The radius or radial bone is one of the two large
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
s of the
forearm
The forearm is the region of the upper limb between the elbow and the wrist. The term forearm is used in anatomy to distinguish it from the arm, a word which is most often used to describe the entire appendage of the upper limb, but which in ...
, the other being the
ulna. It extends from the
lateral side of the
elbow to the
thumb side of the
wrist
In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carp ...
and runs parallel to the ulna. The ulna is usually slightly longer than the radius, but the radius is thicker. Therefore the radius is considered to be the larger of the two. It is a
long bone,
prism-shaped and slightly curved longitudinally.
The radius is part of two
joints: the
elbow and the
wrist
In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carp ...
. At the elbow, it joins with the
capitulum of the humerus, and in a separate region, with the ulna at the
radial notch. At the wrist, the radius forms a joint with the ulna bone.
The corresponding bone in the
lower leg is the
fibula
The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity ...
.
Structure
The long narrow
medullary cavity is enclosed in a strong wall of
compact bone. It is thickest along the interosseous border and thinnest at the extremities, same over the cup-shaped articular surface (fovea) of the head.
The
trabeculae of the spongy tissue are somewhat arched at the upper end and pass upward from the compact layer of the shaft to the ''fovea capituli'' (the
humerus'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 in
radial aplasia.
The radius has a body and two extremities. The
upper extremity of the radius consists of a somewhat cylindrical head articulating with the ulna and the humerus, a neck, and a
radial tuberosity. The
body of the radius is self-explanatory, and the
lower extremity of the radius is roughly quadrilateral in shape, with articular surfaces for the
ulna,
scaphoid and
lunate bone
The lunate bone (semilunar bone) is a carpal bone in the human hand. It is distinguished by its deep concavity and crescentic outline. It is situated in the center of the proximal row carpal bones, which lie between the ulna and radius and the h ...
s. The distal end of the radius forms two palpable points, radially the
styloid process and
Lister's tubercle on the ulnar side. Along with the
proximal
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position ...
and
distal radioulnar articulations, an
interosseous membrane
An interosseous membrane is a thick dense fibrous sheet of connective tissue that spans the space between two bones, forming a type of syndesmosis joint.
Interosseous membranes in the human body:
* Interosseous membrane of forearm
* Interosseous ...
originates medially along the length of the body of the radius to attach the radius to the ulna.
Near the wrist
The distal end of the radius is large and of quadrilateral form.
;Joint surfaces
It is provided with two articular surfaces – one below, for the
carpus, and another at the medial side, for the
ulna.
* The ''carpal'' articular surface is triangular, concave, smooth, and divided by a slight antero-posterior ridge into two parts. Of these, the lateral, triangular, articulates with the
scaphoid bone; the medial, quadrilateral, with the
lunate bone
The lunate bone (semilunar bone) is a carpal bone in the human hand. It is distinguished by its deep concavity and crescentic outline. It is situated in the center of the proximal row carpal bones, which lie between the ulna and radius and the h ...
.
* The articular surface for the ''ulna'' is called the
ulnar notch
The articular surface for the ulna is called the ulnar notch (sigmoid cavity) of the radius; it is in the distal radius, and is narrow, concave, smooth, and articulates with the head of the ulna forming the distal radioulnar joint
The distal ra ...
(''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.
* The ''volar surface'', rough and irregular, affords attachment to the
volar radiocarpal ligament
The palmar radiocarpal ligament (anterior ligament, volar radiocarpal ligament) is a broad membranous band, attached above to the distal end of the radius, and passing downward to the scaphoid, lunate, triquetrum and capitate of the carpal bones ...
.
* The ''dorsal surface'' is convex, affords attachment to the
dorsal radiocarpal ligament, and is marked by three grooves. Enumerated from the lateral side:
** The ''first'' groove is broad, but shallow, and subdivided into two by a slight ridge: the lateral of these two, transmits the tendon of the
extensor carpi radialis longus muscle
The extensor carpi radialis longus is one of the five main muscles that control movements at the wrist. This muscle is quite long, starting on the lateral side of the humerus, and attaching to the base of the second metacarpal bone (metacarpal of ...
; the medial, the tendon of the
extensor carpi radialis brevis muscle.
** The ''second'' 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 the
extensor pollicis longus muscle.
** The ''third'' is broad, for the passage of the tendons of the
extensor indicis proprius and
extensor digitorum communis.
* The ''lateral 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 the
radial collateral ligament of wrist joint. The lateral surface of this process is marked by a flat groove, for the tendons of the
abductor pollicis longus muscle and
extensor pollicis brevis muscle.
Body
The body of the radius (or shaft 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
The volar border (''margo volaris; anterior border; palmar'';) extends from the lower part of the
tuberosity above to the anterior part of the base of the
styloid process below, and separates the
volar 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 the
flexor digitorum superficialis muscle (also ''flexor digitorum sublimis'') and
flexor pollicis longus muscle; the surface above the line gives insertion to part of the
supinator muscle
In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius. Its function is to supinate the forearm.
Structure
Supinator consists of two planes of fibers, between whi ...
. The middle third of the volar border is indistinct and rounded. The lower fourth is prominent, and gives insertion to the
pronator quadratus muscle, and attachment to the
dorsal carpal ligament; it ends in a small tubercle, into which the tendon of the
brachioradialis muscle
The brachioradialis is a muscle of the forearm that flexes the forearm at the elbow. It is also capable of both pronation and supination, depending on the position of the forearm. It is attached to the distal styloid process of the radius by way o ...
is inserted.
The dorsal border (''margo dorsalis; posterior border'') begins above at the back of the neck, and ends below at the posterior part of the base of the
styloid process; it separates the posterior from the lateral surface. is indistinct above and below, but well-marked in the middle third of the bone.
The interosseous border (''internal border; crista interossea; interosseous crest;'') begins above, at the back part of the
tuberosity, 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 the
ulnar notch
The articular surface for the ulna is called the ulnar notch (sigmoid cavity) of the radius; it is in the distal radius, and is narrow, concave, smooth, and articulates with the head of the ulna forming the distal radioulnar joint
The distal ra ...
. To the posterior of the two ridges the lower part of the
interosseous membrane
An interosseous membrane is a thick dense fibrous sheet of connective tissue that spans the space between two bones, forming a type of syndesmosis joint.
Interosseous membranes in the human body:
* Interosseous membrane of forearm
* Interosseous ...
is attached, while the triangular surface between the ridges gives insertion to part of the
pronator 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 a
syndesmosis joint.
;Surfaces
The volar surface (''facies volaris; anterior surface'') is concave in its upper three-fourths, and gives origin to the
flexor pollicis longus muscle; it is broad and flat in its lower fourth, and affords insertion to the
Pronator 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 the
volar radiocarpal ligament
The palmar radiocarpal ligament (anterior ligament, volar radiocarpal ligament) is a broad membranous band, attached above to the distal end of the radius, and passing downward to the scaphoid, lunate, triquetrum and capitate of the carpal bones ...
. At the junction of the upper and middle thirds of the volar surface is the nutrient foramen, which is directed obliquely upward.
The dorsal surface (''facies dorsalis; posterior surface'') is convex, and smooth in the upper third of its extent, and covered by the
Supinator. Its middle third is broad, slightly concave, and gives origin to the
Abductor pollicis longus above, and the
extensor 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.
The lateral surface (''facies lateralis; external surface'') is convex throughout its entire extent and is known as the convexity of the radius, curving outwards to be convex at the side. Its upper third gives insertion to the
supinator muscle
In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius. Its function is to supinate the forearm.
Structure
Supinator consists of two planes of fibers, between whi ...
. About its center is a rough ridge, for the insertion of the
pronator teres muscle. Its lower part is narrow, and covered by the tendons of the
abductor pollicis longus muscle and
extensor pollicis brevis muscle.
Near the elbow
The upper extremity of the radius (or proximal extremity) presents a head, neck, and tuberosity.
* The radial ''head'' has a cylindrical form, and on its upper surface is a shallow cup or fovea for articulation with the
capitulum (or capitellum) of the
humerus. The circumference of the head is smooth; it is broad medially where it articulates with the
radial notch of the ulna, narrow in the rest of its extent, which is embraced by the
annular 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 the ''neck'', on the back of which is a slight ridge for the insertion of part of the
supinator muscle
In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius. Its function is to supinate the forearm.
Structure
Supinator consists of two planes of fibers, between whi ...
.
* Beneath the neck, on the medial side, is an eminence, the ''
radial tuberosity''; its surface is divided into a posterior, rough portion, for the insertion of the tendon of the
biceps brachii muscle, and an anterior, smooth portion, on which a
bursa
( grc-gre, Προῦσα, Proûsa, Latin: Prusa, ota, بورسه, Arabic:بورصة) is a city in northwestern Turkey and the administrative center of Bursa Province. The fourth-most populous city in Turkey and second-most populous in the ...
is interposed between the
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
and the bone.
Development
The radius is
ossified from ''three'' 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 of
fetal life.
Ossification commences in the lower end between 9 and 26 months of age. The ossification center for the upper end appears by the fifth year.
The upper
epiphysis 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 the
radial tuberosity, appears about the fourteenth or fifteenth year.
Function
Muscle attachments
The
biceps
The biceps or biceps brachii ( la, musculus biceps brachii, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join t ...
muscle inserts on the
radial tuberosity of the upper extremity of the bone. The upper third of the body of the bone attaches to the
supinator, the
flexor digitorum superficialis
Flexor digitorum superficialis (''flexor digitorum sublimis'') is an extrinsic flexor muscle of the fingers at the proximal interphalangeal joints.
It is in the anterior compartment of the forearm. It is sometimes considered to be the deepest p ...
, and the
flexor pollicis longus
The flexor pollicis longus (; FPL, Latin ''flexor'', bender; ''pollicis'', of the thumb; ''longus'', long) is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus. This muscle is un ...
muscles.
The middle third of the body attaches to the
extensor ossis metacarpi pollicis,
extensor primi internodii pollicis, and the
pronator teres muscles.
The lower quarter of the body attaches to the
pronator quadratus muscle and the
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
of the
supinator longus.
Clinical significance
Radial aplasia refers to the congenital absence or shortness of the radius.
Fracture
Specific
fracture
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displ ...
types of the radius include:
*Proximal radius fracture. A fracture within the capsule of the
elbow joint results in the
fat pad sign or "sail sign" which is a displacement of the
fat pad at the elbow.
**
Essex-Lopresti fracture – a fracture of the
radial head with concomitant dislocation of the
distal radio-ulnar joint with disruption of the
interosseous membrane
An interosseous membrane is a thick dense fibrous sheet of connective tissue that spans the space between two bones, forming a type of syndesmosis joint.
Interosseous membranes in the human body:
* Interosseous membrane of forearm
* Interosseous ...
.
[Essex Lopresti fracture](_blank)
at Wheeless' Textbook of Orthopaedics online
*Radial shaft fracture
*
Distal radius fracture
**
Galeazzi fracture – a fracture of the radius with dislocation of the
distal radioulnar joint
**
Colles' fracture
A Colles' fracture is a type of fracture of the distal forearm in which the broken end of the radius is bent backwards. Symptoms may include pain, swelling, deformity, and bruising. Complications may include damage to the median nerve.
It typi ...
– a distal fracture of the radius with dorsal (posterior) displacement of the wrist and hand
**
Smith's fracture
A Smith's fracture, is a fracture of the distal radius.
Although it can also be caused by a direct blow to the dorsal forearm or by a fall with the wrist flexed, the most common mechanism of injury for Smith's fracture occurs in a palmar fall wi ...
– a distal fracture of the radius with volar (ventral) displacement of the wrist and hand
**
Barton's fracture – an
intra-articular fracture of the distal radius with dislocation of the
radiocarpal joint.
History
The word ''radius'' is
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
for "ray". In the context of the radius bone, a ray can be thought of rotating around an axis line extending diagonally from center of
capitulum to the center of distal
ulna. While the
ulna is the major contributor to the elbow joint, the radius primarily contributes to the
wrist
In human anatomy, the wrist is variously defined as (1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand; "The wrist contains eight bones, roughly aligned in two rows, known as the carp ...
joint.
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 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.
Animals
In four-legged animals, the radius is the main load-bearing bone of the lower forelimb. Its structure is similar in most terrestrial
tetrapods, but it may be fused with the ulna in some
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
s (such as
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...
s) and reduced or modified in animals with flippers or vestigial forelimbs.
Gallery
References
{{Authority control
Long bones
Bones of the upper limb