What Is the Ulna? Definition, Diagram & Key Points for Students
The ulna is a long bone that is found in the forearm. It starts from the elbow and then stretches to the smallest finger in the hand. Ulna runs parallel to the radius. The ulna is thinner and slightly longer than the radius bone. But because the radius is thicker in nature than the ulna, the radius is considered longer than the ulna.
Ulna Anatomy
The ulna is a long bone in the forearm that runs from the elbow to the smallest finger and is located on the medial side of the forearm. Close to the elbow, the ulna is wider and then narrows as it approaches the wrist. The ulna has a bony process called the olecranon process that is located just below the elbow. The olecranon process is a hook-like structure on the humerus that fits into the olecranon fossa. The trochlea of the humerus forms a hinge joint with this, preventing hyperextension.
Structure of Ulna
Olecranon process and coronoid process are the two processes that are present in Ulna. They are present close to the elbow.
The olecranon process is a hook-like structure. This structure fits in the olecranon fossa that is present in the humerus.
From the anterior portion of the ulna, the coronoid process bulges out. It is triangular in shape. It helps in stabilizing the elbow joint. This further helps in preventing hyperflexion.
With the help of the olecranon and coronoid process, a semilunar notch is formed. It is a large depression and helps in the articulation of the trochlea of the humerus.
The upper part of the ulna is prismatic in shape. The central part is straight and the lower part is rounded.
Function of Ulna
The ulna contributes to the formation of the wrist and elbow joints. In a hinge joint, the ulna articulates with the humerus at its most proximal point, forming the elbow. The trochlea of the humerus is found to sit with the semi-lunar notch of the ulna to form a joint.
Pronation (from the proximal joint) and supination (from the distal joint) of the forearm are caused by the ulna articulating with the radius proximally and distally.
The interosseous membrane of the ulna articulates with the radius in a syndesmosis joint. A syndesmosis joint is formed when two adjacent bones are linked together by strong ligaments.
Fractured Ulna
The fractures of ulna occur due to fragility and falling from high ground on the hand. The different types of fractures of the ulna are listed below:
Monteggia fracture: In this fracture, the ulnar shaft and the radial head are involved. These two structures are dislocated from their places.
Hume fracture: This fracture involves the dislocation of the radial head and the fracture of the olecranon process.
Galeazzi's fracture: This fracture occurs when the distal radius gets fractured and this results in the dislocation of the ulnar head.
Conclusion
The ulna is found in the forearm and it is one of the long bones. It starts from the elbow and then stretches to the smallest finger. The radius is slightly smaller than the Ulna but the radius is thicker than Ulna. The elbow and wrist joints are formed by the Ulna. It also helps in forming the elbow. There are different types of fractures that can occur in the ulna and thus care should be taken.
FAQs on Ulna Bone: Structure, Function & Importance
1. What is the ulna bone and where is it located in the body?
The ulna is one of the two long bones in the forearm. It is located on the side of your little finger (the medial side) and runs parallel to the other forearm bone, the radius, extending from the elbow to the wrist.
2. What are the main parts of the ulna?
The ulna consists of three main sections:
- The proximal end (near the elbow), which features the hook-like olecranon process that forms the bony point of the elbow.
- The shaft, which is the long, triangular middle section of the bone.
- The distal end (near the wrist), which includes the head of the ulna and a small point called the ulnar styloid process.
3. What is the primary function of the ulna bone?
The ulna's main function is to stabilise the forearm. It forms the major, hinge part of the elbow joint, allowing the arm to bend and straighten. It also acts as a fixed point that the radius pivots around to allow for hand rotation.
4. How do the ulna and radius work together to move the hand?
The ulna and radius work as a team to create complex movements. While the ulna provides a stable anchor at the elbow, the radius is designed to rotate around it. This rotation allows for pronation (turning the palm to face down) and supination (turning the palm to face up).
5. What is the key difference between the ulna's role at the elbow and its role at the wrist?
The ulna's role is much more significant at the elbow than at the wrist. At the elbow, it forms the main, stabilising part of the joint with the humerus. In contrast, at the wrist, the radius is the primary bone that connects with the wrist bones to bear weight. The ulna's role at the wrist is more for providing stability and serving as an attachment point for cartilage.
6. Why is an isolated ulna fracture sometimes called a 'nightstick fracture'?
This term describes a specific type of break in the middle of the ulna. It is called a 'nightstick fracture' because it commonly occurs when a person raises their forearm to defend against a direct blow from a long, hard object, like a stick or pipe. The ulna takes the full force of the impact and breaks.
7. What important muscles are attached to the ulna?
Several important muscles that control movement in the arm and hand attach to the ulna. Key examples include the triceps brachii, which attaches to the olecranon to straighten the elbow, and the flexor and extensor muscles, which originate on the ulna and control the bending and straightening of the wrist and fingers.
8. Why is the ulna fused with the radius in some animals like frogs, but not in humans?
In animals like frogs, the ulna and radius are fused into a single, strong bone called the radio-ulna. This provides extra rigidity needed to absorb the shock of landing after a jump. Humans require separate ulna and radius bones to perform fine, rotational movements of the hand and wrist, which a fused bone would not allow.
