The shoulder joint comprises of three bones: the humerus (upper arm bone), the clavicle (collar bone), and the scapula (shoulder blade) which are interlinked by muscles, tendons and ligaments. It is a very complex joint, the most mobile of the body and enable a wide range of movements.
The humeral head is ball shaped that partially fits into a cavity or socket called glenoid (the joint component of the scapula). The glenoid is quite shallow allowing for the wide range of movement at this joint. Stability is achieved by connecting ligaments and the shoulder labrum, a special type of cartilage which provides a greater surface area for the bones to form continuity with one another. The humerus remains close to the glenoid thanks to a tight fibrous cuff, comprised of the joint capsule, and the stabilising action of the powerful rotator cuff muscles surrounding it. Above the humeral head is a bony prominence arising from the scapula, called acromion, which in the space below (subacromial space) flow the tendons of the rotator cuff muscles. In this space the shoulder tendons pass over each other during shoulder movements.
The efficiency of these movements is achieved due to the presence of bursae (naturally lubricated linings between adjacent structures), with the subacromial bursa being the largest in this region. In addition to providing stability, they permit the lifting and rotating of the arm itself. The two tendons of the biceps and of the pectoralis major muscles insert near the humeral head. This whole complex is covered by the deltoid muscle.
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