The ulna (''pl''. ulnae or ulnas) 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 bone, short, Flat bone, flat, Irregular bone, irregular and Sesamoid bone, sesamoid. Long bones, especially the femur and tibia, are subj ...

found in the forearm that stretches from the elbow to the smallest finger, and when in

anatomical position The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages of animals with r ...

, is found on the medial side of the forearm. That is, the ulna is on the same side of the forearm as the little finger. It runs parallel to the radius, the other long bone in the forearm. 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.

Structure

The ulna is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. It is broader close to the elbow, and narrows as it approaches the wrist. Close to the elbow, the ulna has a bony process, the olecranon process, a hook-like structure that fits into the olecranon fossa of the

humerus The humerus (; ) 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 and ulna, and consists of three sections. The humeral upper extremity consists of a roun ...

. This prevents hyperextension and forms a

hinge joint A hinge joint (ginglymus or ginglymoid) is a bone joint in which the articular surfaces are molded to each other in such a manner as to permit motion only in one plane. According to one classification system they are said to be uniaxial (having ...

with the trochlea of the humerus. There is also a

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 bac ...

for the head of the radius, and the ulnar tuberosity to which muscles attach. Close to the wrist, the ulna has a styloid process.

Near the elbow

Near the elbow, the ulna has two curved processes, the olecranon and the

coronoid process The Coronoid process (from Greek , "like a crown") can refer to: * The coronoid process of the mandible, part of the ramus mandibulae of the mandible * The coronoid process of the ulna The coronoid process of the ulna is a triangular process proj ...

; and two concave, articular cavities, the

semilunar Semilunar can refer to: * Semilunar valves * Semilunar ganglion, or the trigeminal ganglion * An older name for the Lunate bone The lunate bone (semilunar bone) is a carpal bone in the human hand. It is distinguished by its deep concavity and cr ...

and

es. The olecranon is a large, thick, curved eminence, situated at the upper and back part of the ulna. It is bent forward at the summit so as to present a prominent lip which is received into the olecranon fossa of the humerus in extension of the forearm. Its base is contracted where it joins the body and the narrowest part of the upper end of the ulna. Its posterior surface, directed backward, is triangular, smooth, subcutaneous, and covered by a bursa. Its superior surface is of quadrilateral form, marked behind by a rough impression for the insertion of the triceps brachii; and in front, near the margin, by a slight transverse groove for the attachment of part of the posterior ligament of the elbow joint. Its anterior surface is smooth, concave, and forms the upper part of the semilunar notch. Its borders present continuations of the groove on the margin of the superior surface; they serve for the attachment of ligaments: the back part of the ulnar collateral ligament medially, and the posterior ligament laterally. From the medial border a part of the flexor carpi ulnaris arises; while to the lateral border the

anconeus The anconeus muscle (or anconaeus/anconæus) is a small muscle on the posterior aspect of the elbow joint. Some consider anconeus to be a continuation of the triceps brachii muscle. Some sources consider it to be part of the posterior compartment ...

is attached. The

is a triangular eminence projecting forward from the upper and front part of the ulna. Its base is continuous with the body of the bone, and of considerable strength. Its apex is pointed, slightly curved upward, and in flexion of the forearm is received into the coronoid fossa of the humerus. Its upper surface is smooth, concave, and forms the lower part of the semilunar notch. Its antero-inferior surface is concave, and marked by a rough impression for the insertion of the

brachialis The brachialis (brachialis anticus), also known as the Teichmann muscle, is a muscle in the upper arm that flexes the elbow. It lies deeper than the biceps brachii, and makes up part of the floor of the region known as the cubital fossa (elbow pi ...

. At the junction of this surface with the front of the body is a rough eminence, the tuberosity of the ulna, which gives insertion to a part of the brachialis; to the lateral border of this tuberosity the oblique cord is attached. Its lateral surface presents a narrow, oblong, articular depression, the

. Its medial surface, by its prominent, free margin, serves for the attachment of part of the ulnar collateral ligament. At the front part of this surface is a small rounded eminence for the origin of one head of 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 pa ...

; behind the eminence is a depression for part of the origin of the flexor digitorum profundus; descending from the eminence is a ridge which gives origin to one head of 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 attachments, to t ...

. Frequently, the flexor pollicis longus arises from the lower part of the

by a rounded bundle of muscular fibers. The semilunar notch is a large depression, formed by the olecranon and the coronoid process, and serving as articulation with the trochlea of the humerus. About the middle of either side of this notch is an indentation, which contracts it somewhat, and indicates the junction of the olecranon and the coronoid process. The notch is concave from above downward, and divided into a medial and a lateral portion by a smooth ridge running from the summit of the olecranon to the tip of the coronoid process. The medial portion is the larger, and is slightly concave transversely; the lateral is convex above, slightly concave below. The

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 backward, and its prominent extremities serve for the attachment of the annular ligament.

Body

The body of the ulna at its upper part is prismatic in form, and curved so as to be convex behind and lateralward; its central part is straight; its lower part is rounded, smooth, and bent a little lateralward. It tapers gradually from above downward, and has three borders and three surfaces. ;Borders * The volar border (''margo volaris; anterior border'') begins above at the prominent medial angle of the

, and ends below in front of the styloid process. Its upper part, well-defined, and its middle portion, smooth and rounded, give origin to the flexor digitorum profundus; its lower fourth serves for the origin of the

pronator quadratus 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 the ...

. This border separates the volar from the medial surface. * The dorsal border (''margo dorsalis; posterior border'') begins above at the apex of the triangular subcutaneous surface at the back part of the olecranon, and ends below at the back of the styloid process; it is well-marked in the upper three-fourths, and gives attachment to an

aponeurosis An aponeurosis (; plural: ''aponeuroses'') is a type or a variant of the deep fascia, in the form of a sheet of pearly-white fibrous tissue that attaches sheet-like muscles needing a wide area of attachment. Their primary function is to join musc ...

which affords a common origin to the flexor carpi ulnaris, the extensor carpi ulnaris, and the flexor digitorum profundus; its lower fourth is smooth and rounded. This border separates the medial from the dorsal surface. * The interosseous crest (''crista interossea; external or interosseous border'') begins above by the union of two lines, which converge from the extremities of the

and enclose between them a triangular space for the origin of part of 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. Its function is to supinate the forearm. Structure Supinator consists of two planes of fibers, between whi ...

; it ends below at the head of the ulna. Its upper part is sharp, its lower fourth smooth and rounded. This crest gives attachment to 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 ...

, and separates the volar from the dorsal surface. ;Surfaces * The volar surface (''facies volaris; anterior surface''), much broader above than below, is concave in its upper three-fourths, and gives origin to the flexor digitorum profundus; its lower fourth, also concave, is covered by the

. The lower fourth is separated from the remaining portion by a ridge, directed obliquely downward and medialward, which marks the extent of origin of the pronator quadratus. At the junction of the upper with the middle third of the bone is the nutrient canal, directed obliquely upward. * The dorsal surface (''facies dorsalis; posterior surface'') directed backward and lateralward, is broad and concave above; convex and somewhat narrower in the middle; narrow, smooth, and rounded below. On its upper part is an oblique ridge, which runs from the dorsal end of the radial notch, downward to the dorsal border; the triangular surface above this ridge receives the insertion of the

Anconæus The anconeus muscle (or anconaeus/anconæus) is a small muscle on the posterior aspect of the elbow joint. Some consider anconeus to be a continuation of the triceps brachii muscle. Some sources consider it to be part of the posterior compartment ...

, while the upper part of the ridge affords attachment 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. Its function is to supinate the forearm. Structure Supinator consists of two planes of fibers, between whi ...

. Below this the surface is subdivided by a longitudinal ridge, sometimes called the perpendicular line, into two parts: the medial part is smooth, and covered by the extensor carpi ulnaris; the lateral portion, wider and rougher, gives origin from above downward to the

, the abductor pollicis longus, the

extensor pollicis longus 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 with ...

, and the extensor indicis proprius. * The medial surface (''facies medialis; internal surface'') is broad and concave above, narrow and convex below. Its upper three-fourths give origin to the Flexor digitorum profundus; its lower fourth is subcutaneous.

Near the wrist

The head of ulna presents an articular surface, part of which, of an oval or semilunar form, is directed downward, and articulates with the upper surface of the triangular articular disc which separates it from the wrist-joint; the remaining portion, directed lateralward, is narrow, convex, and received into 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 The radius or radial bone is one of the two large bones of the forearm, the other being the ulna. It extends from the latera ...

of the radius. Near the wrist, the ulnar, with two eminences; the lateral and larger is a rounded, articular eminence, termed the head of the ulna; the medial, narrower and more projecting, is a non-articular eminence, the styloid process. * The ''head'' presents an articular surface, part of which, of an oval or semilunar form, is directed downward, and articulates with the upper surface of the triangular articular disk which separates it from the wrist-joint; the remaining portion, directed lateralward, is narrow, convex, and received into the ulnar notch of the radius. * The ''styloid process'' projects from the medial and back part of the bone; it descends a little lower than the head, and its rounded end affords attachment to the ulnar collateral ligament of the wrist-joint. The head is separated from the styloid process by a depression for the attachment of the apex of the triangular articular disk, and behind, by a shallow groove for the tendon of the extensor carpi ulnaris. File:Gray336.png, Vertical section through the articulations at the wrist, showing the synovial cavities. File:Gray213.png, Bones of left forearm. Anterior aspect. File:Gray214.png, Bones of left forearm. Posterior aspect. File:Gray334.png, Ligaments of wrist. Anterior view File:Gray335.png, Ligaments of wrist. Posterior view.

Microanatomy

The ulna is a

. The long, narrow medullary cavity of the ulna is enclosed in a strong wall of cortical tissue which is thickest along the interosseous border and dorsal surface. At the extremities the compact layer thins. The compact layer is continued onto the back of the olecranon as a plate of close spongy bone with lamellæ parallel. From the inner surface of this plate and the compact layer below it trabeculæ arch forward toward the olecranon and coronoid and cross other trabeculæ, passing backward over the medullary cavity from the upper part of the shaft below the coronoid. Below the coronoid process there is a small area of compact bone from which trabeculæ curve upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of compact bone. The trabeculæ at the lower end have a more longitudinal direction.

Development

The ulna is ossified from three centers: one each for the body, the wrist end, and the elbow end, near the top of the olecranon. Ossification begins near the middle of the body of the ulna, about the eighth week of fetal life, and soon extends through the greater part of the bone. At birth, the ends are cartilaginous. About the fourth year or so, a center appears in the middle of the head, and soon extends into the ulnar styloid process. About the tenth year, a center appears in the olecranon near its extremity, the chief part of this process being formed by an upward extension of the body. The upper epiphysis joins the body about the sixteenth, the lower about the twentieth year.

Function

Joints

The ulna forms part of the wrist joint and elbow joints. Specifically, the ulna joins ( articulates) with: * trochlea of the

, at the right side elbow as a

with semilunar

trochlear notch The trochlear notch (), also known as semilunar notch and greater sigmoid cavity, is a large depression in the upper extremity of the ulna that fits the trochlea of the humerus (the bone directly above the ulna in the arm) as part of the elbow jo ...

of the ulna. * the radius, near the elbow as a

pivot joint In animal anatomy, a pivot joint (trochoid joint, rotary joint or lateral ginglymus) is a type of synovial joint whose movement axis is parallel to the long axis of the proximal bone, which typically has a convex articular surface. According to o ...

, this allows the radius to cross over the ulna in

pronation Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

. * the distal radius, where it fits into the

. * the radius along its length via the

that forms a syndesmosis joint

Muscle attachments

Clinical significance

Fractures

Specific types of

ulna fracture An ulna fracture is a broken bone, break in the ulna bone, one of the two bones in the forearm. It is often associated with a fracture of the other forearm bone, the radius fractures, radius. An ulna fracture can be a single break as in a so cal ...

include: *

Monteggia fracture The Monteggia fracture is a fracture (bone), fracture of the Anatomical terms of location#Proximal and distal, proximal third of the ulna with dislocation of the proximal head of radius, head of the radius. It is named after Giovanni Battista Mont ...

- a fracture of the proximal third of the ulna with the dislocation of the head of the radius * Hume fracture - a fracture of the olecranon with an associated

anterior 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 prov ...

dislocation of the

radial head The head of the radius has a cylindrical form, and on its upper surface is a shallow cup or fovea for articulation with the capitulum of the humerus. The circumference of the head is smooth; it is broad medially where it articulates with the rad ...

Conservative management Conservative management is a type of medical treatment defined by the avoidance of invasive measures such as surgery or other invasive procedures, usually with the intent to preserve function or body parts. For example, in appendicitis, conservati ...

is possible for ulnar fractures when they are located in the distal two-thirds, only involve the shaft, with no shortening, less than 10° angulation and less than 50% displacement.Page 158
/ref> In such cases, a cast should be applied that goes above the elbow.

Other animals

Quill knobs on Mirarce eatoni and Pelecanus occidentalis

In four-legged animals, the radius is the main load-bearing bone of the lower forelimb, and the ulna is important primarily for muscular attachment. In many mammals, the ulna is partially or wholly fused with the radius, and may therefore not exist as a separate bone. However, even in extreme cases of fusion, such as in horses, the olecranon process is still present, albeit as a projection from the upper radius. In

birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

and other

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

the ulna forms a surface of attachment for the

secondary feathers Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges (), singular remex (), while those on the tail ...

. These often leave osteological evidence in the form of quill knobs, allowing for identification of feathers in fossils that otherwise lack integumentary information.

Gallery

File:Ulna - animation.gif, Position of ulna (red). Animation File:Human ulna.stl, 3D image File:RightHumanPosteriorRadiusUlna.jpg, Bones of the right arm, showing the ulna, radius, wrist and humerus File:Gray417 color.PNG, Cross-section through the middle of the forearm, showing the two bones and the muscles, nerves and blood vessels surrounding them. File:Ulna Anatomy by Jason Christian.webm, Ulna Anatomy

References

External links

Bone and Spine : Musculoskeletal-Anatomy
{{Authority control Long bones Bones of the upper limb