The clavicle, commonly known as the collarbone, is a slender, S-shaped bone that connects the torso and the upper limb. Although classified as a long bone, it lies horizontally, unlike most others in the body. Its primary function is to support the shoulder, serving as a strut that maintains the distance between the shoulder joint and the chest. This structural role is accomplished through two distinct articulation points: one on the medial side toward the center of the body, and the other on the lateral side toward the shoulder.
Connection to the Breastbone
The medial end of the clavicle forms a joint with the manubrium, the uppermost section of the sternum (breastbone). This articulation is named the sternoclavicular joint, and it represents the only direct bony connection between the upper extremity and the axial skeleton. The joint is a saddle-type synovial joint, allowing for considerable movement in three axes, including elevation, depression, and rotation.
Despite its mobility, the sternoclavicular joint is strong and stable, largely due to a dense capsule and reinforcing ligaments. The costoclavicular ligament, which anchors the clavicle to the first rib, is the main stabilizing force for this joint. An articular disc of fibrocartilage resides within the joint space, helping to absorb forces and enhance movement capabilities. Due to the strength of this articulation, the clavicle is more likely to fracture than the joint is to dislocate.
Connection to the Shoulder Blade
The lateral, flattened end of the clavicle articulates with the acromion, a bony projection of the scapula (shoulder blade). This junction is termed the acromioclavicular joint, often referred to as the AC joint. Classified as a plane synovial joint, it primarily allows for gliding movements between the two bones.
The movements permitted at the acromioclavicular joint are small but essential for the overall range of motion of the shoulder complex. Stability is provided by the acromioclavicular ligaments, which directly connect the two bones, and the coracoclavicular ligament. The coracoclavicular ligament is composed of two distinct bands—the conoid and trapezoid ligaments—which connect the clavicle to the coracoid process of the scapula.
Role in Shoulder Stability and Movement
The clavicle’s connections to the breastbone and the shoulder blade allow it to function as a rigid strut necessary for wide-ranging arm motion. This strut-like action keeps the scapula and the shoulder joint positioned away from the thoracic cage, maximizing the arm’s range of motion. Without this structural separation, the arm’s ability to move, especially to raise overhead, would be severely limited.
The bone also plays a significant role in force transmission, channeling forces from the upper limb into the central axial skeleton. The clavicle provides protection for neurovascular structures, such as major blood vessels and nerves, that supply the arm. When the continuity of the clavicle is disrupted, such as by a fracture, the shoulder kinematics change, causing reduced rotation and tilting of the scapula during arm elevation.