Unicondylar knee arthroplasty (UKA) is a proven procedure in patients suffering from unicompartmental knee osteoarthritis. Two different types of UKA design are available - both with different pros and cons. The so-called 'mobile-bearing UKA' requires the removal of an additional amount of cartilage and bone close to the implantat. It is unclear whether this is of clinical relevance.

We speculate that the amount of cartilage/bone removal is related to the clinical outcome of the UKA-surgery.

In patients receiving a mobile-bearing UKA that amount of cartilage/bone removal is measured during the surgery. The clinical success of the procedure will be determined with certain knee questionnaires (Kujala & KOOS)preoperatively, 3 months postoperatively, 6 months postoperatively and 1 year postoperatively. Additionally, the so-called bone-remodelling around that area of resected cartilage and bone will be measured (SPECT-CT) postoperatively.

The three variables (removed amount of cartilage/bone; clinical success with regard to the questionnaires; bone remodelling) are then analyzed with statistical methods to find out potential assoziations among them.

Inclusion Criteria:

- osteoarthritis or osteonecrosis in the medial or lateral tibiofemoral compartment

Exclusion Criteria:

• failed upper tibial osteotomy
• insufficiency of the collateral or anterior cruciate ligaments
• a fixed varus or valgus deformity (not passively correctable) above 15°
• a flexion deformity greater than 15°
• rheumatoid arthritis

consecutive patients scheduled for unicondylar knee arthroplasty at the Dept. for orthopaedic surgery of our medical university