The purpose of this study was to see if MRI has a role in pre-operative assessment of patients for unicompartmental knee replacement. Until now, surgeons have been unable to predict whether a patient is suitable until the operation itself when the anterior cruciate ligament is inspected. We found that 33% of patients with anteromedial osteoarthritis had a degenerate anterior cruciate ligament according to magnetic resonance imaging, compared to only 13% on surgical inspection. We conclude that MRI is too sensitive to changes of the anterior cruciate ligament to be of much practical value.