The elbow is not a single joint but a complex designed for both stability and movement, composed of three distinct articulations. An articulation is the meeting point between two bones. The elbow complex allows for a range of motion essential for daily function. The elbow has three articulations, which collectively permit the forearm to bend, straighten, and rotate. These three joints work together within a single, shared capsule to achieve the elbow’s mechanical function.
The Three Bones That Form the Elbow Complex
The elbow complex is formed by three bones meeting at this junction. The humerus is the single long bone of the upper arm, and its distal end forms the upper half of the elbow assembly. Below the humerus are the two bones of the forearm: the ulna and the radius.
The ulna is positioned on the medial side of the forearm, aligning with the pinky finger, and is the larger of the two forearm bones at the elbow. The radius is the smaller forearm bone, located on the lateral side toward the thumb. This arrangement of one upper bone meeting two lower bones necessitates multiple points of contact. These three bony components must fit together precisely to ensure both mobility and structural stability.
Identifying the Three Primary Articulations
The three bones create three distinct articulations within the elbow complex: the humeroulnar joint, the humeroradial joint, and the proximal radioulnar joint. The humeroulnar joint is formed by the trochlea of the humerus fitting into the trochlear notch of the ulna, creating the primary hinge mechanism of the elbow. This tight, wrench-like fit provides the main source of bony stability for the complex.
The humeroradial joint involves the capitulum of the humerus articulating with the head of the radius. This joint contributes to the hinge motion alongside the humeroulnar joint. Its structure also supports the rotation required for turning the palm. Although it resembles a ball-and-socket joint, its function is restricted by surrounding ligaments and the ulna, preventing the full range of motion of a true ball-and-socket joint.
The proximal radioulnar joint is a pivot joint formed by the head of the radius articulating with the radial notch of the ulna. This articulation is contained within the same capsule as the other two joints, making it a functional part of the elbow complex. It allows the radial head to spin against the ulna, a movement separate from bending and straightening. The annular ligament encircles the radial head, holding it securely against the ulna to allow rotation while maintaining stability.
Distinct Movements Enabled by the Articulation Complex
The three articulations work in concert to facilitate two different types of movement. The first is the simple hinge movement, which involves bending and straightening the arm. This motion includes flexion (bending the forearm toward the upper arm) and extension (the straightening motion).
Flexion and extension primarily occur at the humeroulnar joint, which functions like a door hinge, allowing movement in a single plane. The humeroradial joint assists in this action, providing support and contributing to the overall bending and straightening range. The ability to bend the arm by approximately 140 degrees is important for tasks like eating and bringing objects close to the body.
The second type of motion is rotation, which allows the palm to turn over. This pivot motion is facilitated by the humeroradial and proximal radioulnar joints. Turning the palm upward is called supination, and turning it downward is called pronation.
The radial head rotating within the annular ligament at the proximal radioulnar joint makes forearm rotation possible. This complexity allows the hand to be positioned in space not only by moving it closer or further away from the body but also by rotating it. This dual-plane capability—bending and rotation—makes the elbow complex effective for the wide range of activities performed by the hand.