Quick Guide: Distinguishing the Radius vs Ulna in Human Anatomy
The long bones constituting the region between the wrist and the elbow of our forearm are called radius and ulna. The radius is the lateral one whereas the ulna is the medial part among the two bones present in this antebrachial region.
These bones have evolved physiologically to support the unique movements that our forearms can make using the adjacent muscles in this region and other parts of the musculoskeletal system. There are 20 muscles supporting these two bones resulting in a coordinated movement and assisting to perform the essential functions of the forearm.
Radius and Ulna Structure and Features
The two prime bones present in the upper part of our arms in between the wrist and elbow are called radius and ulna. These two bones constitute the upper part of the arm that helps us to work and use our hands. If we look closely then the radius bone supports the thumb or lateral region of our forearm whereas the ulna supports the medial side of the forearm. In this article, we will discuss the physiology, features, and functions of these two bones.
Radius and Ulna Bones Anatomy
Let us check the key factors of the ulna and radius bone anatomy to understand the configuration of these two bones with the adjacent muscles in the forearm.
1.Radius Bone
Proximal End
Radius is the smaller one among the two bones. It also runs parallel to the ulna and is the thicker one. It has a slight longitudinal curve. There are two joints formed by this bone. At the elbow, it forms a joint with the capitulum of the humerus and with ulna in the radial notch in another section. In the wrist, both these bones form a joint that supports the wrist bones.
There are two extremities in this bone. The head is proximally located and is called caput radii. The proximal end of this bone forms a part of the upper surface of the joint to fit the capitulum of the humerus and a radial notch for the accommodation of one end of the ulna. It is a prism-shaped long bone present at the lateral side of the forearm between the wrist and the elbow.
Neck
The neck region of this bone is where it narrows down till the bicipital tuberosity. The radial tuberosity can be located between the neck and head of this bone. It is an oval-shaped structure where the biceps brachii muscles fit or insert.
Radial Shaft
The radius bone shaft is long with a convex shape at the lateral side but shows enlargement when we move in the direction of the wrist. There are three borders called the interosseous anterior, and posterior along with lateral, anterior, and posterior surfaces. The anatomical structure of these surfaces and borders will show several junctions where the tendons insert and join with this bone.
Two muscles are attached to this bone. They are supinator muscles in the radial head region and the pronator teres muscle that adheres to the radial shaft. The latter muscle attaches to the pronator tuberosity.
There are two other muscles attached to this bone are flexor digitorium superficialis and flexor pollicis longus. The distal end of this shaft is covered by the pronator quadratus muscle. To its opposite side, we find the insertion region for the brachioradialis muscle. It is located just above the styloid process.
Distal Radius
In this region, it has bony developments. The dorsal tubercle is located in the posterior aspect of the radius bone anatomy lying between the grooves of muscle tendons of brevus and extensor carpi radialis longus. It also homes the notch for the tendons of extensor pollicis longus.
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2. Ulna Bone
Proximal End
This part of the ulna bone consists of the olecranon, the trochlear notch, the coronoid process, and the radial notch. The coronoid process of ulna is the triangular eminence that projects forward from the front upper part of this bone.
Ulnar shaft
The anatomy of the shaft part shows tapering in the distal region and increased thickness in the neck region. The lateral part is sharper and has the interosseous membrane, also called the interosseous border.
Below the coronoid process, the brachialis muscle attachment region or the ulnar tuberosity is found. The supinator fossa is the concave region that holds the supinator muscle. Another crest is found right beneath this muscle that holds the flexor pollicis longus muscle.
Distal Ulna
In this part, the articular circumference connects with the wrist bones. Any projection is present at the posterior part called the styloid process. The medial aspect of this bone homes the attachment of the pronator quadratus, the muscle that runs amidst the radius and ulna.
Interosseous Membrane
It is a membrane made of dense and fibrous connective tissue spanning between the ulna and radius. It forms a syndesmosis joint.
This is the anatomy of the two bones' radius and ulna forming the integral part of the forearm between the elbow and the wrist along with 20 different muscles. Study the different parts of these bones and refer to a proper radius and ulna diagram to understand how these bones attach to the muscles and function.
FAQs on Radius and Ulna Bones: Structure, Function & Comparison
1. What are the radius and ulna, and where are they located in the human body?
The radius and ulna are the two long bones of the forearm, located between the elbow joint and the wrist. In the standard anatomical position (palms facing forward), the radius is on the lateral, or thumb side, of the forearm, while the ulna is on the medial, or pinky finger side.
2. What are the main functions of the radius and ulna bones?
The radius and ulna work together to provide structure to the forearm and enable complex movements. Their primary functions include:
Movement: The ulna forms the primary hinge joint of the elbow for bending and straightening the arm, while the radius's rotation around the ulna allows for pronation (turning the palm down) and supination (turning the palm up).
Support: They form the articulation points for the wrist and elbow joints, providing stability and support for the hand.
Muscle Attachment: They serve as attachment points for numerous muscles that control movements of the elbow, wrist, hand, and fingers.
3. How can one easily differentiate between the radius and the ulna?
You can differentiate the radius from the ulna based on their size, shape, and position:
Position: The radius is on the thumb side (lateral), while the ulna is on the pinky side (medial).
Size at Ends: The ulna is larger and more prominent at the elbow, forming the bony point (olecranon process). The radius is wider and thicker at the wrist, where it primarily articulates with the carpal bones.
Shape: The proximal end of the ulna has a distinctive C-shaped or U-shaped trochlear notch that articulates with the humerus. The head of the radius is a circular, disc-like shape.
4. What are the joints formed by the articulation of the radius and ulna?
The radius and ulna articulate with each other at two key locations, forming the radioulnar joints:
Proximal Radioulnar Joint: Located near the elbow, this pivot joint is formed by the head of the radius articulating with the radial notch of the ulna. It allows the head of the radius to rotate.
Distal Radioulnar Joint: Located near the wrist, this joint is where the ulnar notch of the radius rotates around the head of the ulna. This joint works in conjunction with the proximal joint to produce forearm rotation.
5. What is the interosseous membrane and its importance for the forearm?
The interosseous membrane is a strong, flexible sheet of fibrous connective tissue that spans the space between the shafts of the radius and ulna. Its importance is threefold: it securely binds the two bones together, it transfers compressional forces from the radius to the ulna (distributing weight-bearing stress), and it provides a broad surface for the attachment of deep forearm muscles.
6. How do the radius and ulna work together to allow for complex forearm movements like rotation?
The unique rotational movement of the forearm, known as pronation and supination, is possible because the radius and ulna function as a unit. The head of the radius pivots within the radial notch of the stationary ulna at the elbow. Simultaneously, the wide distal end of the radius swings around the head of the ulna at the wrist. This coordinated pivot mechanism at both ends allows the entire radius bone, and thus the hand, to rotate over the ulna without requiring movement at the elbow joint itself.
7. Why is the radius thicker at the wrist while the ulna is larger at the elbow? How does this structural difference relate to their functions?
This structural difference is a classic example of anatomy supporting function. The ulna's large proximal end (with its olecranon and trochlear notch) forms a stable, strong hinge joint with the humerus, making it the primary bone for elbow flexion and extension. In contrast, the radius's wider, thicker distal end forms the main articulation with the carpal bones of the wrist. This makes the radius the primary weight-bearing bone of the forearm at the wrist and allows for a wide range of wrist movements.
8. What makes the radius and ulna susceptible to common injuries like fractures?
The radius and ulna are among the most frequently fractured bones in the body, primarily due to their position and function. When a person falls, the natural instinct is to stretch out a hand to break the fall (an action known as FOOSH, or Fall on an Outstretched Hand). This action transmits the entire impact force up through the wrist and directly into the radius and ulna. The distal radius is particularly vulnerable, leading to common injuries like a Colles' fracture. Their long, relatively thin structure also makes them susceptible to fractures from direct blows.
