The elbow is a complex joint connecting the upper arm to the forearm, allowing for a wide range of movements necessary for daily life. It functions as a load-bearing connection point that must be stable enough to transmit force and mobile enough to position the hand in space. This joint is composed of three distinct bones that meet and interact within a single joint capsule. These three bones—one from the upper arm and two from the forearm—allow the arm to bend, straighten, and rotate.
The Humerus: Foundation of the Elbow
The humerus is the bone of the upper arm, establishing the foundation for the entire elbow joint structure. Its distal end flares out to create two specialized surfaces that articulate directly with the forearm bones. On the inner, medial side, the structure is the trochlea, which resembles a pulley shape.
The trochlea is the primary partner for the main hinge of the elbow, allowing for bending and straightening movements. On the outer, lateral side, the humerus presents a rounded, dome-like surface known as the capitulum. This structure interacts with the second forearm bone, contributing to the joint’s rotation capabilities.
Above these articulating surfaces are depressions called fossae, which accommodate parts of the forearm bones when the elbow is fully bent or straightened. For instance, the olecranon fossa on the back of the humerus receives the prominent, pointy part of the elbow bone when the arm is fully extended.
The Ulna and Radius: Forming the Forearm
The ulna is the main contributor to the elbow’s hinge function. At its proximal end, the ulna features a large, C-shaped indentation called the trochlear notch. This notch interlocks with the trochlea of the humerus, creating a stable joint.
The bony prominence felt as the “point” of the elbow is the olecranon process, which is part of the ulna. This process fits into the olecranon fossa of the humerus when the arm is straight, preventing hyperextension. This close fit provides the elbow with its bony stability.
The radius is the second forearm bone, and its primary role involves the rotational movement of the hand and wrist. The top of the radius is the radial head, a disc-shaped structure that articulates with the capitulum of the humerus. The radial head also pivots against the ulna at the proximal radioulnar joint.
The Mechanics of the Joint
The elbow is functionally classified as a trochoginglymus joint, combining the actions of both a hinge joint and a pivot joint. The hinge component allows for flexion (bending) and extension (straightening), primarily achieved through the articulation between the ulna and the humerus. The tight engagement of the ulna’s trochlear notch around the humerus’s trochlea dictates this single plane of movement.
The pivot component allows the forearm to rotate, enabling the palm to turn up (supination) or down (pronation). This rotational movement occurs at two locations: the connection between the radial head and the humerus’s capitulum, and the proximal radioulnar joint. As the forearm rotates, the radial head spins within a ligamentous ring, while the ulna remains relatively stationary.
The ulna provides stability for bending, and the radius contributes mobility for rotation. When the arm is bent, the radial head fits into the radial fossa on the humerus, while the coronoid process of the ulna enters the coronoid fossa.