Forearm Anatomy: Interosseous Membrane, Radioulnar Ligaments, and TFCC

What is the ligament between the radius and the ulna?

The primary ligamentous structure connecting the radius and ulna along their shafts is the interosseous membrane, a strong fibrous sheet that plays a crucial role in forearm stability, force transmission, and muscle attachment.

Introduction to the Forearm Bones

The forearm is composed of two long bones: the radius and the ulna. These bones run parallel to each other, extending from the elbow to the wrist.

• The radius is located on the lateral (thumb) side of the forearm and is the larger of the two bones at the wrist, forming the primary articulation with the carpal bones.
• The ulna is located on the medial (little finger) side and is the larger of the two bones at the elbow, forming the primary articulation with the humerus.

While they articulate directly at their proximal and distal ends (forming the proximal and distal radioulnar joints), their shafts are also connected by a robust fibrous structure, essential for their coordinated movement and the overall function of the upper limb.

The Interosseous Membrane: The Primary Ligament

The most prominent and functionally significant ligamentous connection between the shafts of the radius and ulna is the interosseous membrane.

• Anatomy:

  • This is a broad, flat, and strong fibrous sheet that stretches obliquely downwards and medially from the interosseous border of the radius to the interosseous border of the ulna.
  • Its fibers are predominantly directed from the radius distally and medially to the ulna.
  • It contains small apertures proximally and distally to allow passage of blood vessels (e.g., anterior interosseous artery).

• Function: The interosseous membrane serves multiple critical functions:

  • Force Transmission: Its oblique fiber orientation is optimally designed to transmit axial compressive forces from the hand, through the radius (which bears the majority of load at the wrist), to the ulna, and then proximally to the humerus at the elbow. This effectively distributes the load across both bones, preventing excessive stress on the radius alone.
  • Binding the Bones: It firmly binds the radius and ulna together along their lengths, maintaining their relative positions and ensuring stability of the forearm.
  • Muscle Attachment: It provides extensive surface area for the origin of several muscles of the forearm, including the deep flexors (flexor pollicis longus, flexor digitorum profundus) and extensors (abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis).
  • Limiting Movement: While not its primary role, it contributes to limiting extreme ranges of pronation and supination, especially at the mid-forearm.

Other Important Ligaments and Structures

While the interosseous membrane connects the shafts, other critical ligaments stabilize the articulations between the radius and ulna at their ends:

• At the Proximal Radioulnar Joint (Elbow):

  • Annular Ligament: This strong, fibrous band encircles the head of the radius, holding it firmly against the radial notch of the ulna. It allows the radial head to rotate freely during pronation and supination while maintaining stability.

• At the Distal Radioulnar Joint (Wrist):

  • Palmar (Volar) Radioulnar Ligament: Connects the distal radius to the distal ulna on the palmar aspect.
  • Dorsal Radioulnar Ligament: Connects the distal radius to the distal ulna on the dorsal aspect.
  • These ligaments, along with the articular disc, are integral components of the Triangular Fibrocartilage Complex (TFCC).
  • Triangular Fibrocartilage Complex (TFCC): This is a complex of structures located at the ulnar side of the wrist. It includes the articular disc, the palmar and dorsal radioulnar ligaments, and several other structures. The TFCC is crucial for:
    • Stabilizing the distal radioulnar joint.
    • Distributing compressive forces across the wrist.
    • Providing a smooth gliding surface for the ulnar carpal bones.

Clinical Significance and Injuries

The integrity of these ligaments is vital for proper forearm and wrist function. Injuries can significantly impair movement and strength.

• Forearm Fractures:

  • Fractures of the radius and/or ulna (e.g., Monteggia fracture involving ulnar fracture and radial head dislocation, Galeazzi fracture involving radial fracture and distal radioulnar joint dislocation) often involve associated ligamentous disruption.
  • The interosseous membrane can be torn in severe forearm trauma, leading to instability and difficulty with load bearing.

• Radial Head Subluxation (Nursemaid's Elbow):

  • Common in young children, this occurs when a sudden pull on the extended arm causes the radial head to slip out from under the annular ligament. The ligament can become entrapped, causing pain and inability to move the arm.

• TFCC Injuries:

  • Common in athletes (e.g., gymnasts, tennis players) and individuals involved in repetitive wrist movements or falls onto an outstretched hand.
  • Can lead to ulnar-sided wrist pain, clicking, instability, and weakness, particularly with gripping and rotational movements.

• Chronic Instability:

  • Damage to the interosseous membrane or the ligaments of the radioulnar joints can lead to chronic instability, pain, and degenerative changes over time if not properly managed.

Conclusion

The connection between the radius and ulna is far more intricate than a simple articulation. The interosseous membrane forms the primary fibrous link along their shafts, critical for distributing forces and providing muscle attachment. Complementary ligaments like the annular ligament at the elbow and the components of the TFCC at the wrist ensure the stability and coordinated movement of these two bones. Understanding these structures is fundamental for comprehending the biomechanics of the forearm and diagnosing injuries that can severely impact upper limb function.