lumbar plexus is embedded within the muscle and its branches emerge from it 3:
- anterior aspect: genitofemoral nerve
- lateral border: iliohypogastric, ilioinguinal, lateral femoral cutaneous and femoral nerves
- medial border: obturator nerve and lumbosacral trunk
The psoas muscle is enclosed by the psoas fascia and it is this that retains the pus in a psoas abscess 3. The psoas fascia (part of the iliac fascia) invests the surface of the muscle, attached to the vertebral bodies, fibrous arches, and the transverse process, and extends along the pelvic brim attached to the iliopubic eminence at the margins of the muscle. The lateral edge blends with the anterior layer of the lumbar fascia (over quadratus lumborum).
The muscle comes to lie medial to and fuses with the iliacus muscle, such that inferiorly the two are often referred to together as the iliopsoas muscle 1,2.
Innervation
The psoas muscle is innervated by the lumbar plexus via branches from L1-L3 (mainly L2) 3.
Arterial Supply
- upper part: lumbar arteries
- middle part: iliolumbar artery (main artery to muscle), deep circumflex iliac and external iliac arteries
- lower part: branches of the femoral artery
Action
The action of this muscle is complex, acting to both laterally flex the lumbar spine as well as stabilize and flex the thigh 1,2. The antagonist is the gluteus maximus.
Relations
Lower abdomen:
- lumbosacral plexus embedded within the muscle
- anteriorly: genitofemoral nerve
- laterally: iliohypogastric, ilioinguinal, lateral femoral cutaneous and femoral;
- medially: obturator nerve and lumbosacral trunk
- posteriorly: part of the external vertebral venous plexus (in front of the transverse process), four lumbar arteries and veins pass beneath the four arches and run laterally behind the psoas accompanied by the sympathetic rami
Pelvis and lower limb:
- behind the inguinal ligament
- femoral nerve lying on the iliacus
- lateral to the contents of the femoral triangle
Lumbosacral triangle of Marcille:
- contents from medial to lateral: obturator nerve, iliolumbar artery, lumbosacral trunk, sympathetic trunk
Variant anatomy
- asymmetry of the psoas major muscle, which is usually not clinically significant 4
- associated with psoas minor - origin from T12 and L1 vertebrae and inserted on the arcuate line and iliopubic eminence
History and etymology
From the Greek "psoa" meaning "loin" 3. The psoas muscle is referred to as the tenderloin by butchers.
Related pathology
Related Radiopaedia articles
Anatomy: Abdominopelvic
- skeleton of the abdomen and pelvis
- muscles of the abdomen and pelvis
- spaces of the abdomen and pelvis
- anterior abdominal wall
- posterior abdominal wall
-
abdominal cavity
-
peritoneum
- peritoneal ligaments
- mesentery
-
peritoneal spaces
-
supramesocolic space
-
right supramesocolic space
- right subphrenic space
-
right subhepatic space
- anterior right subhepatic space
- posterior right subhepatic space (Morison pouch)
-
lesser sac
- epiploic foramen (of Winslow)
- left supramesocolic space (left perihepatic space)
-
right supramesocolic space
- inframesocolic space
-
supramesocolic space
- inguinal canal (mnemonic)
- Hesselbach triangle
- umbilical folds
- retroperitoneum
-
peritoneum
- pelvic cavity
- perineum
- abdominal and pelvic viscera
- gastrointestinal tract
- spleen
- hepatobiliary system
-
endocrine system
-
adrenal gland
- adrenal vessels
- chromaffin cells
- variants
- pancreas
- organs of Zuckerkandl
-
adrenal gland
-
urinary system
-
kidney
- renal pelvis
- renal sinus
- avascular plane of Brodel
-
variants
- number
- fusion
- location
- shape
- ureter
- urinary bladder
- urethra
- embryology
-
kidney
- male reproductive system
-
female reproductive system
- vulva
- vagina
- uterus
- uterine tubes
- ovaries
- broad ligament (mnemonic)
- variant anatomy
- embryology
- blood supply of the abdomen and pelvis
- arteries
-
abdominal aorta
- inferior phrenic artery
- celiac artery
- superior mesenteric artery
- middle suprarenal artery
- renal artery (variant anatomy)
- gonadal artery (ovarian artery | testicular artery)
- inferior mesenteric artery
- lumbar arteries
- median sacral artery
-
common iliac artery
- external iliac artery
-
internal iliac artery (mnemonic)
- anterior division
- posterior division (mnemonic)
- variant anatomy
-
abdominal aorta
- portal venous system
- veins
- anastomoses
- arterio-arterial anastomoses
- portal-systemic venous collateral pathways
- watershed areas
- arteries
- lymphatics
- innervation of the abdomen and pelvis
- lumbar plexus
-
sacral plexus
- lumbosacral trunk
- sciatic nerve
- superior gluteal nerve
- inferior gluteal nerve
- nerve to piriformis
- perforating cutaneous nerve
- posterior femoral cutaneous nerve
- parasympathetic pelvic splanchnic nerves
- pudendal nerve
- nerve to quadratus femoris and inferior gemellus muscles
- nerve to internal obturator and superior gemellus muscles