Ulna is a long bone in the forearm. It lies medially and parallel to the radius, the other bone in the forearm
Ulna acts as stabilizing bone, with the radius pivoting to produce pronation and supination movement of the forearm.
The ulna bone participates in formation of elbow joint by articulating with humerus. Distally, the ulna articulates with the radius, forming the distal radio-ulnar joint.
The ulna is the medial bone of the forearm, and is homologous with the fibula of the lower limb. This along with other bone called radius connects elbow with radius. Ulna has upper and lower ends and a shaft.
The Upper End
The upper end of ulna presents the olecranon and coronoid processes, and the trochlear and radial notches.
The olecranon process projects upwards from the shaft. It has superior, anterior, posterior, medial and lateral surfaces. The anterior surface is articular. It is concave and forms forms the upper part of the trochlear notch. The posterior surface forms a triangular subcutaneous area which is separated from the skin by a bursa.
Inferiorly, it is continuous with the posterior border of the shaft of the ulna. The upper part forms the point of the elbow. The medial surface of the shaft. The lateral surface is continuous inferiorly with the posterior surface of the shaft.
Coronoid process projects forwards from the shaft just below the olecranon and has four surfaces, superior, anterior, medial and lateral. The supeiror surface forms the lower part of the trochlear notch. The anterior surface is triangular and rough. Its lower corner forms the ulnar tuberosity.
The upper part of its lateral surface is marked by the radial notch for the head of the radius. The annular ligament is attached to the anterior and posterior margins of the notch. The lower part of the lateral surface forms a depressed area (to accommodate the radial tuberosity). It is limited behind by a ridge called the supinator crest.
It forms an articular surface that articulates with the trochlea of the humerus to form the elbow joint.
The radial notch articulates with the head of the radius to form the superior radioulnar joint.
The Shaft of Ulna
The Shaft of ulna has three borders and three surfaces.
The interosseus (or lateral) border is sharpest in its middle two fourths. Inferiorly it can be traced to the lateral side of the head. Superiorly it is continuous with the supinator crest.
The anterior border of shaft of ulna is thick and rounded. It begins above on the medial side of the ulnar tuberosity, passes backwards in its lower one third, and terminates at the medial side of the styloid process.
Ulna has three borders anterior, posterior and medial.
Distal Portion of the Ulna
The distal end of the ulna is much smaller in diameter that the proximal end. It terminates in a rounded head which also has a distal projection – the ulnar styloid process.
The head articulates with the ulnar notch of the radius to form the distal radio-ulnar joint.
Attachments on Ulna
Triceps brachii muscle
Common tendon of triceps brachii muscle inserts on posterior part of superior surface of Olecranon process
Anconeus muscle inserts on lateral aspect of olecranon process.
Brachialis muscle inserts on to interior surface of the coronoid process of the ulna
Pronator teres muscle originates from medial surface on middle portion of coronoid process. It shares origin with medial epicondyle of the humerus.
Flexor carpi ulnaris muscle
This muscle takes origin from olecranon process and posterior surface of ulna. It also shares origin with medial epicondyle of the humerus.
Flexor digitorum superficialis and profundus muscles takes origin from coronoid process and anteromedial surface of ulna
It originates fromdistal portion of anterior ulnar shaft.
Extensor carpi ulnaris
It originates from posterior border of ulna [and lateral epicondyle of the humerus]
Originates from proximal ulna [along with lateral epicondyle of the humerus]
Abductor pollicis longus
This muscle originates from posterior surface of ulna [and also from posterior surface of the radius bone]
Extensor pollicis longus
It originates from dorsal shaft of ulna [and also from dorsal shaft of the radius and the interosseous membrane)
It originates from posterior surface of distal ulna (and interosseous membrane)
Clinical Significance of Anatomy of Ulna
Knowing and understanding ulna anatomy provides us with knowledge to communicate the sites of affection.
An isolated fracture of ulna occurs when ulna is hit by an object. The shaft is the most likely site of fracture.
Olecranon process gets fractured when patient falls on a flexed elbow. The triceps brachii can displace part of the fragment proximally.
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