The shoulder is a complex ball-and-socket joint that relies on a group of four muscles and their tendons, collectively known as the rotator cuff, for stability and movement. This flexibility allows for an immense range of motion but also makes the area susceptible to injury. Two common sources of shoulder discomfort are a torn rotator cuff and shoulder impingement, both involving the soft tissues. While they share similar symptoms, the underlying physical damage and necessary treatments are distinct. Understanding the specific mechanism of injury for each condition is the first step toward accurate diagnosis and effective recovery.
Defining the Mechanics of Injury
A torn rotator cuff is a structural injury involving the fraying or ripping of one or more of the four tendons that attach the muscles to the upper arm bone (humerus). These tendons—the supraspinatus, infraspinatus, teres minor, and subscapularis—can suffer a partial-thickness tear, where the tendon is damaged but still attached. A more severe injury is a full-thickness tear, which involves the complete separation or detachment of the tendon from the bone.
Shoulder impingement, also known as subacromial impingement syndrome, is a compression injury rather than a tear. It occurs when the rotator cuff tendons, particularly the supraspinatus, and the fluid-filled sac called the bursa are pinched. This pinching happens in the narrow subacromial space, beneath the acromion, the bony roof of the shoulder. The injury is characterized by inflammation and irritation as the soft tissues are repeatedly squeezed during arm movement, potentially leading to tendonitis or bursitis. This mechanical compression can, over time, lead to a secondary rotator cuff tear.
Key Differences in Symptoms and Causes
The subjective experience of pain and the origin of the injury differ significantly. Rotator cuff tears often present with acute weakness, a major distinguishing factor. A person with a tear may have difficulty lifting their arm against gravity or holding it steady in an elevated position, a sign of structural failure. The pain is frequently described as sharp and may be centered on the front of the shoulder, often radiating down the side of the upper arm.
In contrast, shoulder impingement typically causes a chronic, dull ache localized to the top and outer side of the shoulder. While movement is painful, strength is generally preserved, meaning the person can usually hold their arm up. They may experience a painful arc of motion when lifting the arm between 60 and 120 degrees. A common complaint with impingement is pain at night, especially when lying directly on the affected shoulder.
The causes also follow distinct patterns. Rotator cuff tears can result from a single acute traumatic event, such as a fall or a sudden, heavy lifting motion. More commonly, tears are degenerative, occurring over time due to age-related wear and chronic overuse.
Impingement is most often caused by repetitive overhead activities, such as those performed by swimmers or painters, which repeatedly stress the subacromial space. Structural factors can also predispose someone to impingement, such as an anatomically “hooked” shape of the acromion bone or the formation of bone spurs, which physically narrow the space. Impingement may also arise from muscle weakness or poor mechanics that cause the humerus to shift upward, further reducing the subacromial space.
Diagnostic Confirmation and Treatment Paths
Initial assessment for both conditions involves a physical examination where a clinician uses specific tests to reproduce the patient’s pain. For impingement, tests like Neer’s Sign and the Hawkins-Kennedy test are used to compress the soft tissues under the acromion, confirming mechanical irritation. Rotator cuff tears are assessed with strength tests, such as the “empty can” test, which evaluates the integrity of the supraspinatus tendon.
Definitive diagnosis requires medical imaging. X-rays are used to look for bone spurs or abnormal acromion shapes that contribute to impingement. A Magnetic Resonance Imaging (MRI) scan is the gold standard for confirming a torn rotator cuff, as it clearly shows the degree of soft tissue damage, distinguishing between partial and full-thickness tears.
Treatment approaches are tailored to the specific diagnosis, starting with conservative management for both conditions. Impingement syndrome is primarily treated non-surgically with physical therapy focused on strengthening the rotator cuff and improving shoulder mechanics. Nonsteroidal anti-inflammatory drugs (NSAIDs) and corticosteroid injections into the subacromial bursa are effective for reducing inflammation and pain.
For a torn rotator cuff, treatment depends on the tear’s severity and the patient’s activity level. Partial tears are often managed with physical therapy, similar to impingement. However, large or full-thickness tears frequently require surgical repair to re-anchor the torn tendon back to the bone. Surgery for impingement, known as subacromial decompression, is reserved for cases that have failed a minimum of four to six months of conservative treatment.