Humerus: Understanding the Capitulum and Trochlea of the Elbow Joint

What is the relationship between the capitulum and the trochlea of the humerus?

The capitulum and trochlea are distinct yet complementary articular surfaces located at the distal end of the humerus, forming the primary components of the elbow joint and dictating its complex range of motion for the forearm.

Anatomy of the Distal Humerus

The distal end of the humerus, the long bone of the upper arm, flares out to form a broad, flattened structure designed to articulate with the bones of the forearm—the radius and ulna. This region is critical for the intricate movements of the elbow joint.

• The Capitulum:

  • Located on the lateral (outer) side of the distal humerus.
  • It is a rounded, knob-like projection, resembling a small head (hence "capitulum," Latin for "little head").
  • Its smooth, convex surface is specifically designed to articulate with the head of the radius.
  • The capitulum's spherical shape allows for both rotation and a degree of gliding, facilitating pronation and supination of the forearm in conjunction with elbow flexion and extension.

• The Trochlea:

  • Positioned medial (inner) to the capitulum, separated by a shallow groove.
  • It is a spool-shaped or pulley-like structure, with a deep central groove and prominent medial and lateral lips. The term "trochlea" is derived from the Greek word for "pulley."
  • Its unique shape is perfectly adapted to articulate with the trochlear notch of the ulna.
  • The trochlea extends further distally on its medial side than on its lateral side, contributing to the "carrying angle" of the elbow.

• Associated Structures: While not directly articulating surfaces, the medial and lateral epicondyles are bony prominences flanking the trochlea and capitulum, respectively. These serve as attachment points for muscles and ligaments that stabilize and move the elbow and wrist.

Articulation and Function at the Elbow Joint

The relationship between the capitulum and trochlea is defined by their distinct roles in forming the compound elbow joint, which is comprised of three articulations within a single synovial capsule:

• Humeroulnar Joint (Trochlea and Ulna):

  • This is the primary articulation for flexion and extension of the elbow.
  • The trochlea's deep groove and the corresponding trochlear notch of the ulna create a highly stable hinge joint. This interlocking mechanism limits movement primarily to a single plane, ensuring strong and precise bending and straightening of the arm.
  • Its design provides significant bony stability against valgus and varus stresses.

• Humeroradial Joint (Capitulum and Radius):

  • The capitulum articulates with the concave radial head.
  • This joint also contributes to flexion and extension of the elbow, as the radial head spins within the capitulum's surface during these movements.
  • Crucially, this articulation is vital for pronation and supination of the forearm. As the radius rotates around the ulna, the radial head spins on the capitulum, allowing the palm to turn up (supinate) or down (pronate).

In essence, the trochlea provides the main hinge for elbow flexion/extension, offering robust stability, while the capitulum allows for the necessary rotational movements of the radius, enabling pronation/supination alongside flexion/extension. Their close proximity and shared joint capsule underscore their integrated function.

Biomechanical Implications

The precise anatomical relationship between the capitulum and trochlea has profound biomechanical consequences:

• Joint Stability: The deep interdigitation of the trochlea and ulnar trochlear notch provides inherent bony stability to the elbow, making it a very strong hinge. The capitulum's more forgiving articulation with the radial head allows for the necessary rotational freedom.
• Defined Range of Motion: The specific shapes of these articular surfaces dictate the types and limits of movement possible at the elbow. The trochlea's pulley shape ensures a predictable arc of flexion and extension, while the capitulum's spherical nature accommodates the spinning of the radial head.
• Carrying Angle: The oblique orientation of the trochlea, extending further distally on its medial side, is the primary anatomical basis for the elbow's "carrying angle." This slight valgus (outward) angulation of the forearm relative to the upper arm, particularly noticeable when the arm is extended and supinated, helps clear the forearm from the hips during walking and carrying objects.
• Load Distribution: During activities involving the upper limb, forces are transmitted through both the humeroulnar and humeroradial joints. The large surface area of the trochlea helps distribute compressive forces efficiently.

Clinical Relevance and Injury Considerations

Understanding the relationship between the capitulum and trochlea is vital in clinical practice:

• Fractures: Fractures of the distal humerus, involving the capitulum or trochlea, can severely impair elbow function and require precise anatomical reduction for optimal recovery.
• Dislocations: While relatively stable, elbow dislocations often involve disruption of the humeroulnar and humeroradial articulations.
• Osteochondritis Dissecans (OCD): Repetitive stress, particularly in throwing athletes, can lead to damage of the articular cartilage, notably on the capitulum.
• Rehabilitation: Exercise prescription for elbow injuries or post-surgical recovery must consider the specific roles of these structures to restore full range of motion, strength, and stability, ensuring proper joint mechanics.

Conclusion

The capitulum and trochlea are not merely two bumps on the end of the humerus; they are intricately shaped, specialized articular surfaces that, in concert, form the cornerstone of the elbow joint. The trochlea provides the primary hinge for powerful flexion and extension, offering robust stability, while the capitulum facilitates the crucial rotational movements of the forearm. Their distinct yet harmonious relationship underscores the elegant design of the human musculoskeletal system, enabling the diverse and complex actions of the upper limb essential for daily life and athletic performance.