Syndesmosis Joint: Definition, Locations, Function, and Injuries

What is a syndesmosis joint also known as?

A syndesmosis joint is a specific type of fibrous joint, characterized by bones united by a sheet of fibrous tissue, either a ligament or an interosseous membrane. While it doesn't have a common, simple synonym, it is fundamentally classified under the broader category of fibrous joints within the structural classification of joints.

Understanding Syndesmosis Joints

Joints, or articulations, are crucial connections between bones that allow for movement and provide stability. Structurally, joints are classified based on the type of material binding the bones together. Fibrous joints are one such category, where bones are connected by dense fibrous connective tissue, typically allowing for very little or no movement. Syndesmosis joints fall squarely into this category.

Unlike other fibrous joints like sutures (found in the skull, immovable) or gomphoses (tooth in its socket), a syndesmosis is distinguished by:

• Connecting Material: The bones are joined by a cord or band of fibrous tissue, known as a ligament, or a broad sheet of fibrous tissue, called an interosseous membrane.
• Limited Movement: While largely immobile, syndesmosis joints allow for a small degree of movement, primarily to accommodate forces and movements in adjacent, more mobile joints. This slight give is crucial for shock absorption and distributing stress.

Key Locations of Syndesmosis Joints

Syndesmosis joints are strategically located where strong, stable connections with minimal movement are required. The most prominent examples include:

• Distal Tibiofibular Syndesmosis: This is perhaps the most well-known syndesmosis, located at the ankle. It connects the distal ends of the tibia and fibula via several strong ligaments (anterior inferior tibiofibular ligament, posterior inferior tibiofibular ligament, and the interosseous membrane). This joint is critical for the stability of the ankle mortise, which is the socket for the talus bone. Injury to this syndesmosis is often referred to as a "high ankle sprain."
• Radioulnar Syndesmosis: Found in the forearm, this joint connects the shafts of the radius and ulna via the interosseous membrane of the forearm. This broad, strong sheet of fibrous tissue provides a critical attachment point for many muscles and plays a vital role in transmitting forces between the radius and ulna, especially during pronation and supination of the forearm.
• Tibiofibular Syndesmosis (Proximal): While less prominent than its distal counterpart, the proximal ends of the tibia and fibula are also connected by a smaller syndesmosis, though it often has a synovial component as well.
• Sacroiliac Joint (Partially): While primarily a synovial joint, the sacroiliac joint also has strong ligamentous connections that resemble syndesmosis, providing immense stability to the pelvic girdle.

Functional Significance and Biomechanics

The unique structure of a syndesmosis joint serves critical biomechanical functions:

• Stability: The strong fibrous connections provide immense stability, preventing excessive separation or displacement of the bones. This is crucial in areas like the ankle, where significant forces are transmitted during weight-bearing activities.
• Force Transmission: In the forearm and lower leg, the interosseous membranes facilitate the transmission of forces between the two long bones. For instance, in the forearm, forces applied to the hand (via the radius) are effectively distributed to the ulna, which articulates with the humerus at the elbow.
• Limited Accommodation: The slight flexibility allows for subtle adjustments and movements, which can help absorb shock and accommodate the more extensive movements of adjacent synovial joints. For example, the distal tibiofibular syndesmosis allows for slight widening of the ankle mortise during dorsiflexion, accommodating the broader anterior part of the talus.

Clinical Relevance and Injuries

Given their role in stability and force transmission, syndesmosis joints are susceptible to specific types of injuries:

• Syndesmotic Sprains (High Ankle Sprains): Injuries to the distal tibiofibular syndesmosis are common, particularly in sports involving rotational forces or extreme dorsiflexion of the ankle (e.g., football, basketball, skiing). Unlike typical ankle sprains that affect lateral ligaments, syndesmotic sprains involve the ligaments connecting the tibia and fibula. These injuries often require longer recovery times due to the critical role of the syndesmosis in ankle stability and the complex nature of the injury.
• Diastasis: Severe injuries can lead to a "diastasis," which is a pathological widening or separation of the bones at the syndesmosis. This is a serious condition that can severely compromise joint stability and may require surgical intervention to reduce the separation and restore integrity.
• Fractures: Fractures involving the tibia or fibula, especially near the ankle, can also disrupt the syndesmosis, leading to instability and requiring specific management to ensure proper healing and function.

Conclusion

A syndesmosis joint is a vital, albeit subtly moving, component of the human musculoskeletal system. While primarily known as a type of fibrous joint, its specific anatomical configuration—bones united by a ligament or interosseous membrane—endows it with unique biomechanical properties. Understanding these joints, particularly their locations and functional significance, is crucial for appreciating the intricate design of the human body and for effectively managing injuries that compromise their integrity.