The shoulder joint is a complex structure, allowing for an extensive range of motion. This mobility stems from its unique design as a ball-and-socket joint, where the head of the humerus (upper arm bone) articulates with the shallow glenoid cavity of the scapula (shoulder blade). Surrounding this articulation is a network of muscles, tendons, and ligaments that work together to facilitate movement and provide stability. Within this intricate system lies an important component: the labrum.
Anatomy and Location of the Shoulder Labrum
The shoulder labrum, also known as the glenoid labrum, is a ring of fibrocartilaginous tissue. It is attached to the rim of the glenoid cavity, which is the shallow, cup-like socket of the shoulder blade. This structure deepens the glenoid socket, much like a washer, to provide a more secure fit for the humeral head. The glenoid fossa, the bony part of the socket, is shallow and covers only about one-third of the humeral head, making the labrum’s role in enhancing depth important.
The labrum is triangular in cross-section, with its base fixed to the circumference of the glenoid cavity and a thinner, sharper free edge. Its composition of fibrocartilage provides a combination of flexibility and strength, allowing it to conform to the curvature of the humeral head. This anatomical arrangement increases the contact area between the humeral head and the scapula, contributing to the joint’s overall structure.
Role of the Labrum in Shoulder Stability and Movement
The glenoid labrum plays an important role in maintaining shoulder stability. By deepening the glenoid socket, it acts as a static stabilizer, keeping the humeral head centered within the cavity. This enhanced congruency is important given the glenoid fossa’s shallow nature.
The labrum also serves as an attachment point for structures, contributing to shoulder stability. Ligaments, such as the glenohumeral ligaments (superior, middle, and inferior), directly attach to the labrum, reinforcing the joint capsule. The long head of the biceps tendon also blends into the superior aspect of the labrum, adding dynamic support to the shoulder joint. These attachments work together to resist excessive movement, preventing dislocations and subluxations, especially at the extremes of motion.
Common Labral Injuries and Their Impact
The labrum is susceptible to injury, often from trauma or repetitive stress. Common mechanisms of injury include falls onto an outstretched arm, direct blows to the shoulder, sudden forceful pulling motions, or shoulder dislocations. Repetitive overhead movements, frequently seen in sports like baseball or swimming, can also lead to wear and tear or tearing of the labrum.
Two common types of labral tears are SLAP (Superior Labrum Anterior to Posterior) tears and Bankart lesions. SLAP tears occur in the upper part of the labrum where the biceps tendon attaches, often due to a “peel-back” mechanism from overhead activities. Bankart lesions, conversely, are associated with anterior shoulder dislocations, involving a tear of the lower front portion of the labrum. These injuries can lead to symptoms including a dull, throbbing ache, particularly with overhead movements or specific activities. Individuals may also experience clicking, popping, grinding, or catching sensations within the joint, along with feelings of instability, weakness, or a decreased range of motion.