Triathlon® Partial Knee Replacement (PKR) Outcomes Study

Inclusion Criteria: * The subject is a male or non-pregnant female 21-75 years of age at the time of enrollment. * The subject requires a primary cemented unicompartmental knee replacement. * The subject has a diagnosis of osteoarthritis (OA) or posttraumatic arthritis (TA). * The subject has clinically intact cruciate and collateral ligaments and no ligamentous instability is present. * The subject has less than 10 degrees of flexion contracture and greater than 90 degrees of flexion. * The subject's preoperative mechanical alignment is less than 15 degrees of varus and 15 degrees of valgus. * The subject has signed the Institutional Review Board (IRB) approved study specific Informed Patient Consent Form. * The subject is willing and able to comply with postoperative scheduled clinical and radiographic evaluations and rehabilitation. Exclusion Criteria: * The subject has inflammatory arthritis or avascular necrosis(AVN). * The subject is obese, BMI \> 35. * The subject has a history of total or unicompartmental (contralateral compartment and/or patellofemoral joint) reconstruction of the affected joint. * The subject has a history of anterior cruciate ligament (ACL) reconstruction. * The subject has had a high distal femoral, or proximal tibial osteotomy. * The subject has a mental, neuromuscular or neurosensory disorder, which would create an unacceptable risk of prosthesis instability, prosthesis fixation failure, or complications in post-operative care and/or limit the ability to assess the performance of the device. * The subject has a systemic or metabolic disorder leading to progressive bone deterioration that the surgeon feels would affect the overall outcome of the study. * The subject is immunologically suppressed, or is receiving chronic steroids (\>30 days duration). * The subject has a known sensitivity to device materials. * The subject's bone stock is compromised by disease and/or infection which cannot provide adequate support and/or fixation to the prosthesis. * The subject's bone stock is compromised by a prior implantation which cannot provide adequate support and/or fixation to the prosthesis. * The subject has an active or suspected latent infection in or about the knee joint. * The subject is a prisoner.