Ulna Definition

The ulna is a long bone found in the forearm that stretches from the elbow to the smallest finger. When it is in the anatomical position, it is found on the medial side of the forearm. Ulna runs parallel to the radius and the other long bone in the forearm. The ulna is found to be slightly longer than the radius and thinner compared to the forearm and because the radius is thicker than the ulna, the radius is considered to be the larger one among the two. The ulna is found to be present in all the land vertebrates. In few reptiles, the radius and the ulna do not articulate. The elbow joint was first found to be evolved among birds and mammals. In the birds, the radius is slightly slender but the ulna is more often reduced in mammals, especially in those who have adapted for running. 

This article will discuss ulna bone anatomy, structure, location, and various functions. 

Ulna Anatomy

Let us take a look at the ulna anatomy. The ulna is a long bone in the forearm that runs from the elbow to the smallest finger and is located on the medial side of the forearm when in its anatomical position. Close to the elbow, the ulna is wider, then narrows as it approaches the wrist. The ulna has a bony process called the olecranon process that is located just below the elbow. The olecranon process is a hook-like structure on the humerus that fits into the olecranon fossa. The trochlea of the humerus forms a hinge joint with this, preventing hyperextension. A radial notch for the head of the radius and the ulnar tuberosity, to which the muscles attach, are also present.

Structure of Ulna 

  • The ulna has two curved processes near the elbow, the olecranon, the coronoid process, two concave, articular cavities, the semilunar, and radial notches.

  • The olecranon is a broad, smooth, curved eminence that protrudes from the upper and back portions of the ulna. It is curved forward at the top, forming a prominent lip that is obtained into the olecranon fossa of the humerus during forearm extension. Where it joins the body and the narrowest portion of the upper end of the ulna, the base is contracted. A bursa surrounds the backward-facing posterior surface, which is flat, straight, and subcutaneous. It has a quadrilateral superior surface with a rough impression for the triceps brachii's insertion behind it and a small transverse groove at the margin for the attachment of a portion of the posterior ligament of the elbow joint.

  • The olecranon has a flat, concave anterior surface that forms the upper part of the semilunar notch. Its edges have continuations of the groove on the superior surface's margin; they act as attachment points for ligaments such as the back portion of the ulnar collateral ligament medially and the posterior ligament laterally. A portion of the flexor carpi ulnaris emerges from the medial boundary, while the anconeus is bound to the lateral border.

  • The coronoid process is a triangular eminence that projects out from the ulna's upper and front sections. Its foundation is both continuous with the bone's body and extremely strong. Its apex is pointed, gently curved outward, and it is obtained into the coronoid fossa of the humerus during forearm flexion. It has a flat, concave upper surface that forms the lower portion of the semilunar notch. It has a concave anterior inferior surface with a rough impression for the brachialis insertion. The tuberosity of the ulna, which gives insertion to a segment of the brachialis, is a rugged eminence at the intersection of this surface and the front of the body; the oblique cord is attached to the lateral boundary of this tuberosity.

  • The radial notch is a small, oblong articular depression on its lateral surface. Part of the ulnar collateral ligament is attached to its medial surface, which has a prominent, open margin. A slightly rounded eminence at the front of this surface gives origin to one head of the flexor digitorum superficialis; a depression behind the eminence gives origin to part of the flexor digitorum profundus slope that descends from the eminence gives origin to one head of the pronator teres. A circular bundle of muscular fibers often emerges from the lower part of the coronoid process to form the flexor pollicis longus.

  • The semilunar notch is a large depression created by the olecranon and coronoid process that serves as articulation with the humerus's trochlea. The junction of the olecranon and the coronoid process is shown by an indentation in the center of each side of the notch, which contracts it slightly. A smooth ridge running from the summit of the olecranon to the tip of the coronoid process divides the notch into a medial and a lateral part, which is concave from above downward. The medial section is broader and transversely slightly concave and the lateral portion is convex above and slightly concave below.

  • On the lateral side of the coronoid process, the radial notch is a short, oblong articular depression that receives the circumferential articular surface of the head of the radius. It is concave from the front to the rear, with prominent extremities that act as attachment points for the annular ligament.

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Function of Ulna 


  • The ulna contributes to the formation of the wrist and elbow joints. In a hinge joint, the ulna articulates with the humerus at its most proximal point, forming the elbow. The trochlea of the humerus is found to sit with the semi-lunar notch of the ulna to form a joint.

  • Pronation (from the proximal joint) and supination (from the distal joint) of the forearm are caused by the ulna articulating with the radius proximally and distally.

  • The interosseous membrane of the ulna articulates with the radius in a syndesmosis joint. From the radial notch proximally to the head of the ulna, this runs the length of the shaft. This also helps to separate the forearm's dorsal and volar sides.

Fractured Ulna

The fracture of the ulna occurs as a result of fragility and normally from following a fall such as a fall on an outstretched hand. Special types of the broken ulna or the fractured ulna are:

Monteggia fracture-This occurs when there is a fracture of the proximal one-third of the ulnar shaft which is accompanied by the radial head. 

Hume fracture occurs when there is a fracture of the olecranon accompanied by the anterior dislocation of the radial head. 

Galezzi’s fracture- At the distal radio-ulna junction, Galeazzi’s fracture occurs where the distal radius is fractured and the ulnar head dislocates.

FAQs (Frequently Asked Questions)

Question 1: What is Proximal Ulna?

Ans: The proximal ulna is hook-like in a form that joins or articulates with the trochlea of the humerus to create a hinge joint of the elbow. This articulation is formed due to the olecranon and the coronoid process. 

Question 2: Where is the Head of Ulna?

Ans: The anterior, distal end of the ulna is known as the head of the ulna. It connects to the ulnar notch on the radius and the triangular articular disc of the wrist joint.

Question 3: What are Humerus and Ulna?

Ans: Humerus: The sole bone in the upper arm is the humerus. The head of the humerus is at its proximal end. This is the medially facing broad, oval, smooth area. The glenohumeral (shoulder) junction is formed by the head articulating with the scapula's glenoid cavity. The anatomical collar of the humerus is located at the edge of the smooth region of the head. The greater tubercle is a bony expansion on the lateral side of the proximal humerus. On the anterior side of the humerus, there is a smaller tubercle called the lesser tubercle. Muscles that act around the shoulder joint connect to the greater and lesser tubercles. The deltoid tuberosity is a roughened, V-shaped area in the center of the humerus shaft on the lateral side. It is the deltoid muscle's attachment point, as the name implies.

Ulna: The ulna is the forearm's medial bone. It extends parallel to the radius, which is the forearm's lateral bone. With its broad, C-shaped trochlear notch, the proximal end of the ulna resembles a crescent wrench. As part of the elbow joint, this area articulates with the trochlea of the humerus. The coronoid process of the ulna, a conspicuous bone lip, forms the inferior margin of the trochlear notch. The ulnar tuberosity is a roughened patch on the anterior ulna just below this. The radial notch of the ulna is a narrow, smooth region on the lateral side and marginally inferior to the trochlear notch. The proximal radioulnar joint is formed by the articulation of the proximal radius and the ulna in this location. The olecranon process, which forms the bony tip of the elbow, is made up of the posterior and superior parts of the proximal ulna.

Q4:What is a Distal Ulna Fracture

Ans: The distal ulna fractures are very rare. They're frequently discovered in association with distal radius fractures, and the distal ulna's interplay with the radioulnar joint and triangular fibrocartilage complex makes distal ulna fractures difficult to explain and treat. Fixation of distal ulna fractures might be difficult due to comminution, which makes reduction difficult. Because of the thin soft tissue, hardware prominence might occur, necessitating implant removal.

Q5: What is Caput Ulnae Syndrome ?

Ans: A caput ulnae syndrome arises when the joints of the head of the ulna lose their rigidity due to trauma or persistent inflammation.

Q6: What is an Ulna Fracture or the Left Ulna Fracture?

Ans: A break in the ulna bone, one of the two bones of the forearm, which can be either the left ulna bone or the right ulna bone is known as an ulna fracture. It is frequently linked with a radius fracture, the other forearm bone.

An ulna fracture can be a single break, as in a so-called nightstick fracture, which occurs when someone is stuck on the inside of the forearm with a stick, especially when putting their arm up to protect their head from damage.

Falling on the forearm or on an outstretched arm can potentially cause the ulna bone to shatter.

Ulna fractures are more likely in men before the age of 40 and in women beyond the age of 60. Adolescents that participate in athletics are at an increased risk.

Q7: What is a Radio Ulna Bone?

Ans: The radio ulna is a single bone that is present in the forelimbs of an amphibian such as a frog. The radio ulna bone represents the unique fusion of the separate radius and the ulna of the higher vertebrate forms.