A SLAP tear (Superior Labrum Anterior to Posterior) is an injury to the superior labrum of the shoulder. This injury affects the ring of cartilage surrounding the shoulder socket, specifically where the long head of the biceps tendon attaches. A tear in this area can cause deep shoulder pain and a sensation of catching or popping, particularly during overhead movements. SLAP tears result from various mechanisms, including sudden trauma, chronic overuse, and the natural process of aging.
The Role of the Shoulder Labrum and Basic Mechanism
The glenoid labrum is a fibrous rim of cartilage attached to the shoulder socket (glenoid). It acts like a gasket, deepening the socket and stabilizing the upper arm bone. The long head of the biceps tendon blends directly into the superior labrum, forming the biceps anchor.
This attachment makes the superior labrum vulnerable to forces transmitted through the biceps tendon. One primary mechanism is the “peel-back” effect, which occurs when the arm is in extreme external rotation and abduction. The biceps tendon twists, creating a torsional force that pulls the labrum off the bone.
Other mechanisms involve compression and shear forces. When the arm is forcefully pushed upward, the upper arm bone crashes into the superior labrum, causing a crushing injury. A sudden traction force, or a forceful pull on the arm, can also strain the biceps anchor, causing it to tear away.
Causes Related to Acute Trauma
Acute traumatic events involve a single, high-energy impact or sudden movement. The most common acute cause is a Fall Onto an Outstretched Hand (FOOSH injury), which rapidly transfers impact force up the arm and compresses the humerus head into the superior labrum.
Other sudden events include the forceful movement of the arm above shoulder level, such as attempting to catch a heavy falling object. A direct, forceful blow to the shoulder, often sustained during a collision or contact sport, can also create the necessary shear force.
A shoulder dislocation, where the upper arm bone is pulled out of the socket, frequently causes a SLAP tear due to the extreme traction and twisting placed on the biceps anchor.
Causes Related to Chronic Repetitive Stress
Chronic repetitive stress, or microtrauma, is a frequent cause of SLAP tears, especially in athletic and occupational groups. This mechanism involves the accumulation of small injuries over time rather than a single catastrophic event.
Athletes who engage in repetitive overhead motions, such as baseball pitchers, tennis players, swimmers, and volleyball players, are highly susceptible. The throwing motion is an extreme example of chronic stress.
During the late cocking phase, the arm reaches maximal external rotation, repeatedly utilizing the peel-back mechanism to stress the biceps anchor. The biceps tendon is also subjected to immense eccentric load during the deceleration phase. This constant tension and torsional force gradually weakens the labrum until it tears.
Causes Related to Age and Degeneration
SLAP tears can develop due to biological changes associated with aging, independent of significant trauma or intense athletic activity. The labral cartilage naturally experiences “wear and tear,” losing elasticity and structural integrity. This degenerative process causes the labrum to fray or thin, making it brittle and vulnerable to injury from minimal force.
Tears resulting from age are often classified as Type I SLAP lesions, characterized by simple fraying. While these degenerative tears may be asymptomatic, they increase the risk of a more extensive tear from a minor event. These tears are often found alongside other degenerative changes, such as rotator cuff tears.