The syndesmosis refers to a wide sheet of a ligament that connects the fibula to the tibia. When the foot is planted firmly on the ground and a rotational force is exerted on the leg which twists the leg outward (called external rotation) there is abnormal stress on the inner ligament of the ankle called the deltoid ligament. If this ligament tears, then the force on the ankle continues in an outward direction, and either the syndesmosis tears, the fibula fractures, or both may occur. The combination of a tear of the deltoid ligament and the presence of a syndesmosis ligament tear is often referred to as a “high ankle sprain”. As can be seen in the photographic examples below, the mechanism of injury as well as the implications for treatment are completely different for the two injuries. In fact, the common ankle sprain shown below requires little more than support of the ankle, often with a removable brace for a few weeks, followed by exercise and rehabilitation.
|This is the mechanism of injury with a typical inversion ankle sprain. The ankle turns inwards, and the ligaments on the outside of the ankle (called the talofibular and the calcaneofibular) are torn
The syndesmosis injury however has a far more serious implication, since the ankle is now “loose” or unstable. Interestingly, if there is a fracture of the fibula associated with the syndesmosis ligament injury, this fracture does not always require surgery to correct it. What is far more important, is to recognize that the outer or lateral side of the ankle is unstable, and needs a screw(s) to hold the fibula to the tibia and give the syndesmosis ligament a chance to heal.
These two photos show the force on the leg to produce the syndesmosis injury. The foot rolls inward, and the leg rotates outward (external rotation). The arrows show the type of force with this injury.
These are three XR images of a patient with an acute left ankle injury. On the far left picture, the XR looks quite normal, with a normal space on both the inner (medial) and outer (lateral) side of the ankle. The picture in the middle however shows the same ankle while the ankle is being twisted slightly outwards, and the space between the tibia and the fibula opens up because the syndesmosis ligament is torn. The XR on the far right, shows a fracture of the fibula which is present not at the level of the ankle, but in fact is just below the knee. In order to fracture the fibula in this location, the entire syndesmosis must rupture as the fibula twists outwards.
Compare these two XR’s. The one on the left demonstrates medial (inner) ankle instability, associated with a tear of the deltoid ligament, and the XR on the right shows a tear of the lateral (outside) ankle ligament. Both of these XR’s were taken while applying manual stress or force to the ankle to demonstrate the extent of the instability.
|This XR demonstrates marked widening on the inner side of the ankle. This is the diagnosis of a rupture of the deltoid ligament. The talus has been pushed outward towards the fibula and there is a large gap between the fibula and the edge of the tibia.this is the result of a tear of the syndesmosisligament between the tibia and the fibula
The treatment of these acute syndesmosis injuries
are determined by the amount of instability of the ankle, the location
of the fracture of the fibula, and the extent of widening of the
syndesmosis. In most individuals, the syndesmosis must be stabilized
with a screw or screws inserted from the fibula across the tibia.
The syndesmosis ligament is very wide and long, and extends far
up the outer side of the leg, and it cannot be repaired. The healing
of the syndesmosis therefore takes place “indirectly”
as long as the space between the tibia and the fibula is stable.
In some individuals where the instability is significant, we use
more than one screw, and place the screws through a plate on the
fibula directly into the tibia to hold the two together. Generally,
no weight on the ankle is permitted for 4 weeks following repair
of the syndesmosis, and rehabilitation with physical therapy and
exercise begins at that time. It is important to avoid any twisting
of the ankle for 6-8 weeks to avoid further loosening of the syndesmosis
ligament tear, as well as allow sufficient time for the deltoid
ligament to heal. Return to athletic activity takes anything from
8 to 12 weeks following surgical repair.