Radius (bone)
One of the two large bones of the forearm.
The radius, also called the radial bone, is one of the two large bones in the forearm, alongside the ulna. It runs parallel to the ulna, stretching from the outer side of the elbow to the thumb side of the wrist. Though shorter than the ulna, the radius is thicker. This long bone is prism-shaped and has a slight longitudinal curve. It participates in three joints: the elbow and wrist (both synovial joints) and the radioulnar joint (a syndesmosis). The radioulnar joint enables supination and pronation of the forearm. At the elbow, the radius connects with the humerus’s capitulum and, separately, with the ulna at the radial notch. At the wrist, it forms a joint with the ulna. The tibia is the corresponding bone in the leg.
The radius has a body and two extremities. Its upper extremity includes a cylindrical head that articulates with the ulna and humerus, a neck, and a radial tuberosity. The body, or shaft, is prismoid, narrower above than below, and slightly curved outward. The lower extremity is roughly quadrilateral, with articular surfaces for the ulna, scaphoid, and lunate bones. At the distal end, two palpable points are present: the styloid process on the radial side and Lister’s tubercle on the ulnar side. An interosseous membrane runs medially along the body’s length, attaching the radius to the ulna, alongside the proximal and distal radioulnar articulations.
Inside, the long, narrow medullary cavity is surrounded by a strong wall of compact bone. This wall is thickest along the interosseous border and thinnest at the extremities, particularly over the cup-shaped articular surface (fovea) of the head. The trabeculae of the spongy tissue are arched at the upper end, passing upward from the compact layer to the fovea capituli, and are crossed by others parallel to the fovea’s surface. A similar arrangement exists at the lower end. This structure is absent in radial aplasia.
Near the wrist, the distal end is large and quadrilateral. It has two articular surfaces: one below for the carpus, and another on the medial side for the ulna. The carpal articular surface is triangular, concave, and smooth, divided by a slight ridge into a lateral part (articulating with the scaphoid) and a medial part (articulating with the lunate). The ulnar notch (sigmoid cavity) is narrow, concave, and smooth, articulating with the head of the ulna. A prominent ridge separates these surfaces, and the base of the triangular articular disk attaches here, separating the wrist joint from the distal radioulnar articulation.
The distal end also has three non-articular surfaces: volar, dorsal, and lateral. The volar surface is rough and irregular, attaching the volar radiocarpal ligament. The dorsal surface is convex, attaches the dorsal radiocarpal ligament, and has three grooves. From the lateral side: the first groove is broad but shallow, subdivided into two by a slight ridge—the lateral part transmits the extensor carpi radialis longus tendon, the medial part the extensor carpi radialis brevis tendon. The second groove is deep but narrow, bounded laterally by a sharp ridge, and transmits the extensor pollicis longus tendon. The third groove is broad, for the extensor indicis proprius and extensor digitorum communis tendons. The lateral surface extends obliquely downward into the styloid process, a strong conical projection. Its base attaches the brachioradialis tendon, its apex attaches the radial collateral ligament of the wrist. The lateral surface of the process has a flat groove for the abductor pollicis longus and extensor pollicis brevis tendons.
The body has three borders and three surfaces. The volar border runs from the lower part of the tuberosity to the anterior part of the styloid process base, separating the volar from the lateral surface. Its upper third is prominent and oblique, called the oblique line, giving origin to the flexor digitorum superficialis and flexor pollicis longus muscles; the surface above it gives insertion to part of the supinator. The middle third is indistinct and rounded. The lower fourth is prominent, gives insertion to the pronator quadratus, and attaches the dorsal carpal ligament; it ends in a small tubercle for the brachioradialis tendon. The dorsal border starts at the back of the neck and ends at the posterior part of the styloid process base, separating the posterior from the lateral surface; it is indistinct above and below but well-marked in the middle third. The interosseous border begins at the back part of the tuberosity, is rounded and indistinct above, then becomes sharp and prominent, dividing at its lower part into two ridges that continue to the anterior and posterior margins of the ulnar notch. The posterior ridge attaches the lower part of the interosseous membrane.
- type
- Bone
- location
- Forearm (lateral side)
- joints
- Elbow, wrist, radioulnar joint
- shape
- Long, prism-shaped, slightly curved
- articulations
- Capitulum of humerus, ulna, scaphoid, lunate
Lore & Background
The radius has a body and two extremities. The upper extremity consists of a cylindrical head articulating with the ulna and humerus, a neck, and a radial tuberosity. The head has a shallow cup (fovea) for articulation with the capitulum of the humerus, and its circumference is smooth, broad medially where it articulates with the radial notch of the ulna. The deepest point in the fovea is not axi-symmetric with the long axis of the radius, creating a cam effect during pronation and supination. The radial tuberosity beneath the neck has a posterior rough portion for insertion of the biceps brachii tendon and an anterior smooth portion for a bursa.
Reader's Guide
The radius is a critical bone in the forearm, enabling complex movements of the wrist and forearm. Its structure includes a long narrow medullary cavity enclosed in compact bone, thickest along the interosseous border and thinnest at the extremities. The distal end of the radius is large and quadrilateral, with articular surfaces for the carpus (scaphoid and lunate bones) and for the ulna via the ulnar notch. The distal end also features the styloid process and Lister's tubercle as palpable points. The body of the radius is prismoid, narrower above than below, and slightly curved convex laterally, with three borders and three surfaces that provide attachments for multiple muscles including the flexor pollicis longus, pronator quadratus, supinator, and pronator teres. The interosseous border gives attachment to the interosseous membrane, connecting the radius to the ulna in a syndesmosis joint. The radius ossifies from three centers: one for the body and one for each extremity.
Did You Know?
- The radius is thicker than the ulna, though the ulna is longer.
- The deepest point in the fovea of the radial head is not axi-symmetric with the long axis, creating a cam effect during pronation and supination.
- The distal end of the radius has a triangular carpal articular surface divided by a ridge into two parts: one for the scaphoid and one for the lunate bone.
- The corresponding bone in the leg is the tibia.
The Sigmoid Cavity of the Distal Radius
The ulnar notch, also known as the sigmoid cavity, represents a specifically designated articular surface on the radius. Located within the distal portion of the bone, this feature serves as the precise point of contact where the radius meets the ulna. Three defining physical characteristics set this surface apart: it is narrow in its dimensions, concave in its curvature, and smooth in its texture. These qualities are not incidental but functional, as they allow the surface to receive and accommodate the head of the ulna in a controlled, low-friction manner. The narrowness suggests a limited area of contact, while the concavity provides a cradle-like geometry that stabilizes the articulating partner. The smoothness, of course, is essential for the gliding motions that occur at this junction. Together, these attributes make the sigmoid cavity a highly specialized region of the distal radius, shaped by bone structure to serve one particular mechanical purpose in the forearm.
Forming the Distal Radioulnar Joint
The primary functional role of the ulnar notch is to participate in what is termed the distal radioulnar joint. This joint is created specifically by the articulation between the sigmoid cavity of the radius and the head of the ulna. The relationship is one of a concave, narrow, smooth surface receiving a rounded bony projection — the head of the ulna — in a manner that permits defined movement while maintaining structural connection. The term "distal" in the joint's name anchors it to the end of the radius farthest from the elbow, emphasizing that this is not the proximal connection between the two forearm bones but rather the more terminal one. The smoothness of the sigmoid cavity is particularly significant here, as it ensures that the head of the ulna can move against it with minimal resistance, a prerequisite for the movements that the distal radioulnar joint facilitates. In essence, this single articular surface is the radius's contribution to a joint that links the two forearm bones at their distal ends.
Dual Nomenclature: Ulnotch and Sigmoid Cavity
Anatomical structures often carry more than one name, and the articular surface on the distal radius is a clear example. It is referred to both as the ulnar notch and as the sigmoid cavity. The first name, "ulnar notch," immediately communicates its functional relationship: it is the notch or indentation that receives the ulna. The second name, "sigmoid cavity," draws attention to the shape of the surface. The term "sigmoid" evokes a curved form, while "cavity" reinforces the concave, recessed nature of the structure. Together, the two names paint a more complete picture than either could alone — one emphasizes the partner bone involved, and the other emphasizes the geometric character of the surface itself. This dual naming convention reflects the broader tradition in anatomy where a single structure is described from multiple perspectives: its relational role to neighboring bones and its intrinsic morphological qualities. For students and practitioners, knowing both terms ensures that references in different texts can be cross-referenced without confusion, even when one author prefers the relational name and another favors the descriptive one.
A Legacy in Public-Domain Anatomy
The description of the ulnar notch and the distal radioulnar joint that appears in many modern references traces, at least in part, to a much older source. Because this edition has entered the public domain, its language and descriptions are freely available for reuse, adaptation, and redistribution without copyright restriction. This means that the precise characterization of the sigmoid cavity as narrow, concave, and smooth — and its identification as the articular surface for the ulna's head — has been carried forward from early twentieth-century anatomical scholarship into contemporary digital encyclopedias. The fact that this particular passage remains in use today speaks to the enduring clarity and accuracy of the original description, which required no substantial revision to remain relevant more than a century later.
Frequently Asked Questions
Who is Radius (bone)?
The radius is the forearm bone on the thumb side, running parallel to the ulna from the lateral elbow down to the wrist. It's a long, slightly curved, prism-shaped bone that's a touch thicker than the ulna but a bit shorter in overall length.
What are Radius (bone)'s powers/role?
The radius is the star of forearm rotation, pivoting around the ulna at the radioulnar joints to let you flip your hand palm-up or palm-down. It also anchors the wrist joint and helps form the elbow, making it a three-joint workhorse.
How does Radius (bone) connect to its neighbors?
At the top it meets the capitulum of the humerus and the ulna, while at the bottom it locks in with the scaphoid and lunate carpal bones of the wrist. In between, its shaft stays snug against the ulna, enabling that smooth rotational movement.
Why is Radius (bone) important?
Without the radius, you'd lose the ability to rotate your forearm, which would wreck everything from turning a doorknob to catching a ball. It's also a major load-distributing bone through the wrist, so its structural integrity is critical for everyday hand function.
What's the difference between Radius and Ulna?
The radius sits on the lateral (thumb) side and is slightly thicker but shorter, while the ulna runs along the medial (pinky) side and is a bit longer and thinner. The radius does most of the rotational work, whereas the ulna acts more as a stable track for that rotation.
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