This study will be a prospective, non-randomized evaluation of the change between preoperative and postoperative outcomes for those receiving the Triathlon® TS Total Knee System for a revision knee operation. The mean total Knee Society Score (for pain, motion and function) change is not 10% worse than, or is superior to, the expected change according to published revision total knee replacement data, for cases implanted with the Triathlon® TS Total Knee System as compared from preoperative to 2 years postoperative.