Causes / typical injury mechanism: The ankle syndesmosis (also referred to as the distal tibiofibular syndesmosis) is a group of soft tissue structures that provide support to and resist rotational, axial, and translational forces on the distal tibiofibular joint. These structures include the anterior-inferior tibiofibular ligament (AITFL), the posterior-inferior tibiofibular ligament (PITFL), the interosseus ligament (IOL), the interosseus membrane, and the inferior transverse ligament (ITL).

The most common mechanism is acute ankle external rotation injuries. Syndesmotic injury itself comprises a spectrum of disease ranging from mild sprain to complete tear, and may or may not include associated fractures. The classic "high ankle sprain" is a lower grade injury (grade I / II) and involves more isolated injury to the AITFL ligament. Complete tears are rare in isolation and are more commonly found in conjunction with fractures (most common are Weber C fibula fractures and Maisonneuve proximal third fibula fractures).

If accurately diagnosed and treated appropriately, syndesmotic injuries can have excellent functional outcomes, so a high index of suspicion must be kept. Missed or delayed diagnosis can lead to chronic ankle pain, functional limitations, and potentially ankle arthritis.

Classic history and presentation: The condition presents as pain just proximal to the ankle joint that is worse with activity. Anterolateral pain is most common, and swelling is present if the injury is acute. Patients will usually describe a specific injury, which is most commonly an external rotation force on a dorsiflexed ankle.

The typical patient is a skeletally mature athlete with anterolateral ankle pain / swelling after an external rotation ankle injury to a dorsiflexed foot and an inability to bear weight.

Prevalence: Injuries to the ankle syndesmosis occur at a rate of approximately 15:100 000 in the general population, with the incidence being even higher in the athletic population. They comprise up to 12% of sprains about the ankle. Syndesmosis injuries may occur in isolation or may be accompanied by ankle sprain or fracture. They can occur in up to 13% of all ankle fractures and 0.5% of all ankle sprains.

• Age – Skeletally mature individuals. Syndesmosis injuries are rare in pediatric patients because the ligaments of the syndesmosis are stronger than the growth plates (which tend to give first when the ankle is stressed). In children with this mechanism of external rotation injury, growth plate fractures of distal fibula or distal tibia usually occur rather than isolated ligamentous injury.
• Sex – Men are 3 times more likely than women to injure the syndesmosis, but this condition can occur in any adult.

Risk factors: Risk factors include participation in high impact sports (eg, ice hockey, football, soccer, rugby), sports played on uneven terrain or artificial surfaces, previous syndesmosis injury, and obesity.

Pathophysiology: External rotation of the foot forces the talus to rotate laterally into the fibula, pushing the fibula away from the tibia and causing distraction across the distal tibiofibular joint, resulting in injury to the syndesmosis. Of these structures that make up the syndesmosis, the AITFL is the weakest (and most commonly injured) and the PITFL is the strongest (and least commonly injured).

Grade / classification system: Grade I and II injuries are the more common injury patterns and are often referred to as a "high ankle sprain."

• Grade I: Mild sprain; stable syndesmosis with normal radiographic findings, mild to moderate pain and swelling.
• Grade II: Moderate sprain; partial tear of syndesmosis ligaments with some potential instability and range of motion loss, moderate pain / swelling.
• Grade III: Severe; full thickness tear of syndesmosis ligaments, severe pain / swelling; rare in isolation and usually accompanied by fracture and/or deltoid ligament tears. Highest association with Weber C fibular fractures and Maisonneuve proximal third fibula fractures.