3D model.Full anterior view of right radiusFull posterior view of right radiusFull medial view of right radiusFull lateral view of right radius
The long narrowmedullary cavity is enclosed in a strong wall ofcompact bone. It is thickest along the interosseous border and thinnest at the extremities, same over the cup-shaped articular surface (fovea) of the head.
Thetrabeculae of the spongy tissue are somewhat arched at the upper end and pass upward from the compact layer of the shaft to thefovea capituli (thehumerus's cup-shaped articulatory notch); they are crossed by others parallel to the surface of the fovea. The arrangement at the lower end is somewhat similar. It is missing inradial aplasia.
Thedistal end of the radius is large and of quadrilateral form.
Joint surfaces
It is provided with two articular surfaces – one below, for thecarpus, and another at the medial side, for theulna.
Thecarpal articular surface is triangular, concave, smooth, and divided by a slight antero-posterior ridge into two parts. Of these, the lateral, triangular, articulates with thescaphoid bone; the medial, quadrilateral, with thelunate bone.
The articular surface for theulna is called theulnar notch (sigmoid cavity) of the radius; it is narrow, concave, smooth, and articulates with the head of the ulna.
These two articular surfaces are separated by a prominent ridge, to which the base of the triangular articular disk is attached; this disk separates the wrist-joint from the distal radioulnar articulation.
Other surfaces
This end of the bone has three non-articular surfaces – volar, dorsal, and lateral.
Thesecond is deep but narrow, and bounded laterally by a sharply defined ridge; it is directed obliquely from above downward and lateralward, and transmits the tendon of theextensor pollicis longus muscle.
Thelateral surface is prolonged obliquely downward into a strong, conical projection, the styloid process, which gives attachment by its base to the tendon of the brachioradialis, and by its apex to theradial collateral ligament of wrist joint. The lateral surface of this process is marked by a flat groove, for the tendons of theabductor pollicis longus muscle andextensor pollicis brevis muscle.
Thebody of the radius (orshaft of radius) is prismoid in form, narrower above than below, and slightly curved, so as to be convex lateralward. It presents three borders and three surfaces.
Borders
Thevolar border (margo volaris; anterior border; palmar;) extends from the lower part of thetuberosity above to the anterior part of the base of thestyloid process below, and separates thevolar from the lateral surface. Its upper third is prominent, and from its oblique direction has received the name of the oblique line of the radius; it gives origin to theflexor digitorum superficialis muscle (alsoflexor digitorum sublimis) andflexor pollicis longus muscle; the surface above the line gives insertion to part of thesupinator muscle. The middle third of the volar border is indistinct and rounded. The lower fourth is prominent, and gives insertion to thepronator quadratus muscle, and attachment to thedorsal carpal ligament; it ends in a small tubercle, into which the tendon of thebrachioradialis muscle is inserted.
Thedorsal border (margo dorsalis; posterior border) begins above at the back of the neck, and ends below at the posterior part of the base of thestyloid process; it separates the posterior from the lateral surface. is indistinct above and below, but well-marked in the middle third of the bone.
Theinterosseous border (internal border; crista interossea; interosseous crest;) begins above, at the back part of thetuberosity, and its upper part is rounded and indistinct; it becomes sharp and prominent as it descends, and at its lower part divides into two ridges which are continued to the anterior and posterior margins of theulnar notch. To the posterior of the two ridges the lower part of theinterosseous membrane is attached, while the triangular surface between the ridges gives insertion to part of thepronator quadratus muscle. This crest separates the volar from the dorsal surface, and gives attachment to the interosseous membrane. The connection between the two bones is actually a joint referred to as asyndesmosis joint.
Surfaces
Thevolar surface (facies volaris; anterior surface) is concave in its upper three-fourths, and gives origin to theflexor pollicis longus muscle; it is broad and flat in its lower fourth, and affords insertion to thePronator quadratus. A prominent ridge limits the insertion of the Pronator quadratus below, and between this and the inferior border is a triangular rough surface for the attachment of thevolar radiocarpal ligament. At the junction of the upper and middle thirds of the volar surface is the nutrient foramen, which is directed obliquely upward.
Thedorsal surface (facies dorsalis; posterior surface) is convex, and smooth in the upper third of its extent, and covered by theSupinator. Its middle third is broad, slightly concave, and gives origin to theAbductor pollicis longus above, and theextensor pollicis brevis muscle below. Its lower third is broad, convex, and covered by the tendons of the muscles which subsequently run in the grooves on the lower end of the bone.
Thelateral surface (facies lateralis; external surface) is convex throughout its entire extent and is known as theconvexity of the radius, curving outwards to be convex at the side. Its upper third gives insertion to thesupinator muscle. About its center is a rough ridge, for the insertion of thepronator teres muscle.[3] Its lower part is narrow, and covered by the tendons of theabductor pollicis longus muscle andextensor pollicis brevis muscle.
Theupper extremity of the radius (orproximal extremity) presents a head, neck, and tuberosity.
The radialhead has a cylindrical form, and on its upper surface is a shallow cup or fovea for articulation with thecapitulum (or capitellum) of thehumerus. The circumference of the head is smooth; it is broad medially where it articulates with theradial notch of the ulna, narrow in the rest of its extent, which is embraced by theannular ligament. 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 head is supported on a round, smooth, and constricted portion called theneck, on the back of which is a slight ridge for the insertion of part of thesupinator muscle.
Beneath the neck, on the medial side, is an eminence, theradial tuberosity; its surface is divided into a posterior, rough portion, for the insertion of the tendon of thebiceps brachii muscle, and an anterior, smooth portion, on which abursa is interposed between thetendon and the bone.
The radius isossified fromthree centers: one for the body, and one for each extremity. That for the body makes its appearance near the center of the bone, during the eighth week offetal life.
Ossification commences in the lower end between 9 and 26 months of age.[citation needed] The ossification center for the upper end appears by the fifth year.
The upperepiphysis fuses with the body at the age of seventeen or eighteen years, the lower about the age of twenty.
An additional center sometimes found in theradial tuberosity, appears about the fourteenth or fifteenth year.
Proximal radius fracture. A fracture within the capsule of theelbow joint results in thefat pad sign or "sail sign" which is a displacement of thefat pad at the elbow.
The wordradius isLatin for "ray". In the context of the radius bone, a ray can be thought of rotating around an axis line extending diagonally[clarification needed] from center ofcapitulum to the center of distalulna. While theulna is the major contributor to the elbow joint, the radius primarily contributes to thewrist joint.[5]
The radius is named so because the radius (bone) acts like the radius (of a circle). It rotates around the ulna and the far end (where it joins to the bones of the hand), known as the styloid process of the radius, is[clarification needed] the distance from the ulna (center of the circle) to the edge of the radius (the circle). The ulna acts as the center point to the circle because when the arm is rotated the ulna does not move.
In four-legged animals, the radius is the main load-bearing bone of the lower forelimb. Its structure is similar in most terrestrialtetrapods, but it may be fused with the ulna in somemammals (such ashorses) and reduced or modified in animals with flippers or vestigial forelimbs.[6]
