The shoulder has 3 bones namely the clavicle, scapula, and the humerus. These bones together form the ball and socket joint. The head of the humerus fits into a cavity present on the scapula, known as the glenoid cavity or glenoid fossa. The term glenoid is Greek in origin, where glenoid means a socket. It is a very important part of the entire elbow joint.
The shoulder consists of two major joints; the glenohumeral and the sternoclavicular joint. The latter is a joint between the clavicle and the manubrium that is the head of the sternum, more commonly known as the collar joint. The glenohumeral joint is the one between the humerus and the scapula.
The shoulder joint is stabilized by a process above the glenoid known as the coracoid process of the scapula. Another process, called the Acromion process, an extension of the scapula, which extends over the shoulder joint.
The glenoid cavity of scapula is present on its lateral side. It is pear shaped or more often called a pyriform surface. It is hollow, but shallow with gentle concave shape. It is lined by the hyaline cartilage. A fibrous cartilage, known as the glenoid labrum, lines the rim of this cavity.
The glenoid labrum is a triangular fix at the base of the cavity and it deepens the cavity. Above it, is the supraglenoid tubercle and below is the ridge of the bone, known as the infraglenoid tubercle. The cavity articulates with the humerus in such a way, that the movement of the hand becomes quite flexible.
The following movements of the shoulder are possible due to the nature of the shoulder joint, mostly the humerus and the glenoid cavity:
- Scapular retraction
- Scapular protraction
- Scapular depression
- Arm abduction
- Arm adduction
- Arm flexion
- Arm extension
- Medial rotation of the arm
- Lateral rotation
- Arm circumduction
All these motions make this joint one of the most flexible in the human body, however, this flexibility also makes it more prone to dislocation and injuries. The loose articulation is responsible for the fragility of this joint. The ball and socket joint is one of the most fragile joints of the human skeletal system.
This is the function of the cavity to allow smooth movements of the shoulder. Maintaining the stability is also one of its functions, however, dislocations and fractures of this joint are quite common.
The ball and socket joint is quite fragile. Injuries of the glenoid cavity are commonly seen in sportsmen, especially baseball pitchers. Dislocations of the glenohumeral joint may also cause a fracture on the rim of the cavity.
Another type of fracture of the glenoid is the Bankart lesion, which is also caused due to the anterior dislocation of the humeral head. Hill Sachs Lesion is also a fracture caused due to the impact of the humerus on the rim of the cavity.
This bone fracture is linked with the anterior dislocation of the humerus that causes a compression fracture of the humeral head. These fractures are best diagnosed by CT scanning and MRI.
The glenoid cavity, though a very small part of the human anatomy, is extremely vital to humans. Easy and flexible movement of the limbs is one of the main characteristics that distinguishes us from other animals (other than primates of course).