While most congenital differences of the upper extremity are assessed with plain radiographs, MRI is a useful adjunct in some instances. MRI is particularly useful in the pediatric population as the skeletal structures may still be cartilaginous and are therefore much better seen with MRI as opposed to computed tomography (CT) or plain x-ray (Strouse 2008).

An important entity diagnosed easily on MRI is glenohumeral dysplasia, which may occur in isolation or in association with mucopolysaccharidoses, mucolipidoses, or skeletal dysplasias. Injury to the brachial plexus during birth, with resulting paralysis of the shoulder girdle musculature, can also inhibit normal development of the glenoid and humeral head and thus lead to glenohumeral dysplasia (Gudinchet et al. 1995). The glenoid articular surface is deficient, particular posteriorly, and the humeral head is flattened and small. Furthermore, the humeral head is often posteriorly dislocated because of the glenoid deficiency (Fig. 12). With advances in imaging, MR neurography can be used to depict normal and abnormal peripheral nerves (Figs. 13 and 14) (Chhabra et al. 2011).

Osseous injury may result from a single traumatic event or due to repetitive stresses. Although conventional radiographs are the first modality utilized for evaluating osseous trauma, many acute and chronic osseous injuries are not detectable on plain radiographs. MRI is exquisitely sensitive for detection of radiographically occult injuries. A normal MRI essentially excludes the presence of an osseous injury.

Radiographically occult fracture. It is well known that MRI is able to demonstrate radiographically occult fractures in a rapid, cost-effective manner to enable optimal patient management. In such cases, in addition to demonstrating marrow edema, a fracture line is usually identified as a linear or curvilinear focus of low signal intensity on T1W images that may demonstrate low or high signal on STIR images (Fig. 15).

MRI is also useful in the evaluation of fracture complications. For example, in the setting of a prior scaphoid fracture, MRI will show changes of avascular necrosis before plain radiographs (Fig. 16).

Avulsion fracture. Avulsion fractures typically demonstrate abnormal marrow signal at the site of the injury, but the extent of marrow abnormality generally is limited and appears less robust than with impaction injuries.

Repetitive trauma. Stress injuries to bone typically are divided into two categories: insufficiency and fatigue fractures (Daffner and Pavlov 1992). Fatigue fractures result from abnormal stress to normal bone and, therefore, are seen commonly in the pediatric population. Chronic avulsive injuries such as shin splints can be difficult to diagnose on plain radiographs, but can be detected with MRI.

Physeal injury . In a skeletally immature patient, trauma to the physes can result in premature closure of a portion of the physis, which results in differential growth and subsequent deformity. The diagnosis of osseous bridging can be challenging on plain radiographs. MRI with its gradient echo images, which are designed to assess the cartilage, is the gold standard for the assessment of such injuries (Carey et al. 1998).

The physes may be susceptible to chronic repetitive trauma in athletes, most characteristically the distal radius and olecranon in gymnasts and the proximal humerus in baseball pitchers. The characteristic findings on MRI include physeal widening and T2-hyperintensity at the junction of the physis and metaphysis.

Epiphyseal injury . Osteochondritis dissecans (OCD) is fragmentation or separation of a portion of subchondral bone along the articular surface. Two types of OCD have been described: the juvenile form, occurring before physeal closure, and the adult type. Common joints involved include the knee, ankle, and elbow. In the elbow, there is involvement of the capitellum along its convex anterior surface. MRI is used in the evaluation for instability, as well as osseous or cartilaginous loose bodies within the joint. Linear T2-hyperintense signal between the osseous fragment and the parent bone is suggestive of instability (Fig. 17). The capitellum is also susceptible to avascular necrosis, called Panner’s disease. This disease is most commonly seen in children with repeated valgus stress to the elbow, such as baseball players and gymnasts. The entire capitellum is affected, becoming diffusely T1-hypointense on MRI. Children with Panner’s disease are typically younger than those with an osteochondral injury.

Soft tissue injuries. An added advantage of MRI is its ability to demonstrate soft tissue injuries, including injuries to the ligaments, tendons, and cartilage that may mimic a fracture clinically (El-Khoury et al. 1996). MR arthrography refers to distention of the joint with a solution containing dilute gadolinium (contrast agent). It is extremely useful in assessing intra-articular soft tissue injuries in the shoulder and hip (Waldt et al. 2005). A common labral injury seen in the setting of an anterior shoulder dislocation is the Bankart tear (Stoller 1997) (Fig. 18).

Some other uses of MRI in the setting of trauma include ligament tears in the elbow and wrist. Scapholunate ligament and triangular fibrocartilage (TFCC) tears are optimally evaluated for on MRI (Fig. 19).

Musculoskeletal infections can affect bones, soft tissues, and joints. Although MRI diagnosis of osteomyelitis does not require contrast, use of contrast does increase the confidence of the radiologist in providing the diagnosis and aids in the assessment of complications such as abscesses (Averill et al. 2009). The extent of infection and the presence of abscess formation are best assessed with fat-suppressed T2-weighted, STIR, and post-gadolinium fat-suppressed T1-weighted images (Averill et al. 2009).