The Shoulder Joint
The glenohumeral or commonly referred to as shoulder joint is a synovial, ball and socket type, multiaxial articulation between the head of the humerus and the glenoid fossa of the scapula. It offers the greatest mobility or range or motion throughout the body, capable of performing all terms of movement from extension down to pronation. The glenoid fossa is actually just a shallow articulation and the danger of dislocation always lurks in the background but thanks to the labrum found around its rim, it deepens the connection of the glenoid fossa to the humerus. To prevent damage by friction to the two bones, a thin layer of hyaline cartilage covers both the glenoid fossa and the head of the humerus. The resulting connection is further secured in place by a fibrous joint capsule lined with synovial membrane that houses synovial fluid. Bursae or sacs of synovial fluid are found between the tendons and the muscles. The subacromial bursase is of particular importance to medical professionals.
The tendons add further stability to the mobile shoulder joint as it helps glue the muscles to the bones. Ensheathed by synovium, the tendon passes over the head of the humerus within the fibrous capsule and emerges below the capsule in the intertubercular groove to join the long head of biceps brachii. The tendon originates from the tubercle of the supraglenoid found in the scapula located directly above the glenoid labrum.
The fibrous capsule is lenient yet incorporates thickened bands of glenohumeral ligaments. This capsule’s task is to protect the ball and socket joint by surrounding it and keeping the synovial fluid in place. Another vital task that this joint capsule is entrusted to is to help keep the shoulder properly aligned and in the correct position. Because it is used very often and sometimes near it’s threshold in terms of tension and usage, the joint capsule suffers from overuse. Capsules become abnormally lax, the labrum becomes torn from its attachments, the tendon of biceps become frayed and torn and the cavity of the joint may communicate with various bursae. Repetitive dislocations of the humeral head may induce articular cartilage damage. A condition known as multidirectional instability happens when there is little or not enough tension to hold the shoulder joint in place. On the other hand, if too much pressure exists, a condition known as frozen shoulder happens.
To be worthy of being referred as the joint in the body that has the greatest range of motion, the shoulder joint then needs to be separated from the body. It is classified as an “unstable” joint or a joint that supports without attaching to a parent bone. However, several ligaments exists to make sure that the shoulder joint does not fall out of place.