Ligamentous Support of the AC Joint
The coracoclavicular (CC) ligament serves as a primary stabilizer of the AC joint, playing a crucial role in restricting excessive motion and maintaining overall joint integrity. This strong ligamentous complex consists of two distinct components:
Trapezoid Ligament
- Originates from the superior surface of the coracoid process.
- Extends laterally and slightly superiorly to attach to the trapezoid line of the clavicle.
- Restricts excessive lateral displacement of the clavicle.
Conoid Ligament
- Originates from the base of the coracoid process, closer to the midline.
- Attaches almost vertically to the conoid tubercle of the clavicle.
- Restricts superior displacement of the clavicle and contributes to posterior rotation during shoulder elevation.
Biomechanical Properties of the CC Ligament
- Both trapezoid and conoid ligaments exhibit similar lengths, cross-sectional areas, stiffness, and tensile strength.
- The CC ligament is significantly stronger than many other shoulder ligaments, allowing it to absorb greater amounts of force before rupture.
- The nearly vertical orientation of the conoid ligament plays a critical role in scapular stability and controlled clavicular motion.
AC Joint Disc and Articular Cartilage
- The articular surfaces of the AC joint are lined with fibrocartilage, increasing resilience against repetitive stress.
- A complete or partial intra-articular disc (articular disc) separates the joint space.
- Studies of 223 AC joints found that only 10% contained a fully developed articular disc, while most exhibited incomplete discs with signs of degeneration or wear.
- According to DePalma’s research, the presence of an incomplete disc is not considered a structural defect but rather a natural degenerative process due to the repetitive stress experienced by the AC joint.
Kinematics of the AC Joint
SC Joint vs. AC Joint Function
- The sternoclavicular (SC) joint allows large, global movements of the clavicle, establishing the general movement trajectory of the scapula.
- The AC joint, in contrast, facilitates smaller, refined motions, enabling precise scapular adjustments to optimize biomechanics and maintain joint congruency with the thorax.
Degrees of Freedom in AC Joint Motion
- The relative motion between the scapula and the lateral clavicle within the AC joint occurs in a three-dimensional space, allowing for three degrees of freedom:
- Primary Motion: Upward and Downward Rotation
- Upward rotation: Occurs as the scapula elevates (arm abduction/flexion).
- Downward rotation: Occurs as the scapula returns to its resting position.
- Secondary Adjustments: Rotational Fine-Tuning
- These include subtle rotational shifts in the transverse and sagittal planes, ensuring optimal positioning of the scapula against the thorax.
- Scapular Tilting and Protraction/Retraction
- Anterior-posterior tilting fine-tunes scapular alignment.
- Internal-external rotation helps maintain congruency with the ribcage.
- Primary Motion: Upward and Downward Rotation
Challenges in Measuring AC Joint Motion
- Due to its small amplitude and complex movement patterns, precise measurement of individual AC joint motions is technically challenging.
- In clinical practice, direct quantification of AC joint motion is rarely performed due to motion overlap with SC joint and scapulothoracic mechanics.
Functional Significance
The AC joint serves as a critical pivot point in scapular motion, allowing for necessary micro-adjustments that facilitate full arm elevation and optimal force distribution. Understanding its ligamentous stability and kinematics is essential in diagnosing AC joint injuries, including sprains, separations, and degenerative changes that can significantly impact shoulder function and upper limb biomechanics.