Ulna

1. Overview

The ulna is one of the two long bones in the forearm, the other being the radius. It is located on the medial (pinky-finger) side of the forearm and plays a crucial role in forming the elbow joint and stabilizing the forearm. The ulna primarily facilitates movement at the elbow and wrist joints and serves as a key attachment site for muscles.

2. Location

The ulna is found in the medial aspect of the forearm:

• Proximally: Articulates with the humerus at the elbow joint and with the head of the radius.

• Distally: Articulates with the radius and indirectly with the carpal bones via an articular disc at the wrist joint.

• Medial to: The radius throughout most of its length.

3. Structure

The ulna is a long bone with distinct proximal, shaft, and distal regions:

• Proximal end:

  • Olecranon: Forms the bony prominence of the elbow and serves as the insertion for the triceps brachii.

  • Coronoid process: Projects anteriorly and stabilizes the elbow joint.

  • Trochlear notch: Articulates with the trochlea of the humerus.

  • Radial notch: Site of articulation with the head of the radius.

• Shaft (body): Slightly curved and triangular in cross-section, with interosseous border facing laterally for attachment of the interosseous membrane.

• Distal end:

  • Head of ulna: Rounded structure that articulates with the radius and triangular articular disc (part of the wrist joint).

  • Styloid process: A small projection providing ligamentous attachment to the wrist joint.

4. Function

The ulna contributes to several mechanical and structural functions:

• Elbow joint formation: Provides a hinge-like articulation with the humerus for flexion and extension.

• Forearm stabilization: Acts as the main stabilizing bone of the forearm during movements and load transfer.

• Muscle attachment: Serves as an origin and insertion point for muscles involved in flexion, extension, and rotation of the forearm and hand.

• Assist in pronation/supination: Though the radius primarily rotates, the ulna contributes by maintaining joint stability during movement.

5. Physiological role(s)

In addition to its biomechanical roles, the ulna supports several physiological functions:

• Hematopoiesis: In early life, the ulna’s marrow cavity is involved in the production of blood cells.

• Mineral storage: Acts as a reservoir for calcium and phosphate, contributing to systemic mineral balance.

• Neurovascular pathway: Forms part of the pathway for nerves (e.g., ulnar nerve) and vessels that travel along the forearm.

6. Clinical Significance

The ulna is commonly involved in trauma and musculoskeletal conditions:

• Fractures:

  • Nightstick fracture: An isolated midshaft fracture caused by direct trauma to the forearm.

  • Monteggia fracture-dislocation: Proximal ulna fracture with dislocation of the radial head—requires urgent management.

• Ulnar impaction syndrome:

  • Excessive contact between the ulna and carpal bones, often due to a longer ulna (positive ulnar variance), causing wrist pain and degeneration.

• Ulnar styloid process fracture:

  • Common in distal radius fractures; may impact wrist stability if the ulnar collateral ligament is involved.

• Osteoarthritis:

  • Degeneration of distal radioulnar or elbow joints can involve the ulna, especially in aging populations or after injury.

• Surgical relevance:

  • The subcutaneous border of the ulna makes it a common site for internal fixation (plates/screws) in fractures.