Migration and Survival of All-polyethylene Tibial All Poly Components Compared to the Metal-backed Modular Components of the Triathlon CS Total Knee System. A Single Center RSA Study

1\. The subject is morbidly obese, defined as Body Mass Index (BMI) of \> 40. 2. Patient has a flexion contracture of 15° and more. 3. Patient has a varus/valgus contracture of 15° and more. 4. Patients with a pre-operative knee score of \>70. 5. The subject has a history of total or unicompartmental reconstruction of the affected joint. 6. The subject will be operated bilaterally. 7. Patients who had a Total Hip Arthroplasty (THA) on contralateral and/or ipsilateral side within the last year that is considered to have an unsatisfactory outcome (Patients with contralateral and/or ipsilateral THA \> 1 year ago with good outcome can be included in the study). 8. Patients who had a Total Knee Arthroplasty (TKA) on contralateral side within the last 6 months that is considered to have an unsatisfactory outcome. (Patients with contralateral TKA \> 6 months ago with good outcome can be included in the study). 9. The subject has an active or suspected latent infection in or about the knee joint. 10. Osteomyelitis 11. Sepsis 12. Patient who is expected to need lower limb joint replacement for another joint within one year. 13. The subject has a neuromuscular or neurosensory deficiency, which would limit the ability to assess the performance of the device. 14. The subject has a systemic or metabolic disorder leading to progressive bone deterioration. 15. The subject is immunologically suppressed or receiving steroids in excess of normal physiological requirements. 16. The subject's bone stock in compromised by disease or infection which cannot provide adequate support and/or fixation to the prosthesis. 17. The subject has had a knee fusion to the affected joint. 18. Female patients planning a pregnancy during the course of the study. 19. The patient is unable or unwilling to sign the Informed Consent specific to this study