A shoulder dislocation happens when force pushes the upper arm bone (humerus) out of the shallow socket it sits in on the shoulder blade. Over 95% of shoulder dislocations are anterior, meaning the ball of the joint slips forward and out. The shoulder is the most mobile joint in the body, which also makes it the most vulnerable to dislocation. Understanding the specific positions and forces that cause this injury helps explain why certain activities carry more risk than others.
Why the Shoulder Is So Easy to Dislocate
The shoulder socket, called the glenoid, is remarkably shallow compared to a joint like the hip. Think of it as a golf ball sitting on a tee. A ring of cartilage called the labrum deepens the socket slightly, and a group of ligaments and the rotator cuff muscles work together to hold everything in place. In the mid-range of motion, when those ligaments are naturally slack, stability depends almost entirely on the rotator cuff muscles compressing the ball into the socket and on the slight concavity of the socket itself.
This design gives you an extraordinary range of motion. You can reach overhead, behind your back, and across your body in ways no other joint allows. The tradeoff is that relatively modest force, applied in the wrong direction at the wrong time, can overcome those stabilizers and pop the joint out.
The Classic Anterior Dislocation
The most common scenario involves the arm being forced into a combination of abduction (raised out to the side) and external rotation (rotated outward). Picture a quarterback with their arm cocked back to throw, or someone bracing a fall with an outstretched hand. In this position, the front of the joint capsule is stretched tight, and a violent force levers the humeral head forward and out of the socket.
As the ball exits the socket, it often tears the labrum off the front of the glenoid. This injury, called a Bankart lesion, shows up in about 73% of dislocations. The back of the humeral head can also collide with the rim of the socket on its way out, creating a dent in the bone known as a Hill-Sachs lesion, found in roughly 84% of cases. These two injuries frequently occur together and play a significant role in whether the shoulder dislocates again later.
Someone with an anterior dislocation typically holds their arm slightly away from the body and rotated outward. The normal rounded contour of the shoulder looks flattened, and attempting to move the arm causes intense pain.
Posterior and Inferior Dislocations
Posterior dislocations, where the ball slips backward, account for a small fraction of cases but have a distinctive cause. Seizures are a well-known trigger. During a generalized seizure, the powerful internal rotator muscles of the shoulder (including the large chest and back muscles) contract violently and simultaneously, forcing the humeral head backward over the glenoid rim. Electrocution can produce the same pattern of muscle contraction. A direct blow to the front of the shoulder can also drive the joint posteriorly.
Inferior dislocations are the rarest type. They happen when the arm is forced into extreme overhead extension, essentially hyperabducted beyond its normal range. The person typically presents with their arm locked overhead and their hand behind their head, unable to bring it down. A direct downward force applied to the top of the shoulder can also cause this pattern.
Who Is Most at Risk
Age is the single strongest risk factor. People between 15 and 19 years old are roughly 7 times more likely to experience a first-time dislocation compared to those 35 and older. Young men in their twenties carry the highest sex-specific risk, with men in the 20 to 29 age group about 7.5 times more likely to dislocate than women of the same age. Contact sports, overhead athletics, and high-energy activities explain much of this disparity.
Joint hypermobility, the kind of looseness where your joints bend further than average, also increases risk. People who score high on clinical hypermobility scales are nearly 3 times more likely to experience a first-time traumatic dislocation. When that hypermobility is combined with more than 85 degrees of outward shoulder rotation, the risk climbs to about 3.6 times higher. Connective tissue conditions that cause widespread joint laxity can amplify this vulnerability further.
Older adults face a different risk profile. While dislocations are less common after 65, they tend to cause more associated fractures and a higher rate of nerve injury when they do occur.
Nerve Damage During Dislocation
The axillary nerve runs directly beneath the shoulder joint, and it’s the nerve most commonly injured during a dislocation. This nerve controls the main muscle responsible for lifting your arm out to the side and provides sensation to a patch of skin on the outer shoulder.
The risk of axillary nerve injury rises sharply with age. Roughly 65% of patients over 40 who dislocate their shoulder show evidence of axillary nerve damage on nerve conduction testing. The hallmark symptoms are weakness when trying to lift the arm away from the body and numbness on the outside of the shoulder. Most axillary nerve injuries from dislocation recover on their own over weeks to months, but the initial weakness can be significant.
How the Shoulder Gets Put Back In
Relocating a dislocated shoulder, called reduction, needs to happen as soon as possible. The longer the joint stays out, the more the surrounding muscles spasm, making the procedure harder and more painful. You’ll typically receive pain medication and sometimes sedation before the maneuver begins.
Several techniques exist, and the choice often depends on the clinical setting and practitioner preference. One common approach has you lie face-down on a table with your arm hanging off the edge. Gentle downward traction is applied for 10 to 20 minutes, either manually or with a small weight, allowing the muscles to relax until the ball slides back into the socket. Another method involves a series of controlled rotations performed while you lie on your back with your elbow bent at 90 degrees. The practitioner slowly rotates the arm outward, lifts it, then rotates it inward to guide the humeral head home.
After reduction, X-rays confirm the joint is properly seated and check for fractures. A standard set of images includes a front-facing view, a side view, and a specialized view to assess for the bony dents that commonly accompany dislocation.
Recurrence and Long-Term Outlook
The younger you are at your first dislocation, the more likely it is to happen again. In patients managed without surgery, those aged 16 to 20 have a recurrent instability rate of about 47% within 10 years. Patients 15 and younger show rates near 39% over the same period. Each repeat dislocation tends to further damage the labrum and deepen the bony dent on the humeral head, making the next episode easier to trigger with less force.
Being under 30 at the time of your first dislocation is the strongest predictor of recurrence, with some studies showing the odds of redislocation are 20 or more times higher compared to older patients. This is partly because younger people tend to return to higher levels of physical activity, but also because the structural damage from the first event creates a mechanical vulnerability. The torn labrum no longer deepens the socket effectively, and the bony dent can catch on the socket rim during certain movements, causing the joint to slip out again.
For people with high recurrence risk, surgical repair of the torn labrum can significantly reduce the chance of future episodes. The decision typically depends on age, activity level, the extent of bone and soft tissue damage, and how many dislocations have already occurred.