Shoulder instability occurs when the head of the upper arm bone (humerus) moves beyond its normal position within the shallow glenoid socket. The shoulder is the most mobile joint in the body, which inherently makes it less stable than others. Instability manifests as either a partial slipping (subluxation) or a complete separation (dislocation). Chronic instability results when the soft tissues—including the ligaments, joint capsule, and labrum—become damaged or stretched. The causes are generally grouped into three categories: acute traumatic injury, damage from repetitive motions, or underlying joint looseness.
Instability Caused by Acute Traumatic Injury
The most common cause of shoulder instability is a single, forceful event, such as a fall onto an outstretched arm or a direct blow during sports. This trauma forces the humerus out of the socket, causing immediate structural damage to the joint’s static stabilizers. The direction of the force dictates the type of injury; the humerus typically dislocates forward (anteriorly) in over 95% of cases.
When the humerus exits the front of the socket, it often tears the ring of cartilage (labrum) and associated ligaments from the bone. This soft tissue injury removes a bumper from the socket’s rim, increasing the risk of future instability. The impact of the humerus against the edge of the socket can also cause a compression fracture, creating a dent in the back of the humerus head.
These injuries compromise the shoulder’s ability to remain centered, making it vulnerable to repeated episodes of slipping or dislocating. The combination of soft tissue and bony defects diminishes the joint’s natural restraints. Once the shoulder has dislocated, especially in young, active individuals, the probability of recurrence is increased.
Instability Stemming from Repetitive Overuse
Instability can also develop gradually from chronic, repeated stress, rather than a single traumatic event. This is frequently observed in overhead athletes, such as baseball pitchers, swimmers, or tennis players, whose sports require extreme and repeated motions above the shoulder. The repetitive microtrauma slowly stretches and attenuates the joint capsule and ligaments.
This persistent stretching causes the supportive tissues to become elongated and loose, a state known as acquired laxity. The static restraints can no longer hold the humerus securely, leading to subtle, repeated partial slippages (subluxations). Over time, the dynamic stabilizers (muscles surrounding the joint) become fatigued or ineffective at compensating for the structural looseness, resulting in chronic instability.
Instability Due to Underlying Joint Laxity
In some individuals, shoulder instability occurs without any history of significant injury or chronic overuse. This atraumatic instability is rooted in an inherent looseness of the joints throughout the body, known as generalized joint laxity or hypermobility. This predisposition results from structural differences in the body’s connective tissue, often related to the composition of collagen.
When the collagen forming the ligaments and joint capsule is naturally more elastic, the supportive structures provide less constraint. The shoulder is inherently less stable, making it susceptible to slipping out of place with minimal force or during normal daily activities. In cases of extreme hypermobility, such as Ehlers-Danlos syndrome, abnormal collagen production leads to severe looseness, making stability management challenging.
Recognizing the Associated Symptoms
Recognizing shoulder instability involves identifying specific feelings and sensations that indicate the humerus is not staying centered. A common complaint is the feeling that the shoulder is “slipping out” of the socket, or the sensation of the arm bone feeling loose. This may be accompanied by a clicking or grinding sound within the joint during certain movements.
Individuals often experience apprehension when placing the arm in vulnerable positions, such as reaching back or throwing overhead. This feeling of impending instability signals underlying structural compromise. Chronic aching pain and noticeable weakness in the affected arm are also frequent symptoms.