The shoulder is often thought of as a single anatomical structure, but it is actually a highly mobile complex known as the pectoral girdle or shoulder girdle. It consists of several bones that connect the upper limb to the trunk of the body. This unique skeletal configuration allows for the tremendous range of motion that makes the human arm versatile. Understanding the shoulder requires identifying its bony components and how they interact to achieve both mobility and stability.
The Three Primary Bones
The structure of the shoulder is based on three major bones: the clavicle, the scapula, and the humerus. These bones form the framework for all shoulder movement.
The clavicle, or collarbone, is the only long bone in the body that lies horizontally. It acts as a horizontal strut, connecting the sternum (breastbone) to the shoulder blade. This connection keeps the shoulder away from the chest, allowing the arm to swing freely. Its S-shape allows it to transmit forces from the arm to the rest of the skeleton.
The scapula, or shoulder blade, is a large, flat, triangular bone situated on the back of the rib cage. It serves as an anchor point for many muscles that initiate movement and provide stabilization. Two projections on the scapula are the acromion, which forms the highest point of the shoulder, and the glenoid fossa, a shallow, cup-like depression.
The humerus is the long bone of the upper arm; only its proximal end contributes to the shoulder complex. The spherical head of the humerus fits directly into the scapula’s glenoid fossa, creating the primary articulation. This ball-shaped head is responsible for the wide, circular movements that characterize the arm’s mobility.
How the Shoulder Bones Connect
The unique mobility of the shoulder comes from the articulation of its bones across three distinct joints. These joints coordinate their movements to provide the arm’s full functional range.
The primary point of movement is the glenohumeral joint, the classic ball-and-socket joint formed by the head of the humerus fitting into the glenoid fossa of the scapula. This joint is the most mobile joint in the entire body, allowing rotation and movement in multiple planes. The high range of motion comes at the expense of stability because the socket is shallow.
The acromioclavicular (AC) joint is located at the top of the shoulder, connecting the lateral end of the clavicle to the acromion process of the scapula. This joint is a plane synovial joint, allowing only slight gliding and rotational motions. Its role is to help position the scapula and transmit forces from the arm to the clavicle.
The sternoclavicular (SC) joint is the only bony connection between the entire upper limb and the axial skeleton (the chest and spine). It is where the medial end of the clavicle meets the sternum (breastbone). Its movements of elevation, depression, protraction, and retraction are crucial for allowing the scapula to move across the chest wall.
Common Injuries of the Shoulder Bones
Because the shoulder’s design prioritizes mobility, its bony structure is susceptible to specific types of trauma. These injuries often result from falls, direct impact, or high-energy accidents.
A clavicle fracture is one of the most common bone breaks, frequently occurring when a person falls onto an outstretched hand or directly onto the shoulder. The fracture most often occurs in the middle third of the S-shaped bone, its weakest point. A break in the collarbone can cause the shoulder to slump downward and forward due to the loss of the clavicle’s strut function.
A proximal humerus fracture involves a break near the top of the upper arm bone, close to the shoulder joint. This injury is common in older adults, especially those with osteoporosis, but it can also result from high-impact trauma in younger individuals. These fractures can compromise the ball-and-socket mechanics of the glenohumeral joint.
When the head of the humerus is forced out of the glenoid fossa, it results in a shoulder dislocation. This is the most frequently dislocated major joint in the body due to the shallow nature of its socket. A dislocation is a serious injury that can damage the surrounding joint capsule and ligaments, potentially leading to chronic instability if not properly treated.
An AC joint separation is a common injury where the ligaments connecting the clavicle to the acromion are torn. This separation is not a true dislocation of the main shoulder joint, but rather a disruption of the joint that helps stabilize the shoulder girdle. The injury typically causes a noticeable bump above the shoulder as the clavicle is pushed upward.