Safety and Efficacy of the Oxford Cementless Partial Knee System

Inclusion Criteria: * Patients with a preoperative Knee Society Assessment Score of \<70 * Patients undergoing primary partial knee arthroplasty as unilateral arthroplasty or bilateral arthroplasty, simultaneously or otherwise * Patients diagnosed with osteoarthritis or avascular necrosis limited to the medial compartment of the operative knee joint * Male or female patients who are at least 21 years of age at the time of surgery * Patients with full thickness cartilage loss, with or without bone loss in the medial compartment * Patients with functionally intact Anterior Cruciate Ligament (ACL) and Posterior Cruciate Ligament (PCL) * Patients who needs to obtain relief of pain and/or improved function in their knee * Patients with fixed flexion deformity \<15 degrees * Patients who are able to follow postoperative care instructions * Patients who are willing and able to return for scheduled follow-up evaluations * Patients in which natural alignment can be restored * Patients who have completed a valid, Institutional Review Board (IRB) approved Informed Consent Form * Patients with child-bearing potential who voluntarily agree to prevent pregnancy for 2 years following device implantation Exclusion Criteria: * Patients with a preoperative Knee Society Assessment Score of greater than or equal to 70 * Patients in which the device would be used to revise a failed prosthesis * Patients who are less than 21 years of age, at the time of surgery * Disease or damage to the lateral part of the knee that in the investigator's opinion contraindicates a partial knee replacement * Patients diagnosed with rheumatoid arthritis or other forms of inflammatory joint disease * Patients diagnosed with a failed upper tibial osteotomy in the operative knee * Patients diagnosed with post-traumatic arthritis after tibial plateau fracture * Patients who have had a patellectomy * Patients with a flexion deformity \> 15 degrees * Patients with a fixed varus deformity \> 15 degrees * Patients who have rapid joint destruction, marked bone loss or bone resorption apparent on roentgenogram * Patients with a fused knee on operative side(s) * Patients who have active or suspected infection, local or systemic that, in the opinion of the investigator, may put patients at undue risk * Patients with pre-existing condition(s) that may interfere with the survival of the implants or their outcomes, including: * Sickle Cell Anemia * Lower extremity muscular atrophy * Neuromuscular disease * Vascular insufficiency * Metabolic Disorders which impair bone formation * Paget's Disease * Charcot's Disease * Osteomalacia * Severe Osteoporosis * Patients with clinical conditions that may limit follow-up (in the opinion of the investigator) including: * Immunocompromised conditions (i.e. HIV) * Hepatitis * Tuberculosis * Neoplastic disease such as cancer of the prostate, lung, stomach, cervix, etc. * Chronic renal failure * Organ transplant (i.e. heart, liver, lung, etc.) recipients * Known disease process that in the opinion of the investigator may limit long term (4 year) follow-up (i.e. multiple sclerosis, leukemia, lymphoma, etc.) * Patients diagnosed with Parkinson's or Alzheimer's Disease * Patients who have had an above-knee amputation in the contralateral leg * Instability or deformity of the ligaments and/or surrounding soft tissue, which would preclude stability of the prostheses * Patients with a known metal allergy * Prisoners or, individuals who are known to be abusing drugs or alcohol or are mentally incompetent * Patients who have received systemic steroids within the past 6 months or steroid injections into the affected knee within the previous 6 weeks prior to enrollment * Patients who are pregnant * Patients with severe valgus or varus knees (valgus or varus angulation of more than 20 degrees) where collateral ligament, iliotibial band, or popliteal release is required * Patients who refuse to sign the IRB approved Informed Consent Form * Participation in an interventional clinical research study procedure, other than a bilateral knee arthroplasty in this study, within the past 12 months * Patients with a history of osteomyelitis or sepsis of the index knee * Patients who require patellar resurfacing * Patients who are not skeletally mature * Patients who have had a total hip replacement procedure \< 18 months prior to entering the study * Patients who have had a contralateral non-study knee replacement procedure \< 18 months prior to entering the study * Patients who are found intraoperatively to have inadequate bone stock or other conditions that contraindicate a partial knee replacement