The shoulder is the most mobile joint in the human body, allowing for an extensive range of motion in multiple directions. The joint operates like a ball-and-socket, where the head of the upper arm bone, the humerus, rests within the shallow socket of the shoulder blade, known as the glenoid. Shoulder instability occurs when the humerus slips partially or completely out of its normal resting position in the socket. This condition ranges from a subtle feeling of looseness to a complete dislocation, and it happens when the structures responsible for keeping the joint centered become compromised.
Acute Traumatic Injuries
The most dramatic and common cause of shoulder instability is a single, high-energy event that forcefully separates the joint. This kind of injury often results from a fall onto an outstretched arm, a direct blow to the shoulder, or a sudden, violent tackle during contact sports. The force pushes the humerus head out of the socket, resulting in either a complete separation, known as a dislocation, or a partial separation, called a subluxation.
When a dislocation occurs, the soft tissues that provide static stability—including the ligaments and the labrum—are stretched or torn. Tearing of the labrum from the edge of the socket is particularly damaging, as this structure acts like a bumper. The initial traumatic event permanently compromises the joint’s passive restraints.
This initial damage is the primary reason for recurrence. Patients who suffer a traumatic dislocation when they are young face a significantly high risk of the shoulder slipping out again. The joint remains vulnerable to subsequent instability events, sometimes requiring only minimal force to cause another episode.
Repetitive Strain and Microtrauma
Instability can also develop gradually over time due to repeated, low-level stresses. This mechanism is frequently observed in athletes whose sports demand overhead motions, such as baseball pitchers, swimmers, or volleyball players. The repeated force of the arm moving into extreme ranges of motion causes cumulative strain on the joint’s static stabilizers.
Over months or years, this constant microtrauma leads to a chronic stretching of the joint capsule and the ligaments. This allows for excessive, subtle movement of the humerus head within the socket. This excessive motion is often termed “microinstability” and occurs without the severity of a single, acute injury.
The gradual loosening of the capsule and ligaments reduces the joint’s passive stability, requiring the surrounding muscles to work harder to keep the shoulder centered. This increased demand can cause secondary issues like pain and weakness, even if the shoulder never completely dislocates. Unlike the instability caused by a single traumatic tear, this type of instability results from the slow attenuation of the supporting structures.
Structural and Genetic Predispositions
Some individuals are inherently more susceptible to shoulder instability due to their underlying biology or structural issues. One such factor is generalized ligamentous laxity, often described as being “double-jointed”. People with this condition have naturally stretchier, looser ligaments throughout their body, meaning their shoulders start with less passive stability than others.
This inherent flexibility can predispose a person to shoulder instability. The shoulder may be prone to slipping out in multiple directions, known as multidirectional instability, because the ligaments are uniformly less restrictive. Identifying this hyperlaxity is an important consideration, as it suggests the instability is rooted in a systemic characteristic rather than just localized tissue damage.
Furthermore, a shoulder that has experienced a traumatic dislocation often sustains lasting damage to the bone, not just the soft tissues. When the humerus head is forced out, it can impact the rim of the socket, chipping away a portion of the bone. It can also create a compression fracture on the humerus head as it slams against the socket edge. These bony defects reduce the effective surface area of the joint, creating a mechanical vulnerability that makes recurrent instability more likely, even if the soft tissue tears are repaired.