The shoulder (Mercier 28-29) is composed of three bones: the scapula,, the clavicle and the humerus. The scapula is a thin bone that articulates widely and closely with the posterior chest wall. It also articulates with the humerus by way of a small, shallow, glenoid cavity and with the clavicle at the acromion process. The clavicle and scapula are suspended from the cervical and thoracic vertebrae by the trapezius, levator scapula, and rhomboid muscles.
Four articulations constitute the shoulder joint: the glenohumeral, scapulothoracic, acromioclavicular, and sternoclavicular joints. The stability of these joints is provided by a series of ligaments and muscles.
Motion of the arm results from the coordinated efforts of several muscles. With the irritation of shoulder motion, the scapula is first stabilized. The muscles of the rotator (musculotendinous) cuff then steady the humeral head in the glenoid cavity and cause it to descent. Elevation of the arm results from a combination of scapulothoracic and glenohumeral joint movements. One third of total shoulder abduction is provided by forward and lateral movement of the scapula. The remaining two thirds occurs at the glenohumeral joint through progressively increasing activity of the deltoid and supraspinatus muscles. Thus, even in the complete absence of glenohumeral motion, scapulothoracic movement can still abduct the arm approximately 600 to 700.
The muscle of the rotator cuff (supraspinatus, teres minor, infraspinatus, subscapularis) are separated from the overlying "coracoacromial arch" by two bursae, the subdeltoid and the subcoracoid. These bursae frequently communicate and are affected by lesions of the musculotendinous cuff, acromioclavicular joint, and adjacent structures. They are frequently
referred to as the subacromial bursa. Primary diseases of this bursa are rare, although secondary involvement is quite common.
The shoulder (Matt, July 23, 2002) is made up of three bones: the scapula (shoulder blade), the humerus (upper arm bone), and the clavicle (collar bone).
The rotator cuff connects the humerus to the scapula. The rotator cup is formed by the tendons of four muscles: the supraspinatus, infraspinatus, teres minor, and subscapularis.
Tendons attach muscle to the bones. Muscles move the bones by pulling on the tendons. The rotator cuff helps raise and rotate the arm.
As the arm is raised, the rotator cuff also keeps the humerus tightly in the socket of the scapula. The upper part of the scapula that makes up the roof of the shoulder is called acromion,
A bursa is located between the acromion and the rotator cuff tendons. A bursa is a lubricated sac of tissue that cuts down on the friction between two moving parts. Bursae are located all over the body where tissue must rub against each other. In this case, the bursa protects the acromion and the rotator cuff from grinding against each other.
The Encyclopedia of Medicine explained the movements of the shoulder joint as follows:
"The shoulder joint is a ball-and-socket joint which allows 3600 of movement to give maximum flexibility. In addition to enabling these movements, the muscles of the pectoral girdle add stability.