Introduction
The shoulder girdle consists of a series of four joints. A total of 26 muscles connect the axial skeleton to the humerus.
Neuromuscular control allows these muscles to function in concert to give accurate positioning.
The glenoid fossa of the scapula is pear shaped and encloses about one-third of the articular cartilage circumference of the humeral head. It is deepened by the glenoid labrum, a fibrocartilage structure similar to the menisci of the knee but firmly attached to the glenoid margin.
To allow external rotation of the shoulder, the anterior ligaments and capsule are lax in all positions except full external rotation. Similarly, the inferior capsule is lax in all positions except overhead abduction and flexion.
The image below shows magnetic resonance (MR) illustrating the depth of the glenoid.
Learning bite
The volume of the shoulder joint capsule is about three times the volume of the humeral head, and consequently the capsule and ligaments do not contribute to stability except at the extremes of movement.