Radiohumeral Joint: Anatomy, Function, and Clinical Relevance

What is the name of the bone that articulates with the capitulum?

The bone that articulates with the capitulum is the radius, specifically its most proximal part known as the radial head.

Understanding the Elbow Joint Complex

The elbow is a complex synovial joint that allows for both flexion/extension of the forearm and pronation/supination of the hand. It is formed by the articulation of three bones: the humerus (upper arm bone), and the radius and ulna (forearm bones). Within this intricate structure, specific anatomical landmarks on each bone are designed for precise articulation, enabling its wide range of motion and stability.

The Capitulum: A Key Humeral Landmark

The capitulum is a smooth, rounded eminence located on the lateral aspect of the distal humerus, just anterior to the lateral epicondyle. Its name, derived from Latin, means "little head." This spherical structure is specifically designed to articulate with another bone, forming a crucial component of the elbow's ability to flex, extend, and rotate. Unlike the trochlea, which articulates with the ulna and is primarily involved in hinge-like motion, the capitulum's rounded shape facilitates both a hinge-like action and a rotational movement.

The Articulating Bone: The Radius

The bone that articulates directly with the capitulum of the humerus is the radius. The radius is the lateral bone of the forearm, running from the elbow to the wrist.

• The Radial Head: The proximal end of the radius expands to form the radial head. This distinctive, disc-shaped structure features a concave depression on its superior surface, known as the fovea or articular facet of the radial head. It is this fovea that perfectly cups the convex surface of the capitulum. The radial head also has a smooth, cylindrical margin that articulates with the radial notch of the ulna, forming the proximal radioulnar joint, which is vital for forearm rotation.

The Radiohumeral Joint: Anatomy and Function

The articulation between the capitulum of the humerus and the fovea of the radial head forms the radiohumeral joint. This joint is a critical component of the elbow complex, contributing significantly to its overall function.

• Joint Type: While the elbow is often described as a hinge joint (ginglymus) due to the humeroulnar articulation, the radiohumeral joint is classified as a condyloid joint or, more specifically, a part of a ginglymoarthrodial joint when considering the entire elbow. Its design allows for both flexion and extension, as well as rotation.
• Movements Facilitated:
  • Flexion and Extension: As the elbow flexes and extends, the radial head glides over the capitulum.
  • Pronation and Supination: During pronation (turning the palm downwards) and supination (turning the palm upwards), the radial head rotates within the annular ligament against the capitulum and the radial notch of the ulna. This rotational movement is essential for orienting the hand in space.

Biomechanical Significance

The radiohumeral articulation plays a vital role in the biomechanics of the upper limb:

• Load Transmission: It helps in transmitting forces from the hand and forearm up to the humerus, especially during activities involving pushing or supporting weight.
• Stability and Mobility: While the humeroulnar joint provides the primary stability for flexion/extension, the radiohumeral joint contributes to stability while also allowing the necessary rotational freedom for forearm movements.
• Synchronized Movement: The coordinated movement of the radial head over the capitulum and within the proximal radioulnar joint is fundamental for the complex and versatile actions of the forearm and hand.

Clinical Relevance and Common Issues

Given its crucial role in elbow function, the radiohumeral joint is susceptible to various injuries and conditions:

• Radial Head Fractures: These are common elbow fractures, often resulting from a fall onto an outstretched hand (FOOSH). They can significantly impair elbow flexion/extension and forearm rotation.
• Elbow Dislocations: While less common than radial head fractures, dislocations can involve disruption of the radiohumeral articulation, often in conjunction with humeroulnar dislocation.
• Osteochondritis Dissecans of the Capitulum: This condition, often seen in young athletes (e.g., baseball pitchers, gymnasts), involves a disruption of blood flow to the capitulum, leading to the formation of loose bodies within the joint and pain.
• Nursemaid's Elbow (Pulled Elbow): This common pediatric injury involves the subluxation (partial dislocation) of the radial head from the annular ligament, often caused by a sudden pull on the child's arm.
• Osteoarthritis: Like other synovial joints, the radiohumeral joint can develop osteoarthritis, leading to pain, stiffness, and reduced range of motion, particularly in individuals with a history of trauma or repetitive stress.

Conclusion

The articulation between the capitulum of the humerus and the radial head of the radius forms the radiohumeral joint, a cornerstone of elbow mechanics. This seemingly simple connection is responsible for a complex interplay of movements, enabling not only the bending and straightening of the arm but also the intricate rotational capabilities of the forearm that are essential for daily activities. Understanding this fundamental anatomical relationship is crucial for comprehending the full scope of upper limb function and pathology.