Ulna

Ulna

Position of ulna (shown in red)

Shown is the right hand, palm down (left) and palm up (right). Ulna is #2
Details
Latin Ulna
Identifiers
Gray's p.214
MeSH A02.835.232.087.090.850
Dorlands
/Elsevier		 u_01/12835497
TA A02.4.06.001
FMA 23466
Anatomical terms of bone

The ulna (/ˈʌlnə/[1][2]) or elbow bone (Latin, "elbow") is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position (i.e. when the arms are down at the sides of the body and the palms of the hands face forward) the ulna is located at the side of the forearm closest to the body (the medial side), the side of the little finger.

Structure

Articulations

The ulna articulates with:

Proximal and distal aspects

The ulna is broader proximally, and narrower distally.

Proximally, the ulna has a bony process, the olecranon process, a hook-like structure that fits into the olecranon fossa of the humerus. This prevents hyperextension and forms a hinge joint with the trochlea of the humerus. There is also a radial notch for the head of the radius, and the ulnar tuberosity to which muscles attach.

At the distal end of the ulna is a styloid process.

The long, narrow medullary cavity is enclosed in a strong wall of compact tissue which is thickest along the interosseous border and dorsal surface. At the extremities the compact layer thins. The compact layer is continued onto the back of the olecranon as a plate of close spongy bone with lamellæ parallel. From the inner surface of this plate and the compact layer below it trabeculæ arch forward toward the olecranon and coronoid and cross other trabeculæ, passing backward over the medullary cavity from the upper part of the shaft below the coronoid. Below the coronoid process there is a small area of compact bone from which trabeculæ curve upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of compact bone. The trabeculæ at the lower end have a more longitudinal direction.[3]

Muscle attachments

Bones of left forearm. Anterior aspect.
The radius and ulna of the left forearm, posterior surface.
Muscle Direction Attachment
Triceps brachii muscle Insertion posterior part of superior surface of Olecranon process (via common tendon)
Anconeus muscle Insertion olecranon process (lateral aspect)
Brachialis muscle Insertion anterior surface of the coronoid process of the ulna
Pronator teres muscle Origin medial surface on middle portion of coronoid process (also shares origin with medial epicondyle of the humerus)
Flexor carpi ulnaris muscle Origin olecranon process and posterior surface of ulna (also shares origin with medial epicondyle of the humerus)
Flexor digitorum superficialis muscle Origin coronoid process (also shares origin with medial epicondyle of the humerus and shaft of the radius)
Flexor digitorum profundus muscle Origin coronoid process, anteromedial surface of ulna (also shares origin with the interosseous membrane)
Pronator quadratus muscle Origin distal portion of anterior ulnar shaft
Extensor carpi ulnaris muscle Origin posterior border of ulna (also shares origin with lateral epicondyle of the humerus)
Supinator muscle Origin proximal ulna (also shares origin with lateral epicondyle of the humerus)
Abductor pollicis longus muscle Origin posterior surface of ulna (also shares origin with the posterior surface of the radius bone)
Extensor pollicis longus muscle Origin dorsal shaft of ulna (also shares origin with the dorsal shaft of the radius and the interosseous membrane)
Extensor indicis muscle Origin posterior surface of distal ulna (also shares origin with the interosseous membrane)

Development

Plan of ossification of the ulna. From three centers.
Epiphysial lines of ulna in a young adult. Lateral aspect. The lines of attachment of the articular capsules are in blue.

The ulna is ossified from three centers: one each for the body, the inferior extremity, and the top of the olecranon.

Ossification begins near the middle of the body, about the eighth week of fetal life, and soon extends through the greater part of the bone.

At birth the ends are cartilaginous.

About the fourth year, a center appears in the middle of the head, and soon extends into the ulnar styloid process.

About the tenth year, a center appears in the olecranon near its extremity, the chief part of this process being formed by an upward extension of the body.

The upper epiphysis joins the body about the sixteenth, the lower about the twentieth year.

Clinical significance

Fractures

Specific fracture types of the ulna include:

In other animals

In four-legged animals, the radius is the main load-bearing bone of the lower forelimb, and the ulna is important primarily for muscular attachment. In many mammals, the ulna is partially or wholly fused with the radius, and may therefore not exist as a separate bone. However, even in extreme cases of fusion, such as in horses, the olecranon process is still present, albeit as a projection from the upper radius.[4]

Gallery

See also

This article uses anatomical terminology; for an overview, see anatomical terminology.

References

This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)

  1. ↑ OED 2nd edition, 1989.
  2. ↑ Entry "ulna" in Merriam-Webster Online Dictionary.
  3. ↑ http://www.innerbody.com/image_skelfov/skel21_new.html
  4. ↑ Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. p. 200. ISBN 0-03-910284-X.

External links

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