The ankle joint is formed where the bones of the lower leg, the tibia, and the fibula, connect above the anklebone, called the talus. The tibia is the main bone of the lower leg. The fibula is the small, thin bone along the outer edge of the tibia.
The ankle joint is a hinge that allows the foot to move up (dorsiflexion) and down (plantarflexion). The ankle is a synovial joint, meaning it is enclosed in a joint capsule that contains a lubricant called synovial fluid.
Strong ligaments surround and support the ankle joint. The ligament that crosses just above the front of the ankle and connects the tibia to the fibula is called the anterior inferior tibiofibular ligament (AITFL). The anterior talofibular ligament (ATFL) supports the outer edge of the ankle. The ATFL goes from the tip of the fibula and angles forward to connect with the talus.
The talus rests on the heelbone (the calcaneus). The joint formed between these two bones is called the subtalar joint. The calcaneus extends backward below the ankle, forming a shelf on which the talus rests.
In some people, the lateral tuberosity is not united to the talus. The separate piece of bone is called an os trigonum. This separation of the os trigonum from the talus is usually not a fracture. About 15 percent of people have an os trigonum. An os trigonum sometimes causes problems of impingement in the back of the ankle.