Elbow Joint: Motions Between Humerus and Radius for Flexion, Extension, Pronation, and Supination

What Motions Occur Between Humerus and Radius?

The humerus and radius primarily facilitate the movements of flexion and extension at the elbow joint, alongside the critical forearm rotational movements of pronation and supination.

Introduction to the Elbow Joint Complex

The elbow is a complex hinge joint that connects the upper arm (humerus) to the forearm (radius and ulna). While the humerus articulates with both the radius and ulna, the specific interactions between the humerus and radius are crucial for both the bending and straightening of the arm, and the unique rotational capabilities of the forearm. Understanding these distinct articulations is key to appreciating the elbow's versatile functionality.

The Humeroradial Joint

The humeroradial joint is the articulation between the capitulum of the humerus (a rounded lateral condyle at its distal end) and the fovea (concave superior surface) of the radial head. Unlike the humeroulnar joint, which is a pure hinge, the humeroradial joint is a plane synovial joint, allowing for both gliding and rotation.

During flexion and extension, the radial head glides over the capitulum. However, its most significant contribution is to pronation and supination, where the radial head spins within the annular ligament against the capitulum. This spinning motion is fundamental to forearm rotation.

The Proximal Radioulnar Joint and Its Humeral Link

While not a direct articulation between the humerus and radius, the proximal radioulnar joint is inseparable from the humeroradial joint's function in forearm rotation. This joint involves the radial head articulating with the radial notch of the ulna, held in place by the annular ligament. As the radius rotates around the ulna for pronation and supination, the radial head simultaneously spins against the capitulum of the humerus. Therefore, the humerus provides the stationary pivot point for the radius's rotational movements.

Primary Motions at the Humeroradial Articulation

The motions occurring between the humerus and radius are components of the larger elbow and forearm movements:

• Flexion: This is the action of bending the elbow, bringing the forearm closer to the upper arm. At the humeroradial joint, the radial head glides anteriorly over the capitulum of the humerus.

  • Muscles Involved: Primarily the biceps brachii, brachialis, and brachioradialis.
  • Range of Motion: Typically from 0 degrees (full extension) to 140-150 degrees.

• Extension: This is the action of straightening the elbow, moving the forearm away from the upper arm. The radial head glides posteriorly over the capitulum.

  • Muscles Involved: Primarily the triceps brachii and anconeus.
  • Range of Motion: From full flexion back to 0 degrees, with some individuals capable of slight hyperextension.

• Pronation: This is the rotation of the forearm that turns the palm posteriorly (if the arm is by the side) or downwards (if the arm is flexed). During pronation, the radius crosses over the ulna. The radial head spins on the capitulum of the humerus and within the annular ligament.

  • Muscles Involved: Primarily the pronator teres and pronator quadratus.
  • Range of Motion: Approximately 70-80 degrees from neutral.

• Supination: This is the opposite rotation of the forearm, turning the palm anteriorly (anatomical position) or upwards. During supination, the radius uncrosses from the ulna and returns to its parallel position. The radial head spins back on the capitulum of the humerus.

  • Muscles Involved: Primarily the supinator and biceps brachii.
  • Range of Motion: Approximately 80-90 degrees from neutral.

Functional Significance

The intricate interplay between the humerus and radius, along with the ulna, provides the arm with remarkable dexterity.

• Flexion and Extension allow us to reach, pull, push, and lift objects in various planes.
• Pronation and Supination are critical for fine motor skills and manipulating objects. These rotations enable us to turn doorknobs, use tools, twist screwdrivers, carry a bowl of soup without spilling, or perform complex athletic movements like throwing a ball or swinging a racket. Without these specific movements at the humeroradial and radioulnar joints, many daily activities would be impossible.

Conclusion

In summary, the humerus and radius articulate directly at the humeroradial joint to facilitate elbow flexion and extension, with the radial head gliding over the humeral capitulum. Crucially, the humerus acts as a stable base for the radial head to spin against during the forearm's rotational movements of pronation and supination, which primarily occur at the proximal radioulnar joint. This combination of hinge and pivot actions underscores the elbow's versatility and its indispensable role in upper limb function.