The study seeks to evaluate the condition of patients 5 years after having undergone partial knee replacement for localized osteoarthritis of the knee. Participating patients will voluntarily undergo detailed evaluation of clinical and radiologic outcomes 5 years after surgery. The aims of the study are to describe the survivorship of the prostheses and the clinical results of patients, and to elucidate and describe the natural history disease of the untreated knee joint compartments after a partial replacement. Patients who were operated by the principal investigator 5 years previously will be invited to participate. The study is observational in nature; there is no element of experimentation or test of an intervention.

Inclusion Criteria:

• All individuals who have undergone unicondylar knee replacement of a single tibio-femoral compartment using iBalance prosthesis (Arthrex, Inc, Lakeland Florida) under the care of the principal investigator between December, 2006 and December, 2008 will be invited to participate in the study.

Exclusion Criteria:

• There are no restrictions to inclusion in the study based on surgical or medical criteria; however, individuals who are pregnant, incarcerated, less than 18 years of age, or unable to give valid informed consent on their own behalf will be excluded from the study.

The potential study population consists of all patients who have undergone implantation of a specific unicondylar knee arthroplasty device under the care of the principal investigator between December, 2006 and December, 2008. This is a clinic-based sample of consecutive cases.