Shoulder instability is the feeling that the shoulder joint is loose or prone to slipping out of place. The shoulder is a ball-and-socket joint where the head of the upper arm bone, the humerus, sits in the shallow socket of the shoulder blade, the glenoid. This anatomical arrangement means the joint relies heavily on surrounding soft tissues for stability, including the labrum, the ligaments, and the joint capsule. The labrum is a rim of cartilage that deepens the glenoid socket, while the ligaments and capsule form a tough, fibrous covering that holds the ball in place. When these stabilizers are damaged or stretched, the humeral head can move excessively, leading to a partial slip (subluxation) or a complete displacement (dislocation).
Acute Traumatic Injuries
The most common cause of shoulder instability is a single, forceful event that physically drives the humeral head out of the glenoid socket. This high-energy trauma, often occurring during sports or a fall onto an outstretched arm, forces the ball to move so abruptly that it damages the joint’s static restraints. When the shoulder dislocates, the forces involved typically tear or stretch the ligaments and the joint capsule. This initial traumatic episode is often the beginning of chronic instability because the damaged structures no longer provide adequate restraint, leaving the joint vulnerable to repeat dislocations.
Bankart and Hill-Sachs Lesions
A frequent consequence of a traumatic anterior dislocation is the development of a Bankart lesion, which is a tear of the anterior-inferior part of the glenoid labrum. The humeral head, as it dislocates forward, peels the labrum away from the rim of the socket. If this tear pulls a fragment of the bone off the glenoid, it is termed a bony Bankart lesion, which significantly reduces the effective surface area of the socket. A Hill-Sachs lesion is an indentation fracture on the back of the humeral head. This lesion occurs because as the humeral head is forced out of the joint, its softer bone impacts and compresses against the hard, bony rim of the glenoid. The presence of both a Bankart and a Hill-Sachs lesion complicates stability, making future dislocations more likely.
Chronic Repetitive Strain
Shoulder instability can also develop gradually from cumulative damage caused by repeated, low-level stresses, a mechanism distinct from a single severe trauma. This process, often referred to as microtrauma, is frequently observed in athletes who participate in overhead or throwing sports, such as baseball, swimming, and volleyball. The repeated motions required in these activities push the joint to its end range of motion. Over time, this continuous stretching causes the shoulder capsule and the ligaments to loosen and become lax. This acquired looseness allows the humeral head to translate excessively within the socket, resulting in symptomatic instability.
Underlying Structural and Ligamentous Factors
In some individuals, instability stems from inherent anatomical or genetic factors rather than injury or overuse. The most common non-traumatic cause is generalized ligamentous laxity, often described as being “double-jointed.” This condition is characterized by naturally looser joints throughout the body due to a more extensible collagen structure in the ligaments and joint capsule. This inherent looseness means the supporting structures of the shoulder are less stiff, predisposing the joint to instability without a traumatic event. This often results in Multidirectional Instability (MDI), where the shoulder can slip out of place in multiple directions—anteriorly, posteriorly, or inferiorly.
Anatomical and Neurological Causes
MDI can also be a component of systemic connective tissue disorders, such as Ehlers-Danlos syndrome. Anatomical variations in the bony structure of the shoulder can also contribute to instability. A congenitally shallow glenoid socket, for instance, provides less surface area for the humeral head to rest against, reducing the joint’s natural bony stability. Functional instability can arise from neurological causes, such as nerve injury that weakens the dynamic stabilizers of the shoulder, like the rotator cuff muscles.