The glenoid labrum is a ring of specialized cartilage within the shoulder joint. It encircles the glenoid cavity, the shallow socket of the shoulder blade. This structure deepens the shoulder socket, enhancing the stability of the glenohumeral joint, where the upper arm bone meets the shoulder blade.
Anatomy and Location
The glenoid labrum is a fibrous cartilage ring that attaches around the rim of the glenoid fossa, the shallow shoulder socket. Composed of fibrocartilage, it effectively deepens the glenoid cavity, which is naturally quite shallow, covering at most only about a third of the humeral head. This deepening effect can increase the socket’s depth by up to 50%.
The labrum is triangular in cross-section, with its base fixed to the circumference of the glenoid cavity and a thin, sharp free edge. Its dimensions are variable, typically around 3 mm thick and 4 mm wide. It integrates with the joint capsule and serves as an attachment point for various ligaments, including the superior, middle, and inferior glenohumeral ligaments. The tendon of the long head of the biceps brachii muscle also attaches to the superior (top) aspect of the labrum.
Role in Shoulder Stability
The glenoid labrum enhances shoulder joint stability. By deepening the naturally shallow glenoid cavity, it creates a more congruent articulation for the humeral head. This increased depth helps to keep the humeral head centered within the socket, reducing the risk of dislocation. The labrum contributes to approximately 10% of the concavity-compression stability of the glenohumeral joint.
The labrum provides a stable attachment site for the glenohumeral ligaments. The long head of the biceps tendon also anchors to the superior labrum, contributing to the dynamic stability of the joint. The labrum aids in distributing forces across the joint during movement and helps create a “suction cup” effect, which contributes to the smooth and controlled movement of the humerus within the glenoid.
Types of Labral Injuries
The glenoid labrum can sustain injuries from acute trauma or repetitive stress. One common type is a Superior Labrum Anterior Posterior (SLAP) tear, which occurs at the top of the labrum where the biceps tendon attaches. SLAP tears often result from repetitive overhead motions, common in sports like baseball or tennis, or from direct trauma such as falling on an outstretched arm.
Another frequent injury is a Bankart lesion, which typically involves a tear to the front and lower portion of the labrum. Bankart lesions are strongly associated with shoulder dislocations, particularly anterior dislocations, where the humeral head shifts forward out of the socket. Other mechanisms of injury for labral tears include direct blows to the shoulder or sudden, forceful movements. While SLAP and Bankart tears are the most common, posterior labral tears can also occur.
Signs of a Labral Injury
Individuals with a glenoid labrum injury often experience a range of symptoms. Shoulder pain is a common indicator, frequently described as a deep ache within the joint. This pain may worsen with specific movements, such as reaching overhead, throwing, or carrying heavy objects.
Mechanical symptoms are also frequently reported, including clicking, popping, grinding, or catching sensations within the shoulder joint during movement. Some individuals may feel a sense of instability or apprehension, a sensation that the shoulder might slip out of place. Decreased range of motion and weakness in the affected shoulder or arm can also be present, impacting daily activities and athletic performance.