Evaluation of Triathlon - A New Total Knee Prosthesis System - Triathlon PA vs. Triathlon Pressfit

Inclusion Criteria: 1. Patients suffering exclusively from OA, Stage II-V \[Ahlbäck, 1968 391\] will be operated. 2. Patients requiring knee prosthesis, suitable for the use of Duracon and Triathlon knee system 3. Patients who understand the conditions of the study and are willing and able to comply with the post-operative scheduled clinical and radiographical evaluations and the prescribed rehabilitation. 4. Patients who signed the Ethics Committee approved Informed Consent Form prior to surgery. Exclusion Criteria: 1. Previous major knee surgery 2. Other significant disabling problems from the muscular-skeletal system than in the knees 3. Obese patients where obesity is severe enough to affect subject's ability to perform activities of daily living (body mass index, kg/m2: BMI above 35). 4. Patients with active or suspected infection. 5. Patients with malignancy - active malignancy. 6. Patients with severe osteoporosis, Paget's disease, renal osteodystrophy. 7. Patients immunologically suppressed, or receiving steroids in excess of physiologic dose requirements. 8. The patient has a neuromuscular or neurosensory deficit which would limit the ability to assess the performance of the device or the patient has a neurological deficit which interferes with the patient's ability to limit weight bearing or places an extreme load on the implant during the healing period. 9. Female patients planning a pregnancy during the course of the study. 10. Patients with systemic or metabolic disorders leading to progressive bone deterioration. 11. Patients, who as judged by the surgeon, are mentally incompetent or are unlikely to be compliant with the prescribed post-operative routine and follow-up evaluation schedule. 12. Patients with other severe concurrent joint involvements, which can affect their outcome. 13. Patients with other concurrent illnesses, which are likely to affect their outcome such as sickle cell anaemia, systemic lupus erythematosus or renal disease requiring dialysis. 14. Patients under the protection of law (e.g. guardianship).