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German: Ulna

1 Definition

The ulna is a longitudinal hollow bone that is located on the medial side of the forearm and is parallel to the radius.

2 Anatomy

The ulna consists of a rhombic body (Latin: corpus ulnae) and a proximal and distal end piece (Latin: extremitas proximalis et distalis). The very pronounced proximal end piece forms the largest part of the elbow joint. The smaller distal end piece is connected to the wrist via a cartilage disk.

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2.1 Proximal end piece (extremitas proximalis)

You can differentiate 4 important anatomical structures at the proximal end piece: the olecranon, the coronoid process, the trochlear incision, and the radial incision.

2.1.1 Olecranon

The olecranon is a broad, thick and beak-like extended bone spur at the proximal end of the ulna. Its prominent end reaches into the fossa olecrani of the humerus (bone of the upper arm). The base is narrowing at the transition to the corpus ulnae.

The posterior surface of the olecranon is of almost triangular shape, smooth and is covered by a synovial bursa (bursa of the elbow), since it is located directly below the skin. The superior surface is of almost rhombic shape and roughened where the tendon of the triceps brachii muscle inserts into the bone. At the anterior edge of the olecranon, you can see a horizontal indentation, which serves as insertion for the posterior fibers of the joint capsule of the elbow joint. The anterior surface is smooth, concave and covered by articular cartilage. It forms the superior part of the trochlear incision. Some fibers of the flexor carpi ulnaris muscle origin from the medial edge of the olecranon. The anconeus muscle inserts at the lateral edge.

2.1.2 Coronoid process

The coronoid process of the ulna is a prominent, triangular bone protuberance below the trochlear incision. Its anteroinferior surface is concave and shows a rough indentation, where fibers of the tendon of the brachialis muscle insert.

The lateral surface shows a trapezoidal articular surface covered by cartilage, which accommodates the head of the radius (caput radii). It is called radial incision. The medial side of the bone process serves as origin for the flexor digitorum superficialis muscle and the pronator teres muscle.

During flexion, the coronoid process of the ulna is accommodated by the coronoid fossa of the humerus.

2.1.3 Trochlear incision

The trochlear incision (semilunar incision of the ulna) is a large, sickle-shaped indentation between the olecranon and the coronoid process. It is covered by joint cartilage and articulates with the trochlea of the humerus. The incision is divided into a lateral and a little larger medial part by a smooth bone crest that reaches from the olecranon to the tip of the coronoid process.

2.1.4 Radial incision

The radial incision is an indentation on the lateral side of the coronoid process, covered by cartilage. It accommodates the head of the radius. In longitudinal view, it is slightly concave. Its slightly protruding edges serve as insertion for the annular ligament of the radius.

2.2 Shaft (corpus ulnae)

The proximal section of the shaft of the ulna has an almost triangular transverse section with three edges and three surfaces separated thereby. In distal direction, it is increasingly round.

2.3 Anterior edge

The anterior (volar) edge runs from the coronoid process into distal direction. Its superior and middle sections serve as origin for the flexor digitorum profundus muscle. In the lower quarter, the fibers of the pronator quadratus muscle have their origin. The volar edge separates the anterior surface from the medial surface.

2.4 Posterior edge

The posterior (dorsal) edge of the ulna begins on the backside of the olecranon and runs into distal direction to the styloid process from there. In the upper three quarters, it is strongly pronounced and serves as insertion for an aponeurosis, which again serves as origin for the flexor carpi ulnaris muscle, the extensor carpi ulnaris muscle, and the flexor digitorum profundus muscle. The volar edge separates the posterior surface from the medial surface.

2.5 Interosseous edge

The medial edge is called interosseous crest of the ulna or interosseous edge. It begins at the connection point between two lines that run from the edges of the radial incision into distal direction. These lines enclose a triangular bone area, the crest of the supinator muscle, which marks a part of the origin of the supinator muscle. The interosseous edge terminates distally at the head of the ulna. The edge is sharp in its upper part, whereas it is smooth and rounded in the lower part. The interosseous membrane of the forearm, which connects the ulna to the radius, is fixed to the interosseous edge. It separates the posterior surface from the anterior surface.

2.5.1 Facies anterior

The proximal part of the anterior surface (volar surface, Latin: facies anterior) is broader than the distal part. In its upper 3/4, the flexor digitorum profundus muscle has its origin. The lower quarter is covered by the pronator quadratus muscle.

2.6 Facies posterior

The upper section of the posterior surface (dorsal surface, Latin: facies posterior) is broad and concave, the middle section convex and a little more narrow, and the distal part is smooth and rounded. There is a small bone ridge passing diagonally across the upper part. In the bone above this ridge, the anconeus muscle has its insertion. Below the bone ridge, the surface is divided into two parts by a longitudinal fine bone crest. The medial part is smooth and covered by the extensor carpi ulnaris muscle. The lateral part is rough and serves as origin for the following muscles - from top to bottom:

2.7 Facies medialis

The medial surface (Latin: facies medialis) is the area of origin of the flexor digitorum profundus muscle in its proximal 3/4. The lower quarter delimitates with the integument.

2.8 Distal end piece (extremitas distalis)

The distal end piece of the ulna is markedly more delicate than the proximal end piece. Here, you can find two prominent bone structures.

2.8.1 Caput ulnae

The laterally positioned head of the ulna (Latin: caput ulnae) is the distal joint head of the ulna. The part of the articular surface pointing into distal direction is in contact to the articular disk, which separates it from the proximal row of the carpal bones. The cartilage zone pointing into lateral direction is accommodated by the ulnar incision of the radius.

2.8.2 Styloid process

Medial to the head of the ulna, you can find the styloid process of the ulna. It reaches further into distal direction than the head. Its rounded end serves as insertion for the ulnar carpal collateral ligament.

3 Development

The perichondral ossification of the corpus ulnae begins in the course of the 7th week of embryonic development. The centers of ossification in the distal epiphysis develop endochonrally between the 4th and 7th year of life, while the centers in the styloid process develop between the 9th and 11th year of life. The proximal epiphysis closes earlier than the distal one.

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