Fragments of bone loose in a joint is a problem called osteochondritis. The talus is a bone in the ankle between the calcaneus (heel) and the two bones of the lower leg (tibia and fibula). Sometimes the talus is referred to as the anklebone but really there are many bones that work together to form ankle motion.
Chronic ankle pain and loss of ankle motion are the two main symptoms of this problem. Severe ankle pain after trauma (such as an ankle sprain) could be caused by problems other than osteochondritis. There could be a disruption of the blood supply, a fracture, infection, nerve damage, or even an unstable (dislocated) ankle.
If there has been a thorough diagnostic workup (and it sounds like there has been), it’s probably clear where is the exact location of the problem and the amount of damage done. If there is a significant amount of damage and/or the joint is unstable, then if it is left untreated, the condition could worsen to the point of needing an ankle replacement sometime down the road.
Sometimes it’s possible to save the joint by doing an ankle fusion called an arthrodesis. This just holds off the inevitable gradual joint destruction requiring a joint replacement anyway. All of that occurs much later but it’s clear that without proper treatment at the outset, the final result is less than satisfactory.