MR imaging sheds light on diverse range of injuries in upper extremity

Patients with compressive or entrapment neuropathies of the elbow, forearm, wrist, or hand may go straight to sonographic examination. In skilled hands, ultrasound can produce images that reveal pathology as well as MR images can. But while the diagnosis of a tendon rupture is a relatively simple matter with ultrasound, to assess specific neurological injuries, such as nerve entrapment and compression, the technique requires considerable experience, expertise, and patience, said Dr. Javier Beltran of the Maimonides Medical Center in Brooklyn, NY.

Radiologists consequently are turning to MRI, which can exquisitely visualize soft-tissue involvement of compression neuropathies in the upper extremity and cover a wide swathe of the anatomic area that is being investigated. MRI is typically used to detect external causes of compression to a nerve, identify patterns of muscle denervation, and exclude other pathologies that may mimic compressive and entrapment neuropathies.

Evaluation of patients with MRI nevertheless requires a thorough knowledge of anatomy and the manifestations of pathology on imaging studies, because findings may be easily overlooked.

"That's why it's important to have a heads-up impression of what you are going to look for so that even when you have the appropriate clinical information, you can focus more on these little nerves that show subtle changes of signal intensity. You have to be prepared to understand the clinical manifestations of these entities and understand the relationship between what is given from the clinical standpoint and what the radiologist needs to look for in the MRI exams," Beltran said.

For example, in an entrapment neuropathy, a nerve that runs along the arm may be entrapped at different points along its course in a normally tight tunnel or space. But clinical manifestations of a sensory or motor deficit are the same whether the entrapment is high up in the elbow or distal in the wrist.

"The radiologist has to be familiar with the anatomical areas where the nerve is typically compressed," he said. "The radiologist also has to be aware that many compression neuropathies are produced by normal variants, such as tight spaces or fibrosis tunnels."

Carpal tunnel syndrome is such a frequent and clear-cut problem that most patients are sent to surgery based only on clinical or electromyographic findings. MRI has a place in the workup of even these patients, when the clinical diagnosis is inconclusive, symptoms are severe, and a tumor, aberrant muscle involvement, or infiltrative process may be present and the surgeon needs to consider whether to extend the scope of surgery or further explore the involved area. In an MRI examination of a patient with carpal tunnel syndrome, radiologists typically look for enlargement of the median nerve proximal to the tunnel, high signal intensity on T2-weighted images, obliteration of the deep fat plane, and bowing of the retinaculum.

Although x-rays are the primary means for investigating possible bone trauma or dislocations of the wrist or elbow, MRI is the imaging method of choice when there is a strong clinical impression of a fracture that radiographs are not detecting. MRI also may show bone bruise, which is not visible on plain x-rays, as the cause of an increased amount of joint fluid, said Dr. Martina Lohman of the Orton Orthopedic Hospital of the Invalid Foundation in Helsinki, Finland.

In general, MRI evaluates chondral defects and loose bodies around the elbow, stress fractures and bone marrow edema at the fracture site, tendon degeneration in tendinosis as well as ligament injuries in posterolateral rotatory instability, and articular erosion in radiocapitellar chondromalacia.

To facilitate interpretation of MR images, the choice of the imaging plane is crucial, according to Lohman. Studies have shown that oblique coronal views are indicated for assessing the ulnar and radial collateral ligaments as well as the ligaments and cartilage of the wrist. Lohman recommends using fluid-sensitive sequences, such as T2-weighted fat suppression or short T1- inversion recovery (STIR). These sequences act like a natural contrast agent in the joint fluid, she said.