Clavicle & Scapula: Understanding Their Attachment, Joint Function, and Common Injuries

What is the attachment of the clavicle to the scapula?

The clavicle (collarbone) attaches to the scapula (shoulder blade) primarily at the acromioclavicular (AC) joint, a synovial joint formed between the lateral end of the clavicle and the acromion process of the scapula, reinforced by several key ligaments.

The Acromioclavicular (AC) Joint: The Primary Connection

The most direct and functionally significant attachment between the clavicle and the scapula occurs at the acromioclavicular (AC) joint. This is a synovial plane joint located at the highest point of the shoulder.

• Articulation: The AC joint is formed by the articulation of the lateral (acromial) end of the clavicle with the medial border of the acromion process of the scapula. The acromion is the bony projection at the top of the shoulder blade.
• Function: While seemingly small, the AC joint is crucial for the complex movements of the shoulder girdle. It allows the scapula to rotate and move relative to the clavicle, which is essential for achieving the full range of motion of the arm, particularly during overhead activities. It also helps to transmit forces from the upper limb to the axial skeleton.

Ligamentous Support of the AC Joint

The stability of the AC joint, despite its small articulating surfaces, is largely dependent on a robust network of ligaments. These ligaments are critical for preventing excessive movement and maintaining the integrity of the joint.

• Acromioclavicular Ligament: This ligament directly reinforces the joint capsule of the AC joint. It consists of superior and inferior bands that run over and under the joint, respectively, strengthening it against anterior and posterior displacement of the clavicle relative to the acromion.
• Coracoclavicular Ligament: This is arguably the most important ligament for the stability of the AC joint, despite not being directly part of the joint capsule. It connects the clavicle to the coracoid process of the scapula (a hook-like projection anterior to the glenoid cavity). The coracoclavicular ligament is composed of two distinct parts:
  • Trapezoid Ligament: Located more laterally, this flat, quadrilateral ligament runs from the superior surface of the coracoid process obliquely upward and laterally to the trapezoid line on the inferior surface of the clavicle. It resists posterior displacement and medial compression forces on the clavicle.
  • Conoid Ligament: Situated more medially, this conical or triangular ligament runs from the base of the coracoid process upward and medially to the conoid tubercle on the inferior surface of the clavicle. It primarily prevents superior displacement of the clavicle relative to the acromion, bearing the brunt of forces that would otherwise cause an AC joint separation.

Together, the trapezoid and conoid ligaments provide strong vertical and horizontal stability to the AC joint, preventing the clavicle from overriding the acromion.

Functional Significance in Movement and Stability

The attachment of the clavicle to the scapula via the AC joint and its strong ligamentous complex is fundamental to the overall function of the shoulder girdle.

• Scapular Mobility: It allows the scapula to rotate, elevate, depress, protract, and retract, movements that are indispensable for orienting the glenoid fossa (the socket for the humerus) and optimizing the leverage of the rotator cuff muscles.
• Arm Range of Motion: Without the coordinated movement at the AC joint, the full range of motion of the humerus at the glenohumeral joint (shoulder joint proper) would be severely limited, especially during overhead activities. The clavicle acts as a strut, keeping the scapula at a fixed distance from the trunk, allowing the arm to move freely.
• Force Transmission: The clavicle, through its attachment to the scapula, plays a crucial role in transmitting forces from the upper limb to the axial skeleton (trunk). This is vital for activities involving pushing, pulling, and carrying loads.

Common Injuries and Clinical Relevance

Due to its exposed position and critical role in force transmission, the AC joint is susceptible to injury, commonly known as an AC joint separation or sprain.

• Mechanism of Injury: These injuries typically result from a direct blow to the top of the shoulder (e.g., during a fall, contact sport collision) or a fall on an outstretched hand.
• Grading: AC joint separations are classified into grades (I-VI) based on the severity of ligamentous damage (AC and coracoclavicular ligaments) and the degree of clavicular displacement relative to the acromion. Higher grades involve complete tearing of both sets of ligaments and significant superior displacement of the clavicle.
• Clinical Implications: Injuries to the AC joint can cause significant pain, tenderness, visible deformity (in higher grades), and limitations in shoulder movement, particularly overhead activities. Rehabilitation often focuses on restoring stability, strength, and range of motion.

Conclusion: A Foundation for Upper Body Function

The intricate attachment of the clavicle to the scapula at the acromioclavicular joint, buttressed by the vital acromioclavicular and coracoclavicular ligaments, forms a cornerstone of upper body biomechanics. This seemingly simple connection is a sophisticated articulation that balances mobility and stability, enabling the vast range of motion of the arm while simultaneously providing a stable base for powerful movements. Understanding this anatomical relationship is paramount for comprehending shoulder function, diagnosing injuries, and designing effective rehabilitation and training protocols.