The glenohumeral joint is the ball-and-socket joint that forms your shoulder. It’s where the rounded top of your upper arm bone (the humerus) meets a shallow socket on your shoulder blade (the scapula). This joint gives your arm its remarkable range of motion, letting you reach overhead, throw a ball, or scratch your back. It’s also the most frequently dislocated major joint in the body, a trade-off for all that mobility.
How the Joint Is Built
The “ball” is the head of the humerus, a smooth, rounded surface roughly the size of a golf ball. The “socket” is the glenoid cavity, a shallow dish on the outer edge of the scapula. Unlike the hip, where the socket wraps deeply around the ball, the glenoid is surprisingly flat. Think of a golf ball sitting on a tee. Smooth articular cartilage covers both surfaces, allowing them to glide against each other with minimal friction.
Because the socket is so shallow, the joint relies on a ring of tough, rubbery tissue called the glenoid labrum to deepen it. The labrum attaches around the entire rim of the socket and increases its depth by about 50%. It also works as a seal, helping maintain pressure inside the joint that keeps the humeral head centered. Remove the labrum surgically and stability drops by roughly 20%, particularly in the downward direction. This is why labral tears are such a common source of shoulder problems.
What Keeps It Stable
A joint this mobile needs layers of support to stay in place. The glenohumeral joint gets that support from both passive structures (things that hold firm without active effort) and active ones (muscles you can strengthen).
The joint capsule is a fibrous envelope that wraps the entire joint, enclosing a small amount of lubricating synovial fluid. In a healthy shoulder, the total internal volume of this capsule is roughly 23 cubic centimeters. Built into the capsule walls are three thickened bands called the glenohumeral ligaments: the superior, middle, and inferior. Each one tightens at different arm positions to resist the humeral head from sliding out of the socket. The inferior ligament, for instance, is the primary restraint when your arm is raised and rotated outward.
The active stabilizers are the four rotator cuff muscles. These muscles originate on the shoulder blade and wrap around the humeral head like a cuff:
- Supraspinatus: sits on top, helps lift and rotate the arm
- Infraspinatus: covers the back, rotates the arm outward
- Teres minor: also on the back, assists with outward rotation
- Subscapularis: covers the front, rotates the arm inward
Together, these muscles compress the humeral head into the glenoid during movement, much like pressing a ball into a dish to keep it from rolling off. This compression, combined with the labrum’s deepened socket, is the primary way the shoulder stays stable during dynamic activity.
Range of Motion
The glenohumeral joint allows movement in more directions than any other joint in the body. You can flex your arm forward, extend it behind you, lift it out to the side (abduction), bring it across your body (adduction), and rotate it both inward and outward. The combination of these movements lets you position your hand almost anywhere in a wide sphere around your body.
This extreme mobility is possible precisely because the socket is so shallow and the capsule is loose enough to allow large excursions. The trade-off is inherent instability. Your shoulder depends heavily on soft tissue, the labrum, ligaments, capsule, and rotator cuff, rather than bony architecture to stay in place. When any of those structures are damaged, the joint becomes vulnerable.
Common Problems
Dislocation
Shoulder dislocations account for 50% of all major joint dislocations, and the glenohumeral joint is almost always the one involved. In up to 97% of cases, the humeral head slips forward (anterior dislocation), typically after a fall on an outstretched hand or a forceful blow with the arm raised and rotated outward. Posterior dislocations, where the ball slips backward, are far less common. After a first dislocation, the labrum and ligaments are often stretched or torn, which raises the risk of it happening again, especially in younger people.
Rotator Cuff Injuries
Because the rotator cuff muscles do so much work stabilizing the joint, they’re prone to wear and tear. Partial or complete tears can develop gradually from repetitive overhead use, or acutely from a single injury. Symptoms typically include pain with overhead reaching, weakness when rotating the arm, and aching that worsens at night.
Frozen Shoulder
Adhesive capsulitis, commonly called frozen shoulder, occurs when the joint capsule becomes inflamed and stiff. It progresses through three stages. The “freezing” stage brings increasing pain and stiffness over six weeks to nine months. The “frozen” stage lasts two to six months, during which pain may ease but the shoulder remains locked. Finally, the “thawing” stage gradually restores motion over six months to two years. The total process can stretch well beyond a year.
Osteoarthritis
Like any joint with cartilage surfaces, the glenohumeral joint can develop osteoarthritis. The cartilage wears thin, bone spurs form around the edges, and the joint space narrows. Early stages may only show up on imaging with minor bone spurs. In advanced cases, the joint space is markedly narrowed, the bone surfaces become hardened and deformed, and overhead motion becomes painful and limited. Glenohumeral arthritis is less common than arthritis in the knee or hip, but it can be just as disabling when it progresses.
How Doctors Assess the Joint
A physical exam of the glenohumeral joint focuses on both range of motion and stability. To test for instability, a clinician may use the anterior apprehension test: with you lying on your back, the examiner positions your arm at 90 degrees out to the side and rotates it outward while applying a gentle forward force on the upper arm. If this reproduces the sensation that your shoulder is about to slip out, the test is positive. A follow-up maneuver called the Jobe relocation test then pushes the humeral head back into place. If that pressure relieves the feeling, it confirms anterior instability.
Another common test is the load-and-shift, where the examiner pushes the humeral head forward and backward within the socket at different arm angles to feel for excessive translation. These tests, combined with imaging when needed, help pinpoint whether the problem lies in the labrum, the ligaments, the rotator cuff, or the joint surfaces themselves.