Postmarketing Study to Determine Performance of the SIGMA HP® PARTIAL KNEE SYSTEM

Inclusion Criteria: * Male or female subjects, aged at least 21 years. * Subjects who are able to give voluntary, written informed consent to participate in this clinical investigation and from whom consent has been obtained. * Subjects, who, in the opinion of the Clinical Investigator, are able to understand this clinical investigation, co-operate with the investigational procedures and are willing to return to the hospital for all the required post-operative follow-ups. * Subject has non-inflammatory degenerative joint disease (NIDJD), including: osteoarthritis, posttraumatic arthritis, avascular necrosis and chondrocalcinosis. * Subject has a functional stable knee. * Subject, in the opinion of the Investigator, is suitable for either a single or bicompartmental knee replacement. * Subject meets the following selected radiographic parameters: * X-ray evaluation confirms the presence of NIDJD * Disease location: 1 to 2 compartments, maximum. Disease can be present in the patellofemoral compartment, the medial tibiofemoral compartment, lateral tibiofemoral compartments alone or in any two compartments. * Disease severity: Kellgren and Lawrence44 Grade II or greater for all compartments replaced. Exclusion Criteria: * Subjects who, in the opinion of the Clinical Investigator, have an existing condition that would compromise their participation and follow-up in this clinical investigation. * Women who are pregnant. * Subjects who are known drug or alcohol abusers or with psychological disorders that could affect follow-up care or treatment outcomes. * Subjects who have participated in a clinical investigation with an investigational product in the last 3 months. * Subjects who are currently involved in any injury litigation claims. * Multiple joint involvement that will compromise rehabilitation (for example: advanced hip, ankle or spine disease). * Known to be a prisoner, mentally incompetent, an alcohol / drug abuser, involved in medical-legal case or worker's compensation * Previous knee arthroplasty (any type) in any one of the three compartments of the knee. * Requires more than a lateral soft tissue release or an anterior cruciate ligament reconstruction. * Uncorrectable anatomical tibio-femoral angle. * Bone deficiency requiring structural bone grafts to support the implants. * Previous patellectomy. * For Subjects indicated for a patellofemoral arthroplasty, Subject has uncorrected patella baja (Caton-Deschamps ratio less than or equal to 0.6). * Tri-compartmental disease process (patella-femoral, medial tibio-femoral and lateral tibio-femoral). * Active bacterial infection that may spread to other areas of the body (e.g., osteomyelitis, pyogenic infection of knee joint, urinary tract infection, etc.). * Significant neurological or musculoskeletal deformity or a disease that may affect gait or weight bearing, (e.g., muscular dystrophy, multiple sclerosis, amputation). * Known allergy to implant materials.