MRI of the Hip - Clinical Tree

1 What is the basic normal anatomy of the hip?

The hip is a ball-and-socket joint formed by the femoral head and the acetabulum, providing a large range of multidirectional motion. The acetabulum is tilted anteriorly, allowing a greater degree of flexion than extension. It covers 40% of the femoral head, and its depth is increased by the acetabular labrum, a horseshoe-shaped fibrocartilagenous structure covering the rim of the posterior, superior, and anterior acetabulum. The proximal femur consists of the head, neck, and lesser and greater trochanters. The mostly spherical femoral head, except for the fovea (an indentation in the medial femoral head), is covered by the articular cartilage, which extends over the epiphyseal part of the head to the head-neck junction. The greater femoral trochanter serves as the site of attachment for the hip abductors and lateral rotators (gluteus medius and minimus, obturator internus and externus, and piriformis muscles). The iliopsoas tendon inserts on the lesser trochanter.

2 What are the indications for magnetic resonance imaging (MRI) of the hip?

Hip MRI is the examination of choice when radiography fails to reveal the cause of hip pain, either traumatic or nontraumatic. Following trauma, some of the common causes of hip pain in patients with normal radiographs are occult proximal femoral (for example, neck or intertrochanteric) fractures, bone contusions, stress or insufficiency fractures, or soft tissue injuries such as labral tears, muscle strains, ligament sprains, and tendon tears. Nontraumatic causes of hip pain include avascular necrosis (AVN), transient osteoporosis, impingement syndromes, bone or soft tissue neoplasms, infection (osteomyelitis, septic arthritis, abscess), and inflammation (bursitis, synovitis, inflammatory arthritis). Most of these entities are typically occult on both radiographs and on computed tomography (CT), whereas MRI is much more sensitive and specific.

3 What is the proper MRI technique used for the evaluation of hip pain?

The exact MR imaging sequence combination depends on the scanner, clinical indication, patient factors, and radiologists' preferences. In the setting of trauma, the entire pelvis and both hips should be included in at least one (coronal and/or axial) plane. For evaluating joint pathology, such as labral tears or cartilage abnormalities, a smaller field of view with a lower slice thickness producing higher spatial resolution images should be used. Both T1-weighted (for anatomic detail) and fluid-sensitive sequences such as fat-suppressed T2-weighted, fat-suppressed proton density (PD)-weighted, or short tau inversion recovery (STIR) images (for detection of bone marrow edema) are necessary.

4 When should intravenous contrast material be administered for hip MRI?

In general, intravenous contrast material is not needed, except to differentiate solid from cystic soft tissue masses or to localize an abscess prior to drainage. Occasionally, it is used in cases of osteonecrosis to differentiate enhancing viable from nonenhancing necrotic bone. Intraarticular contrast for MR arthrography is very useful to assess for presence of labral tears.

5 What is the significance of the bone marrow edema pattern in the proximal femur?

Bone marrow edema, manifested on MRI as hypointensity on T1-weighted images and hyperintensity on fluid-sensitive images, is a nonspecific finding observed in a variety of disease entities, including osteonecrosis, idiopathic transient osteoporosis of the hip (ITOH), transient bone marrow edema syndrome, and infiltrative neoplasms. Somewhat more characteristic and specific patterns of bone marrow edema can be seen in association with fractures, stress injuries, and infectious and inflammatory diseases.

6 What is AVN of the hip?

AVN is a form of osteonecrosis that occurs in the epiphyseal or subchondral region, and in the hip typically involves the anterolateral aspect of the femoral head. Unilateral AVN can be a result of trauma, such as a displaced femoral neck fracture, while bilateral AVN occurs due to nontraumatic causes seen in a large variety of clinical conditions, the more common of which include hemoglobinopathies, corticosteroid use, metabolic and endocrine diseases, and alcoholism. The presumed mechanism is failure to repair necrotic fractured trabeculae.

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