Ulna and Radius: Function, Structure, Development, Clinical Significance and History

The arm consists of three large bones where the humerus bone forms the upper arm.

The radius and ulna are the two bones that make up the lower arm. You have a radius and an ulna on each arm.

An easy way to remember which bone is the radius, and which is the ulna, is that the radius connects to the thumb side of the wrist.

The radius is larger and longer than the ulna which is on the inner, or medial, side of the forearm closest to the body. The bony tip of the elbow, which most people consider the elbow, is actually the tip of the ulna bone.

The radius and ulna connect to the humerus bone of the upper arm at the elbow joint.

The elbow joint consists of a series of muscles or ligaments that act like a hinge so that you can bend and straighten your arm. This is bending and straightening is called bending and extending the arm, respectively.

The elbow also provides rotation of the lower arm by twisting the radius and ulna. This twist allows you to turn the palm of your hand up or down.

Rotation of the lower arm is called pronation or supination depending on the direction of movement. Normally, the radius and the ulna are parallel to each other.

During pronation, the radius rolls around the ulna at both the wrist and the elbow. In this position, the radius and ulna appear crossed.

Here is an experiment. In fact, you can feel the radius and ulna rotate. Support your right forearm with your left hand.

Then turn your right wrist up and then your palm down. You will feel the radius and ulna bones in your right arm twist and turn. Read on to learn more about the radius and ulna, and how your forearm works.

The cube

It runs parallel to the radius, the other long bone in the forearm, and is the larger and longer of the two. The bone can break due to excessive weight or impact.



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

The trochlea of ​​the humerus, on the right side elbow as a hinge joint with a semilunar trochlear notch of the ulna.

The radius, near the elbow as a pivot joint, allows the radius to cross over the ulna in pronation. The distal radius, where it fits into the ulnar notch.

The radius along its length through the interosseous membrane that forms a syndesmosis joint


Specific types of ulna fracture include:

Monteggia fracture: a fracture of the proximal third of the ulna with dislocation of the head of the radius.

Hume’s fracture: a fracture of the olecranon with an associated anterior dislocation of the radial head.

Conservative management is possible for ulnar fractures when they are located in the distal two thirds, only affecting the stem, without shortening, less than 10 ° angulation and less than 50% displacement. In such cases, a cast that goes above the elbow should be applied.

Other animals

In four-legged animals, the radius is the main load-bearing bone of the lower forelimb, and the ulna is important primarily for muscle attachment.

In many mammals, the ulna partially or fully fuses with the radius and therefore cannot exist as a separate bone.

However, even in extreme cases of fusion, as in horses, the olecranon process is still present, albeit as a projection from the superior radius.

The radio

The radius or radial bone is one of the two large bones of the forearm, the other is the ulna. It runs from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna. The radius is shorter and smaller than the ulna.

The radius is part of two joints: the elbow and the wrist. 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 leg is the tibia.


The long, narrow medullary canal is enclosed in a strong wall of compact bone. It is thickest along the interosseous border and thinner in the extremities, the same on the articular cup-shaped 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 capital (the cup-shaped articulatory notch of the humerus); they are crossed by others parallel to the surface of the fovea.

The arrangement at the lower end is somewhat similar. Missing in radial aplasia.

The upper extremity of the radius consists of a somewhat cylindrical head that articulates with the ulna and humerus, a neck, and a radial tuberosity.

The body of the radius is self-explanatory, and the lower extremity of the radius is approximately quadrilateral in shape, with articulating surfaces for the ulna, scaphoid, and lunate bones.

The distal end of the radius forms two palpable points, radially the styloid process and the tubercle of Lister on the ulnar side.

Along with the proximal and distal radioulnar joints, an interosseous membrane originates medially along the body of the radius to attach the radius to the ulna.

Close to the wrist

The distal end of the radius is large and quadrilateral in shape.

Articular surfaces

It is provided with two articular surfaces, one below, for the carpus, and another on the medial side, for the ulna.

The articular surface of the carpus is triangular, concave, smooth and divided by a slight anteroposterior ridge into two parts. Of these, the lateral, triangular, articulates with the scaphoid bone; the medial, quadrilateral, with the lunate bone.

Other surfaces

The dorsal surface is convex, adheres to the dorsal radiocarpal ligament, and is marked by three grooves.

The second is deep but narrow, and is delimited laterally by a very well defined ridge; it runs obliquely from top to bottom and to the sides, and transmits the tendon of the extensor pollicis longus muscle.


Its upper third is prominent, and from its oblique direction it has received the name of the oblique line of the radius.

It gives rise to the superficial flexor digitorum muscle (also flexor digitorum sublimis) and the flexor longus muscle of the thumb; the surface on the line gives insertion to part of the supinator muscle.

The middle third of the palmar border is indistinct and rounded.

The interosseous border (medial border, interosseous crystal, interosseous ridge) begins above, at the back of the tuberosity, and its upper part is rounded and indistinct.

It becomes sharp and prominent as it descends, and in its lower part it divides into two ridges that continue to the anterior and posterior margins of the ulnar notch.

The lower part of the interosseous membrane joins the posterior part of the two ridges, while the triangular surface between the ridges gives insertion to part of the pronator quadratus muscle.

This ridge separates the palmar surface from the dorsal one, and gives fixation to the interosseous membrane. The connection between the two bones is actually a joint known as a syndesmosis joint.


The lateral surface ( facies lateralis , external surface) is convex throughout and is known as the convexity of the radius, curving outward to be convex on the side.

Close to the elbow

The upper extremity of the radius (or proximal extremity) has a head, neck, and tuberosity.

The deepest point of the fovea is not axiometric with the long axis of the radius, creating a chamber effect during pronation and supination.


The radius ossifies from three centers: one for the body and one for each limb. That for the body makes its appearance near the center of the bone, during the eighth week of fetal life.

Ossification begins at the lower end between 9 and 26 months of age. The center of ossification for the upper end appears in the fifth year.

The upper epiphysis merges with the body at the age of seventeen or eighteen, the lowest around the age of twenty.

An additional center, sometimes found on the radial tuberosity, appears around the fourteenth or fifteenth year.


Muscle attachments

The biceps muscle attaches to the radial tuberosity of the upper extremity of the bone. The upper third of the body of the bone joins the supinator, the superficial flexor of the fingers, and the long flexor muscles of the thumb.

The middle third of the body joins the extensor ossis metacarpi pollicis, extensor primi internodii pollicis, and the pronator teres to the muscles. The lower quarter of the body joins the pronator quadratus muscle and the long supinator tendon.

Clinical significance

Radial aplasia refers to the congenital absence or insufficiency of the radius.


Specific types of radius fractures include:

Proximal Radius Fracture: A fracture within the capsule of the elbow joint results in the fat pad sign or “candle sign,” which is a displacement of the fat pad at the elbow.

Essex-Lopresti fracture: fracture of the radial head with a concomitant dislocation of the distal radioulnar joint with rupture of the interosseous membrane.

Galeazzi fracture: fracture of the radius with dislocation of the distal radioulnar joint.

Colles’ fracture – A dorsally (posterior) displaced fracture of the radius of the wrist and hand.

Smith’s fracture – a palmar (ventral) displaced distal radius fracture of the wrist and hand.

Barton’s fracture: an intra-articular fracture of the distal radius with dislocation of the radiocarpal joint.


The word radio is Latin for “ray.” In the context of the radius of the bone, a ray can be thought of as rotating around an axis line that runs diagonally from the center of the chapter to the center of the distal ulna.

While the ulna is the main contributor to the elbow joint, the radius contributes primarily to the wrist joint.

The radius is so named because the radius (bone) acts like the radius (of a circle).

It rotates around the ulna and the end (where it joins the bones of the hand), known as the styloid process of the radius, requires the distance from the ulna (center of the circle) to the edge of the ulna. the radius (the circle).

The ulna acts as the center point of the circle because when the arm is rotated, the ulna does not move.