Axial Rotation of the Clavicle
The third degree of freedom in the sternoclavicular (SC) joint involves the axial rotation of the clavicle around its longitudinal axis. This rotational movement is strongly coupled with shoulder movements, particularly during arm abduction and flexion.
Key Characteristics of Clavicular Axial Rotation
- As the arm moves away from the body, the superior surface of the clavicle rotates posteriorly by approximately 20–35°.
- During arm lowering, the clavicle reverses its rotation, returning to its anatomical position.
- This motion occurs primarily as a spin movement at the sternal end of the clavicle, relative to the articular disc of the SC joint.
Functional Integration
- Clavicular axial rotation is not an independent motion; it is intrinsically linked to global shoulder mechanics.
- It cannot occur in isolation; it only happens as part of coordinated shoulder movements such as arm elevation.
- Without clavicular rotation, full arm elevation (above 120°) would be significantly restricted.
Acromioclavicular (AC) Joint
The acromioclavicular (AC) joint is a critical articulation that connects the lateral end of the clavicle to the acromion of the scapula. This joint allows for scapular motion relative to the clavicle, contributing to the overall mobility and stability of the shoulder girdle.
Anatomical Features of the AC Joint
- The clavicular facet on the acromion is oriented medially and slightly superiorly, ensuring optimal contact with the acromial facet of the clavicle.
- The AC joint often contains an articular disc, though its size and presence vary significantly among individuals.
- The joint surfaces are typically planar, classifying the AC joint as a gliding or planar synovial joint. However, variations exist, with some joints displaying slight concave or convex contours.
Kinematics of the AC Joint
- Unlike other synovial joints, the AC joint does not follow the traditional roll-and-glide mechanics due to its flat joint surfaces.
- Movements primarily consist of slight gliding and rotational adjustments, accommodating scapular movement relative to the clavicle.
Ligamentous and Muscular Support of the AC Joint
The stability of the AC joint is reinforced by strong capsular and extracapsular ligaments, as well as muscular attachments.
Primary Stabilizing Structures
- Superior and Inferior AC Joint Capsular Ligaments
- Coracoclavicular Ligament (Trapezoid and Conoid Ligaments)
- Articular Disc (if present)
- Muscular Reinforcement by Deltoid and Upper Trapezius
The superior AC ligament is further reinforced by muscle fibers from the deltoid and upper trapezius, providing additional dynamic stability.
Functional Importance
The AC joint plays a key role in scapular mobility and clavicular rotation, ensuring full range of motion for the shoulder complex. Understanding its mechanics is crucial in diagnosing injuries, such as AC joint sprains, osteoarthritis, and clavicular dysfunctions, which can significantly impair shoulder function and upper limb mobility.