Syndesmosis injury typically refers to an injury to the ankle syndesmosis, a joint between the tibia and fibula bones found just above the talocrural or ankle joint. This syndesmosis is also known as the inferior tibiofibular joint, the lower of two joints between the two bones. It is classified as a syndesmosis because the bones are joined by a fibrous, collagen-based membrane rather than a fluid-filled capsule like many movable joints, and the flexibility of this membrane allows for a very small degree of rotation between the bones. This makes the joint stable yet not too rigid and therefore able to withstand forces directed at the ankle, though it is susceptible to such syndesmosis injury as the high ankle sprain, which is often accompanied by damage to the ligaments around the ankle joint.
At the ankle are three joints, or articulations between bones. From top to bottom these are the tibiofibular syndesmosis; the talocrural or ankle joint between the lower ends of the tibia and fibula and the talus bone of the foot; and the subtalar joint between the talus and the calcaneus beneath it, also known as the heel bone. While the syndesmosis does not permit much movement between the leg bones, the talocrural joint allows the ankle to hinge from front to back, and the subtalar joint allows it to tilt from side to side.
Due to the motion allowed at the ankle, a type of connective tissue known as a ligament is needed to prevent the joint from bending too far in any direction, thus limiting the chance of ankle or syndesmosis injury. Made up of the same types of fibers as the interosseous membrane between the tibia and fibula, several ligaments hold the two bones to one other. Aside from the interosseous membrane, the syndesmosis is united on its front side by the anterior inferior tibiofibular ligament and on its back side by the posterior inferior tibiofibular ligament. Additional ligaments hold the two bones to the foot bones below.
These lower ankle joints are more susceptible to injuries like bone fracture and ligament sprain because they move and therefore are less stable, as demonstrated by the common injury of spraining the ankle ligaments by rolling the joint to the outside. Often, however, forces directed at the ankle are absorbed by the more rigid tibiofibular joint, leading to syndesmosis injury. For instance, the anterior or posterior tibiofibular ligament can be sprained, a type of injury known as a high ankle sprain. Additionally, the lower ends of the fibula and tibia can be fractured, a syndesmosis injury that is often accompanied by a sprain of either the tibiofibular ligament or the interosseous membrane.