Spontaneous reduction of a posteriorly dislocated mobile-bearing insert in a medial unicompartmental knee replacement: A case report

Posterior dislocation of the mobile polyethylene bearing is an uncommon complication of medial Unicompartmental Knee Replacement (UKR) that is diagnosed radiographically and typically requires manipulation under anaesthesia or revision surgery. We report a 67-year-old man with a 7-year history of bilateral medial UKR who presented with a locked, painful knee at 60° of flexion after a twisting injury. Radiographs demonstrated posterior dislocation of the bearing, with collapse of the medial joint space and metal-on-metal contact between the femoral and tibial components. The 3 radiopaque metallic markers in the polyethylene bearing helped to visualise the location of the bearing behind the medial femoral condyle. While the patient awaited manipulation under anaesthesia, spontaneous relocation occurred overnight during sleep in a lateral position. Repeat radiographs on the following morning confirmed anatomical reduction of the meniscus bearing, with clinical resolution of symptoms. Awareness of this radiographic pattern, and of its potential for spontaneous resolution, can help radiologists and referring orthopaedic clinicians avoid unnecessary further investigations and urgent surgical intervention.