The rotator cuff is a collective of four muscles and their tendons that surround the shoulder joint, connecting the upper arm bone (humerus) to the shoulder blade (scapula). This group stabilizes the highly mobile ball-and-socket joint and enables a wide range of motion, including lifting and rotation. Because these tendons operate in a narrow space and are subject to repetitive stress and wear, they are frequently prone to injury, leading to tears. Magnetic Resonance Imaging (MRI) is a standard tool for diagnosing these injuries due to its exceptional ability to visualize soft tissues like tendons and muscles in detail.
Understanding the MRI View
MRI technology utilizes strong magnetic fields and radio waves to generate detailed cross-sectional images of the body, unlike X-rays, which rely on ionizing radiation. The machine effectively aligns the protons, primarily those in water molecules, within the body’s tissues. When a radiofrequency pulse is applied, the subsequent energy release as they “relax” back to their original state is measured.
Different tissues relax at different rates, allowing the MRI to differentiate between them based on their signal intensity, or how bright or dark they appear. T2-weighted images are designed to make fluid appear bright (hyperintense). Since tears, inflammation, and joint fluid all contain high amounts of water, they generally display a bright signal on T2-weighted scans, making this sequence crucial for identifying pathology.
Conversely, tissues with low water content, such as dense tendons or cortical bone, generally appear dark (hypointense) on most sequences.
The Healthy Rotator Cuff on MRI
Interpreting a torn rotator cuff requires establishing a visual baseline for a normal tendon. On both T1- and T2-weighted MRI sequences, a healthy rotator cuff tendon exhibits a uniformly dark, or hypointense, signal. This dark appearance results from the tendon’s dense, highly organized fibrous collagen structure, which contains very little free water.
The four tendons should all appear continuous and uniformly dark as they attach to the humerus. A normal tendon has an intact attachment to the bone, with no breaks or gaps visible at the insertion site.
Any area of intermediate or slightly increased signal within the tendon that does not extend all the way through is often considered degenerative change (tendinosis) rather than a tear.
Key Visual Signs of a Rotator Cuff Tear
The most direct and definitive sign of a rotator cuff tear on MRI is the visual presence of tendon discontinuity, which is a clear gap or break in the normal dark signal of the tendon. This gap represents the rupture of the tendon fibers and is the primary focus for diagnosis. In acute or symptomatic tears, the space left by the torn tendon is often filled with fluid, which appears bright or hyperintense on T2-weighted images.
The tear’s extent is classified by its thickness, which significantly impacts treatment decisions. A full-thickness tear extends completely through the tendon, creating a direct communication between the joint space and the subacromial-subdeltoid bursa. Visually, this is seen as a bright fluid signal that breaches the entire tendon thickness. Conversely, a partial-thickness tear involves only a portion of the tendon’s thickness, appearing as a focal fluid signal that does not traverse the entire tendon.
Radiologists also assess the degree of tendon retraction, which describes how far the torn tendon end has pulled back from its bony attachment. The greater the distance of this retraction, the more challenging the tear may be to repair surgically. Tears are measured in centimeters and classified as small (less than 1 cm), medium (1 to 3 cm), or large (greater than 3 cm), based on the distance from the tendon stump to its original insertion site.
Assessing Tear Severity and Associated Damage
Beyond the immediate tear, MRI is crucial for identifying secondary findings that determine the overall severity and prognosis of the injury.
Muscle Atrophy and Fatty Infiltration
A particularly concerning sign in chronic tears is muscle atrophy, which is the shrinking of the muscle belly due to disuse. This muscle volume loss is often accompanied by fatty infiltration, where healthy muscle tissue is replaced by fat. Fatty infiltration appears bright on T1-weighted MRI sequences and is a critical prognostic factor, as muscle with extensive fat replacement is less likely to regain strength even after surgical repair. Radiologists utilize grading scales, such as the Goutallier classification, to systematically assess the amount of fat within the muscle. This classification compares the proportion of fat streaks to muscle fibers, with higher grades indicating a more severe, irreversible condition.
Associated Degenerative Changes
Associated changes also provide context for the tear, including the presence of tendinosis in the remaining tendon tissue. This appears as a mild, intermediate signal intensity within the tendon adjacent to the tear, but not as bright as the fluid in the tear itself. Furthermore, a tear commonly causes inflammation of the subacromial-subdeltoid bursa, known as bursitis. Bursitis is seen as an accumulation of bright fluid signal in the bursa on T2-weighted images. Chronic stress from the tear can also lead to bone changes, such as the formation of bone spurs or subchondral cysts, which indicate long-term mechanical stress on the joint.