Ulna

Ulna
Position of ulna (shown in red)
Shown is the right hand, palm down (left) and palm up (right). Ulna is #2
Details
Identifiers
Latin	Ulna
MeSH	A02.835.232.087.090.850
TA	A02.4.06.001
FMA	23466
Anatomical terms of bone [edit on Wikidata]

The ulna (/ˈʌlnə/^[1][2]) is one of the two long bones in the forearm (the other is the radius). It is on the side opposite the thumb, extends from the elbow to the wrist, and runs parallel to the radius, which it exceeds in length and size. In anatomical position, 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 bone may break due to excessive weight or impact.

Structure

The ulna is broader close to the elbow, and narrows as it approaches the wrist.

Close to the elbow, 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.

Close to the wrist, the ulna has a styloid process.

Histology

The ulna is a long bone. The long, narrow medullary cavity of the ulna is enclosed in a strong wall of cortical 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]

Development

Plan of ossification of the ulna. From three centers.

Epiphyseal 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 of the ulna, 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 or so, 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.

Function

Joints

The ulna forms part of the wrist joint and elbow joints. Specifically, the ulna articulates with:

• trochlea of the humerus, at the right side elbow as a hinge joint with semilunar trochlear notch of the ulna.
• the radius, near the elbow as a pivot joint, this allows the radius to cross over the ulna in pronation.
• the distal radius, where it fits into the ulnar notch.
• the radius along its length via the interosseous membrane that forms a syndesmosis joint

Muscle attachments

Bones of left forearm. Anterior aspect.

The radius and ulna of the left forearm, posterior surface.

Clinical significance

Fractures

Specific fracture types of the ulna include:

• Monteggia fracture - a fracture of the proximal third of the ulna with the dislocation of the head of the radius
• Hume fracture - a fracture of the olecranon with an associated anterior dislocation of the radial head

Conservative management is possible for ulnar fractures when they are located in the distal two-thirds, only involve the shaft, with no shortening, less than 10° angulation and less than 50% displacement.^[4] In such cases, a cast should be applied that goes above the elbow.^[4]

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.^[5]

Gallery

• 
  Position of ulna (shown in red). Animation
• 
  Right posterior human radius and ulna
• 
  Human arm bones diagram
• 
  Cross-section through the middle of the forearm.

See also

• Bone terminology
• Terms for anatomical location
• Body of ulna

References

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

External links

Wikimedia Commons has media related to Ulna.

• Bone and Spine : Musculoskeletal-Anatomy