LEGION Hinge Safety and Efficacy Study

Inclusion Criteria: * Subject has gross knee instability resulting from loss of collateral ligament function, gross bone loss, comminuted fractures of the proximal tibia or distal femur, rheumatoid arthritis, post-traumatic arthritis, osteoarthritis, degenerative arthritis, failed osteotomies, unicompartmental replacement or total knee replacement, or absent or incompetent posterior cruciate ligament and one or both of the collateral ligaments * Subject has a failed primary or revision knee replacement * Subject is 18-80 years of age * Subject is skeletally mature in Investigator's judgment i.e., subject is not actively growing or does not have immature bones for any reason * Subject has met an acceptable preoperative medical clearance and is free of or treated for cardiac, pulmonary, hematological, infection or other conditions that would pose excessive operative risk * Subject is willing to sign and date an IRB/EC-approved consent form * Subject plans to be available through the five (5) year postoperative follow-up * If of child bearing potential, Subject reports she is not pregnant nor plans to become pregnant during the study * Subject agrees to follow the study protocol Exclusion Criteria: * Subject is receiving the study device as a primary knee replacement * Subject has presence of malignant tumor, metastatic, or neoplastic disease * Subject is not expected to return to normal ambulatory function (i.e., morbid obesity or other limiting co-morbidities) * Subject is pregnant or plans to become pregnant during the course of the study * Subject has conditions that may interfere with the revision arthroplasty survival or outcome (i.e., Paget's or Charcot-Marie-Tooth disease, vascular insufficiency, muscular atrophy, uncontrolled diabetes, moderate to severe renal insufficiency or neuromuscular disease) * Subject has known (Subject reported) metal hypersensitivity * Subject has an emotional or neurological condition that would pre-empt their ability or willingness to participate in this study * Subject has BMI\>45 * Subject is participating in any other pharmaceutical, biologic, or medical device clinical investigation or has been treated with an investigational product in the past 30 days * Subject is facing current or impending incarceration * Subject is not a good candidate for the study based on Investigator opinion