Humerus and Radius: Attachment, Joint Type, and Functional Significance

What is the attachment of the humerus and radius?

The humerus and radius articulate at the elbow joint, forming the humeroradial joint, a crucial articulation that enables both flexion and extension of the elbow, as well as forearm pronation and supination.

Anatomy of the Elbow Joint Complex

The elbow is a complex synovial joint, comprising three distinct articulations within a single joint capsule. While the query specifically focuses on the humerus and radius, it's essential to understand its relationship within the broader elbow structure, which also involves the ulna. The three articulations are:

• Humeroulnar Joint: The articulation between the trochlea of the humerus and the trochlear notch of the ulna. This is the primary hinge joint responsible for most elbow flexion and extension.
• Humeroradial Joint: The articulation between the capitulum of the humerus and the head of the radius. This is the direct answer to the query.
• Proximal Radioulnar Joint: The articulation between the head of the radius and the radial notch of the ulna. This joint is critical for forearm rotation (pronation and supination).

The Humeroradial Joint: Detailed Attachment

The humeroradial joint is the specific articulation between the distal end of the humerus and the proximal end of the radius.

• Articulating Surfaces:

  • Humerus: The capitulum, a rounded, lateral eminence on the distal humerus, articulates with the radius.
  • Radius: The head of the radius, a disc-shaped structure with a concave superior surface, articulates with the capitulum.

• Joint Type: Functionally, the humeroradial joint is considered a pivot joint within the context of forearm rotation, allowing the radial head to spin on the capitulum. However, its contribution to elbow flexion and extension gives it characteristics of a hinge joint as well. It's often described as a plane synovial joint due to the relatively flat surfaces, but its movement is more complex.

Supporting Structures and Stability

The stability of the humeroradial attachment and the entire elbow complex is maintained by a robust network of ligaments and the joint capsule.

• Joint Capsule: A fibrous capsule encloses all three elbow articulations, providing general containment and stability.
• Ligaments:
  • Radial (Lateral) Collateral Ligament (RCL/LCL): Originates from the lateral epicondyle of the humerus and blends with the annular ligament and supinator crest of the ulna. It provides lateral stability to the elbow and resists varus stress.
  • Annular Ligament: A strong, fibrous band that encircles the head of the radius, holding it firmly against the radial notch of the ulna. This ligament is crucial for the stability of the proximal radioulnar joint and allows the radial head to rotate smoothly during pronation and supination. While not directly attaching the humerus to the radius, it is vital for the radius's overall stability and function at the elbow.

Functional Significance in Movement

The humeroradial attachment plays a vital role in the complex movements of the elbow and forearm:

• Elbow Flexion and Extension: During these movements, the concave head of the radius glides over the convex capitulum of the humerus. While the humeroulnar joint is the primary contributor to the range of motion, the humeroradial joint ensures smooth, coordinated movement, especially during full extension.
• Forearm Pronation and Supination: This is where the humeroradial joint's pivot function is paramount. The head of the radius spins within the annular ligament and against the capitulum of the humerus.
  • Pronation: The palm turns downwards/backwards, achieved by the radius crossing over the ulna.
  • Supination: The palm turns upwards/forwards, with the radius and ulna lying parallel.

Clinical Relevance

Understanding the attachment of the humerus and radius is crucial for diagnosing and treating various conditions:

• Radial Head Fractures: Common injuries, often resulting from a fall onto an outstretched hand (FOOSH), affecting the articulation with the capitulum.
• Nursemaid's Elbow (Pulled Elbow): A common childhood injury where the radial head subluxes (partially dislocates) from under the annular ligament due to a sudden pull on the forearm.
• Elbow Dislocation: While rare, severe trauma can disrupt the entire elbow joint, including the humeroradial attachment.
• Osteoarthritis: Degenerative changes can affect the articular cartilage of the humeroradial joint, leading to pain and reduced range of motion.

Conclusion

The attachment of the humerus and radius forms the humeroradial joint, an integral part of the elbow complex. This articulation, alongside the humeroulnar and proximal radioulnar joints, facilitates the intricate movements of elbow flexion and extension, and critically, the forearm's ability to pronate and supinate. Its unique structure, supported by key ligaments, allows for both stability and the diverse range of motion essential for upper limb function in daily activities and athletic performance.