The elbow is a complex synovial joint connecting the upper arm to the forearm, providing stability and a wide range of motion. It allows the arm to bend, straighten, and enables the forearm to rotate the hand. The joint is formed by the interaction of three distinct bones, creating a functional unit composed of both hinge and pivot mechanics.
The Three Bony Components
The structure of the elbow begins with the humerus, the single bone of the upper arm, which provides the superior articulating surface of the joint. The distal end of the humerus features two specialized structures: the spool-shaped trochlea on the inner side and the rounded capitulum on the outer side.
The ulna, located on the little finger side of the forearm, is the primary bone forming the hinge component of the elbow. Its most prominent feature is the large, C-shaped trochlear notch, which firmly cups the trochlea of the humerus. This notch is defined by the olecranon, the bony point felt at the back of the elbow, and the coronoid process, a smaller projection at the front.
Parallel to the ulna, the radius is the forearm bone situated on the thumb side. Its disc-shaped radial head articulates with the humerus. This radial head is flatter and allows for spinning motion rather than a simple hinge movement.
Articulations of the Elbow Complex
The elbow complex is formed by three distinct articulations. The first and largest is the humeroulnar joint, a pure hinge joint located between the humerus’s trochlea and the ulna’s trochlear notch. This articulation is responsible for the majority of the elbow’s stability and its capacity for bending and straightening the arm.
A secondary connection occurs at the humeroradial joint, where the humerus’s capitulum meets the head of the radius. This joint is structurally a gliding type, allowing the radius to slide and spin against the humerus, facilitating rotational movement.
The third articulation is the proximal radioulnar joint, which enables forearm rotation. The radial head pivots within a shallow socket on the ulna known as the radial notch. This arrangement allows the radius to cross over the ulna, which is fundamental to turning the palm up and down.
Primary Functions and Movements
The coordinated mechanics of the three articulations permit two main categories of movement at the elbow complex. The hinge-like function allows for movement in a single plane, characterized by flexion and extension. Flexion is the action of bending the arm, while extension is the motion of straightening the arm.
The second category involves rotational movements of the forearm, made possible by the pivot action of the radius around the ulna. Pronation is the turning of the forearm to face the palm downward, and supination is the reverse action, turning the palm upward.