Anatomy: Joints on the pectoral girdle

Joints on Pectoral Girdle

The human pectoral girdle contains several important joints that play a key role in shoulder mobility and stability. Two primary types of joints located within the pectoral girdle are:

  1. Sternoclavicular Joint
  2. Acromioclavicular Joint

Shoulder Joints

The shoulder comprises three major joints that allow for a wide range of motion:

  1. Sternoclavicular Joint
  2. Acromioclavicular Joint
  3. Glenohumeral Joint

Sternoclavicular Joint

1. Articular Components

The sternoclavicular joint is formed between the following structures:

  • The sternal end of the clavicle.
  • The clavicular notch of the manubrium of the sternum.
  • The adjoining first costal cartilage.

2. Classification

  • This joint is classified as a synovial joint, specifically a planar joint.

3. Movements

The sternoclavicular joint allows for various movements, including:

  • Gliding movements, which permit limited motion in nearly every direction, including:
    • Backward
    • Upward
    • Forward
    • Downward
    • Circumduction

Stabilization of sternoclavicular joint

The stability of the sternoclavicular joint is ensured by a series of ligaments including:

  • Articular Capsule: surrounding the articulation, varying in thickness and strength; thicker anteriorly and posteriorly, while thinner above and below.
  • Anterior Sternoclavicular Ligament: a broad band of fibers on the anterior surface attached to the upper front part of the sternal end of the clavicle and the front of the upper part of the manubrium sternum.
  • Posterior Sternoclavicular Ligament: similar to the anterior ligament, it covers the posterior surface attaching at the upper back part of the sternoclavicular joint.
  • Interclavicular Ligament: a flattened band that varies in form and size, running from the upper part of one clavicle to the other and fixed to the upper margin of the sternum.
  • Costoclavicular Ligament: a flat, rhomboid-form ligand attached to the first rib's cartilage, ascending obliquely to the costal tuberosity under the clavicle. Nerve supply is provided by the medial supraclavicular nerves (C3, C4) from the cervical plexus.

Movements of the Joint

The sternoclavicular joint allows for several important movements:

  • Elevation: shrugging the shoulder, causing upward movement of the sternal end of the clavicle.
  • Depression: downward movement of the sternal end of the clavicle.
  • Forward and Backward Movements: squaring the shoulders adjusts movements at the sternal end.
  • Rotary Movements: abduction of the arm above the head involves upward rotation of the scapula. The acromioclavicular joint enables this by transmitting forces through the coracoclavicular ligaments without direct involvement from the rotator cuff muscles.

Joint Stability

The integrity of the sternoclavicular joint is maintained primarily by the costoclavicular ligament, which helps to transmit stress from the clavicle to the first costal cartilage. This primary cartilaginous joint is immovably fixed to the manubrium, ensuring unusual occurrences of dislocation as the clavicle tends to fracture before dislocating.

Acromioclavicular Joint

1. Articular Components

The acromioclavicular joint is formed by the following components:

  • The acromion process of the scapula.
  • The acromial end of the clavicle.

2. Classification

  • This joint is also categorized as a synovial joint, specifically a planar joint.

3. Movements

The acromioclavicular joint allows:

  • Gliding and Rotation: of the scapula on the clavicle, facilitating reciprocal movements forward and backward.

Stabilization of Acromioclavicular joint

The acromioclavicular joint is strengthened by a series of ligaments, including:

  • Acromioclavicular ligaments: providing overall stability across the joint.
  • Coracoclavicular ligaments: comprising:
    • Conoid Ligament: extending from the coracoid process to the conoid tubercle of the clavicle.
    • Trapezoid Ligament: attached to the trapezoid ridge on the undersurface of the clavicle and provides additional stability.
  • Coracoacromial Ligament: controls anterior and posterior translation of the lateral clavicle.

Function of Ligaments

The coracoclavicular ligament plays a crucial role in maintaining vertical stability and restrains superior and anterior displacements. The acromioclavicular ligament prevents posterior translation and displacement of the clavicle. Together, these ligaments form the joint capsule, which is a watertight sac enclosing the joint and lubricating fluids.

Movements of the Joint

The movements allowed at the acromioclavicular joint include:

  • Protraction and Retraction: around the chest wall.
  • Elevation and Depression: of the scapula.
  • Rotation: essential during scapular movements.
Stability Understanding

The stability of the acromioclavicular joint is significantly supported by the coracoclavicular ligament, allowing the upper limb to be suspended from the clavicle. Forces generated in the upper limb are transmitted medially through the trapezoid ligament and then to the first rib through the costoclavicular ligament.

Additional Notes

Nerve supply to the acromioclavicular joint is facilitated by the suprascapular nerve (C5, 6).

Conclusion

The understanding of these joints—the sternoclavicular and acromioclavicular—is critical for comprehending shoulder mechanics and hence, pivotal for both anatomical studies and practical applications such as rehabilitation and physical therapy.