The radius is one of the two long bones that make up the forearm, running from the elbow to the wrist on the thumb side. At its upper end, this bone features a distinct, disk-shaped structure known as the radial head. This specialized shape allows for a unique range of rotational movement. The radial head functions as the upper anchor of the forearm, connecting it to the upper arm bone. It is the site of two distinct articulations, which contribute to the overall mobility and function of the elbow joint complex.
The Articulation with the Humerus
The first connection is established at the elbow joint, where the radial head meets the rounded surface of the upper arm bone, forming the humeroradial joint. The shallow, cup-like depression on the superior surface of the radial head fits against the capitulum of the humerus. The capitulum is a smooth, ball-like structure located on the lateral side of the distal humerus. This articulation is structurally classified as a limited ball-and-socket joint, though its function is primarily like a hinge.
This hinge function allows the forearm to bend and straighten (flexion and extension). The radial head glides smoothly over the capitulum during these actions. The humeroradial joint also contributes to the stability of the elbow when the arm is fully extended. The precise fit between the two surfaces allows for the simultaneous rotation necessary for the forearm’s pivoting capabilities.
The Articulation with the Ulna
The radial head forms a separate joint with the ulna, known as the proximal radioulnar joint. The smooth, cylindrical side of the radial head articulates within a specialized depression on the ulna called the radial notch. This establishes a pivot joint separate from the main elbow hinge.
A strong, fibrous band of connective tissue, called the annular ligament, stabilizes this pivot joint. This ligament completely encircles the radial head, holding it securely against the radial notch like a collar. The inner surface is lined with a synovial membrane, creating a friction-reducing environment. This arrangement allows the radial head to spin freely within the ligament and the notch.
Movements Enabled by the Radial Head
The unique design of the radial head’s two articulations allows for the specialized forearm movements of pronation and supination. Pronation turns the palm downward, while supination turns the palm upward. These actions are defined by the rotation of the forearm, which changes the orientation of the palm.
These movements occur as the radial head spins within the ring formed by the annular ligament and the radial notch. During this rotation, the radial head simultaneously pivots against the capitulum of the humerus. This coordinated action involves the radius crossing over the ulna during pronation. The ability of the radial head to rotate in two different joint sockets makes grasping, carrying, and manipulating objects possible.
Common Injuries and Clinical Significance
Given the radial head’s role as a rotational hub, it is a common site for trauma and instability. Radial head fractures frequently result from a fall onto an outstretched hand, transmitting force up the forearm into the elbow joint. These fractures range from simple cracks to complex breaks that disrupt the articular surface, severely limiting forearm rotation.
Radial head subluxation, commonly known as “Nursemaid’s elbow,” is seen primarily in young children between one and four years old. This injury occurs when a child’s hand is suddenly pulled, causing a longitudinal traction force on the arm. The pull causes the annular ligament to slip over the radial head and become trapped, partially displacing the bone. The child will hold the arm close to the body and refuse to use it due to pain.