The ulna or elbow bone 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 the ulna is located at the side of the forearm closest to the
body, the side of the little finger.
Structure Articulations
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 syndesmoses joint
it is also called the poisidion 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.
Muscle attachments Development
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.
Fracture 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
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.
Gallery
See also This article uses anatomical terminology;
for an overview, see anatomical terminology.
Bone terminology Terms for anatomical location
Ossification of ulna References
This article incorporates text from a public domain edition of Gray's Anatomy.
External links Bone and Spine : Musculoskeletal-Anatomy