Capitulum of the Humerus: Function, Anatomy, and Clinical Significance

What is the function of the capitulum of the humerus?

The capitulum of the humerus serves as a crucial articular surface on the distal end of the humerus, primarily facilitating the articulation with the head of the radius to allow for elbow flexion, extension, and forearm rotation (pronation and supination).

Anatomy of the Distal Humerus

The distal end of the humerus, the long bone of the upper arm, flares out to form the medial and lateral epicondyles and two distinct articular surfaces: the trochlea and the capitulum. These structures are vital for forming the elbow joint, a complex hinge and pivot joint.

• Location: The capitulum is located on the lateral (outer) side of the distal humerus, anterior to the lateral epicondyle.
• Shape: It is a smooth, rounded, knob-like projection, resembling a small head (capitulum means "small head" in Latin). Its spherical shape is perfectly adapted for articulation.
• Composition: Like other articular surfaces, it is covered with hyaline cartilage, providing a low-friction surface for smooth movement.

The Capitulum's Articulation: The Humeroradial Joint

The primary function of the capitulum is its articulation with the head of the radius, the smaller of the two forearm bones. This specific articulation forms the humeroradial joint, which is one of the three articulations comprising the elbow joint complex (the others being the humeroulnar joint and the proximal radioulnar joint).

• The fovea (a shallow, cup-shaped depression) on the superior surface of the radial head perfectly conforms to the spherical shape of the capitulum.
• This articulation allows for both gliding and rotation movements, essential for the diverse functions of the elbow and forearm.

Primary Function: Elbow Flexion and Extension

While the humeroulnar joint (trochlea of humerus with trochlear notch of ulna) is the primary articulation responsible for the hinge-like movements of elbow flexion (bending) and extension (straightening), the humeroradial joint plays a significant supporting role.

• During elbow flexion, the radial head glides along the surface of the capitulum and enters the radial fossa, a depression superior to the capitulum, allowing for full flexion.
• During elbow extension, the radial head glides out of the fossa, and the joint reaches its maximal extension range.
• The smooth interaction between the capitulum and radial head ensures stable and coordinated movement throughout the range of motion for these actions.

Secondary Function: Forearm Pronation and Supination

Beyond flexion and extension, the capitulum is critically involved in the movements of forearm pronation (turning the palm downwards/backwards) and supination (turning the palm upwards/forwards).

• During pronation and supination, the head of the radius pivots and rotates against the stationary capitulum.
• The annular ligament encircles the radial head, holding it firmly against the ulna and allowing it to spin freely against the capitulum. This unique rotational capability at the humeroradial joint is fundamental for manipulating objects, reaching, and performing countless daily activities.

Clinical Significance and Injury

Given its integral role in elbow mechanics, the capitulum can be involved in various injuries and conditions, impacting overall upper limb function.

• Fractures: Fractures of the capitulum, often resulting from falls onto an outstretched hand, can be challenging due to their intra-articular nature and potential impact on joint congruity and movement.
• Osteochondritis Dissecans (OCD): This condition, more common in throwing athletes, involves a disruption of blood supply to a segment of the capitulum, leading to the formation of a loose fragment of cartilage and bone. This can cause pain, clicking, and limited range of motion.
• Dislocations: While less common for the humeroradial joint alone, radial head dislocations (often in conjunction with other elbow injuries or in children, known as "nursemaid's elbow") disrupt the articulation with the capitulum.

Conclusion: A Pivotal Structure for Upper Limb Function

The capitulum of the humerus, though seemingly small, is a highly specialized anatomical structure. Its precise articulation with the radial head is indispensable for the full range of motion at the elbow joint, facilitating not only the powerful hinge actions of flexion and extension but also the intricate rotational movements of the forearm. Understanding its function is key to appreciating the biomechanical complexity and efficiency of the human upper limb.