Ulna is a long bone—a bone with a shaft and expanded ends—in the forearm, on the little-finger side when the palm faces forwards. It extends from the elbow towards the wrist, forming a stable connection with the upper arm and supporting forearm rotation alongside the radius, the other forearm bone.
Where is the ulna located?
In anatomical position, the body stands upright with the palms facing forwards. The ulna lies medially, meaning towards the body's midline, while the radius lies laterally, meaning away from the midline. These relationships describe anatomical position; the bones change their relationship when the forearm rotates.
The left ulna and right ulna are matching bones on opposite sides of the skeletal system, the body's bony framework. Each has a broad upper end near the elbow and a smaller lower end near the wrist. The humerus, the upper-arm bone, lies above it, while the carpal bones, the bones of the wrist, lie beyond its lower end.
What are the main parts of the ulna?
The ulna has a proximal end, a shaft and a distal end. Proximal means nearer the attachment of the limb to the trunk; distal means farther from that attachment. The proximal end is therefore at the elbow, and the distal end is at the wrist.
The shaft, or central elongated part, has a roughly triangular cross-section through much of its length. Its surfaces are named anterior, posterior and medial: towards the front, towards the back and towards the body's midline, respectively. Its borders are named anterior, posterior and interosseous.
The interosseous border is the sharp edge facing the radius. It gives attachment to the interosseous membrane, a sheet of strong connective tissue spanning between the forearm bones. The posterior border can be felt beneath the skin along much of the forearm.
What are the landmarks at the elbow end?
The proximal ulna forms the prominent point of the elbow and a deep joint surface that receives the lower end of the humerus. A process is a projecting part of a bone; a notch is an indentation in its outline. Several proximal landmarks use these terms.
| Landmark | Location and anatomical role |
|---|---|
| Olecranon | Large posterior projection forming the point of the elbow and providing attachment for the triceps brachii muscle. |
| Coronoid process | Anterior projection below the main elbow joint surface that contributes to the stability of the joint. |
| Trochlear notch | Deep curved surface between the olecranon and coronoid process that receives the trochlea of the humerus. |
| Radial notch | Small joint surface on the lateral side of the proximal ulna that receives the head of the radius. |
| Ulnar tuberosity | Roughened area below the coronoid process that provides attachment for the brachialis muscle. |
The trochlea is the spool-shaped joint surface at the lower end of the humerus. Its fit within the trochlear notch guides bending and straightening at the elbow. The triceps brachii is the main muscle that straightens the elbow, while the brachialis bends it.
What are the landmarks at the wrist end?
The distal ulna has a rounded head and a styloid process, a pointed projection extending from its distal end. The head has a joint surface that meets the ulnar notch, an indentation at the lower end of the radius.
The styloid process lies on the posteromedial side, meaning towards the back and towards the body's midline. It provides attachment for supporting connective tissues around the distal forearm and wrist.
The ulna does not directly articulate with the carpal bones: to articulate means to meet another structure at a joint. An articular disc, a plate of fibrous cartilage, separates its distal end from the wrist bones.
Which joints does the ulna form?
The ulna participates in the following synovial joints at the elbow and at both ends of the radius:
- Humeroulnar joint: the trochlear notch meets the trochlea of the humerus, allowing elbow bending and straightening.
- Proximal radioulnar joint: the radial notch meets the head of the radius near the elbow, allowing rotation.
- Distal radioulnar joint: the head of the ulna meets the ulnar notch of the radius near the wrist, also allowing rotation.
At the proximal radioulnar joint, the annular ligament—a strong connective-tissue band surrounding the radial head—holds it against the radial notch while permitting movement. The interosseous membrane links the shafts as part of the middle radioulnar syndesmosis, a fibrous joint; it does not form a synovial joint surface.
What does the ulna do during movement?
The ulna provides a stable framework for the forearm and forms an important part of the elbow's hinge mechanism. Flexion means bending the elbow; extension means straightening it. The olecranon gives the triceps brachii an attachment through which it can produce extension.
Muscles of the muscular system, the body's muscles considered together, attach to the ulna to help move the elbow, forearm, wrist and fingers. The shaft and its borders provide broad attachment areas, while the interosseous membrane also provides muscle attachment and helps transfer forces between the forearm bones.
Pronation turns the palm backwards from anatomical position, or downwards when the elbow is bent. Supination returns it forwards, or upwards with the elbow bent. During these movements, the radius rotates around the relatively stable ulna; in pronation, the radius crosses in front of it.
How can you find the ulna in the 3D model?
In the 3D model next to this article, rotate the view, select the ulna and isolate it. Zoom in for a closer view, then zoom out and rotate again to change the viewing angle. You can also hide or fade selected structures and switch layers while adjusting the view.
To change the presentation further, cut a section along the sagittal, coronal or transverse plane, or pull parts apart. Combine these actions with rotation, zooming and selection to explore the model from different views.