Clavicle: Anatomy, Biomechanics, and Common Injuries

What is the biomechanics of the clavicle?

The clavicle, or collarbone, is a slender, S-shaped bone that serves as the only bony connection between the axial skeleton and the upper limb, playing a critical role in shoulder girdle mobility, stability, and force transmission.

The Clavicle: An Anatomical Overview

The clavicle is a long bone, although it lacks a medullary cavity like typical long bones. Its unique S-shape allows it to function effectively as a strut, keeping the upper limb away from the trunk. It has two distinct ends:

• Sternal (Medial) End: This rounded, triangular end articulates with the manubrium of the sternum to form the sternoclavicular (SC) joint.
• Acromial (Lateral) End: This flattened end articulates with the acromion of the scapula to form the acromioclavicular (AC) joint.
• Shaft: The body of the clavicle, which is convex anteriorly at its medial two-thirds and concave anteriorly at its lateral one-third. It serves as an attachment point for various muscles and ligaments.

Crucial Role in Shoulder Girdle Function

The clavicle's biomechanical significance stems from its multifaceted roles within the shoulder complex:

• Spacer Function: It acts as a rigid strut, holding the scapula and the upper limb laterally away from the thorax. This provides a wide range of motion for the upper extremity, preventing impingement against the rib cage during arm movements.
• Lever Arm and Muscle Attachment: It provides extensive surface area for the attachment of numerous muscles that move the shoulder and neck, including the deltoid, pectoralis major, sternocleidomastoid, trapezius, and subclavius. These muscles leverage the clavicle to produce movement and stability.
• Protection: Positioned superior to vital neurovascular structures (brachial plexus, subclavian artery and vein), the clavicle offers a degree of protection, though it can also be a point of compression in conditions like thoracic outlet syndrome.
• Force Transmission: It efficiently transmits forces from the upper limb through the shoulder girdle to the axial skeleton. For instance, when pushing or pulling, forces generated by the arm are transferred via the clavicle to the trunk.

Key Articulations and Their Biomechanical Significance

The clavicle forms two critical joints, each contributing uniquely to shoulder kinematics:

• Sternoclavicular (SC) Joint:

  • Type: Classified as a saddle joint, but functionally acts more like a ball-and-socket joint due to its extensive range of motion. It is the only direct bony articulation between the upper limb and the axial skeleton.
  • Movements: The SC joint allows for three primary movements of the clavicle:
    • Elevation and Depression: Approximately 45-60 degrees of elevation and 10 degrees of depression.
    • Protraction and Retraction: Approximately 15-30 degrees of movement in the horizontal plane.
    • Posterior Rotation: Approximately 30-50 degrees of rotation along its longitudinal axis, which is crucial for full arm elevation.
  • Stability: Highly stable due to strong ligaments (anterior and posterior sternoclavicular ligaments, interclavicular ligament, and the powerful costoclavicular ligament) and an articular disc that improves congruence and acts as a shock absorber.

• Acromioclavicular (AC) Joint:

  • Type: A plane (gliding) synovial joint, allowing for subtle but essential movements.
  • Movements: Primarily allows for small gliding movements and rotations that fine-tune the position of the scapula relative to the clavicle:
    • Upward/Downward Rotation: Accommodates scapular rotation.
    • Internal/External Rotation: Small rotational adjustments.
    • Anterior/Posterior Tipping: Minor tipping movements.
  • Stability: Primarily supported by the acromioclavicular ligaments (superior and inferior) and, more importantly, the strong coracoclavicular ligaments (trapezoid and conoid ligaments). These ligaments are vital for preventing superior displacement of the clavicle relative to the acromion and limit excessive rotation.

Clavicular Movement and Scapulohumeral Rhythm

Clavicular motion is an integral component of scapulohumeral rhythm, the coordinated movement of the scapula and humerus that allows for full range of motion of the arm. For every 3 degrees of arm abduction or flexion, 2 degrees occur at the glenohumeral joint and 1 degree at the scapulothoracic joint (which involves movement of the scapula on the thorax).

The clavicle's role in this rhythm is critical:

• During arm elevation (abduction or flexion), the clavicle first elevates at the SC joint, then retracts, and most importantly, posteriorly rotates along its longitudinal axis. This posterior rotation allows the conoid ligament to tighten, pulling the inferior angle of the scapula medially and contributing to the upward rotation of the scapula. Without this clavicular rotation, full arm elevation would be significantly limited (to approximately 90-120 degrees).
• During arm depression, the reverse occurs: the clavicle depresses, protracts, and anteriorly rotates.

This intricate coordination ensures optimal positioning of the glenoid fossa for the humeral head, maximizing stability and preventing impingement.

Clinical Biomechanics and Common Injuries

Given its exposed position and critical biomechanical roles, the clavicle is susceptible to injury:

• Clavicle Fractures: These are among the most common fractures, often occurring in the mid-shaft due to direct trauma (e.g., a fall directly onto the shoulder) or an indirect fall on an outstretched hand. Fractures can disrupt the continuity of the shoulder girdle, impacting range of motion and force transmission.
• Acromioclavicular (AC) Joint Separations: Also known as "shoulder separations," these injuries involve damage to the AC ligaments and/or the coracoclavicular ligaments, typically from a fall directly onto the acromion. The severity is graded based on the extent of ligamentous disruption, affecting the stability of the AC joint and the position of the clavicle relative to the acromion.
• Sternoclavicular (SC) Joint Dislocations: These are relatively rare due to the inherent stability of the joint but can be serious, especially posterior dislocations that may impinge on vital structures in the mediastinum.
• Thoracic Outlet Syndrome (TOS): The clavicle forms part of the boundaries of the thoracic outlet. Anomalies or injuries involving the clavicle can narrow this space, compressing the neurovascular bundle (brachial plexus, subclavian artery and vein) and leading to symptoms in the upper limb.

Conclusion: The Unsung Hero of the Shoulder Girdle

The clavicle, often overlooked in its complexity, is a biomechanical marvel. Its unique S-shape, two distinct articulations, and intricate ligamentous support enable it to serve as a vital link between the trunk and the upper limb. Understanding its movements—elevation, depression, protraction, retraction, and especially posterior rotation—is fundamental to comprehending the full range and efficiency of shoulder motion. From protecting delicate neurovascular structures to facilitating powerful overhead movements, the clavicle is indeed an unsung hero, crucial for the stability, mobility, and overall function of the human upper extremity.