A novel radiographic technique to assess 180° rotational spin of the Oxford unicompartmental knee mobile bearing

Abstract

Introduction: The recognition of anteromedial knee arthritis as a distinct early clinicopathological entity has led to a resurgence in medial unicompartment knee arthroplasty (UKA). Symptomatic knee pain caused by 180° rotational spin of the mobile bearing of the Oxford Knee is an unrecognized and therefore under-reported complication of UKA. Whilst the post-operative radiographic criteria for optimal positioning of UKA is well described in the available literature, this isn't the case for assessing antero-posterior (AP) orientation of the mobile-bearing. Methods: Following a literature review, we describe a novel radiographic technique that can consistently assess AP orientation, and as a result, diagnose 180° rotational spin of the mobile-bearing. This technique overcomes the radiological challenge of superimposition of the radiopaque markers with the lateral edge of the tibial tray. Results: The modified oblique view results in clear visualization of the metallic rod embedded in the polyethylene, away from the lateral edge of the tibial tray. An anteriorly viewed metallic rod would indicate a well oriented mobile bearing. However, if the metallic beads are visualized anteriorly without dislocation, the component would have spun 180°. Conclusion: Clinicians should have a high index of suspicion for 180° spin to have occurred in patients with posterior dislocation with or without spontaneous reduction. We recommend bearing exchange ± revision arthroplasty for symptomatic patients. The modified oblique view is now part of our immediate post-operative XR protocol and repeated for any patient who re-presents symptomatically at any stage following the index procedure.