Radiohumeral Joint: Anatomy, Function, and Clinical Relevance

Where does the radius articulate with the humerus?

The radius, one of the two bones of the forearm, articulates with the humerus, the upper arm bone, at the elbow joint. Specifically, the concave head of the radius connects with the rounded capitulum of the humerus.

The Elbow Joint: A Complex Tri-Articular Structure

The elbow is a remarkable joint, often primarily viewed as a simple hinge, but it's far more intricate. It is a compound joint formed by the articulation of three bones: the humerus (upper arm bone), and the ulna and radius (forearm bones). Within this single joint capsule, three distinct articulations occur:

• Humeroulnar Joint: The trochlea of the humerus articulates with the trochlear notch of the ulna. This is the primary hinge articulation responsible for flexion and extension of the elbow.
• Radiohumeral Joint: This is the specific articulation between the humerus and the radius, which is the focus of this discussion.
• Proximal Radioulnar Joint: The head of the radius articulates with the radial notch of the ulna. This joint, along with the distal radioulnar joint, is crucial for forearm rotation (pronation and supination).

The Radiohumeral Articulation: A Precise Connection

The articulation between the radius and the humerus occurs at the lateral (outer) aspect of the elbow joint.

• The Humeral Component: The capitulum is a rounded, knob-like eminence located on the lateral epicondyle of the distal humerus. Its spherical shape is perfectly adapted for the rotational movements of the radius.
• The Radial Component: The head of the radius is a cylindrical structure with a shallow, cup-shaped depression on its superior surface, known as the fovea. This fovea is designed to articulate precisely with the capitulum of the humerus.

When the arm is extended, the fovea of the radial head is in full contact with the capitulum. As the elbow flexes, the radial head glides and rotates against the capitulum, and in full flexion, it enters the radial fossa of the humerus.

Anatomical Structures Supporting the Articulation

The stability and smooth function of the radiohumeral joint, and indeed the entire elbow, depend on a network of supporting structures:

• Articular Cartilage: Both the capitulum of the humerus and the fovea of the radial head are covered with smooth, low-friction articular (hyaline) cartilage. This cartilage allows for effortless gliding movement and absorbs compressive forces during activity.
• Joint Capsule: A fibrous capsule encloses the entire elbow joint, encompassing all three articulations. It is reinforced by various ligaments.
• Ligaments:
  • Annular Ligament: This strong, fibrous band encircles the head of the radius, holding it firmly against the radial notch of the ulna. While primarily stabilizing the proximal radioulnar joint, it indirectly contributes to the stability of the radiohumeral joint by keeping the radial head properly aligned.
  • Lateral (Radial) Collateral Ligament: This fan-shaped ligament originates from the lateral epicondyle of the humerus and blends with the annular ligament. It provides lateral stability to the elbow, preventing excessive varus (outward) angulation.
  • Medial (Ulnar) Collateral Ligament: While primarily stabilizing the humeroulnar joint, this robust ligament (composed of anterior, posterior, and oblique bands) is critical for overall elbow stability, particularly against valgus (inward) stress.

Functional Significance in Movement

The radiohumeral articulation plays a vital role in two primary movements of the forearm:

• Elbow Flexion and Extension: While the humeroulnar joint is the main contributor to the hinge-like motion of the elbow, the radiohumeral joint also participates. As the elbow flexes, the radial head glides anteriorly on the capitulum.
• Forearm Pronation and Supination: This is where the radiohumeral joint truly shines. During pronation (turning the palm downwards) and supination (turning the palm upwards), the head of the radius rotates around its longitudinal axis against the capitulum of the humerus and within the annular ligament. This complex rotation allows for the versatile positioning of the hand and wrist, crucial for countless daily activities and athletic movements.

Clinical Relevance and Common Issues

Understanding the radiohumeral articulation is essential for diagnosing and treating various conditions:

• Radial Head Fractures: These are common injuries, often resulting from a fall onto an outstretched hand. They can significantly impair elbow and forearm movement.
• Nursemaid's Elbow (Radial Head Subluxation): This is a common injury in young children, where a sudden pull on the extended arm (e.g., swinging a child by their arms) causes the radial head to slip out from under the annular ligament.
• Osteoarthritis: Degenerative changes can occur in the articular cartilage of the radiohumeral joint, leading to pain, stiffness, and reduced range of motion, particularly in individuals with a history of trauma or repetitive stress.
• Ligamentous Injuries: Sprains or tears of the collateral ligaments or the annular ligament can compromise the stability of the elbow joint, affecting the proper articulation of the radius and humerus.

Conclusion

The articulation between the radius and the humerus occurs at the radiohumeral joint, where the head of the radius meets the capitulum of the humerus at the elbow. This specific connection, facilitated by precise bony geometry, smooth articular cartilage, and strong surrounding ligaments, is fundamental not only for elbow flexion and extension but, critically, for the complex and versatile movements of forearm pronation and supination. A thorough understanding of this intricate joint is paramount for anyone involved in human movement, from fitness enthusiasts to clinical professionals.