Anatomical Structures of Glenohumeral Joint
Designed for Movement
Each joint in the body is structurally designed for its specific function. Some joints are designed to be very stable (hip joint) whereas others are designed for less stability but for more movement (shoulder joint). Some are designed to have limited motion (humeroulnar joint can only flex and extend), while others can move in multiple directions (glenohumeral joint).
Bony Structure of Glenohumeral Joint
The glenohumeral joint (primary shoulder joint between the head of the humerus and the glenoid fossa of the scapula) is a shallow joint with little bone to bone contact between the two articulating surfaces. Although known as a “ball and socket” joint, this specific joint has a shallow “socket” lending itself to greater mobility but less stability.
A “ball and socket” joint is one classification of joints. It is designed to provide movement in a number of directions including flexion and extension (front and back), abduction and adduction (out to the side and back), internal and external rotation, and diagonal movements. This structure allows the glenohumeral joint to move in multiple directions for the purpose of performing many types of activities and motions.
Because the glenohumeral joint has a shallow bone-to-bone surface, there are numerous connective tissue structures designed to enhance the stability of the joint. If any of these soft tissue structures are compromised, then the stability of the joint is also compromised.
Connective Tissue Structures of Glenohumeral Joint
Moving from the inside of the joint outwards, the deepest connective tissue designed to enhance stability is the labrum. The labrum is a fibrocartilaginous ring that surrounds the shallow glenoid fossa deepening the articular surface.
Reinforcing the anterior portion of the joint are the glenohumeral ligaments. They are three ligaments (superior, middle, and inferior) that span the entire front of the joint providing a thick band of resistance. Adding to this support is the coracohumeral ligament which provides stability to the anterosuperior portion of the joint.
The joint capsule is a dense network of connective tissue that surrounds the entire joint. The outer layer is a tough fibrous capsule designed to withstand tensile forces in multiple directions while the inner layer is a synovial membrane producing synovial fluid for the purpose of lubricating the joint. The glenohumeral joint capsule is reinforced by the ligaments.
Function of the Rotator Cuff
The last line of defense is the deep muscle group surrounding the joint specifically known as the rotator cuff. These four muscles are designed to stabilize the head of the humerus within the glenoid fossa as well as produce rotatory movement of the shoulder (internal and external rotation).
These muscles include the infraspinatus and teres minor (moving from the back of the scapula to the front of the humerus), supraspinatus (moving from the top of the scapula to the front of the humerus), and the subscapularis (moving from the front of the scapula to the front of the humerus). These muscles co-contract to keep the head of the humerus centered within the glenoid fossa.
When Stabilizing Structures are Compromised
Each structural layer provides an additional stabilizing line of defense for the purpose of keeping the head of the humerus stabilized within the glenoid fossa. When any of these structures are compromised through tears (labrum, capsule), sprains (ligaments), or strains (muscles), the stability of the glenohumeral joint is also compromised placing the structures around the shoulder at further risk of injury.
Any shoulder injury that reduces the mobility of the joint (pain during movement of the shoulder) needs to be seen by a physician for diagnosis and appropriate treatment.
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