Radius and ulna
The radius and ulna are long bones that make up the forearm, extending from the elbow to the wrist. In the anatomical position, the radius is found in the lateral forearm, while the ulna is found in the medial forearm.
The radius is shorter than the ulna and has a small proximal end that articulates with the humerus, and a broad distal end that articulates with the carpal bones at the wrist. Compared to the radius, the dimensions of the ulna are reversed because it has a large proximal end which articulates with the humerus and a small distal end.
The design of the radius and ulna as well as the joints between them enable pronation and supination of the forearm and hand. Both bones participate in joints and provide surfaces for the attachment of several upper limb muscles which control the various movements of the upper limb. Thus, they are essential for the proper mobility of the upper extremity.
This article will discuss the anatomy and function of the radius and ulna.
Proximal end: Head, neck, radial tuberosity
Shaft (body): Anterior, posterior and interosseous borders; anterior, posterior and lateral surfaces
Distal end: Ulnar notch, radial styloid process, dorsal tubercle
Proximal end: Olecranon, trochlear notch, coronoid process, radial notch, ulnar tuberosity and sublime tubercle
Shaft (body): Anterior, posterior and interosseous borders; anterior, posterior and lateral surfaces
Distal end: Head, ulnar styloid process
Humeroulnar: Trochlear notch of ulna + trochlea of humerus
Humeroradial: Head of radius + capitulum of humerus
Proximal radioulnar: Head of radius + radial notch of ulna
Middle radioulnar: Shaft of radius + shaft of ulna (interosseous membrane)
Distal radioulnar: Ulnar notch of radius + head of ulna
Radiocarpal: Distal end of radius + scaphoid, lunate and triquetrum bones
|Blood supply||Radial, ulnar, anterior and posterior interosseous arteries|
|Functions||Structural support, muscle attachment, formation of joints that enable movement|
- Blood supply
- Fractures of radius and ulna
The radius is the lateral bone of the forearm. It is a long bone that has three main parts: a proximal end, shaft and a distal end. The proximal end has a head which articulates with both the distal humerus and the proximal ulna, while the distal end articulates with the head of the ulna and carpal bones at the wrist. The shaft (body) is firmly connected to that of the ulna by dense connective tissue called the interosseous membrane.
The proximal end of the radius bears the head, neck and radial tuberosity. The disc-shaped head of the radius bears a concave superior surface which articulates with the capitulum of the humerus and forms part of the compound elbow joint. Additionally, the peripheral aspect of the radial head, called the articular circumference of the head of the radius, is placed within the radial notch of the ulna and enwrapped with the annular ligament, forming the proximal radioulnar joint.
The narrow portion of the radius distal to the head forms the neck. Just below the neck is the oval-shaped, medially oriented radial (bicipital) tuberosity onto which the biceps brachii muscle inserts.
The shaft of the radius is a long section of bone that continues distally from the neck and radial tuberosity. It is narrow proximally but enlarges towards the wrist, where it broadens to form the distal end of radius. The radial shaft has a slight lateral curvature and is triangular in cross-section for most of its length. It has three borders: an anterior, posterior and interosseous border.
The anterior border lies on the medial aspect of the bone. It starts just distal to the radial tuberosity and crosses diagonally to the lateral aspect of the shaft. The posterior border lies on the posterior aspect of the radius and is most visible in the midsection of the shaft. The sharp interosseous border faces the ulna medially. This border is connected to the interosseous border of the ulna via the fibrous interosseous membrane, forming the middle radioulnar joint.
The radial shaft also bears three surfaces: an anterior, posterior and lateral surface. Its anterior surface lies between the anterior and interosseous borders, while its posterior surface lies between the interosseous and posterior borders. The lateral surface is slightly convex and lies between the anterior and posterior borders.
The shaft of the radius expands to form a wide rectangular distal end which extends beyond the distal end of the ulna and is four sided in cross-section.
The anterior surface of the distal radius is smooth, concave and is angled anteriorly. The medial surface bears the ulnar notch, a concavity which receives the head of the ulna to form the distal radioulnar joint. The lateral surface of the distal radius, on the other hand, is rough and projects inferiorly as the radial styloid process. The inferior surface (carpal articular surface) bears two facets which articulate with the scaphoid and lunate bones of the carpus. Lastly, the distal radius has a prominent bony projection on its posterior surface called the dorsal tubercle (Lister’s tubercle), which sits between the grooves that transmit the tendons of forearm muscles.
The ulna is the medial bone of the forearm and the longer of the two parallel forearm bones. Like the radius, the ulna also has three main parts: a proximal end, shaft and a distal end. The proximal end articulates with the distal humerus and the head of the radius. The distal end, on the other hand, has a head which articulates with the distal radius. The interosseous membrane joins the shaft (body) of the ulna to the shaft of the radius.
The proximal ulna is a large hook-shaped structure which articulates with the distal humerus and the head of the radius. It bears the olecranon, trochlear notch, coronoid process, radial notch, sublime tubercle and ulnar tuberosity.
The olecranon is a prominent, proximal projection from the posterior aspect of the proximal ulna and serves as the point of insertion for the triceps brachii muscle. Its topmost portion bends anteriorly like a beak, which fits into the olecranon fossa of the distal humerus during extension, preventing hyperextension of the elbow joint. The anterior surface of the olecranon is concave and forms the c-shaped trochlear notch. This notch is covered with articular cartilage and articulates with the trochlea of the distal humerus in a manner similar to the jaws of a wrench, creating a hinge that permits flexion and extension movements at the elbow.
The lower lip of the trochlear notch projects anteriorly as the coronoid process, which is accommodated by the coronoid fossa on the anterior aspect of the distal humerus during flexion. The medial border of the coronoid process bears a small tubercle called the sublime tubercle on its proximal part, which is the site of attachment of the anterior band of the ulnar collateral ligament. Just distal to the coronoid process, the proximal ulna bears the ulnar tuberosity onto which the brachialis muscle inserts.
On the lateral aspect of the proximal ulna and just distal to the trochlear notch lies the shallow, rounded depression called the radial notch. The radial notch articulates with the circumference of the radial head and forms the proximal radioulnar joint.
To learn more about the anatomy of radius and ulna explore our articles, quizzes, video tutorials and labeled diagrams.
The shaft of the ulna is broader around the proximal portion and tapers distally toward the head of the ulna. Like the radius, the ulnar shaft is also triangular in cross-section for most of its length and has three borders (anterior, posterior and interosseous).
The anterior border is rounded and smooth. It begins just medial to the tuberosity of the ulna and descends along the anteromedial shaft. Distally, the anterior border crosses to the posterior aspect of the shaft, terminating close to the base of the styloid process. The posterior border runs from the posterior aspect of the olecranon to the styloid process, and can be palpated subcutaneously along the entire length of the forearm. Similar to the radial shaft, the medially projecting interosseous border of the ulna is sharp and serves as a surface of attachment for the fibrous interosseous membrane, which connects the ulna to the radius.
The ulnar shaft bears three surfaces: an anterior, posterior and medial. The anterior surface lies between the interosseous and anterior borders and is largely smooth, apart from some roughening at its distal end that marks the attachment of the pronator quadratus muscle. The region of the ulnar shaft between the posterior and interosseous borders forms the posterior surface. This surface bears linear markings which depict the different areas of muscle attachment. The medial surface of the ulnar shaft is located between the anterior and posterior borders and is smooth and convex transversely.
The distal ulna consists of a small rounded head and an ulnar styloid process. The lateral convex articular surface of the head of the ulna articulates with the ulnar notch of the distal radius to form the distal radioulnar joint. The inferior surface of the head of the ulna articulates with an articular disc called the triangular fibrocartilage (TFC), which separates the head from the carpal bones. Consequently, the head of the ulna does not directly contribute to the formation of the wrist joint. The short, round ulnar styloid process projects distally from the posteromedial aspect of the distal ulna and is palpable at the dorsomedial aspect of the wrist.
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Fractures of radius and ulna
Fractures are the most common pathological condition that directly affects the radius or the ulna. Depending on the mechanism of injury, fractures may involve both bones or may be isolated. Due to the fact that the shafts of both bones are strongly connected by the fibrous interosseous membrane, a fracture of one bone often results in the dislocation of the nearest joint. Examples of these fractures include:
- Monteggia fracture refers to a fracture of the upper portion of the ulna along with the dislocation of the radial head (proximal radioulnar joint).
- Galeazzi’s fracture refers to a fracture of the distal third of the radial shaft along with the dislocation of the distal radioulnar joint.
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