Humerus: Capitulum Articulation, Radiohumeral Joint Function, and Clinical Importance

What does the capitulum of the humerus articulate with?

The capitulum of the humerus, a rounded prominence on the lateral aspect of the distal humerus, articulates specifically with the head of the radius, which is the superior, disc-shaped end of the radial bone in the forearm.

Understanding the Humerus and Elbow Joint

The humerus is the single bone of the upper arm, extending from the shoulder to the elbow. Its distal (lower) end broadens and features two prominent articular surfaces that engage with the bones of the forearm (radius and ulna) to form the complex elbow joint. This joint is not merely a simple hinge; it allows for both flexion and extension of the forearm, as well as the crucial pronation and supination movements of the hand and forearm.

The Capitulum: A Key Articular Surface

The capitulum is located on the lateral (thumb-side) aspect of the distal humerus, anterior to the trochlea. It presents as a smooth, rounded, knob-like projection. Its name, derived from Latin, means "little head," aptly describing its appearance. This convex articular surface is specifically designed to fit into a complementary concave surface on one of the forearm bones, facilitating precise movement and stability.

The Articulating Partner: The Radial Head

The bone that articulates with the capitulum is the radius, the more lateral of the two forearm bones (the other being the ulna). Specifically, it is the head of the radius that forms this articulation. The radial head is a distinctive disc-shaped structure located at the proximal (upper) end of the radius. Its superior surface is concave, creating a shallow cup-like depression perfectly shaped to receive the convex capitulum of the humerus.

The Radiohumeral Joint: Function and Movement

The articulation between the capitulum of the humerus and the head of the radius forms the radiohumeral joint. This is a synovial joint, classified functionally as a limited ball-and-socket joint or, more accurately in terms of its primary motion, a pivot joint. While it contributes to the overall elbow's flexion and extension, its most significant role lies in facilitating the pronation and supination of the forearm.

• Flexion and Extension: As the elbow flexes and extends, the concave head of the radius glides over the convex capitulum.
• Pronation and Supination: During pronation (turning the palm down) and supination (turning the palm up), the radial head spins within the annular ligament against the capitulum and the radial notch of the ulna. This rotational movement is essential for the versatile positioning of the hand.

Clinical Significance and Injury Considerations

Understanding the articulation of the capitulum is vital for diagnosing and treating elbow injuries, as well as for designing effective exercise programs.

• Radial Head Fractures: These are common injuries, often resulting from a fall onto an outstretched arm. The articulation with the capitulum is directly affected, impacting elbow and forearm movement.
• Distal Humerus Fractures: Fractures involving the capitulum itself can be complex, requiring precise surgical repair to restore the smooth articular surface necessary for proper joint function.
• Joint Stability: The congruency between the capitulum and radial head contributes to the overall stability of the elbow joint, particularly against valgus (outward) stress.
• Exercise Implications: For fitness professionals and enthusiasts, knowledge of this articulation helps in understanding the mechanics of exercises like bicep curls, hammer curls, and triceps extensions, ensuring proper form to protect the joint and optimize muscle activation.

Conclusion: The Elbow's Intricate Design

In summary, the capitulum of the humerus articulates exclusively with the head of the radius. This specific bone-to-bone connection forms the radiohumeral joint, a critical component of the elbow's complex structure. It not only contributes to the hinge-like movements of the elbow but, more uniquely, enables the rotational movements of pronation and supination, which are fundamental to the vast dexterity of the human upper limb. This intricate anatomical design underscores the precision required for fluid and functional movement.