Radius (bone)

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.