Humerus: Function, Anatomy, and Role in Elbow Joint Movements

What is the function of the humerus in the elbow joint?

The humerus serves as the crucial proximal bone of the elbow joint, providing the primary articulating surfaces—the trochlea and capitulum—that facilitate the joint's complex movements of flexion, extension, pronation, and supination by interfacing with the ulna and radius.

Anatomy of the Humerus and Elbow Joint

The elbow joint is a complex synovial joint primarily responsible for adjusting the length of the upper limb during activities, allowing for precise placement of the hand in space. It is formed by the articulation of three bones: the humerus (upper arm bone), and the ulna and radius (forearm bones).

The humerus is the single bone of the upper arm. Its distal (lower) end is specifically designed to interact with the forearm bones to form the elbow joint. Key features of the distal humerus relevant to the elbow include:

• Trochlea: A spool-shaped articular surface located medially, which articulates with the trochlear notch of the ulna. This articulation primarily forms the hinge component of the elbow.
• Capitulum: A rounded, lateral articular eminence, which articulates with the concave fovea (head) of the radius. This articulation allows for rotation of the radius.
• Medial and Lateral Epicondyles: Prominent bony projections on either side of the distal humerus, serving as crucial attachment points for ligaments and muscles that cross the elbow joint.
• Olecranon Fossa: A deep depression on the posterior aspect of the humerus, superior to the trochlea, which accommodates the olecranon process of the ulna during full elbow extension.
• Coronoid Fossa: A smaller depression on the anterior aspect of the humerus, superior to the trochlea, which accommodates the coronoid process of the ulna during full elbow flexion.
• Radial Fossa: A shallow depression on the anterior aspect of the humerus, superior to the capitulum, which accommodates the head of the radius during full elbow flexion.

Primary Function: Articulation and Movement

The humerus's fundamental function in the elbow joint is to provide the stable, articulating surfaces that allow for a wide range of motion.

• Humeroulnar Joint: This is the primary articulation responsible for the hinge-like movements of the elbow. The trochlea of the humerus fits precisely into the trochlear notch of the ulna, forming a strong, stable joint that primarily permits flexion (bending the arm) and extension (straightening the arm). The snug fit ensures that these movements occur in a single plane.
• Humeroradial Joint: This articulation, between the capitulum of the humerus and the head of the radius, contributes to both flexion/extension and the rotational movements of the forearm. While the humerus itself does not rotate, its capitulum provides the fixed spherical surface around which the radial head can pivot, enabling pronation (turning the palm downwards) and supination (turning the palm upwards) of the forearm.

Role in Specific Elbow Movements

The unique morphology of the distal humerus dictates the specific movements possible at the elbow:

• Flexion and Extension: These movements occur predominantly at the humeroulnar joint.
  • During flexion, the coronoid process of the ulna and the radial head move into the coronoid and radial fossae of the humerus, respectively, allowing the forearm to move closer to the upper arm.
  • During extension, the olecranon process of the ulna moves into the olecranon fossa of the humerus, limiting hyperextension and providing bony stability.
  • The trochlea's slightly oblique nature contributes to the "carrying angle" of the elbow, which helps the arms clear the hips during walking.
• Pronation and Supination: While these movements primarily involve the proximal radioulnar joint, the humerus's capitulum provides the stationary axis around which the radial head rotates. The smooth articular cartilage of the capitulum minimizes friction, allowing the radius to spin freely during these rotational actions.

Structural Stability and Ligamentous Attachments

Beyond providing articular surfaces, the humerus plays a critical role in the structural integrity of the elbow joint through its bony prominences that serve as anchor points for crucial ligaments and muscles.

• Ligamentous Attachments: The medial epicondyle and lateral epicondyle are vital attachment sites for the collateral ligaments of the elbow:
  • Medial (Ulnar) Collateral Ligament (MCL/UCL): Attaches from the medial epicondyle to the ulna, providing significant resistance to valgus stress (force from the outside, pushing the forearm outward).
  • Lateral (Radial) Collateral Ligament (LCL/RCL): Attaches from the lateral epicondyle to the annular ligament (which encircles the radial head) and ulna, providing resistance to varus stress (force from the inside, pushing the forearm inward). These ligaments, anchored to the humerus, are essential for maintaining the stability of the elbow joint, preventing excessive movement and dislocation.
• Muscle Attachments: The epicondyles also serve as origins for many of the muscles responsible for forearm and hand movements.
  • The medial epicondyle is the common origin for most of the forearm flexor muscles.
  • The lateral epicondyle is the common origin for many of the forearm extensor muscles. This arrangement allows the humerus to act as a stable base from which these muscles can exert force, contributing to the powerful and intricate movements of the hand and wrist.

Clinical Significance and Injury Considerations

The humerus's critical role in the elbow joint makes its integrity paramount for arm function. Injuries to the distal humerus or its associated structures can severely impact elbow mobility and stability. Common clinical considerations include:

• Fractures: Supracondylar fractures of the humerus (above the epicondyles) are common in children and can have serious implications for neurovascular structures. Fractures of the epicondyles can compromise ligamentous and muscular attachments.
• Epicondylitis: Conditions like "golfer's elbow" (medial epicondylitis) and "tennis elbow" (lateral epicondylitis) involve inflammation or degeneration of the tendons originating from the respective epicondyles, directly affecting the humerus's function as a muscle attachment site.
• Dislocations: While the ulna and radius dislocate from the humerus, the stability provided by the humerus's articular surfaces and ligamentous attachments is crucial in preventing such events.

Conclusion

The humerus is far more than just a connecting bone in the elbow; it is the fundamental structural and functional anchor. Its precisely shaped trochlea and capitulum provide the necessary articular surfaces for the ulna and radius, enabling the complex hinge and pivot movements of flexion, extension, pronation, and supination. Furthermore, its epicondyles serve as critical attachment points for the ligaments that stabilize the joint and the muscles that power the forearm and hand. Understanding the multifaceted role of the humerus is essential for comprehending the biomechanics, stability, and potential pathologies of the elbow joint.