Book : Human Anatomy
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Posted by: CHELSEA
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Writer CHELSEA

Radius and Ulna

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Overview:

The radius or radial bone is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally. 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 ulna is one of the two long bones in the forearm (the other is the radius). It is on the side opposite the thumb, extends from the elbow to the wrist, and runs parallel to the radius, which it exceeds in length and size. In anatomical position, when the arms are down at the sides of the body and the palms of the hands face forward, the ulna is located at the side of the forearm closest to the body (the medial side). The bone may break due to excessive weight or impact.

The lower arm consists of two bones extending from the elbow to the wrist, running parallel to each other. The radius is on the outside, or lateral side, of the elbow. It connects to the thumb side of the wrist. The radius is bigger and longer than the ulna which is on the inside, or medial side, of the forearm closest to the body. The bony point of the elbow that most people think of as the elbow is actually the tip 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 acting like a hinge so you can bend and straighten your arm. This is bending and straighten is called flexing and extending the arm, respectively.

The elbow also provides for rotation of the lower arm through twisting of the radius and ulna. This twisting allows you to turn your hand palm up or palm down. The rotation of the lower arm is called pronation or supination depending on the direction of movement. Normally, the radius and 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. You can actually feel the radius and ulna rotating. Hold your right forearm with your left hand. Then rotate your right wrist palm up and then palm down. You will feel the radius and ulna bones in your right arm twisting and rotating.

The Ulna:

Anatomically, the ulna is located medially to the radius, placing it near the little finger. The ulna is slightly larger than the radius. 

Proximally, there are five key regions of the ulna: 

The olecranon is a projection of bone that extends proximally from the ulna.

The triceps brachii muscle attaches to the ulna superiorly. 

The cornoid process, together with the olecranon, forms the trochlear notch where it articulates with the trochlea of the humerus. 

Laterally to the trochlear notch lies the radial notch, which articulates with the head of the radius to form the proximal radioulnar joint. 

Immediately distal to the coronoid process is the tuberosity of ulna, to which the brachialis muscle attaches.

The shaft of the ulna is triangular and numerus muscles involved in pronation and flexion of the forearm attach to its surface.

Distally, the ulna is much smaller and terminates with a rounded head that articulates with the ulnar notch of the radius to form the distal radioulnar joint. The styloid process of the ulna extends distally and is the site of attachment for ligaments found in the wrist.

The Radius:

Anatomically, the radius is located laterally to the ulna placing it near the thumb. The radius is slightly smaller than the ulna and pivots around the ulna to produce movement at the proximal and distal radioulnar joints. 

Proximally, the radius terminates with a disk-shaped head that articulates with the capitulum of the humerus and the radial notch of the ulna. Immediately below the head lies the radial tuberosity to which the biceps brachii attaches. As with the ulna, the shaft of the radius is triangular in shape and numerous muscles, including the protonator teres, attach to it.

Distally the radius expands, medially the ulnar notch articulates with the head of the ulnar. Immediately adjacent to the ulnar notch, the radius articulates with the scaphoid and lunate carpal bones to form part of the wrist.


The lower arm consists of two bones extending from the elbow to the wrist, running parallel to each other. The radius is on the outside, or lateral side, of the elbow. It connects to the thumb side of the wrist. The radius is bigger and longer than the ulna which is on the inside, or medial side, of the forearm closest to the body. The bony point of the elbow that most people think of as the elbow is actually the tip the ulna bone.

The radius and ulna connect to the humerus bone of the upper arm at the elbow joint. Thehe elbow joint consists of a series of muscles or ligaments acting like a hinge so you can bend and straighten your arm. This is bending and straighten is called flexing and extending the arm, respectively.

The elbow also provides for rotation of the lower arm through twisting of the radius and ulna. This twisting allows you to turn your hand palm up or palm down. The rotation of the lower arm is called pronation or supination depending on the direction of movement. Normally, the radius and 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. You can actually feel the radius and ulna rotating. Hold your right forearm with your left hand. Then rotate your right wrist palm up and then palm down. You will feel the radius and ulna bones in your right arm twisting and rotating.



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