Comparison of two unicompartmental knee arthroplasties (UKA), to see if Sigma UKA performs equally good as the Oxford UKA. The study focuses on how well the arthroplasty is fixated to the bone and on the difference in wear of the plastic insert. Finally, the functional outcome after surgery is compared between the two arthroplasties.

Hypothesis:

H1: The Sigma UKA has an equal migration pattern compared to the Oxford UKA with a follow up of two years - that is, no significant difference in migration between 1-2 years follow-up.

H2: The Oxford and Sigma UKA's have comparable polyethylene wear with a follow up of 5 years.

H3: The Oxford and Sigma UKA's have comparable patient perceived outcome measures (PROM) and leg extension power (LEP) postoperative.

H4: BMD of the proximal tibia does not influence migration of the tibial component

Inclusion Criteria:

• All eligable patients with medial osteoarthritis of the knee.

Exclusion Criteria:

• - Inflammatory arthritis.
• Patients who are not able to participate in the fast-track surgery program.
• Patients with a contralateral knee arthroplasty.
• Patients who are unable to provide informed consent.
• Disseminated malignant disease (cancer) and treatment with radiation or chemotherapy.
• Serious systemic disease (e.g.. hemiparesis and severe parkinsonism)
• On-going case involving work injury of the knee.
• Female patients with a wish of becoming pregnant in the study period.