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The tibiofibular syndesmosis is the joint just above the ankle where the two long bones of the shin, the tibia and fibula, come together at their lower ends. While there are two tibiofibular joints in the leg, one found where the bones meet at their top ends below the knee and one where they meet at their bottom ends above the ankle, only the lower joint is classified as a syndesmosis. A syndesmosis is a joint that does not allow the bones to move much relative to each other. Instead, they are held together by a fibrous membrane between the two bones called an interosseous membrane. In addition, ligaments to the outside of the joint hold the two bones together and contribute to the stability of the ankle as a whole.
What is referred to as the ankle is actually a complex of three joints contributing to ankle movement: the tibiofibular syndesmosis, the talocrural joint, and the subtalar joint. The highest of these is the tibiofibular syndesmosis at the bottom of the shin bones, and below that is the talocrural joint. Located where the bottom ends of the tibia and fibula meet the talus bone of the foot, the talocrural joint is the main joint of the ankle. Below the talocrural joint between the talus and the calcaneus, or heel bone, is the subtalar joint. While the talocrural and subtalar joints allow the ankle to flex, extend, and roll inward and outward, the syndesmosis acts to stabilize the ankle as a whole.
Like the syndesmosis found between the radius and ulna bones in the arm, the tibiofibular syndesmosis is a type of joint called an amphiarthrosis. Joints may be categorized according to their structure or according to the amount of movement they permit between bones, and an amphiarthrosis is a functional classification. This type of joint makes possible a very small degree of motion between the bones it links, generally a slight rotation past one another. In doing so, the syndesmosis lends stability to the ankle as a whole, as it permits some flexibility between the bones at the bottom of the leg so that they may receive various forces without the bones breaking.
The tibiofibular syndesmosis may also be distinguished according to its structure. Unlike the talocrural joint, a synovial joint that hinges the ankle from front to back, the syndesmosis does not contain cartilage or fluid. Instead, the bones are connected by a sheet of tissue, the interosseous membrane. Though it is thin, it consists of strong elastic fibers that hold the bones together while also allowing for some movement at the joint. In addition, several ligaments hold the tibia and fibula to each other and to the bones of the foot, further contributing to the structure and stability of the tibiofibular syndesmosis: the anterior inferior tibiofibular ligament (AITFL), the posterior inferior tibiofibular ligament (PITFL), and the transverse ligament.
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