Ankle Replacement. The bottom line should be....when it interferes with your normal and daily life activity. If painful when walking or standing for any period of time. If you are a candidate, total ankle replacement is an excellent choice (TAR). There are other option, Fusion and and non-surgical bracing. Bracing should be your first choice well constructed brace (Arizona or Ritchie) before surgery.
Severe pain. Replacement of any joint should be considered only for painful joint that as not responded to non operative management such as therapy and medications.
Ankle arthritis. It depends on your level of pain, disability, and changes in quality of life. This is a personal decision. In general, you want to wait as long as possible.