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The shoulder joint (glenohumeral joint) is a ball-and-socket joint formed by the glenoid (scapula) and the humerus.

It is the major joint connecting the upper limb to the trunk. The top of the humerus (humeral head) is a relatively larger ball that articulates with the glenoid, a relatively smaller socket formed by the scapula. Because the ball is larger than the socket, the shoulder joint is very mobile but not very stable.

A fibrocartilage structure known as the glenoid labrum is attached to the periphery of the glenoid, which increases the overall size of the socket and therefore improves stability of the joint. Tearing of the glenoid labrum from its attachment to the glenoid can result in instability and pain of the shoulder.

The shoulder joint is surrounded by a capsule. A number of ligaments traverse the bones and further add stability to the joint.

The bony surfaces of the shoulder joint are covered by hyaline cartilage, which allows for smooth, frictionless movement. Damage and degeneration of the cartilage results in osteoarthritis, which is characterised by pain, stiffness and restriction of movement.

There are four main muscles which surround the shoulder joint, collectively known as the rotator cuff group. These muscles (supraspinatus, infraspinatus, subscapularis and teres minor) are important in guiding movement of the shoulder joint, whilst also providing stability. Tears or damage to any or all of the rotator cuff muscles reduces stability and is associated with pain and limitation of movement.