Introduction

The iliopsoas musculotendinous unit (IPMU), composed of the psoas major, psoas minor, and iliacus muscles, functions as the primary flexor of the thigh and contributes to external rotation and stabilization of the hip and lumbar spine (see Image. Right Hip and Femoral Muscles, Anterior View). The psoas major originates from the lumbar vertebrae, while the iliacus arises from the iliac fossa. Both insert onto the lesser trochanter. Blood supply derives primarily from branches of the external iliac and lumbar arteries, with venous drainage to the external iliac vein. Innervation arises from the lumbar plexus, with contributions from L1 to L3 to the psoas muscles and the femoral nerve (L2–L4) to the iliacus. Notable variants include absence of the psoas minor, accessory muscular slips, and bifid or multiple tendinous insertions.

The IPMU has clinical significance due to its proximity to abdominal and pelvic viscera, variable anatomy, and susceptibility to conditions such as tendinopathy, bursitis, hematoma, abscess, and snapping hip, which may mimic intra-abdominal or neuropathic pathology. Within surgical contexts, operations involving the hip and retroperitoneum may present complications, including tendon impingement following arthroplasty, abscess requiring drainage, or contracture requiring tenotomy. Knowledge of iliopsoas anatomy and function enables accurate diagnosis of hip and groin pain, guides appropriate imaging and management strategies, and helps prevent iatrogenic injury during surgical and interventional procedures.

Structure and Function

Anatomy 

The IPMU consists of 3 muscles: the iliacus, psoas major, and psoas minor. The IPMU is part of the inner hip muscles and contributes to the posterior abdominal wall, positioned posteriorly within the retroperitoneum.

The psoas major has a fusiform shape and originates from the transverse processes and lateral surfaces of the bodies of the first 4 lumbar vertebrae, including the transverse process and body of the last thoracic vertebra. The fibers of this muscle also arise from intervertebral discs. The muscle bundles run inferiorly, parallel to the lumbar vertebrae, reaching the iliac fossa where they merge with the bundles of the iliacus. The psoas major and iliacus bundles converge deep to the inguinal ligament and insert via a robust tendon onto the lesser trochanter of the femur. Typically, the right psoas major is larger than the left.

The psoas minor lies anterior to the psoas major, originating from the last thoracic vertebra and the 1st lumbar vertebra. This muscle is present in 60% to 65% of the population. Distally, the psoas minor converges with the iliac fascia and the psoas major tendon, inserting onto the iliopectineal eminence in approximately 90% of individuals. The psoas minor contributes to the action of the IPMU.

The iliacus has a fan-shaped morphology and originates from the upper 2/3 of the iliac fossa and the lateral aspects of the sacral wing. Iliacus bundles pass deep to the inguinal ligament anterior to the hip joint together with psoas major fibers. The muscle bundles merge with the psoas major tendon and attach to the lesser trochanter. Small lateral fibers (infratrochanteric portion) extend along the iliac bone, increasing the contact surface.

The iliopectineal bursa is located beneath the IPMU tendon. This bursa separates the tendon from the bone surface of the lesser trochanter and the proximal femur.[1]

Fascia 

The IPMU has extensive anatomical relationships through the surrounding fascial system. Superiorly, the IPMU fascia merges with the chest fascia (endothoracic fascia) and posteriorly with the arcuate ligament of the diaphragm. Anteriorly, the IPMU fascia merges with the fascia covering the kidneys, pancreas, descending aorta, inferior vena cava, ascending and descending colon, duodenum, and cecum. Inferiorly, the fascia merges with the fascia lata of the thigh and the pelvic floor fascia. Fusion with the latter links the psoas fascia to the fascial system of the abdominal muscles. Posteriorly, the IPMU fascia merges with the fascia of the quadratus lumborum.[2]

Function

The IPMU is the primary flexor of the thigh and contributes to adduction and external rotation of the coxofemoral joint. With the lower limb fixed, contraction of the iliopsoas produces trunk flexion and ipsilateral inclination.

The muscles can act independently. The iliacus stabilizes the pelvis and facilitates effective hip flexion during running. The psoas major stabilizes the lumbar spine during sitting and assists with thigh flexion in the supine and standing positions. The psoas major also stabilizes the femoral head within the acetabulum during the initial 15° of movement. The psoas minor contributes to trunk flexion and can tension the iliac fascia.

Embryology

Detailed studies on the embryological development of the muscles comprising the IPMU are limited. The IPMU derives from the paraxial mesoderm. The musculature is already formed by 8 weeks of gestation, at a length of 3 cm.[3] The tendinous component uniting the 3 muscles of the IPMU likely develops within the structure that will become the inguinal ligament.[4]

Blood Supply and Lymphatics

The common iliac artery gives off small branches to the psoas major and minor, while the primary blood supply to the IPMU arises from the external iliac artery, a branch of the common iliac, which sends vascular branches to the iliacus along its course. The external iliac vein drains blood from the psoas major, psoas minor, and iliacus and continues as the femoral vein (see Image. Superficial and Deep Structures of the Femoral Region).[5] The lymphatic drainage of the IPMU occurs via the external iliac lymphatic plexus, which subsequently merges into the common iliac plexus.[6]

Nerves

The short collateral branches of the lumbar plexus (L1–L3) innervate the psoas major and minor. The iliacus is innervated by the femoral nerve, the terminal branch of the lumbar plexus (L1–L4).

Muscles

In a human model, psoas major muscle fibers consist primarily of anaerobic or type IIA fibers (approximately 60%), with the remainder composed of aerobic or type I fibers (approximately 40%). Fiber distribution varies along the muscle. Red fibers predominate at the lumbar origin, whereas white fibers predominate near the hip joint. The lumbar portion performs primarily static functions related to posture, while the hip portion executes more dynamic movements.[7]

In an animal model, the psoas minor contains aerobic and anaerobic fibers in roughly equal proportions.[8] The iliacus predominantly contains anaerobic or type IIB fibers, with the proportion of these fibers increasing toward the hip joint.[9]

Physiologic Variants

Numerous anatomical variations of the IPMU are described in scientific literature. In a study, the iliopsoas tendon, formed by the psoas major and iliacus, presented as a bifid tendon unilaterally in approximately 26% of cases.[10] Bilateral bifid tendons are rare, and cases with 3 tendons have also been reported.[11] The anomalous tendon is a potential site of pain and hip joint dysfunction.

The psoas major may contain accessory muscle bodies, typically on the left side, innervated by lumbar branches. Additional fascicles may be present, although the muscle remains a single entity. The tendon may exist separately from the iliacus or connect with the quadratus lumborum.[12] The psoas minor is bilaterally absent in roughly 40% of the population.

Anatomical variants of the iliacus have been described. A supernumerary iliacus may arise on the left side from the middle 1/3 of the iliac crest, separate from the main iliacus muscle, consistently innervated by the femoral nerve, and inserting with the tendon of the IPMU.[13] An accessory muscle may originate from the iliolumbar ligament, course anterior to the iliacus and lateral to the psoas major, and merge into the fibers of the IPMU. This accessory muscle, termed "iliacus minimus" or "iliocapsularis," is innervated by the femoral nerve.[14] The muscle insertion can involve the proximal 1/3 of the femur or the iliofemoral ligament.

Innervation of the IPMU also exhibits variability. The lumbar plexus and femoral nerve typically pass posterior to the psoas major. In some cases, the lumbar plexus penetrates the psoas major, producing classic nerve entrapment symptoms.[15] The femoral nerve may traverse the iliacus.[16] An accessory psoas major can split the femoral nerve, which subsequently resumes its normal course deep to the inguinal ligament.[17]

Surgical Considerations

Psoas Abscesses

Abscess formation within the psoas major is rare but requires prompt aspiration. Etiologies include invasion from adjacent visceral or lymphatic structures, direct trauma with hematoma, and predisposing conditions such as diabetes, intravenous drug use, renal failure, and HIV infection. Clinical presentation may include back pain, inguinal pain, hip pain, and fever, with a higher incidence in men. Common infectious agents include Staphylococcus aureus, streptococci, and Escherichia coli. Accurate diagnosis relies on computed tomography (CT) and blood tests. Management involves abscess drainage, debridement of necrotic tissue when necessary, and targeted antibiotic therapy.[18]

Impingement of the Iliopsoas Tendon

Following femoral head replacement, movement of the artificial head during hip extension can irritate the surrounding soft tissues, including the IPMU tendon. Surgical review of the space between the femoral head and the tendon may be necessary in such cases.[19]

Complete replacement of the coxofemoral joint may result in iliopsoas impingement, causing pain and functional limitation in approximately 4.3% of procedures.[20] Contributing factors include malpositioned prostheses, protruding screws, or cement debris. Excessive anteversion of the total hip acetabular component can produce anterior hip pain and patient-reported audible "snapping" or "popping" during hip movement from flexion to extension.[21]

Arthroscopic tendon release is generally well tolerated with few adverse effects.[22] Impingement may also arise from tendon abnormalities or morphological changes at the upper lip of the hip joint capsule, causing pain and reduced function. Conservative management is typically preferred, with arthroscopic tenotomy reserved for refractory cases after 3 months of unsuccessful treatment.[23]

Iliopsoas Bursitis

Bursitis affecting the IPMU tendon is characterized by inflammation that enlarges the bursa and produces pain during movement. The condition is multifactorial and not fully understood, but possible etiologies include trauma, inflammatory arthritis, repetitive tendon use in sports, or prior hip surgery. Diagnosis relies on ultrasonography. Treatment involves aspiration of excess fluid, often combined with corticosteroid and anesthetic injection to reduce inflammation and pain.[24]

Other Surgical Considerations

Distal tenotomy is indicated for spasticity due to neurological disorders in the pediatric population when significant contractures impair walking, as seen in cerebral palsy. This procedure improves posture and facilitates greater independence. Surgical excision of IPMU tumors is rare due to their deep pelvic location.

Clinical Significance

The iliopsoas lies adjacent to multiple viscera, and certain pathologies involving the muscle can mimic internal organ disease. Lesions of the right muscle may produce pain in the right lower quadrant of the abdomen, resembling appendicitis. Muscle hypertrophy can obstruct the normal passage of colonic fecal material. Such hypertrophy may compress the femoral nerve and the lateral cutaneous nerve of the thigh at the lacuna musculorum and the genitofemoral nerve at the lacuna vasculorum, deep to the inguinal ligament.

Diagnosing specific dysfunctions of the iliopsoas complex remains challenging. The following sections outline evaluation procedures designed to identify recurrent functional impairments.

Iliopsoas Hematoma

A hematoma of the iliopsoas can result from direct trauma or occur spontaneously. Spontaneous hematomas may arise from hemophilia, liver disease (cirrhosis), or errors in coagulation management, such as after cardiac valve interventions. Minor trauma, improperly repaired muscle fibers following physical activity, or rupture of small vessels may also cause hematoma formation.

Clinical signs include sudden back pain unrelated to strain and flank pain on the affected side. Massive retroperitoneal hematomas may produce a Grey Turner sign—ecchymosis visible on the flank. Compression of the femoral nerve can result in thigh involvement and muscular hypoesthesia of the leg. Pain is often relieved by assuming a flexed-leg position while lying down. Surgical intervention is reserved for life-threatening complications, including abdominal compartment syndrome, hypovolemia, or femoral nerve compromise.[25]

Iliopsoas Snapping

Iliopsoas snapping, or coxa saltans, is a dysfunction of the iliopsoas complex that produces audible and palpable noise during active hip movements, with or without pain. Three types of iliopsoas snapping exist: external, internal, and intra-articular.

The external type results from the iliotibial band and the gluteus maximus tendon moving over the greater trochanter, producing noise and pain. The internal type involves the iliopsoas tendon sliding over the anterior capsule of the femoral head, the iliopectineal eminence (muscle pulley), or exostoses of the lesser trochanter. The intra-articular type arises from intra-articular bodies, labral tears, synovial chondromatosis, or a chronically subluxated hip, as seen in dancers or children. Other contributing factors include soft tissue abnormalities, an accessory strip of the psoas major, paralabral cysts, tendon inflammation, bursitis, or movement of the iliacus tendon against the pubic ramus. Approximately 50% of iliopsoas snapping cases follow hip trauma.

A general palpatory inspection is performed after an accurate history to identify anomalies. The active iliopsoas snapping test follows. From the supine Fabere test position, the operator places a hand on the inguinal region while the patient returns the lower limb to a neutral position (abduction, external rotation, extension, and rest). Palpation may reveal or reproduce the characteristic noise, often accompanied by pain at 30° to 45° of flexion.

For differential diagnosis, the bicycle test evaluates external-type iliopsoas snapping. The patient lies in lateral decubitus, flexing and actively extending the leg of the nonsupporting side, allowing identification of abnormal sliding or noise over the greater trochanter. The FADIR (flexion, adduction, and internal rotation) test assesses intra-articular iliopsoas snapping. The supine patient actively performs flexion, internal rotation, and adduction to detect lesions, such as labral tears.[26]

Instrumental examinations confirm the diagnosis. Radiographs evaluate bone alterations. Ultrasound assesses tendon movement and inflammation, while magnetic resonance imaging (MRI) identifies intra-articular lesions. Therapy is primarily conservative.

Iliacus Hematoma

Hematoma of the iliacus is an infrequent event. As with the psoas major, a hematoma can result from disorders of blood coagulation, either hereditary or acquired. Trauma or surgery may also cause hematoma formation. Common symptoms include groin and hip pain, abdominal pain, and quadriceps weakness. Severe cases may present with tachycardia, hypovolemia, and decreased hematocrit values. Extensive hemorrhage can rarely cause femoral nerve neuropathy.[27] Abdominal CT is recommended to determine and confirm the diagnosis. Small hematomas without neurological symptoms may be managed conservatively. A hematoma may result in femoral nerve paralysis in severe cases related to trauma or surgery.[28][29]

Facioscapulohumeral Muscular Dystrophy

MRI in patients with facioscapulohumeral muscular dystrophy demonstrates that the iliopsoas is less affected in 67% of cases, showing slower strength loss over time. Muscle tone and size are generally preserved, but functional use is impaired due to reduced neuromuscular coordination, particularly during walking. Similar patterns of preservation with functional limitation are observed in other muscular dystrophies, such as dystrophinopathies.

Groin Pain

Groin pain may result from iliopsoas tendinitis, arising from previous surgery or functional overload. Endoscopic iliopsoas tenotomy is generally indicated following surgery, while reducing muscle workload is recommended in cases of functional overload.[30][31]

Recent studies have identified an additional cause of hip or inguinal pain in athletes. Ultrasound evaluation revealed fascial hypertrophy in the medial fibers of the iliacus, impairing iliopsoas function, causing stiffness, and negatively affecting joint mechanics. Improvement occurred with stretching and high-energy laser therapy.[32]

A rare cause of groin pain is spontaneous rupture of the iliopsoas tendon. Diagnosis requires imaging tools, such as MRI or CT, in addition to assessment of movement-related pain. Conservative management may be effective.[33]

Other Issues

Conservative therapy for iliopsoas snapping relies primarily on physiotherapeutic interventions. Rest and anti-inflammatory drugs are the cornerstones of therapy in the acute phase. Once approved by the treating clinician, physiotherapy includes exercises, such as stretching of the iliopsoas complex, active and passive hip joint mobilization, and activities to improve lumbar lordosis posture. Muscle strengthening is indicated for weakened muscles, not only the psoas major but also all muscles involved in hip, lower limb, and lumbar movement. Mirror-assisted exercises improve neuromotor control. Following a structured rehabilitation program, motor function typically recovers without pain within 6 to 12 months.[34][35]

The iliopsoas may compress the femoral nerve, producing knee pain. Stretching exercises to reduce muscle tension should precede surgical intervention. Surgery is generally avoidable if physiotherapy protocols are followed. Alternative approaches include acupuncture, which has demonstrated efficacy, and osteopathic manual therapy, which supports physiotherapy, particularly postsurgically, by reducing pain and addressing deep muscle–joint areas with soft techniques and minimal side effects.[36]

The hip-external rotation-flexion-ceiling (HEC) test is a novel, noninvasive procedure designed to identify iliopsoas tendon inflammation (tendinopathy). The test involves 2 movements: hip flexion and external rotation. The patient lies supine while the operator stands beside the table. The operator flexes the hip, keeping the foot in contact with the table surface, until the thigh is fully elevated. Pain at the muscle insertion site indicates a positive test result.[37]

The iliopsoas can contribute to nonspecific low back pain. MRI demonstrates a direct relationship between reduced muscle volume and pain, likely due to altered hip arthrokinematics and impaired muscle strength management.[38] Trigger points in the muscle represent another source of pain.[39][40] Symptom relief may be achieved through dry needling performed by trained personnel.[41]

Pelvic organ prolapse (POP) can affect women after pregnancy. Weakness and decreased myofibrillar density of the iliopsoas, a change resembling ageing, may contribute to prolapse. No definitive preventive treatment for this condition is currently known.[42]

A pilot study demonstrated that in patients with an unstable ankle, the iliopsoas on the side of the weak ankle shows reduced activity at the end of inspiration compared with the contralateral side. This difference does not occur during other phases of respiration. The phenomenon appears related to subclinical diaphragm weakness. Neurophysiological mechanisms underlying this adaptation remain unclear. Diaphragm strengthening could induce a favorable adjustment in iliopsoas activity and support ankle stability.[43]

Animal studies indicate that the iliopsoas increases muscle tone to help produce body heat under hypothermic conditions. Further research is required to determine the clinical relevance of this mechanism, particularly in older adults or individuals with iliopsoas dysfunction, and to explore potential applications.[44]

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Disclosure: Bruno Bordoni declares no relevant financial relationships with ineligible companies.

Disclosure: Matthew Varacallo declares no relevant financial relationships with ineligible companies.