The Ulna: Its Articulations, Functions, and Clinical Relevance in the Forearm

What Does the Ulna Articulate With?

The ulna, the medial bone of the forearm, primarily articulates with the humerus at the elbow joint, and with the radius at both its proximal and distal ends to facilitate forearm rotation.

Understanding the Ulna: An Overview

The ulna is one of the two long bones forming the forearm, positioned on the medial side (pinky finger side) when in anatomical position. While often overshadowed by the radius in terms of direct wrist articulation, the ulna is the primary stabilizing bone of the elbow joint and plays a crucial role in forearm rotation and force transmission. Its unique shape, with a large proximal end and a smaller distal end, dictates its specific articulation points and functional contributions.

Proximal Articulations: The Elbow Joint

The most significant and stable articulations of the ulna occur proximally, forming the core of the elbow joint.

• Humeroulnar Joint: This is the primary articulation of the elbow, forming a robust hinge joint.
  • Trochlear Notch of Ulna and Trochlea of Humerus: The large, C-shaped trochlear notch of the ulna precisely fits around the trochlea (a spool-shaped condyle) of the humerus. This articulation allows for flexion and extension of the forearm. The olecranon process (the prominent bony projection at the back of the elbow) and the coronoid process (a smaller projection on the anterior aspect) of the ulna engage with the olecranon and coronoid fossae of the humerus, respectively, to limit the range of motion during extension and flexion.
• Proximal Radioulnar Joint: While the ulna itself doesn't move significantly in this articulation, it provides the stable pivot point for the radius.
  • Radial Notch of Ulna and Head of Radius: The radial notch, a shallow depression on the lateral side of the coronoid process of the ulna, articulates with the head of the radius. This articulation, along with the annular ligament, forms a pivot joint that allows the radius to rotate around the ulna, enabling pronation and supination of the forearm.

Distal Articulations: The Wrist and Distal Radioulnar Joint

The distal end of the ulna is much smaller than its proximal end and has distinct articulations.

• Distal Radioulnar Joint: Similar to the proximal joint, this is a pivot joint crucial for forearm rotation.
  • Head of Ulna and Ulnar Notch of Radius: The rounded head of the ulna articulates with the ulnar notch, a shallow concavity on the medial aspect of the distal radius. This joint works in conjunction with the proximal radioulnar joint to allow the radius to cross over the ulna during pronation and uncross during supination.
• Indirect Articulation with Carpal Bones: Unlike the radius, the ulna does not directly articulate with the carpal bones of the wrist.
  • Articular Disc (Triangular Fibrocartilage Complex - TFCC): Instead, an articular disc, part of the Triangular Fibrocartilage Complex (TFCC), intervenes between the head of the ulna and the carpal bones (specifically the lunate and triquetrum). This disc acts as a cushion, a stabilizer for the distal radioulnar joint, and helps transmit compressive forces across the wrist.

Mid-Shaft Articulation: The Interosseous Membrane

Along the length of the shafts of both the ulna and radius, a strong fibrous sheet connects them.

• Interosseous Membrane: This dense connective tissue runs obliquely between the interosseous borders of the ulna and radius. It serves several vital functions:
  • Muscle Attachment: Provides broad surface area for the attachment of many forearm muscles.
  • Force Transmission: Crucially, it transmits forces from the radius to the ulna. When a force is applied through the hand and radius (e.g., during pushing), the interosseous membrane helps distribute that load to the ulna, which then transmits it to the humerus at the elbow.
  • Maintains Spacing: Helps to maintain the relative spacing between the two forearm bones.

Functional Significance in Movement

The ulna's articulations are fundamental to the diverse and powerful movements of the upper limb:

• Elbow Stability and Hinge Action: The robust humeroulnar articulation provides the primary stability for the elbow, allowing for powerful pushing, pulling, and lifting actions that rely on flexion and extension.
• Forearm Rotation (Pronation/Supination): The coordinated movement at the proximal and distal radioulnar joints, with the ulna acting as a stable axis, enables the radius to rotate around it. This allows for the crucial movements of pronation (turning the palm down) and supination (turning the palm up), essential for manipulating objects, using tools, and performing daily tasks.
• Load Bearing and Force Distribution: Through the interosseous membrane, the ulna contributes to bearing and distributing compressive forces originating from the hand, protecting the more delicate structures of the wrist and hand.

Clinical Relevance

Understanding ulnar articulations is critical in diagnosing and treating various conditions:

• Elbow Fractures and Dislocations: Fractures of the olecranon or coronoid processes, or dislocations of the humeroulnar joint, can severely compromise elbow stability and function.
• Distal Radioulnar Joint (DRUJ) Instability: Injury to the ligaments or the TFCC can lead to instability or dislocation of the DRUJ, impairing pronation/supination and causing wrist pain.
• TFCC Tears: Tears to the articular disc can result from falls on an outstretched hand or repetitive rotational movements, leading to ulnar-sided wrist pain, clicking, and weakness.

Conclusion

The ulna, though seemingly secondary to the radius in hand articulation, is a cornerstone of upper limb mechanics. Its intricate articulations with the humerus, radius, and the mediating TFCC, alongside the stabilizing interosseous membrane, underscore its indispensable role in elbow stability, forearm rotation, and efficient force transmission. A comprehensive understanding of these anatomical relationships is vital for anyone involved in exercise science, rehabilitation, or clinical practice.