The peroneal tendons run through a groove on the back of the lateral malleolus (outside of the ankle). They are held in the groove by a slip of tough tissue called the peroneal retinaculum. When this tissue gets torn they can pop in and out of the groove causing a painful snapping sensation on the outside of the ankle. The initial damage often occurs as part of an inversion injury to the ankle, such as with rapid changes of direction in ball sports, or running on uneven ground.
You may feel a popping or snapping sensation at the time of an ankle injury, but often this is hard to notice because of the pain and symptoms of the sprained ankle. The tendons can either come out and stay out, or be unstable popping in and out. When the ankle has settled down a bit you may feel snapping as you move the ankle and notice a swelling or altered appearance at the outside of the ankle. Often you can make the tendons pop in and out yourself.
If you think you have this condition it is well worth getting it checked out by Mr Willis-Owen. In the mean time you should avoid any activities which may make things worse or do more damage. You should try to avoid making the tendons pop in and out, and wearing an over the counter ankle brace may well help.
Treatment involves first confirming the diagnosis. This usually means a careful examination and an MRI scan. Surgery is often required to reconstruct the torn structures. Usually intensive physio will be required after surgery to get the best out of the ankle. With prompt appropriate surgery and really good physio you can expect an excellent recovery.