MR imaging illuminates elbow joint disorders

MRI has become the secondary procedure of choice, after plain-film x-ray, for evaluating elbow abnormalities. The modality can visualize ligament and tendon injuries, compressive or entrapment neuropathy, bone injuries, inflammatory and synovial conditions, and soft-tissue masses. Congenital abnormalities in the elbow are rare. The most common is radial head dislocation, which may affect one or both elbows. Another congenital anomaly is synostosis, or osseous fusion of tubular bones between the proximal portions of the radius and ulna. Fusion may be complete or fibrous, and it may limit forearm supination. Synostosis is often bilateral and may be accompanied by other anomalies.2 MRI is rarely used in such cases, but it may be applied to evaluate the relation between soft tissues and osseous structures. Infections to the elbow are most often due to Staphylococcus aureus. Osteomyelitis, for example, results from hematogenous spread of bacteria or bacterial spread from surrounding tissues. The thinness of the soft-tissue layer covering the elbow means that osteomyelitis is commonly caused by direct inoculation from an open fracture or surgery.3 Other causes of elbow infection include septic arthritis, septic olecranon bursitis, and pyomyositis. Infection may appear as areas of cortical interruption or periostitis. Its extension into surrounding soft tissues may result in cellulitis, abscess, phlegmon, and sinus tract formation.4 MRI is useful for diagnosing or excluding osteomyelitis and evaluating its extent. It can also help determine the location and size of abscesses and identify areas of phlegmonous or necrotic tissue that may need surgical management.5 Bone tumors are not common in the elbow, and those that do occur are more often benign than malignant.6 It is extremely important to obtain plain-film x-rays prior to MRI so that suspected tumors can be diagnosed and characterized. MRI is indicated whenever the tumor appears aggressive, has a soft-tissue component, is not clearly seen on x-ray, or requires further anatomic delineation. Intravenous contrast is often helpful but not always necessary. Some benign osseous tumors (e.g., osteoid osteoma, osteochondroma, and enchondroma) have a characteristic appearance on MRI. Other benign tumors found relatively frequently in the elbow are chondroblastoma, fibrous dysplasia (Figure 1), giant cell tumor, aneurysmal bone cyst, and unicameral bone cyst.6 Primary malignant osseous tumors are very rare in the elbow. Plain-film x-ray should always be obtained first. MRI is particularly useful for defining tumor extent and also helps in tumor staging, which can affect prognosis and therapy. Many malignant elbow tumors are metastases, though this is not a common site for them. Primary cancers that frequently metastasize to bone include melanoma and lung, breast, and kidney cancers. MRI is not generally indicated for evaluating a known elbow fracture. Unsuspected fracture or contusion, however, may be demonstrated in patients with elbow pain following trauma. Osteochondritis dissecans of the elbow usually affects the capitellum. This condition, which often occurs in adolescents and may be asymptomatic in its early stages, can be related to repetitive trauma.7 Osteochondritis dissecans primarily affects subchondral bone. The underlying articular cartilage surface is initially preserved, but necrosis and softening of subchondral bone secondary to fracture may cause the articular cartilage to separate. This injury eventually leads to osteoarthritis in more than 50% of patients.