Anatomy, Bony Pelvis and Lower Limb, Psoas Major

The psoas muscle is among the most significant muscles that overlie the vertebral column. It is a long fusiform muscle on either side of the vertebral column and the brim of the lesser pelvis. At its distal end, it combines with the iliacus muscle to form the iliopsoas muscle. The psoas muscle has traditionally described having a deep and superficial segment. The deeper segment of the muscle originates from the first four lumbar vertebrae, while the superficial segment originates along the lateral surface of the distal thoracic vertebrae and from adjacent intervertebral discs. In between the deep and superficial layers, one can find the lumbar plexus. At the distal end, the psoas muscle combines with the iliacus muscle to form the iliopsoas muscle which is encircled by the dense iliac fascia. The common tendon attaches on the lesser trochanter of the femur: the muscle during contraction of the fibers lead to external rotation and abduction of the femur. The orientation of the muscle changes from the superior-inferior direction of the upper portion to a horizontal direction toward anterior of the middle portion reaching the pubic branch and passing under the inguinal ligament, and to an oblique direction toward posterior of the distal portion of the tendon inserting on the lesser trochanter. The inconsistent psoas minor muscle completes the psoas muscle.

The psoas major muscle has a biomechanical and postural function during both moving and static states. It is also involved in the metabolic function used in the clinical assessment of some diseases,[1] as well as mood and stress disorders as a cause of low back pain.[2]

The classical anatomy textbooks, like Testut, describe the psoas major muscle originating at the top of the lateral face and the base of the transverse processes of the first four lumbar vertebrae and from the twelfth thoracic vertebrae and interposed intervertebral discs. The insertion on the discs and on the transverse processes is through short tendon tongues, which insert on the vertebral body with a medially concave arched tendon to allow passage of the lumbar vessels and the communicating branches of the ortho-sympathetic nerves.

From the lumbar spine the numerous muscular bundles move forward, downwards, and laterally to form a single, cylindrical and fusiform muscular body, it presents the maximum thickness at the sacroiliac joint. The muscle crosses the lumbar region and the iliac fossa, from which it emerges through an incisura on the upper margin of the ilio-pubic branch between the inferior anterior iliac spine and the iliopectineal eminence (lacuna musculorum). At this level, the posterior fascicles attach firmly to the pelvic brim, and the muscle fibers change direction taking it posteriorly and laterally to insert itself through a thick tendon on the posterior face of the small trochanter of the femur.[3]

The iliacus muscle has a fan-shaped triangular shape that occupies the internal iliac fossa. It originates on the upper two-thirds of the iliac fossa, on the medial edge of the iliac crest, on the ilium-lumbar ligament, where it blends with the bundles of the quadratus lumborum muscle, on the base of the sacrum, anteriorly on the upper and lower anterior iliac spines, and finally on the anterior capsule of the hip joint. The muscular bundles move towards the lateral side of the psoas major muscle tendon on which they are inserted. Few lateral and inferior muscle bundles are inserted directly on the femur.
The psoas minor muscle is an inconsistent and small muscle bundle that often turns into fibrous tissue, it is present in around 60-65% of the population.[4]. It originates from the corpus of the last thoracic vertebra and from the first lumbar and from the interposed disc and is located anteriorly and medially to the large psoas muscle. It has the shape of a flattened and thin ribbon and fits on the iliopectineal eminence and the iliac fascia.

Psoas bursa. The psoas muscle is separated from the bony margin of the ilium and from the fibrous capsule of the hip joint by a serous fluid bursa (the iliopectineal bursa) of considerable size that sometimes communicates with the synovial space of the hip joint. A smaller bursa is located between the muscle tendon and the anterior surface of the lesser trochanter of the femur.

Fascial Relationship

The iliopsoas muscle has important fascial relationships in its upper or lumbar abdominal portion, in its medial portion in the iliac fossa near the inguinal ligament, and in its femoral portion in the lower limb.

Lumbo-pelvic portion. The psoas major muscle sits in a posterior relationship to the diaphragm, to the psoas minor muscle, to the kidney and the renal vessels, to the ureter, to the gonadal vessels, to the ascending colon on the right and descending to the left. It is positioned anteriorly to the inter-transversal muscles of the transverse apophysis of the vertebrae on which it is inserted, with the quadratus lumborum muscle, from which it is separated from the deep sheet of the thoracolumbar fascia,[5], and from the anterior branches of the lumbar nerves. Its right medial margin corresponds to the inferior vena cava and to the descending portion of the duodenum, the left medial margin is related to the aorta, with the ascending portion of the duodenum and the duodenojejunal flexure. Inferiorly it is crossed by the ureter and the common iliac vessels and runs parallel to the common iliac vessels. The psoas muscle is crossed by the lumbar plexus branches, which emerge from its surface: anteriorly the lateral femoral cutaneous nerve and the genitofemoral nerve; medially the lumbosacral trunk and the obturator nerve; laterally the iliohypogastric nerve, the ilioinguinal nerve, and the femoral nerve. One must be aware that the lumbar plexus and peripheral branches have huge anatomical variability.[6] The iliac muscle is posteriorly positioned to the cecum on the right and the sigmoid colon on the left; its posterior surface rests in the iliac fossa and is related to the fibrous capsule of the sacroiliac joint, the iliolumbar ligaments and the deep branches of the iliac circumflex vessels. The iliolumbar ligaments are an integrative part of the thoracolumbar fascia following the posterior aspect of the psoas major muscle that is covered by the deep layer of the thoracolumbar fascia and continues superiorly with the endo thoracic fascia.[5]

Inguinal portion. The iliopsoas muscle passes under the inguinal ligament, filling all the space between the ligament, the iliopectineal band and the anterior border of the iliac bone: a space called the lacuna musculorum. The iliopectineal band separates it from the artery and femoral vein. The femoral nerve is located on the anteromedial side.

Femoral portion. The lateral surface of the iliopsoas muscle constitutes the lateral portion of Scarpa's triangle. Its posterior side rests above the fibrous capsule of the hip joint. Its medial margin corresponds to the lateral edge of the pectoral muscle with which it forms a groove for passaging the femur artery. Its lateral margin is related to the sartorial muscle and the rectus femoris muscle.

Function

The function of the psoas muscle is to connect the upper body to the lower body, the outside to the inside, the appendicular to the axial skeleton, and the front to the back, with its fascial relationship. Combined with the iliopsoas muscle, the psoas is a major contributor of flexion of the hip joint. Unilateral contraction of the psoas also helps with lateral motions and bilateral contraction can help elevate the trunk from the supine position. The psoas muscle also works in conjunction with the hip flexors to elevate the upper leg towards the body when the body is static or pull the body towards the leg when the leg is in a fixed position.

The biomechanical and postural function of the psoas complex muscle is to flex the hip, to adduct the femur and to externally rotate the hip. In an upright position, the muscle takes a fixed point on the femur and acts on the pelvis and on the lumbar spine. Its action is to flex the spine, to side-bend ipsilateral and to rotate it towards the opposite side. The proper role of the psoas major muscle is to stabilize the lumbar spine in the sitting position and to flex the femur in a supine or standing position.[4] The psoas major acts as a stabilizer of the femoral head in the hip acetabulum in the first 15 degrees of movement, maintains the erector action from 15° to 45°, and it is an effective flexor of the femur from 45° to 60°.[7] It seems that the lower fascicles flex the lower lumbar spine and the upper fascicles to extend the upper lumbar vertebrae: the flexion-extension movement is small, whereas the compression and shear forces are large, collapsing the spine in a sigmoid fashion and forcing it into lordosis while severely straining the lumbosacral segment.[8][9]  An anchoring retinaculum at the level of the lumbosacral junction keeps the psoas muscle in place alongside the lumbar spine, making its action on the increase of the lumbar lordosis independent of the lumbar curvature. During gait movements, the ipsilateral psoas muscle is activated during the onset of the hip flexion and during the latter part of the swing phase.[10]

The psoas major is in relationship with the medial arcuate ligament and continues with the thoracic diaphragm and the endo thoracic fascia: the psoas major muscle seems able to exert influence on the balance of the dynamics of respiratory function and the functional relationship between the diaphragm and the pelvic floor. The fascia of the muscle appears to exhibit continuity with the fascia of the crus of the diaphragm, blending with the anterior longitudinal ligament; while the inferomedial fascia of the psoas muscle becomes thick and is continuous with the deep fascia of the pelvic floor[3]: this forms a link with the conjoint tendon, transversus abdominis muscle, and the internal oblique muscle. From this point of view, the psoas muscle may influence the pelvic floor functions, the balance between the pelvic floor movements with the rhythm of the thoracic diaphragm; it may be involved in the venous and lymphatic drainage of abdomen, pelvis and lower limbs.

Literature does not describe the embryology of the psoas major muscle in detail. A 3-D reconstruction study of 8 weeks gestation embryos shows that the muscles of the iliac region are well recognizable in their spatial organization and orientation, as in adult anatomy. From origin to insertion the major psoas muscle and the iliacus muscle were noticeably separated from each other: in adults specimens, this distinction is not so evident at their inferior end where the tendons fuse to form the iliopsoas muscle.[11]

The psoas major muscle is supported by collateral rami of the lumbar arteries, by the iliolumbar arteries, circumflex iliac artery, obturator artery, and femoral artery.

The psoas major muscle is innervated via the anterior rami of L1-L4, and also receives small branches from the femoral nerve.