Sternoclavicular (SC) Joint: Anatomical Structure and Function

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Sternoclavicular (SC) Joint: Anatomical Structure and Function

General Characteristics and Anatomical Structure

The sternoclavicular (SC) joint is a highly specialized articulation formed between:

  • The medial end of the clavicle,
  • The clavicular facet of the sternum,
  • The superior border of the first rib’s costal cartilage.

This joint serves as a key basilar joint, connecting the appendicular skeleton (upper limb) to the axial skeleton (trunk). Due to its strategic position, the SC joint must fulfill two opposing functional demands:

  1. Provide stability by firmly anchoring the upper limb to the trunk.
  2. Allow mobility, enabling the wide range of motion required for arm movements.

These seemingly contradictory functions are achieved through complex anatomical adaptations, including strong periarticular ligaments and a saddle-shaped articulation.

Joint Surface Configuration

Although individual variations exist, the general structure of the SC joint surfaces follows a reciprocal saddle shape:

  • The medial end of the clavicle is convex vertically and concave transversely.
  • The sternum’s clavicular facet is concave vertically and convex transversely.

This unique curvature enhances both stability and mobility, facilitating smooth multi-directional movements.

Periarticular Ligaments and Stabilizing Structures

The SC joint is reinforced by robust connective tissue structures, including:

  1. Anterior and Posterior Sternoclavicular Ligaments
    • These thickened capsular ligaments secure the joint capsule and prevent excessive anterior-posterior displacement.
  2. Interclavicular Ligament
    • Spanning across the jugular notch, this ligament connects both clavicles, enhancing medial stability.
  3. Costoclavicular Ligament (Rhomboid Ligament)
    • Attaches from the first rib’s cartilage to the costal tuberosity of the clavicle.
    • Composed of two perpendicular fiber bundles:
      • Anterior bundle: Runs superolaterally.
      • Posterior bundle: Runs superomedially.
    • This crisscrossing fiber orientation restricts all movements except clavicular depression, providing dynamic stabilization.

Articular Disc and Shock Absorption

A key stabilizing feature of the SC joint is the articular disc, a fibrocartilaginous structure that:

  • Divides the joint cavity into two compartments (medial and lateral).
  • Increases joint congruency, distributing loads more evenly.
  • Absorbs compressive forces that occur during upper limb activities.

The articular disc attaches:

  • Inferiorly to the sternal facet of the clavicle,
  • Superiorly to the interclavicular ligament and the clavicle’s sternal end,
  • Laterally to the joint capsule’s inner surface.

However, degenerative changes in aging individuals can reduce its shock-absorbing capacity, leading to osteoarthritis of the SC joint.

SC Joint Stability and Injury Considerations

The SC joint’s stability primarily results from:

  • The interlocking structure of the joint surfaces.
  • The extensive reinforcement provided by periarticular ligaments.

Due to its strong ligamentous support, SC joint dislocations are rare. Instead, in cases of high-impact trauma (e.g., sports injuries, car accidents), the force is more likely to cause clavicle fractures rather than SC joint dislocations.

  • Clavicle fractures are particularly common in men under 30, primarily due to contact sports or motor vehicle accidents.
  • SC joint instability may still occur following ligamentous injuries or repetitive strain on the joint.

Summary of SC Joint Stabilizing Structures

  1. Anterior and Posterior Sternoclavicular Ligaments – Prevent excessive anterior-posterior displacement.
  2. Interclavicular Ligament – Connects both clavicles, stabilizing the medial ends.
  3. Costoclavicular Ligament – Provides multi-directional stability through crisscrossing fiber orientation.
  4. Articular Disc – Enhances joint congruency and absorbs shock.
  5. Muscular Reinforcement:
    • Anteriorly: Sternocleidomastoid (SCM) muscle.
    • Posteriorly: Sternothyroid and Sternohyoid muscles.
    • Inferiorly: Subclavius muscle.

Understanding the SC joint’s anatomy and biomechanics is critical for diagnosing shoulder girdle dysfunctions and managing clavicular injuries effectively.