The shoulder joint is known for having the greatest range of motion in the human body, allowing the arm to move almost 360 degrees. This mobility, however, comes at the expense of inherent stability, making the shoulder prone to injury. The glenoid labrum is a specialized, ring-shaped structure that plays a major role in addressing this stability challenge. It is a rim of tough, flexible fibrocartilage that helps to keep the upper arm bone securely centered in the shallow shoulder socket.
Exact Location of the Labrum
The labrum is a ring of fibrocartilage attached to the rim of the glenoid fossa, the shallow socket located on the shoulder blade (scapula). This ring lines the edge of the socket where the head of the humerus (upper arm bone) articulates, forming the glenohumeral joint. The labrum acts much like a washer or an O-ring, lining the edge of the socket.
The bony glenoid socket is naturally small and shallow, covering only about 25% to 30% of the humeral head. The labrum effectively increases the surface area and depth of this socket, creating a more secure fit for the humeral head. While the labrum encircles the entire rim, its structure is triangular in cross-section, with a thin, sharp free edge facing the joint space.
The labrum also serves as an anchor point for soft tissues that stabilize the shoulder. The long head of the biceps tendon attaches directly to the superior part of the labrum. Additionally, the glenohumeral ligaments, which are thickenings of the joint capsule, attach to the labrum, integrating it into the shoulder’s stability system.
Role in Shoulder Stability
The primary function of the labrum is to enhance the passive stability of the glenohumeral joint. By affixing to the bony rim, the fibrocartilaginous ring effectively deepens the shallow glenoid socket by approximately 50%. This deepening creates a concavity that helps the surrounding rotator cuff muscles compress the humeral head into the socket during movement, a concept known as concavity-compression stability.
The labrum also contributes to joint stability by helping to maintain negative pressure within the joint space. It creates a seal around the joint, forming a vacuum effect that resists the humeral head from being pulled away from the glenoid. This mechanism is a component of the shoulder’s stability, working alongside the active forces provided by the surrounding muscles.
The labrum’s attachment site for the long head of the biceps tendon contributes to dynamic stability. When the biceps muscle contracts, it exerts tension that helps to depress the humeral head and keep it centralized within the socket during high-force or overhead movements. Without an intact labrum, the joint becomes more lax, increasing the risk of the humeral head translating excessively, which can lead to instability or dislocation.
Common Injury Patterns
The labrum is susceptible to tearing from acute trauma or repetitive stress. The location of the tear determines its classification, with two common patterns being the SLAP tear and the Bankart lesion. These distinct injury patterns are often associated with different mechanisms of injury.
The SLAP tear (Superior Labrum Anterior to Posterior) involves damage to the top part of the labrum, extending from the front to the back. This tear is characterized by its involvement with the biceps tendon anchor. SLAP tears are frequently caused by chronic repetitive overhead activities, such as throwing, or by an acute event like falling onto an outstretched arm.
A Bankart lesion is a tear of the labrum in the anterior-inferior (front and bottom) portion of the socket. This injury pattern is strongly associated with anterior shoulder dislocation, the most common direction for the shoulder to dislocate. When the humeral head is forcefully pushed out, it tears the labrum and the attached anterior glenohumeral ligaments, often leaving a defect on the bony rim.
General trauma, such such as a direct blow or a sudden, forceful pull on the arm, can cause either type of tear, depending on the direction of the force. Symptoms include deep shoulder pain, a clicking or popping sensation, and a feeling of instability in the joint.