The glenohumeral joint is commonly known as the shoulder joint. Typically, the humerus, or upper arm bone, meets the scapula, or shoulder blade, to form what is classed as a ball and socket joint. This is because the round, ball-shaped head of the humerus fits inside a shallow, socket-shaped cavity in the scapula known as the glenoid cavity. The arrangement permits a large range of motion, but at the same time the joint is not very stable, relying on the surrounding muscles and ligaments to hold everything together and prevent injury.
Generally, the head of the humerus and the inside of the glenoid cavity are covered in a tough, smooth tissue known as cartilage, which enables the surfaces to slide over one another. A lubricating fluid known as synovial fluid can be found in the space between the two. Although the socket part of the joint is shallow, the glenoid cavity has a rim made of a fibrous cartilage, effectively making it deeper. Surrounding the joint is a capsule which is quite thin and does not fit tightly. This allows the glenohumeral joint to move freely.
The group of muscle tendons known as the rotator cuff fuses with the joint capsule. This helps to give the joint some stability and protection. Sometimes the cuff can become damaged and is a frequent cause of shoulder pain.
A common problem is frozen shoulder, or adhesive capsulitis. This is where the shoulder joint capsule becomes thickened and full of fibrous tissue strands, making movement limited and painful. The cause is not always easy to determine, although it can result from rotator cuff injury or long periods of immobility.
The nerve supply to the capsule and ligaments means that the glenohumeral joint is responsive to injury. Typically, the surrounding muscles respond to pain by going into spasm, preventing joint movement, and stopping the discomfort. With some types of disease, such as arthritis or infection, a glenohumeral joint effusion may occur. This is where the joint lining produces excess fluid, which may show up as a swelling between the head of the humerus and the scapula.
Due to the many and varied movements carried out by the shoulder, joint stability may sometimes be compromised. This is why, of all the large joints, the shoulder is probably the one that is most commonly dislocated. Glenohumeral joint dislocation occurs if force is suddenly applied to the humerus so as to lever the head of the humerus down and rupture the capsule. This can happen during a fall where the arm is outstretched.