Normal Anatomy of the Shoulder Joint
The shoulder is the most flexible joint in the body, making it the most susceptible to instability and injury. This mobility provides the upper extremity with tremendous range of motion. The shoulder join is also known as a ‘ball-and-socket’ joint. The ‘ball’ is at the top of the upper extremity, the humerus, fits neatly into a ‘socket’, called the glenoid cavity, which is part of the shoulder blade, scapula.
The shoulder joint is made up several bones and soft tissues. It is comprised of three bones, the collarbone (clavicle), scapula, and humerus. The three articulations of the shoulder joint are the sternoclavicular joint, acromioclavicular joint, and the glenohumeral joint. The soft tissues of the shoulder include the rotator cuff, biceps tendon, coraco clavicular ligament, cromio clavicular ligament, and glenoid labrum.
Clavicle– The S-shaped bone that forms the anterior portion of the shoulder girdle that keeps the arm away from the body (trunk) and has two articulations, the sternoclavicular joint and the acromioclavicular joint. The clavicle provides protection for the subclavian artery and vein, as well as the brachial plexus.
Scapula– A triangular-shaped bone that is the main site for muscle attachment. The four rotator cuff muscles the supraspinatus, infraspinatus, teres minor, and subscapularis take their origin from the scapula.
Humerus– The bone of the upper arm, extending from shoulder to elbow. The upper surface of the humerus is called the humeral head which articulates against the glenoid cavity and provides attachment to muscles of the upper arm.
Sternoclavicular Joint– This is the connection between the manubrium of the sternum and the clavicle.
Acromioclavicular Joint– The only articulation between the clavicle and scapula. It is formed by the distal clavicle articulating with the acromion of the scapula.
Glenohumeral Joint– The glenoid cavity is the smooth shallow depression at the end of scapula that forms the socket of shoulder joint and is the main articulation of the shoulder joint. It is the ball-and-socket joint formed by the articular surfaces of the glenoid cavity and the head of the humerus.