Clavicular Joints: Anatomy, Function, and Common Conditions

What is clavicular joint?

The term "clavicular joint" typically refers to the two main articulations involving the clavicle (collarbone): the sternoclavicular (SC) joint, where the clavicle meets the sternum, and the acromioclavicular (AC) joint, where it meets the scapula.

Understanding the Clavicular Joints

The clavicle, or collarbone, is a slender S-shaped bone that serves as a crucial strut connecting the upper limb to the axial skeleton. It plays a pivotal role in the mobility and stability of the shoulder girdle. This connection is facilitated by two distinct joints, each with unique anatomical features and biomechanical functions: the sternoclavicular (SC) joint and the acromioclavicular (AC) joint. Understanding these joints is fundamental for anyone involved in exercise science, rehabilitation, or advanced fitness training, as they are integral to all upper body movements.

Anatomy of the Clavicular Joints

Sternoclavicular (SC) Joint

The sternoclavicular (SC) joint is located at the medial (inner) end of the clavicle, where it articulates with the manubrium of the sternum (breastbone) and the first costal cartilage.

• Type: Functionally, it is classified as a saddle joint, but anatomically it behaves like a modified ball-and-socket joint due to its extensive range of motion. It is a synovial joint, meaning it has a joint capsule, synovial fluid, and articular cartilage.
• Key Structures:
  • Articular Disc: A fibrocartilaginous disc is present within the joint, completely dividing it into two compartments. This disc acts as a shock absorber, improves congruence between the joint surfaces, and helps prevent medial displacement of the clavicle.
  • Ligaments: Several strong ligaments provide stability:
    • Anterior and Posterior Sternoclavicular Ligaments: Reinforce the joint capsule anteriorly and posteriorly.
    • Interclavicular Ligament: Connects the medial ends of both clavicles, preventing excessive superior displacement.
    • Costoclavicular Ligament: A very strong extracapsular ligament connecting the inferior surface of the clavicle to the first rib and its cartilage. It is the primary restraint against superior displacement of the clavicle.
• Movements: Despite its relatively small size, the SC joint permits a wide range of movements, including elevation, depression, protraction, retraction, and rotation of the clavicle. These movements are essential for positioning the scapula and, consequently, the entire upper limb.

Acromioclavicular (AC) Joint

The acromioclavicular (AC) joint is located at the lateral (outer) end of the clavicle, where it articulates with the acromion process of the scapula (shoulder blade).

• Type: It is a plane synovial joint, designed to allow gliding and rotational movements.
• Key Structures:
  • Articular Disc: A small, often incomplete, fibrocartilaginous disc may be present within the joint, though it is less prominent and consistent than in the SC joint.
  • Ligaments: The AC joint's stability primarily relies on two sets of ligaments:
    • Acromioclavicular Ligaments: Strengthen the joint capsule directly, reinforcing the superior and inferior aspects of the joint.
    • Coracoclavicular Ligaments: These are the primary stabilizers of the AC joint and are located external to the joint capsule. They consist of two distinct bands:
      • Trapezoid Ligament: More lateral and quadrilateral in shape.
      • Conoid Ligament: More medial and conical in shape. These ligaments connect the clavicle to the coracoid process of the scapula, preventing superior displacement of the clavicle relative to the acromion and limiting excessive rotation.
• Movements: The AC joint allows for subtle gliding movements that facilitate scapular rotation and adjustments, essential for full range of motion of the glenohumeral (shoulder) joint. It contributes to the "setting" of the glenoid fossa (shoulder socket) to optimize arm elevation.

Function and Biomechanics

The clavicular joints are integral to the shoulder girdle complex, which comprises the clavicle, scapula, and sternum. Their primary functions include:

• Connecting the Upper Limb to the Axial Skeleton: The SC joint is the only bony attachment of the upper limb to the trunk, making it a critical pivot point for all arm movements.
• Transmitting Forces: Both joints play a vital role in transmitting forces from the upper limb through the clavicle to the axial skeleton. This is particularly evident during activities involving pushing, pulling, or absorbing impact through the arm.
• Maximizing Range of Motion: The coordinated movements at the SC and AC joints allow the scapula to rotate, elevate, depress, protract, and retract, which in turn significantly increases the overall range of motion of the glenohumeral joint. Without this scapulothoracic rhythm, full arm elevation would be impossible.
• Protecting Neurovascular Structures: By acting as a bony strut, the clavicle helps protect the brachial plexus and subclavian vessels that pass beneath it.

Common Conditions and Injuries

Due to their critical roles and exposure to external forces, the clavicular joints are susceptible to various conditions and injuries:

• Acromioclavicular (AC) Joint Separations: Often caused by a direct fall onto the shoulder or a direct blow to the acromion, leading to sprains or tears of the AC and/or coracoclavicular ligaments. These are graded based on severity.
• Sternoclavicular (SC) Joint Dislocations: While less common than AC separations, SC joint dislocations can occur from significant trauma (e.g., motor vehicle accidents). Anterior dislocations are more frequent, but posterior dislocations can be medically urgent due to potential compression of vital structures behind the joint.
• Osteoarthritis: Degenerative changes can occur in both joints, particularly the AC joint, due to repetitive stress, previous injury, or aging. This can lead to pain, stiffness, and reduced function.
• Clavicle Fractures: The clavicle is one of the most commonly fractured bones, often occurring in the midshaft, but fractures can also extend into or affect the articular surfaces of the SC or AC joints.

Importance in Fitness and Movement

For fitness enthusiasts, personal trainers, and kinesiologists, understanding the clavicular joints is paramount:

• Optimizing Movement Patterns: Recognizing how these joints contribute to shoulder mechanics allows for better exercise selection and technique cues. For example, understanding scapular stability (influenced by the AC joint) is crucial for overhead presses or pull-ups.
• Injury Prevention: Proper warm-up, progressive overload, and attention to form can help minimize stress on these joints. Strengthening the muscles that stabilize the shoulder girdle (e.g., trapezius, rhomboids, serratus anterior) indirectly supports these articulations.
• Rehabilitation and Performance: In cases of injury, a comprehensive understanding of these joints guides effective rehabilitation protocols, focusing on restoring stability, mobility, and strength. For athletes, optimizing the function of these joints can directly translate to improved performance in throwing, pushing, and overhead activities.

Conclusion

The clavicular joints, specifically the sternoclavicular (SC) and acromioclavicular (AC) joints, are small but profoundly important articulations. They serve as the critical links between the upper limb and the axial skeleton, enabling the vast and complex movements of the shoulder and arm. Their intricate anatomy, robust ligamentous support, and coordinated biomechanics are essential for both the stability and mobility required for daily activities, athletic performance, and overall upper body function. A thorough understanding of these joints is indispensable for anyone seeking to optimize human movement and maintain musculoskeletal health.