The Purpose of this study is to evaluate compatibility of the Triathlon Partial Knee Replacement (PKR) with the Japanese population. It is expected that patients who receive the Triathlon PKR will achieve excellent clinical results without adverse effects and that the device fits well with Japanese morphology.

Inclusion Criteria:

1. Patient requires a primary unicompartmental knee replacement.
2. Patient is between 20 years old to 80 years old.
3. Patient is diagnosed as osteoarthritis, rheumatoid arthritis, traumatic arthritis or avascular necrosis.
4. Patient can walk independently at least 10m.
5. Patient has signed the Institutional Review Board (IRB) approved, study specific informed patient consent form.
6. Patient is willing and able to comply with post-operative scheduled clinical and radiographic evaluations and rehabilitation.

Exclusion Criteria:

1. Patient requires a revision.
2. Patient is pregnant.
3. Patient has lateral osteoarthritis.
4. Patient has less than 10°of flexion contracture and greater than 90°of flexion.
5. Patient's preoperative mechanical alignment is less than 10° of varus and 15° of valgus.
6. Patient has had high tibial osteotomy, femoral osteotomy and/or joint fusion.
7. Patient has a neuromuscular or neurosensory deficiency that would limit the ability to assess the performance of the device.
8. Patient has a systemic or metabolic disorder leading to progressive bone deterioration.
9. Patient is immunologically suppressed or receiving steroids (>30 days) in excess of normal physiological requirements.
10. Patient has a deformity which will require the use of wedges or augments.
11. Patient has an active or suspected latent infection in or about the knee joint.
12. Patient who is inappropriate for participating in the study by the judgment of the investigator.