The shoulder is comprised of three bones and three joints that work together to ensure a wide range of motion. The humerus, scapula and clavicle are the three bones that make up the shoulder. The humerus is the large bone located in the upper arm. The scapula, also known as the shoulder blade, is the flat triangular bone in the rear (posterior) of the shoulder. The clavicle, commonly referred to as the collar bone, forms the front (anterior) portion of the shoulder, and is the only bony attachment between the trunk and the upper limb. It is a long, thin bone that is positioned between the scapula and the sternum.
The glenohumeral joint is one of the shoulder joints. Unstable and yet the most mobile, it is a common source of dislocation. A ball-and-socket type joint that is formed between the humerus and the scapula, it is housed in the shallow socket of the scapula – called the glenoid fossa. The surface area is covered with a layer of cartilage (labrum cartilage), which surrounds the glenoid fossa and aides in extending the overall size of the socket. The glenoid fossa is surrounded by the joint capsule and the glenohumeral ligaments and allows for articulation and elevation of the arm. The four rotator cuff muscles originate from the scapula and surround the humeral head to assist with rotation of the arm and help prevent dislocation.
The acromioclavicular joint, also referred to as the AC joint, is a joint at the top of the shoulder. It is where the clavicle meets the acromion of the scapula. The AC joint is not very mobile and only moves when the shoulder is overhead or across the chest and plays a role in lifting the arm. The joint is filled with a thick pad of cartilage.
The sternoclavicular joint is the only attachment of the upper limb to the axial skeleton. It is where the other end of the clavicle attaches to the sternum. It is a mobile joint that allows the clavicle to move.