The shoulder is a sophisticated, “ball and socket” joint comprised of three bones (clavicle, humerus and scapula) and four individual joints (glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic). Together, these are known as the shoulder girdle.
The clavicle, also known as the collar bone, is the only bony attachment between the upper limb and the trunk. It articulates on the medial end with the sternum and on the lateral end, with the acromion of the scapula. Furthermore, the clavicle forms the anterior portion of the shoulder girdle.
The glenoid cavity is the shoulder structure that serves as the “socket.” The humeral head fits into the shallow glenoid cavity, providing the “ball” to make up the ball and socket joint. Compared to the hip “socket,” or acetabulum, the lack of depth in the cavity affords the humerus complete rotation about the joint. However, due to its skeletal structure, the shoulder is relatively unstable. The rotator cuff muscles, the labrum, and glenohumeral ligaments provide joint stability.
The scapula is a large, flat, triangular shaped bone with three distinct processes: the acromion, spine and corocoid process. The articulation formed between the acromion and the clavicle makes up the acromioclavicular (AC) joint. The scapula articulates laterally with the humeral head at the glenoid cavity and glides along the back of the chest during arm extension. The coracoid process is on the anterior side and serves as an attachment for ligaments and muscles.
The shoulder also contains a significant amount of soft tissue. It is comprised of several muscles, tendons, ligaments and cartilage. The rotator cuff is a group of four muscles and tendons that originate on the scapula and attach distally on the tuberosities of the humerus. These muscles provide stability by keeping the humeral head centrally located in the glenoid cavity, or fossa by surrounding the joint, creating a cuff to support movement of the ball and socket. The rotator cuff is also essential in moving the arm as it allows the arm to be raised overhead, in front of the body or to the side.
In addition to muscles and tendons, ligaments also provide stability to the shoulder by forming the joint capsule that surrounds the glenohumeral joint. The labrum is a ring of elastic cartilage found within the shoulder socket, surrounding the edge of the glenoid fossa. Comprised of the same biological material as the menisci in the knee joint, the labrum is also found within the acetabulum of the hip. It acts as a container for the humeral head, deepening the socket to maintain stability of the shoulder. It creates a suction seal to further act as a stabilizer of the shoulder joint.