The shoulder joint is formally called the glenohumeral joint. It’s a ball-and-socket joint where the rounded top of your upper arm bone (the humerus) fits into a shallow curve in your shoulder blade (the scapula). This design gives the shoulder a greater range of motion than any other joint in your body, but that flexibility comes at the cost of stability.
Why It’s Called the Glenohumeral Joint
The name comes from the two bones involved. The “glenoid” refers to the glenoid cavity, a shallow, dish-like socket on the outer edge of your shoulder blade. The “humeral” part refers to the humerus, your upper arm bone. Where these two surfaces meet is the glenohumeral joint. You’ll sometimes hear doctors or physical therapists use this term interchangeably with “shoulder joint,” though the full shoulder is actually more complex than a single joint.
Five Joints Make Up the Shoulder Complex
When most people say “shoulder joint,” they mean the glenohumeral joint. But your shoulder actually relies on five distinct joints and gliding surfaces working together:
- Glenohumeral joint: the primary ball-and-socket joint between your upper arm and shoulder blade
- Acromioclavicular joint: where the outer tip of your shoulder blade meets your collarbone
- Sternoclavicular joint: where your collarbone meets your breastbone, the only bony connection between your arm and the rest of your skeleton
- Subacromial space: a gap between the bony roof of your shoulder and the top of the humerus, cushioned by a fluid-filled sac called a bursa
- Scapulothoracic gliding surface: where the flat front of your shoulder blade slides along your rib cage as you move your arm
All five work in coordination. When you raise your arm overhead, the glenohumeral joint provides most of the movement, but your shoulder blade rotates along your rib cage at the same time. Losing motion at any one of these joints changes how the others function.
How the Ball-and-Socket Design Works
The glenoid socket is surprisingly shallow. It only covers about a third of the humeral head at any given time, which is part of why your shoulder can move in so many directions. A ring of tough cartilage called the labrum lines the rim of the socket, acting like a bumper that deepens the cavity and creates a mild suction-cup effect to help keep the ball centered.
This design allows an impressive range of motion. A healthy shoulder can flex forward 150 to 180 degrees, extend backward 45 to 60 degrees, and abduct (lift sideways) about 150 degrees. External rotation reaches around 90 degrees, and internal rotation ranges from 70 to 90 degrees. Few other joints in the body come close to that versatility.
What Holds the Joint Together
Because the socket is so shallow, the glenohumeral joint depends heavily on soft tissue for stability. The rotator cuff is the most important group: four muscles and their tendons that wrap around the joint and hold the head of the humerus firmly in the socket. Each muscle handles a slightly different job:
- Supraspinatus: runs along the top of the shoulder blade and helps you lift and rotate your arm
- Infraspinatus: sits on the back of the shoulder blade and assists with outward rotation
- Teres minor: attaches along the outer edge of the shoulder blade and also helps rotate the arm outward
- Subscapularis: covers the front surface of the shoulder blade and lets you hold your arm away from your body
Beyond the rotator cuff, a set of ligaments reinforces the joint capsule, a watertight sleeve of connective tissue that surrounds the glenohumeral joint. Together, the labrum, ligaments, capsule, and rotator cuff muscles compensate for what the bony structure lacks in depth.
Common Glenohumeral Joint Problems
The same flexibility that makes the shoulder so useful also makes it vulnerable. More than 4 million people seek treatment for shoulder pain each year in the United States alone, and roughly 250,000 undergo rotator cuff repair surgery annually.
Rotator cuff tears are among the most common issues. Small tears sometimes respond well to physical therapy and injections, with about 70 percent of patients reporting improvement without surgery. But untreated full-thickness tears tend to grow: research from Washington University found the risk of a tear enlarging increased by 22 percent within the first two years and reached 60 percent over five years. Interestingly, patients with a painful tear in one shoulder often have an undetected, painless tear in the opposite shoulder. About 30 percent of those eventually develop pain on that side too, especially with age.
Frozen shoulder is another condition specific to the glenohumeral joint. The joint capsule thickens and tightens, progressively limiting movement. It typically unfolds in three stages: a “freezing” phase lasting six weeks to nine months where pain builds and stiffness sets in, a “frozen” phase lasting two to six months where pain eases but the joint stays locked, and a “thawing” phase lasting six months to two years where motion gradually returns. The entire process can stretch well beyond a year, but most people recover full or near-full movement eventually.
Dislocations also disproportionately affect the glenohumeral joint compared to other joints in the body, precisely because the socket is so shallow. The humerus can slip forward, backward, or downward out of the glenoid cavity, often stretching or tearing the labrum and ligaments in the process.