Shoulder (Glenohumeral) Joint
The shoulder joint (glenohumeral joint) is the most flexible joint in the human body. Due to its numerous ligaments and muscles it is also a quite strong and physically powerful ball-and-socket joint (spheroidal joint).
Anatomy of the shoulder joint
The shoulder joint consists of the humeral head and the socket (glenoid cavity). The socket is part of the shoulder blade (scapula) and is enlarged by the glenoid labrum, a “lip” of fibrocartilage. The articular capsule which completely surrounds the cavity is reinforced by several ligaments (coracohumeral ligament, glenohumeral ligaments).
The muscle system is significantly important for the stability and movements in the shoulder joint, especially the rotator cuff. The rotator cuff consists of the following four muscles: subscapularis muscle, teres minor muscle, infraspinatus muscle and supraspinatus muscle. The functions of this muscle group are various. On the one hand they fix the humeral head to the socket; on the other hand they execute different movements. They allow both the internal and external rotation of the humeral head and furthermore the abduction, adduction and retroversion.
The long tendon of the two-headed Biceps brachii muscle originates at the supraglenoid tubercle and passes through the rotator cuff. The origin of the short tendon of the biceps is the coracoid process (“raven's beak” extension). The insertion of the biceps is the radial tuberosity and the antebrachial fascia of the forearm. The contraction of the biceps brachii muscle leads to an abduction, adduction or anteversion, depending on the particular contracting part.
The acromion (highest point of the shoulder) is a bony part of the scapula. Together with the coracoid process it forms a cavity in which the humeral head and the tendon of the supraspinatus muscle moves. This cavity comprises an important synovial bursa (subacromial bursa) which provides a buffer reducing compression and friction forces. However the bursa may also cause problems, e.g. it can become inflamed through overuse or injury (see also: pathology of the shoulder).