The radius or radial bone (: radii or radiuses) 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 used to describe the entire appendage of the upper limb, but which in anatomy, techn ...

, the other being the

ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...

. It extends from the

lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...

side of the

elbow The elbow is the region between the upper arm and the forearm that surrounds the elbow joint. The elbow includes prominent landmarks such as the olecranon, the cubital fossa (also called the chelidon, or the elbow pit), and the lateral and t ...

to the

thumb The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for 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 carpal ...

and runs parallel to the ulna. The ulna is longer than the radius, but the radius is thicker. The radius is a

long bone The long bones are those that are longer than they are wide. They are one of five types of bones: long, short, flat, irregular and sesamoid. Long bones, especially the femur and tibia, are subjected to most of the load during daily activities ...

prism PRISM is a code name for a program under which the United States National Security Agency (NSA) collects internet communications from various U.S. internet companies. The program is also known by the SIGAD . PRISM collects stored internet ...

-shaped and slightly curved longitudinally. The radius is part of two

joints A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

: the

and the

. At the elbow, it joins with the

capitulum of the humerus In human anatomy of the arm, the capitulum of the humerus is a smooth, rounded eminence on the lateral portion of the distal articular surface of the humerus. It articulates with the cup-shaped depression on the head of the radius, and is limi ...

, and in a separate region, with the ulna at the

radial notch The radial notch of the ulna (lesser sigmoid cavity) is a narrow, oblong, articular depression on the lateral side of the coronoid process; it receives the circumferential articular surface of the head of the radius. It is concave from before b ...

. At the wrist, the radius forms a joint with the ulna bone. The corresponding bone in the

lower leg The leg is the entire lower limb (anatomy), limb of the human body, including the foot, thigh or sometimes even the hip or Gluteal muscles, buttock region. The major bones of the leg are the femur (thigh bone), tibia (shin bone), and adjacent f ...

is the

tibia The tibia (; : tibiae or tibias), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two Leg bones, bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outsi ...

Structure

The long narrow

medullary cavity The medullary cavity (''medulla'', innermost part) is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored; hence, the medullary cavity is also known as the marrow cavity. Located in the ma ...

is enclosed in a strong wall of

compact 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, and ...

. 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 A trabecula (: trabeculae, from Latin for 'small beam') is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ. A trabecula generally has a ...

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 The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

'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 Radial aplasia is a congenital defect which affects the formation of the radius bone in the arm. The radius is the lateral bone (thumb side) which connects the humerus of the upper arm to the wrist via articulation with the carpal bones. A child ...

. 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 Beneath the neck of the radius, 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. * an ''anterior, smooth portion'', on ...

. 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

scaphoid The scaphoid bone is one of the carpal bones of the wrist. It is situated between the hand and forearm on the thumb side of the wrist (also called the lateral or radial side). It forms the radial border of the carpal tunnel. The scaphoid bone ...

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 describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

and

distal radioulnar articulation The distal radioulnar articulation (also known as the distal radioulnar joint, or inferior radioulnar joint) is a synovial pivot joint between the two bones in the forearm; the radius and ulna. It is one of two joints between the radius and ulna ...

s, 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 * Interosseo ...

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

. * 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 scaphoid bone is one of the carpal bones of the wrist. It is situated between the hand and forearm on the thumb side of the wrist (also called the lateral or radial side). It forms the radial border of the carpal tunnel. The scaphoid b ...

; the medial, quadrilateral, with the

. * The articular surface for the ''ulna'' is called the ulnar 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. * The ''volar surface'', rough and irregular, affords attachment to the volar radiocarpal ligament. * The ''dorsal surface'' is convex, affords attachment to the

dorsal radiocarpal ligament The dorsal radiocarpal ligament (posterior ligament) is less thick and strong than its volar counterpart, and has a proximal attachment to the posterior border of the distal radius. Its fibers run medially and inferiorly to form a distal attachmen ...

, 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 In human anatomy, extensor carpi radialis brevis is a muscle in the forearm that acts to extend and abduct the wrist. It is shorter and thicker than its namesake extensor carpi radialis longus which can be found above the proximal end of the ext ...

. ** 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 In human anatomy, the extensor pollicis longus muscle (EPL) is a skeletal muscle located dorsally on the forearm. It is much larger than the extensor pollicis brevis, the origin of which it partly covers and acts to stretch the thumb together wit ...

. ** The ''third'' is broad, for the passage of the tendons of the

extensor indicis proprius In human anatomy, the extensor indicis (proprius) is a narrow, elongated skeletal muscle in the deep layer of the dorsal forearm, placed medial to, and parallel with, the extensor pollicis longus. Its tendon goes to the index finger, which it ext ...

and

extensor digitorum communis In anatomy, extension is a movement of a joint that increases the angle between two bones or body surfaces at a joint. Extension usually results in straightening of the bones or body surfaces involved. For example, extension is produced by extend ...

. * 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 radial collateral ligament (external lateral ligament, radial carpal collateral ligament) extends from the tip of the styloid process of the radius and attaches to the radial side of the scaphoid (formerly navicular bone of the hand), immedia ...

. The lateral surface of this process is marked by a flat groove, for the tendons of the

abductor pollicis longus In human anatomy, the abductor pollicis longus (APL) is one of the extrinsic muscles of the hand. Its major function is to abduct the thumb at the wrist. Its tendon forms the anterior border of the anatomical snuffbox. Structure The abducto ...

muscle and

extensor pollicis brevis In human anatomy, the extensor pollicis brevis (EPB) is a skeletal muscle on the dorsal side of the forearm. It lies on the medial side of, and is closely connected with, the abductor pollicis longus. The extensor pollicis brevis belongs to t ...

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 Flexor digitorum superficialis (''flexor digitorum sublimis'') or flexor digitorum communis 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 s ...

(also ''flexor digitorum sublimis'') and

flexor pollicis longus muscle 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 uniq ...

; 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 The supinator consists of two planes of fibers, between ...

. The middle third of the volar border is indistinct and rounded. The lower fourth is prominent, and gives insertion to the

pronator quadratus muscle Pronator quadratus is a square-shaped muscle on the distal forearm that acts to pronate (turn so the palm faces downwards) the hand. Structure Its fibres run perpendicular to the direction of the arm, running from the most distal quarter of th ...

, and attachment to the dorsal carpal ligament; it ends in a small tubercle, into which the tendon of the brachioradialis muscle 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. To the posterior of the two ridges the lower part of the

is attached, while the triangular surface between the ridges gives insertion to part of the

. 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 A syndesmosis (“fastened with a band”) is a type of fibrous joint in which two bones are united to each other by fibrous connective tissue. The gap between the bones may be narrow, with the bones joined by ligaments, or the gap may be wide a ...

joint. ;Surfaces The volar surface (''facies volaris; anterior surface'') is concave in its upper three-fourths, and gives origin to the

; 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. 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 In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius (bone), radius. Its function is to supination, supinate the forearm. Structure The supinator consists of tw ...

. Its middle third is broad, slightly concave, and gives origin to the

Abductor pollicis longus In human anatomy, the abductor pollicis longus (APL) is one of the extrinsic muscles of the hand. Its major function is to abduct the thumb at the wrist. Its tendon forms the anterior border of the anatomical snuffbox. Structure The abducto ...

above, and the

extensor pollicis brevis muscle In human anatomy, the extensor pollicis brevis (EPB) is a skeletal muscle on the dorsal side of the forearm. It lies on the medial side of, and is closely connected with, the abductor pollicis longus. The extensor pollicis brevis belongs to th ...

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

. 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 In human anatomy, the abductor pollicis longus (APL) is one of the extrinsic muscles of the hand. Its major function is to abduct the thumb at the wrist. Its tendon forms the anterior border of the anatomical snuffbox. Structure The abducto ...

and

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 capitulum (plural capitula) may refer to: *the Latin word for chapter ** an index or list of chapters at the head of a gospel manuscript ** a short reading in the Liturgy of the Hours *** derived from which, it is the Latin for the assembly known ...

(or capitellum) of the

. 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

. * Beneath the neck, on the medial side, is an eminence, the ''

''; its surface is divided into a posterior, rough portion, for the insertion of the tendon of the

biceps brachii muscle The biceps or 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 Muscle head, heads of the muscle arise on the scapula and join to form a single ...

, and an anterior, smooth portion, on which a

bursa Bursa () 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 Marmara Region, Bursa is one of the industrial centers of the country. Most of ...

is interposed between the

tendon A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...

and the bone.

Development

The radius is

ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

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 A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a ...

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 An epiphysis (; : epiphyses) is one of the rounded ends or tips of a long bone that ossify from one or more secondary centers of ossification. Between the epiphysis and diaphysis (the long midsection of the long bone) lies the metaphysis, inc ...

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

, appears about the fourteenth or fifteenth year.

Function

Muscle attachments

The

biceps The biceps or 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 to form a single muscle bel ...

muscle inserts on the

of the upper extremity of the bone. The upper third of the body of the bone attaches to the

supinator In human anatomy, the supinator is a broad muscle in the posterior compartment of the forearm, curved around the upper third of the radius (bone), radius. Its function is to supination, supinate the forearm. Structure The supinator consists of tw ...

, the

flexor digitorum superficialis Flexor digitorum superficialis (''flexor digitorum sublimis'') or flexor digitorum communis 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 s ...

, 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 uniq ...

muscles. The middle third of the body attaches to the extensor ossis metacarpi pollicis, extensor primi internodii pollicis, and the

pronator teres The pronator teres is a muscle (located mainly in the forearm) that, along with the pronator quadratus, serves to pronate the forearm (turning it so that the palm faces posteriorly when from the anatomical position). It has two origins, at th ...

muscles. The lower quarter of the body attaches to the pronator quadratus muscle and the

of the supinator longus.

Clinical significance

Radial aplasia Radial aplasia is a congenital defect which affects the formation of the radius bone in the arm. The radius is the lateral bone (thumb side) which connects the humerus of the upper arm to the wrist via articulation with the carpal bones. A child ...

refers to the congenital absence or shortness of the radius.

Fracture

$Pradialheadfrac$ Specific

fracture Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...

types of the radius include: *Proximal radius fracture. A fracture within the capsule of the

joint results in the fat pad sign or "sail sign" which is a displacement of the

fat pad A fat pad (aka haversian gland) is a mass of closely packed fat cells surrounded by fibrous tissue septa.TheFreeDictionary > Fat padCiting: Mosby's Medical Dictionary, 8th edition. 2009 They may be extensively supplied with capillaries and nerve end ...

at the elbow. $Forearm fracture - Radius fracture 2 -- Smart-Servier$ ** Essex-Lopresti fracture – a fracture of the radial head with concomitant dislocation of the

distal radio-ulnar joint The distal radioulnar articulation (also known as the distal radioulnar joint, or inferior radioulnar joint) is a synovial pivot joint between the two bones in the forearm; the radius and ulna. It is one of two joints between the radius and ulna ...

with disruption of the

.Essex Lopresti fracture
at Wheeless' Textbook of Orthopaedics online *Radial shaft fracture *

Distal radius fracture A distal radius fracture, also known as wrist fracture, is a fracture (bone), break of the part of the radius (bone), radius bone which is close to the wrist. Symptoms include pain, bruising, and rapid-onset swelling. The ulna bone may also be br ...

** 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 ty ...

– 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 spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

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

to the center of distal

. While the

is the major contributor to the elbow joint, the radius primarily contributes to the

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.

Other 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 A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...

, but it may be fused with the ulna in some

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

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 mi ...

s) and reduced or modified in animals with flippers or vestigial forelimbs.

Gallery

References

{{Authority control Long bones Bones of the upper limb