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8.2C: Syndesmoses

Syndesmoses are slightly movable joints formed where an interosseous ligament joins two bones.

 

LEARNING OBJECTIVES

 

Describe syndesmoses

 

KEY TAKEAWAYS

Key Points

 

  • A syndesmosis, a subcategory of fibrous joints, is a slightly movable (amphiarthrodial) articulation where the contiguous bony surfaces are united by an interosseous ligament, such as the tibiofibular articulation.
  • Tears in this joint are generally repaired with a syndesmotic screw.
  • Due to the lack of flexibility in these joint structures, ligament injuries in syndesmosis joints are common, particularly at the wrist and ankle.

 

Key Terms

 

  • symphysis: The cartilaginous material that adjoins and facilitates the junction of such bones, with or without synovia.
  • prime mover: A muscle that acts directly to bring about a desired movement.
  • diastasis: A separation between two parts of a bone, without fracture.

A syndesmosis is a type of articulation or joint in which two adjacent bones are joined by an interosseous membrane.

Interosseous Membrane

The interosseous membrane is a type of connective tissue found between certain bones, such as those in syndesmosis joints. The membrane is important in creating compartments to separate different structures, distributing the impact of forces and separating the joints. For example, the long bones of the lower arm and the leg both have attached interosseous membranes. In the leg, the interosseous membrane extends between the tibia and the fibula, running along the crests of the bones. The muscles in the leg are separated into sections in the front and back with this membrane. The strength of the membrane allows absorption and distribution of impacts to either bone.

The interosseous membrane in the lower arm extends between the radius and the ulna. It is involved in the elbow joint and helps to stabilize the lower arm bones for strength, durability, and flexibility. Like other joint tissue, it is designed to be able to deform and flex rather than shred or fracture on impact, allowing the joint to absorb considerable stress before damage occurs.

Syndesmoses and Amphiarthrosis Joints

Along with symphysis joints, syndesmoses are classified as amphiarthrosis joints in that they allow slight movement. Joints of this kind are found at several points in the human body, including the intermediate radioulnar joint where the radius and ulna meet above the wrist, in the spine between the spinous processes of various adjacent vertebra, and above the ankle joint where the tibia and fibula converge.

Image of fibrous joints with the tibiofibular syndesmosis demonstration in figure (b). The diagram includes the suture line, suture, dense fibrous connective tissue, ulna, radius, syndesmosis, antebrachial interosseous membrane, socket, gomphosis, root of tooth, and periodontal ligament.

Fibrous Joints: Image of fibrous joints with the tibiofibular syndesmosis demonstration in figure (b).

Located directly above the ankle joint, which is a synovial hinge joint, the ankle syndesmosis is held together by four ligaments. The anterior inferior tibiofibular ligament crosses in front of the tibia and fibula bones. The posterior inferior tibiofibular ligament and the transverse ligament connect the two bones from behind, and the interosseous ligament runs between the contiguous bony surfaces of the two bones.

Due to the limited flexibility in these joint structures, ligament injuries in syndesmosis joints are common, particularly at the wrist and ankle. When the wrist or ankle joint is bent beyond its normal range of motion, a sprain or even a tear in these ligaments can occur. Mild syndesmosis injury may involve the sprain of a single ligament.

More severe injuries can involve damage to multiple ligaments at once or even the separating of the bones at the joint (known as diastasis). Players of rough sports such as football or rugby have an increased risk of fracturing their fibulas and tearing the interosseous ligament between it and the tibia. When that happens, the surgeon temporarily replaces the ligament with a syndesmotic screw.