Sternoclavicular Joint: Functional Classification, Structure, and Biomechanical Significance

What is the functional classification of the sternoclavicular joint?

The sternoclavicular (SC) joint is functionally classified as a diarthrotic joint, specifically a synovial joint that is multiaxial. This classification signifies its free range of motion across multiple planes, which is crucial for the extensive mobility of the upper limb.

Understanding Joint Classification

Joints, or articulations, are points where two or more bones meet. Their functional classification is based on the degree of movement they permit. There are three primary functional categories:

• Synarthrosis: Immovable joints (e.g., sutures of the skull).
• Amphiarthrosis: Slightly movable joints (e.g., pubic symphysis, intervertebral discs).
• Diarthrosis: Freely movable joints, also known as synovial joints.

The Sternoclavicular (SC) Joint: A Diarthrosis

The sternoclavicular joint unequivocally falls into the diarthrotic category. This means it allows for a significant range of motion, distinguishing it from rigid or semi-rigid connections.

As a diarthrotic joint, the SC joint is also structurally classified as a synovial joint. Key characteristics of synovial joints, all present in the SC joint, include:

• Articular Cartilage: Smooth hyaline cartilage covering the bone ends, reducing friction.
• Joint Capsule: A fibrous capsule enclosing the joint, providing stability.
• Synovial Fluid: A viscous fluid within the capsule that lubricates the joint and nourishes the cartilage.
• Articular Disc: Uniquely, the SC joint contains a complete articular disc that divides the joint into two separate cavities, enhancing congruence and shock absorption.
• Reinforcing Ligaments: Strong ligaments (anterior and posterior sternoclavicular, interclavicular, costoclavicular) provide significant stability.

Specific Functional Type: Multiaxial (Saddle Joint Characteristics)

While structurally often described as a saddle joint due to the reciprocal concave-convex surfaces of the sternal end of the clavicle and the manubrium of the sternum, its functional classification emphasizes its movement capabilities. Functionally, the SC joint is multiaxial, meaning it permits movement in three planes of motion.

The multiaxial nature of the SC joint allows for a complex array of movements, enabling the clavicle to move relative to the sternum. These movements include:

• Elevation and Depression:
  • Elevation: The clavicle moves superiorly (e.g., shrugging the shoulders).
  • Depression: The clavicle moves inferiorly (e.g., pulling shoulders down).
  • These movements occur in the frontal (coronal) plane.
• Protraction and Retraction:
  • Protraction: The clavicle moves anteriorly (e.g., reaching forward).
  • Retraction: The clavicle moves posteriorly (e.g., pulling shoulders back).
  • These movements occur primarily in the transverse (horizontal) plane.
• Rotation:
  • The clavicle can rotate anteriorly and posteriorly along its longitudinal axis, particularly during arm elevation. This rotation is crucial for achieving full range of motion at the glenohumeral joint.

Biomechanical Significance of SC Joint Function

The sternoclavicular joint is the only direct bony articulation between the upper limb and the axial skeleton. Its multiaxial, diarthrotic nature is therefore paramount for the overall mobility and function of the entire shoulder girdle and upper extremity.

• Foundation for Shoulder Movement: The SC joint provides a stable yet mobile base from which the scapula can move, which in turn influences the position and movement of the humerus at the glenohumeral (shoulder) joint. Without its extensive range of motion, activities like reaching overhead, throwing, or lifting would be severely limited.
• Force Transmission: It plays a critical role in transmitting forces from the upper limb to the axial skeleton and vice versa, acting as a crucial link in the kinetic chain.
• Shock Absorption: The articular disc within the joint helps to absorb and dissipate forces, protecting the joint surfaces.

Clinical Relevance

Understanding the functional classification of the SC joint is vital in clinical practice. Its high degree of mobility, combined with its critical role as the sole bony attachment of the upper limb, makes it susceptible to injury, although dislocations are relatively rare due to its strong ligamentous support. When injuries do occur (e.g., sprains, dislocations), they can significantly impair upper limb function, highlighting the joint's indispensable contribution to human movement. Rehabilitation efforts often focus on restoring its specific multiaxial movements to regain full shoulder and arm function.