Iliopsoas musculotendinous unit

Iliopsoas musculotendinous unit comprises two muscles, the Iliacus and Psoas major muscles.

Psoas major

The psoas major muscle originates from the transverse processes of T12 and all the lumbar vertebrae. There are 5 slips that arise from the vertebral bodies and the intervertebral disks. A series of tendinous arches extend across the narrow parts of the bodies of the lumbar vertebrae between the described digitations.¹

Fig. 1: Coronal T2 weighted SPACE image, demonstrating the origin of the psoas major muscle and the slips arising from the lateral lumbar vertebral bodies and intervertebral disks (red arrows). Tendinous arches between the slips (blue arrows). Psoas major (green asterisk), Iliacus muscle (pink asterisk). Anatomical Specimen. Note psoas major and iliacus muscles.
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell and Anatomy Lab at Donald and Barbara Zucker School of Medicine

Four upper lumbar intervertebral foramina lie posterior to the attachment sites of the psoas major muscle. The lumbar nerves course through the psoas major muscle with the lumbar plexus partially embedded in the substance of the psoas major. The lumbar arteries and veins and the sympathetic trunk pass underneath the tendinous arches.¹

Fig. 2: Axial T2 weighted SPACE image. Note the right psoas major muscle and the lumbar nerve root exiting the intervetbral foramen (red arrow) and coursing toward the substance of the muscle (yellow arrow).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

The muscle courses inferiorly along the pelvic brim. It receives fibers from the iliacus muscle on the lateral side and continues posterior to the inguinal ligament and anterior to the hip joint capsule. The Psoas muscle inserts on the to the anteromedial surface of the lesser trochanter.¹ 

Vascular supply:Multiple arteries. Iliolumbar artery is the main artery to the muscle.

Innervation: Lumbar spinal nerves, mainly L1 and 2 with some contribution from L3.¹

Iliacus 

The iliacus muscle is a triangular muscle that arises from the superior two-thirds of the concavity of the iliac fossa and the inner lip of the iliac crest and upper surface of the lateral part of the sacrum.¹

It receives some fibers from the superior capsule of the hip joint. Most of its fibers join the lateral side of the psoas major tendon and muscle then insert together into the lesser trochanter. Iliacus is attached to the medial or anterior surface of the base of the lesser trochanter. Some fibers are attached directly to the femur, inferior and anterior to the lesser trochanter.¹

Vascular supply: Iliac branches of the iliolumbar artery.

Innervation: Branches of the femoral nerve, L2 and L3.

The iliopsoas unit passes anterior to the pelvic brim and hip capsule in a groove between the anterior inferior iliac spine laterally and iliopectineal eminence medially. The musculotendinous junction is consistently found at the level of this groove.

Iliopsoas Tendon

The psoas major tendon rotates through its course, with the ventral surface becoming the medial surface distally. The iliac part of the IP tendon is more lateral relative to the psoas part of the tendon.

The lateral portion of the iliacus muscle attaches directly on the anterior part of the proximal femoral diaphysis. There is a thin intramuscular tendon within the lateral iliacus muscle that is separated from the iliopsoas tendon by a cleft of fatty fascia that is seen as a line of increased T1 signal on MRI.²

Fig. 3: Sagittal oblique proton density image of the left hip demonstrates the iliopsoas tendon (red arrow) at its insertion on the lesser trochanter (blue arrowhead). Note the muscular insertion of the iliacus, anterior and slightly inferior to the lesser trochanter (green asterisk).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Fig. 4: Coronal proton density image of the left hip demonstrates the distal iliopsoas tendon (red arrow) and a cleft of fat signal (yellow arrow) between the iliopsoas tendon and medial aspect the iliacus muscle (blue arrow).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Psoas Minor

The psoas minor is present in 50% of individuals.  Psoas minor lies anterior to psoas major. It arises from the bodies of the T12 and L1 vertebrae, and from the intervertebral disc between them.

It attaches to the iliopectineal line, iliopubic ramus and, laterally, to the iliac fascia.¹

Vascular supply: Lumbar arteries.

Innervation: A branch from L1.

Fig. 5: Anatomical specimen. Note the psoas minor (red arrow) along the lateral margin of the psoas major muscle (yellow arrow).The iliopsoas tendon (blue arrow) courses inferiorly to insert on the lesser trochanter of the femur.
References: Anatomy Lab at Donald and Barbara Zucker School of Medicine

Iliopsoas Bursa

The large subtendinous iliopsoas bursa, communicates with the hip joint in 15% of individuals. The IP bursa is the largest bursa in the body.

Fig. 6: Left hip arthrogram demonstrating communication of the iliopsoas bursa with contrast within the bursa (red arrow) and the hip joint (yellow arrowheads).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

IP tendinopathy and tendon tear

IP tendinosis can occur secondary to repetitive chronic injury or acute traumatic injury.  It is usually asymptomatic in early stages. On MR, there is thickening or attenuation of the tendon with associated increased PD signal with or without edema.

ON US, there is diffuse heterogeneous hypoechoic thickening of the tendon. Recommended sonographic technique by Balius et al. using the FABER maneuver to demonstrate a long view of the distal IP tendon to its insertion on the lesser trochanter.

IP tendon tear is seen in activities with repetitive hip flexion, such as soccer, gymnastics and hockey. Partial IP tendon tear is more common secondary to sport injuries and fall. However, full-thickness tear of IP tendon is seen more in elderly patients with degeneration of the tendon. 

Fig. 7: Axial, sagittal and coronal fat-suppressed proton density-weighted images of the left hip demonstrate avulsion of iliopsoas tendon(red arrows) with Iliacus (pink arrow) and psoas major (red arrow) components of the IP tendon retracted to the level of the femoral head, with associated marked edema/hemorrhage surrounding the tendon (light blue asterisk). Rectus femoris tendon (blue arrowhead).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Iliopsoas bursitis

Fluid distention of the IP bursa may be secondary to overuse, inflammatory arthropathy or hip joint pathology. The IP tendon does not have a tendon sheath, instead is surrounded by a layer of connective tissue or paratenon. Edema of paratenon or paratenonitis can have a similar appearance to the IP bursitis. 

Fig. 8: Axial, sagittal and coronal fat-suppressed proton density-weighted images of the left hip demonstrate fluid (green asterisk) within the iliopsoas bursa consistent with IP bursitis. Note the iliopsoas tendon (red arrow) deep to the distended IP bursa.
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Iliopsoas strain

IP muscle strain is more common at the myotendinous junction. It may or may not be asccoated with tendon tear.

Fig. 9: Iliopsoas muscle strain in a 25-year-old male from a hockey injury one week earlier. Sagittal and axial fat-suppressed proton density-weighted images of the left hip demonstrate feathery pattern of edema (green arrowheads) at medial aspect of the iliopsoas musculotendinous unit consistent with muscle strain. Note iliopsoas tendon (red arrow) and thin intramuscular portion of the iliacus tendon laterally (yellow arrow).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Snapping Iliopsoas tendon

Snapping IP tendon is one of a few causes of an extraarticular snapping hip syndrome with the IP tendon snapping over the iliopectineal eminence. It is more common in young athletes engaged in sport activities involving the extreme hip movements. The audible snapping is usually reproducible during certain hip movements, most commonly when the hip transitions from frogleg to neutral position.³

Fig. 10: 20 year old female with painful snapping sound during hip extension from a flexed position. Axial fat suppressed T2 weighted image of the right hip demonstrates edema surrounding the iliopsoas tendon (red arrow) at the level of iliopectineal eminence (green arrow). Note anterior acetabular labrum (blue arrow). History and findings suggestive of iliopsoas snapping hip.
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Fig. 11: Transverse cine clip of the iliopsoas tendon at the level of the acetabular rim demonstrating an abnormal movement of the IP tendon over the iliopectineal eminence as the patient extends his hip from frog-leg position to neutral position.
References: Cine clip reproduced by kind permission of Dr. Jon Jacobson, University of Michigan

Iliopsoas impingment

IP impingement is characterized by anterior labral tear at 3 O’clock position where the IP tendon crosses over the labrum. On MR, there is a tear of the anterior labrum which does not extend superiorly, other findings such as thickening of the IP tendon, focal edema adjacent to the anterior labrum and focal bursitis can also be seen.⁴

Fig. 12: 35 year-old female with anterior hip pain and positive impingement test. Sagittal T1 weighted MR arthrogram and axial proton density fat suppressed image of the right hip demonstrate an anterior labral tear (yellow arrow) at 3 o’clock position underneath the iliopsoas tendon (red arrow), with associated mild peritendinous edema (blue arrow).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Post arthroplasty

Iliopsoas bursitis and Iliopsoas impingement

IP bursitis and IP impingement are uncommon complications of total hip arthroplasty.  The proposed etiology is altered center of rotation of the acetabular component of the arthroplasty which can be secondary to an oversized or lateral positioning of the acetabular cup. IP bursitis is characterized as fluid distention of the IP bursa.⁵

Fig. 13: Axial and coronal proton density weighted images of the left hip with metal suppression demonstrate fluid distention (green arrowheads) of the iliopsoas bursa consistent with iliopsoas bursitis. Susceptibility artifact is noted arising from the femoral component of the arthroplasty. Note the Iliopsoas tendon (red arrow).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Treatment options and Image guided Interventions

Treatment options for iliopsoas tendinopathy, IP bursitis    and  the painful, snapping iliopsoas tendon include conservative management with  rest, ice, analgesics, physical therapy, injection of corticosteroids into the iliopsoas bursa, and surgical management including  release of the iliopsoas tendon.^6,7

Image-guided iliopsoas bursal  injections can provide relief to most patients with iliopsoas tendinosis/bursitis after hip replacement, however the results of injection are not as successful in cases of idiopathic iliopsoas tendinosis/bursitis.⁸

Ultrasound guided iliopsoas bursa injection of steroid and anesthetic

Ultrasound guided iliopsoas bursa injection is performed with the patient in supine position. Different techniques have been described for IP bursa injection with the final goal being distention of the IP bursa.

Depending on the patient body habitus a linear or curvilinear transducer can be used. The skin over the iliopectineal eminence of the acetabulum is marked, where the IP tendon courses anterior to the femoral head/joint capsule.

The patient’s hip is prepped and draped using sterile technique. After administration of local anesthesia, using a lateral approach, the needle is advanced underneath the IP tendon and between the IP tendon and hip joint capsule, while observing the  IP bursal distention.⁸

Fig. 14: Axial oblique ultrasound images at the level of the acetabular rim (green arrowhead) and femoral head (yellow arrow) demonstrate the needle trajectory (green arrow) from the lateral approach traversing the IP musculotendinous unit (blue arrow) with the tip of the needle deep to the IP tendon (red arrow). Note femoral vascular structures medially (yellow asterisk).
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Fluoroscopic iliopsoas bursa injection of  steroid and anesthetic

The femoral vascular structures are palpated. The skin over the center of the superior acetabulum is marked. The patient’s hip is prepped and draped using sterile technique. After administration of local anesthesia, using a vertical technique the needle is advanced until it reaches the acetabulum. Needle positing is conformed using a small dose of contrast. Once placement is confirmed, a mixture of steroid and anesthetic is injected into the IP bursa.⁹

Fig. 15: Radiograph of the right hip demonstrating the needle tip over the center of the acetabular rim (red arrow) and contrast filling the iliopsoas bursa (green arrowheads) in this patient status post total hip arthroplasty.
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Following the procedure, the patient is monitored for 30 minutes to evaluate for leg weakness secondary to transient femoral nerve palsy which is a potential immediate complication of this procedure.

Differential Considerations:

Femoral neck stress injuries

Fig. 16: Coronal T1 weighted and coronal STIR images of the right hip demonstrate moderate bone marrow edema (red asterisk) along the compressive side of femoral neck with associated mild periosteal edema (red arrow) consistent with a stress injury.
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Femoroacetabular impingement

Fig. 17: Oblique axial fat-suppressed proton density image of the right hip demonstrates a convex bump (red arrow) at the anterior femoral head-neck junction consistent with CAM type morphology.
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell

Ischiofemoral impingment 

Fig. 18: Axial STIR image of the left hip demonstrates a narrow ischiofemoral interval between the ischial tuberosity (green asterisk) and the lesser trochanter (yellow asterisk) with associated moderate edema of the quadratus femoris muscle.
References: Department of Radiology, Donald and Barbara Zucker School of Medicine at Hofstra/Northwell