Radiographs are X-rays usually used to identify fractures after a severe injury or injury that doesn't heal. Stress X-rays look at the function of the bone or bones. Are they lined up correctly when the body part is moving? Sometimes a bone can look perfectly normal at rest. The fracture or displaced bone doesn't show up until the person moves.
In the case of an ankle sprain, stress radiographs are usually looking at the talus, a bone in the ankle. It sits just below the two bones from the lower leg (the tibia and the fibula). The first X-ray is taken with the foot in a non-weight bearing position. The second X-ray is taken with the patient standing on that foot and leg.
Ligaments in the ankle hold the two bones of the lower leg together and keep the talus in line. This connective tissue structure is called the syndesmosis. It is made up of several ligaments in the ankle and a sheet of tissue between the two bones called the interosseus membrane.
A severe ankle sprain can tear the syndesmosis. The syndesmosis keeps the talus in its proper place under the tibia when the ankle is under various loads and weights. It keeps the tibia from sliding to one side or the other.
If the syndesmosis has been damaged and the talus is affected, then the stress X-rays will show it. There will be a greater than normal gap between the bottom of the tibia and the talus below the tibia. If the movement (gap) is too great, then surgery may be needed. Without proper treatment at the right time, the patient can develop severe arthritis later.