What's your diagnosis, and what could be the underlying condition? I'll post the correct answer in a couple of days.
Correct answer: Charcot (neuropathic) joint. It’s caused by loss of pain sensation, leading to mechanical stress (unimpeded by pain) and repeated joint injury. In this case, swelling, instability and eventually deformity have developed. Reasons for loss of pain sensation include e.g. DM, leprosy, spina bidifa and syringomyelia.