Ulna

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

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]

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.

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 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]

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  Position of ulna (shown in red). Animation
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  Right posterior human radius and ulna
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  Human arm bones diagram
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  Bones of left forearm. Anterior aspect.
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  The radius and ulna of the left forearm, posterior surface.
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  Left elbow-joint, showing anterior and ulnar collateral ligaments.
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  Cross-section through the middle of the forearm.
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  The Supinator.
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  Transverse section across distal ends of radius and ulna.
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  Muscles of upper limb.Cross section.
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  Wrist joint. Deep dissection.Anterior, palmar, view.
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  Elbow joint. Deep dissection. Anterior view.
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  Elbow joint. Deep dissection. Posterior view.
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  Elbow joint. Deep dissection. Posterior view.

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

  This article incorporates text in the public domain from page 214 of the 20th edition of Gray's Anatomy (1918)

• Bone and Spine : Musculoskeletal-Anatomy