X-rays are only two-dimensional pictures of bones. They can't always "see" everything that's going on in the ankle joint. The talus is a bone sandwiched in between two other bones (the heel and the tibia forming the lower leg bone).
The talus has two bumps on the back of the bone. These are the medial and lateral processes. X-rays only show four out of 10 cases (40 percent) where the lateral process of the talus is broken off.
When this fracture goes unnoticed, patients may be treated for an ankle sprain. They don't get better and end up back in the doctor's office with chronic pain and swelling. Further imaging is needed to get to the bottom of the problem.