Shoulder

The human shoulder is a complex anatomical region. It allows for an extraordinary range of motion. This region includes several bones, muscles, ligaments, and tendons. The primary bones are the clavicle, or collarbone. There is also the scapula, known as the shoulder blade. Finally, the humerus serves as the upper arm bone. The shoulder is defined as the group of structures in this joint region. It must be mobile enough for wide arm movements. It must also be stable enough for lifting, pushing, and pulling.

The main connection is the glenohumeral joint. This is a ball-and-socket joint. The rounded head of the humerus acts as the ball. The socket is the glenoid cavity of the scapula. This cavity is quite shallow. In fact, there is a 4-to-1 size disproportion between the large humeral head and the small cavity. To help deepen the socket, a fibrocartilaginous ring called the glenoid labrum is present. This labrum is more rigid than the articular cartilage on the bone ends. Articular cartilage allows the bones to glide smoothly.

Stability is provided by a group of four muscles called the rotator cuff. These muscles are the supraspinatus, subscapularis, infraspinatus, and teres minor. These muscles and their tendons surround the joint. They attach to the humeral head and the glenohumeral capsule. This group works to hold the humerus in the glenoid cavity. It prevents the bone from migrating upward when the deltoid muscle pulls. The infraspinatus and teres minor help with external rotation. Their tendons often fuse near their junctions.

Other muscles contribute to movement and stability. The deltoid muscle covers the joint on three sides. It arises from the clavicle, acromion, and the spine of the scapula. The deltoid helps with flexion, abduction, and extension. The teres major assists with medial rotation. Muscles from the chest, like the pectoralis minor, help rotate the scapula. Muscles from the back, like the trapezius, assist with elevation and retraction. The serratus anterior helps fix the scapula to the thoracic wall. Even the neck muscles, like the sternocleidomastoid, play a role in respiration and shoulder movement.

Supporting structures include various ligaments and sacs. The joint is wrapped in a soft tissue envelope called the capsule. This capsule is lined by a smooth synovial membrane. To ensure smooth movement, the body uses bursae. These are small, sac-like structures. They act as cushions between bone, muscle, and tendon. They protect the rotator cuff from the bony arch of the acromion. Ligaments like the coracohumeral and the glenohumeral ligaments strengthen the capsule. These connections help maintain the relationship between the bones during movement.

Vital pathways travel through the armpit, or axilla. The armpit is the space between the shoulder muscles. Nerves and blood vessels pass through this area. The brachial plexus is a major nerve network. It forms from nerve roots C5 through T1. These nerves supply the majority of the shoulder muscles. The axillary artery is a primary blood source for the region. It arises from the subclavian artery. A network of vessels, or anastomosis, exists behind the shoulder. This network helps supply blood even if one artery is compromised.

The shoulder's design involves a significant trade-off. Because the socket is shallow, the arm has tremendous mobility. It can move through a full 360-degree range in the sagittal plane. However, this mobility makes the joint very unstable. The shoulder is much easier to dislocate than most other joints. The loose connections allow for movement but increase the risk of injury. Despite this, the complex coordination of bones and muscles makes the shoulder essential for human activity.