The syndesmosis is the thick band of tissue connecting the two leg bones (tibia & fibula) together just above the ankle. The syndesmosis is crucial in maintaining ankle stability, and tends to be damaged in more severe ankle fractures.
When a big twisting force acts on the foot and ankle, it may stretch and even tear the ligaments in the syndesmosis. When this occurs, pain and swelling will appear above the ankle. The ankle will also feel extremely unstable, and will be prone to dislocation.
In most cases, syndesmosis sprains recover well with physiotherapy and several weeks of rest (possibly non-weight-bearing using crutches). When the syndesmosis has ruptured completely and is not healing with physiotherapy, it may be necessary to perform surgery. In these cases, a large screw is inserted to hold the two leg bones together, and can be removed months later when the bones and ligaments have healed.