The ulna is one of the two long bones in the forearm, the other being the radius. It is the larger and longer bone on the medial side of the forearm, opposite the thumb.[2] The ulna is a crucial component of the forearm’s skeletal structure, primarily responsible for forming the elbow joint with the humerus and contributing to the wrist joint. Its shape and structure allow it to support various movements of the arm and hand.
Location
The ulna is located on the medial side of the forearm, running parallel to the radius. It extends from the elbow, where it articulates with the humerus, to the wrist, where it articulates with the carpal bones and the radius.
Structure and Anatomy
The ulna is a long bone of the forearm, situated on the medial side (the side closest to the body when in anatomical position).[3] It consists of three main parts: the proximal end, the shaft, and the distal end.
Proximal End
Olecranon Process
- Description: A large, curved bony projection at the proximal end of the ulna, forming the prominence of the elbow.
- Articulations: Fits into the olecranon fossa of the humerus during elbow extension.
- Muscle Attachments: Serves as the insertion point for the triceps brachii muscle.
Coronoid Process
- Description: A triangular projection on the anterior side of the proximal ulna.
- Articulations: Fits into the coronoid fossa of the humerus during elbow flexion.[5]
- Muscle and Ligament Attachments: Provides attachment for the brachialis muscle and the ulnar collateral ligament.
Trochlear Notch
- Description: A large, C-shaped notch between the olecranon and coronoid processes.
- Articulations: Articulates with the trochlea of the humerus, forming part of the elbow joint.
Radial Notch
- Description: A shallow, oval depression on the lateral side of the coronoid process.
- Articulations: Articulates with the head of the radius, forming the proximal radioulnar joint.
Shaft
Shape and Cross-Section
- The shaft of the ulna is triangular in cross-section, with three borders (anterior, posterior, interosseous) and three surfaces (anterior, posterior, medial).[7]
Borders
- Anterior Border: Sharp and prominent, running along the anterior surface of the ulna.
- Posterior Border: Rounded and palpable along the back of the forearm.
- Interosseous Border: A sharp edge on the lateral side, where the interosseous membrane connects the ulna to the radius.[8]
Surfaces
- Anterior Surface: Smooth, providing attachment for forearm muscles.
- Posterior Surface: Slightly rough, serving as an attachment site for muscles and ligaments.
- Medial Surface: Faces inward and is generally featureless.
Nutrient Foramen
- Located on the anterior surface, allowing the entry of blood vessels to nourish the bone.
Distal End
Head of the Ulna
- Description: A small, rounded structure at the distal end of the ulna.
- Articulations: Articulates with the ulnar notch of the radius to form the distal radioulnar joint.
Styloid Process
- Description: A pointed bony projection extending from the posteromedial aspect of the ulnar head.
- Significance: Provides attachment for ligaments, contributing to wrist stability.
Articulations
- Elbow Joint
- The ulna articulates with the humerus at the trochlear notch, enabling flexion and extension of the forearm.
- Proximal Radioulnar Joint
- The radial notch of the ulna articulates with the head of the radius, allowing pronation and supination of the forearm.
- Distal Radioulnar Joint
- The head of the ulna articulates with the ulnar notch of the radius, facilitating rotational movements of the forearm.
- Wrist Joint
- The ulna does not directly articulate with the carpal bones but connects via the triangular fibrocartilage complex (TFCC), which stabilizes the wrist.
Vascular Supply
- Arteries: The ulna is supplied by branches of the ulnar and interosseous arteries.
- Nutrient Foramen: Located in the shaft, through which the nutrient artery enters.[1]
Muscular Attachments
- Proximal Attachments:
- Triceps brachii on the olecranon process.
- Brachialis on the coronoid process.
- Shaft Attachments:
- Flexor digitorum profundus on the anterior surface.
- Extensor carpi ulnaris on the posterior surface.
- Distal Attachments:
- Pronator quadratus attaches near the distal end.
Notable Features
- Structural Adaptation
- The ulna tapers distally, contributing less to the wrist joint but maintaining structural integrity for forearm rotation.
- Stabilizing Role
- The ulna provides a stable axis for forearm movements and supports the functional articulation of the radius.[4]
- Bone Composition
- Composed primarily of cortical bone in the shaft for strength and spongy bone at the ends for shock absorption.
Function
Formation of the Elbow Joint
The ulna forms a major part of the elbow joint by articulating with the humerus via the trochlear notch. This allows for flexion and extension of the forearm.
Forearm Stability
The ulna provides structural stability to the forearm, acting as a fixed axis around which the radius rotates during pronation and supination.
Support for Muscle Attachments
The ulna serves as an attachment site for several muscles that facilitate movements of the forearm, wrist, and fingers, such as the triceps brachii, brachialis, and flexor digitorum profundus.
Facilitation of Rotational Movements
The ulna contributes to the proximal and distal radioulnar joints, enabling the rotation of the forearm, such as in turning the palm up (supination) or down (pronation).
Transmission of Forces
The ulna, in conjunction with the radius, transmits forces from the hand and wrist to the humerus during activities like lifting or pushing.
Wrist Support
Although it does not directly articulate with the carpal bones, the ulna supports wrist stability through its connection with the triangular fibrocartilage complex (TFCC).
Clinical Significance
The ulna plays a critical role in forearm and wrist function, making it susceptible to various injuries and disorders:
- Fractures:
- Isolated Ulnar Fractures: Often caused by direct trauma, such as a blow to the forearm (e.g., nightstick fracture).
- Monteggia Fracture: A fracture of the ulna accompanied by dislocation of the radial head, requiring prompt treatment.
- Combined Fractures: Commonly seen with injuries to the radius.[6]
- Dislocations:
- Dislocations at the proximal or distal radioulnar joints can impair forearm rotation and require careful management.
- Compartment Syndrome:
- Fractures or trauma to the ulna may lead to increased pressure in the forearm’s compartments, risking neurovascular damage.
- Arthritis:
- Degenerative changes at the distal radioulnar joint or triangular fibrocartilage complex (TFCC) can result in wrist pain and reduced mobility.
- Nerve Damage:
- Injuries near the ulna may affect nearby nerves, such as the ulnar nerve, causing sensory and motor deficits in the hand.