Last Updated on April 22, 2025
t raefdv cThe ulna is a long bone in the forearm. It lies medially and parallels to the radius, the other bone in the forearm. It is the longer of the two bones. It acts as a stabilizing bone, with the radius pivoting to produce the pronation and supination movement of the forearm.
The ulna participates in the formation of the elbow joint by articulating with the humerus. Distally, it articulates with the radius, forming the distal radio-ulnar joint.
This bone is homologous with the fibula of the lower limb. Along with another bone called the radius, it connects the elbow to the wrist. The shafts of two bones are joined by the interosseous membrane.
The ulna also has three main parts: a proximal end, shaft, and a distal end.
Bony Structures of Ulna
Proximal End or Upper End of Ulna
The proximal end of ulna is a large hook-shaped structure that articulates with the distal humerus and the head of the radius. It has the following parts
- Olecranon process
- Trochlear notch
- Coronoid process
- Radial notch
Olecranon Process
The olecranon process projects upwards from the shaft. It is the prominent proximal projection and can be subcutaneously palpated on the posterior elbow. It serves as an insertion point to the triceps brachii muscle on the posterosuperior aspect. It has superior, anterior, posterior, medial, and lateral surfaces.
The anterior surface is concave and forms the upper part of the c-shaped trochlear notch. The notch is covered by articular cartilage and articulates with the trochlea of the distal humerus to create a hinge joint. The topmost portion has a beak-like projection that fits into the olecranon fossa of the humerus when the elbow is fully extended. This prevents hyperextension of the joint.
The posterior surface forms a triangular subcutaneous area separated from the skin by a bursa. Inferiorly, it is continuous with the posterior border of the shaft. The upper part forms the point of the elbow.
The medial surface is continuous with the coronoid process and the medial surface of the shaft of the ulna. The lateral surface is continuous inferiorly with the posterior surface of the shaft.
Coronoid Process
The lower lip of the trochlear notch projects forward as the coronoid process from just below the olecranon. It has four surfaces, superior, anterior, medial, and lateral. The superior surface forms the lower part of the trochlear notch. The anterior surface is triangular and rough. Its lower corner forms the ulnar tuberosity and serves as the insertion for the brachialis.
The coronoid process is accommodated by the coronoid fossa on the anterior aspect of the distal humerus during flexion.
Sublime tubercle is the bearing on the medial border of the coronoid process on its proximal part. It serves as the site of attachment of the anterior band of the ulnar collateral ligament.
The upper part of its lateral surface is marked by the radial notch for the head of the radius. The lower part of the lateral surface forms a depressed area to accommodate the radial tuberosity.
Shaft
The shaft is broader proximally and tapers distally till before head of ulna. In the cross-section, the shaft appears triangular in most of its length.
Borders
It has three borders
- Anterior border: The Anterior border begins just medial to the ulnar tuberosity in the proximal end and then descends along the anteromedial shaft. Distally. it passes backward in its lower one-third, and terminates at the medial side of the styloid process.
- Posterior border: It begins at the posterior aspect of the olecranon and then descends to the styloid process. This border can be identified subcutaneously by palpating along the entire forearm length.
- Interosseous Border: This border projects laterally and serves as a surface of attachment for the fibrous interosseous membrane between the two bones. The interosseous (or lateral) border is sharpest in its middle half. Inferiorly, it can be traced to the lateral side of the head. Superiorly, it is continuous with the supinator crest. This crest is continuous with the posterior border of the supinator fossa, which is a depression just distal to the radial notch. The ulnar head of the supinator muscle takes origin from the supinator crest and fossa.
Surfaces
The ulnar shaft has three surfaces
- Anterior surface: It lies between the interosseous and anterior borders. It is mostly smooth. and is largely smooth, apart from some roughening at its distal end. This rough area is the attachment of the pronator quadratus muscle.
- Posterior surface: It is the region of the ulnar shaft between the posterior and interosseous borders.
- Medial surface: The medial surface of the ulnar shaft is between the anterior and posterior borders. It is a smooth surface and convex transversely.
Distal End
The distal end is much smaller in diameter than the proximal end. It terminates in a rounded head with a distal projection called the ulnar styloid process.
The head has a lateral convex articular surface that articulates with the ulnar notch of the radius to form the distal radio-ulnar joint.
The inferior surface of the head of the ulna articulates with an articular disc, the triangular fibrocartilage, which separates the head from the carpal bones. The styloid process is a short, rounded projection from the posteromedial aspect of the distal ulna.
Attachments
Muscles
Triceps Brachii
Triceps brachii is a three-headed muscle that spans almost the entire length of the humerus. Its head originate as follows
- Long head – infraglenoid tubercle of the scapula
- Medial head – posterior surface of the humerus, inferior to the radial groove
- Lateral head – posterior surface of the humerus, superior to radial groove
Three heads combine to form a common tendon and insert on the olecranon process of the ulna and adjoining fascia. The main function of triceps brachii is extension of the forearm at the elbow joint. Its long head contributes to the extension and adduction of the arm at the shoulder joint.
The muscle is supplied by the radial nerve (C6-C8). The vascular supply is by the deep brachial artery and the superior ulnar collateral artery.
Anconeus
The anconeus is a small, triangular muscle of the arm at the posterior aspect of the elbow that originates from the dorsal aspect of the lateral epicondyle of the humerus. This origin is just proximal to the common extensor tendon.
The origin is tendinous, and then it forms a muscle belly directed obliquely and medially. The insertion is at the lateral surface of the olecranon of the ulna and the adjoining posterior ulnar shaft.
The muscle is supplied by the radial nerve (C7-C8) and the posterior interosseous recurrent artery.
Anconeus is a weak elbow extensor.
Brachialis Muscle
The brachialis muscle originates from the anterior surface of the distal half of the humerus, medial and lateral intermuscular septae, just distal to the insertion of the deltoid muscle.
Distally, it forms a strong tendon that inserts onto the ulnar tuberosity and a rough depression on the anterior surface of the coronoid process.
It may also send a slip to the radius or the bicipital aponeurosis.
The brachialis is a major flexor of the elbow.
It is innervated by musculocutaneous nere (C5,C6). The vascular supply is by brachial artery and radial recurrent artery.
Pronator Teres
The pronator teres muscle is composed of two heads. The humeral or superficial head originates from the medial supracondylar ridge of the humerus, superior to the medial epicondyle. The ulnar or deep head originates from the coronoid process of the ulna.
The two muscle heads run inferolaterally under the brachioradialis muscle and fuse into a single muscle belly. The muscle inserts as a flat tendon onto the lateral surface of the radius at the rough area at the middle of its shaft. This rough area is also called the pronator tuberosity and lies inferior to the insertion of the supinator muscle.
The muscle acts to cause pronation of the forearm at the proximal radioulnar joint and flexion of the forearm at the elbow joint. It is supplied by the median nerve (C6, C7)
Flexor carpi ulnaris muscle
The muscle originates via two heads joined by a tendinous arch. The humeral head is smaller of the two. It arises from the common flexor origin on the medial epicondyle of the humerus. The ulnar head originates from the olecranon and the proximal part of the posterior border of the ulnar shaft.
Toward the wrist joint, both the heads converge on a long tendon to insert onto the pisiform and hamate carpal bones, and the base of the fifth metacarpal bone. The muscle causes flexion of the wrist and also results in ulnar deviation of the wrist.
It is supplied by the ulnar nerve (C7-T1).
Flexor Digitorum Superficialis or Sublimis
The humeral head of flexor digitorum superficialis (also called flexor digitorum sublimis) takes origin from the medial epicondyle of the humerus via the common flexor tendon and the medial margin of the coronoid process, with some fibers emanating from the anterior band of the ulnar collateral ligament of the elbow joint.
Radial head of flexor digitorum superficialis arises from the upper two-thirds of the anterior border of the radius.
The muscle courses in the anterior forearm are arranged into superficial and deep layers. Around the midforearm region, each layer gives off two tendons. The superficial layer tendons eventually supply digits 3 and 4, and the deep layer supplies digits 2 and 5.
In the hand, the tendons are superficial to the corresponding flexor digitorum profundus tendon. After crossing the metacarpophalngeal joint, the tendons enter the digital flexor sheath. Each tendon of flexor digitorum superficialis bifurcates at the proximal interphalangeal joint so that the corresponding tendon of flexor digitorum profundus can pass through and then reunite again to insert into the palmar surface of the base of the middle phalanx.
The muscle is a finger flexor and is supplied by the median nerve (C8, T1)
Flexor Digitorum Profundus
This muscle has multiple origins. It takes from origin from
- Three-fourths of the anterior surface of the ulna
- Adjacent part of the interosseous membrane
- Coronoid process
- Aponeurosis of the flexor carpi ulnaris muscle
The muscle goes inferiorly and at the level of the distal third of the forearm, it gives off a broad tendon that enters the hand by passing below the flexor retinaculum. Then the tendon divides into to four slips. These slips insert into the palmar surfaces of the bases of the distal phalanges of digits 2-5.
The medial part of the muscle that inserts to the fourth and fifth digits is supplied by the ulnar nerve (C8-T1), whereas the lateral part (inserts to the second and third digits) is supplied by the median nerve, via the anterior interosseous branch (C8-T1).
Flexor digitorum profundus is the flexor of fingers, especially distal phalanges.
Pronator Quadratus
It is a flat, short, quadrilateral muscle that originates from the anterior surface of the distal ulnar shaft and course laterally and distally to insert on anterior surface of radius.
Pronator quadratus is supplied by innervated by the anterior interosseous nerve (a branch of median nerve, C7 and C8).
Pronator quadratus causes forearm pronation by acting on the radioulnar joints.
Extensor Carpi Ulnaris
The extensor carpi ulnaris muscle originates from the lateral epicondyle of the humerus via the common extensor tendon and adjacent fascia.
The muscle fibers curve towards the ulnar side of the hand. Its tendon that passes below the extensor retinaculum lies in the most medial extensor compartment of the wrist. The tendon inserts to the medial aspect of the base of the 5th metacarpal bone.
It is supplied by the posterior interosseous nerve (C7, C8), a branch of the radial nerve.
Extensor carpi ulnaris acts to cause combined extension and ulnar deviation of the hand. Acting along with extensor carpi radialis brevis and longus, it causes extension of wrist. Along with flexor carpi ulnaris, it results in adduction (ulnar deviation) of the hand at the wrist.
Supinator muscle
The supinator muscle has two layers – a superficial layer that has a tendinous origin and a deep layer that has a muscular origin. Both layers originate from many landmarks-
- Lateral epicondyle of humerus
- Radial collateral ligament of humeroulnar joint
- Annular ligament of the superior radioulnar joint
- Supinator crest of ulna
- Adjacent part of the ulnar fossa.
The muscle covers the proximal third of the radius and inserts into the upper third of its lateral, posterior, and anterior surfaces.
The supinator muscle is supplied by the posterior interosseous nerve (C7, C8). Its action causes supination of the forearm
This muscle originates from the posterior surface [and also from the posterior surface of the radius bone]
Extensor pollicis longus
It originates from the middle third of the posterior surface of the ulna along its interosseous border, and the adjacent interosseous membrane. The origin is proximal to the origin of the extensor indicis muscle. The muscle belly obliquely courses to the lateral aspect of the wrist joint to give off a tendon that inserts onto the base of the distal phalanx of the thumb, posterior aspect.
Extensor pollicis longus is supplied by the posterior interosseous nerve (C7 and C8). This muscle is a thumb exentensor mainly. Minor contribution to wrist joint extension is also there.
Extensor Indicis
Extensor indicis originates from the posterior two-thirds of the ulnar distal surface, distal to the extensor pollicis longus muscle and adjoining the inerosseous membrane.
Inferiorly, it narrows into a tendon that passes obliquely over the carpal bones to insert at the extensor expansion on the back of the proximal phalanx of the index finger.
It is supplied by the posterior interosseous nerve, a branch of the radial nerve derived from spinal roots C7 and C8. It extends the index finger at the metacarpophalangeal and interphalangeal joints to extend the index finger.
Ligaments
Annular Ligament
The annular ligament is attached to the anterior and posterior margins of the radial notch.
Ulnar Collateral Ligament
Ulnar collateral ligament of the wrist attaches to the styloid process.
Elbow Capsule or Capsular Ligament of Elbow
The capsular ligament of the elbow joint attaches to the margins of the trochlear notch, i.e. coronoid process and olecranon process.
Other Attachments
Interosseous Membrane
It is a tough membranous structure that connects the radius and ulna. It attaches to the interosseous border or radial border of the ulna
Clinical Significance
Injuries
Forearm bones fracture very commonly. Often, both bones of forearm fracture but isolated fractures of ulna also occur.
The olecranon process gets fractured when a patient falls on a flexed elbow. The triceps brachii can displace part of the fragment proximally.
Dislocation of the elbow occurs when there is a fall on an outstretched hand.
Monteggia’s fracture and Galeazzi’s fracture are other common injuries that involve ulna.
Madelung Disease
It is a deformation of the distal end of the radius and leads to the dorsal subluxation of the ulna.
Rerefences
