A magnetic resonance imaging (MRI) scan of the shoulder creates detailed pictures of the joint’s complex internal structures using a strong magnetic field and radio waves. This imaging method is preferred for the shoulder because it offers exceptional soft tissue contrast, which X-rays and even computed tomography (CT) scans cannot provide effectively. An MRI can clearly distinguish between different tissues, such as tendons, muscles, cartilage, and fluid, making it the superior tool for evaluating a wide range of shoulder problems. The shoulder is the most mobile joint in the human body, relying heavily on a delicate balance of soft tissues for both movement and stability.
Visualizing the Rotator Cuff and Major Tendons
The primary function of the shoulder MRI is often to assess the rotator cuff, a group of four muscles and their tendons that surround the shoulder joint. These tendons—the supraspinatus, infraspinatus, teres minor, and subscapularis—are responsible for stabilizing the upper arm bone (humerus) in the socket and facilitating arm rotation. The scan provides a precise view of the location, size, and severity of any tears within these tendons.
The MRI can clearly differentiate between a partial-thickness tear and a full-thickness tear, where the tendon has completely detached from the bone. The scan also reveals signs of chronic tendon issues, including tendinosis (degeneration within the tendon structure) and calcific tendinitis (the presence of calcium deposits).
Beyond tears, MRI highlights inflammation and fluid dynamics that contribute to shoulder pain, such as in cases of impingement syndrome. Impingement often involves swelling or fluid accumulation in the subacromial-subdeltoid bursa. The long head of the biceps tendon, which runs through the joint, is also closely examined for signs of inflammation, fraying, or dislocation from its groove. Chronic, large tears can lead to muscle atrophy and fatty infiltration within the muscle bellies, which the MRI can quantify for treatment planning.
Assessing Joint Stability and Cartilage Structures
The MRI is highly effective at evaluating the structures responsible for keeping the humeral head centered within the glenoid socket, which are crucial for joint stability. This assessment focuses heavily on the glenoid labrum, a rim of fibrocartilage that deepens the socket. Tears to the labrum, such as Bankart lesions or SLAP lesions, are clearly visible.
The capsule and ligaments that encase the joint are scrutinized for damage, which is common following a dislocation. The glenohumeral ligaments can be stretched or torn, and the MRI shows associated soft tissue damage and bone bruising. The MRI also provides a view of the articular cartilage, the smooth tissue lining the ends of the bones, which is often difficult to see clearly on other scans.
Specialized MRI techniques can detect early changes in the cartilage composition, such as loss of proteoglycan content, indicating damage before structural defects are visible. Assessing the integrity of the labrum, ligaments, and cartilage helps determine the cause of shoulder instability and guides decisions regarding surgical stabilization.
Identifying Bone Marrow and Chronic Conditions
A significant advantage of the MRI is its sensitivity to changes within the bone itself, particularly the bone marrow, which is not visible on standard X-rays. The scan can detect bone marrow edema, which appears as increased fluid signal within the bone, often indicating an occult fracture, a stress injury, or a bone contusion from trauma. This finding is important for identifying injuries that could otherwise be missed in an acute setting.
The MRI is also the preferred modality for diagnosing various chronic and degenerative conditions. It shows the progressive changes of osteoarthritis, including loss of articular cartilage, the formation of osteophytes (bone spurs), and subchondral cysts beneath the joint surface. Furthermore, the scan can reveal signs of infection, such as osteomyelitis, which presents with specific signal changes in the marrow.
Fluid collections outside the joint are easily visualized, including bursitis and joint effusion. The MRI is also used to detect and characterize soft tissue masses, such as tumors or ganglion cysts, by showing their size, precise location, and relationship to surrounding nerves and vessels.