Arthroscopic observation was useful to detect loosening of the femoral component of unicompartmental knee arthroplasty in a recurrent hemoarthrosis

Recurrent hemarthrosis after knee arthroplasty is a rare complication. The occurrence is presumed to be less than 1% in total knee arthroplasty (TKA) [1-9], and it is merely reported after UKA [10,11]. The reported causes of hemarthrosis after TKA are impingement of the fat pad or hypertrophic vascular mass of the synovium, femoral flare eroding through an atherosclerotic superior lateral genicular artery, and pigmented villonodular synovitis [1-9]. The reported lesions in nonprosthetic knees are anterior cruciate ligament tears, major meniscus tears, osteochondral fractures, posterior cruciate ligament tears, and coagulation disorder [12-17].We report a case of recurrent hemarthrosis after UKA caused by loosening of the femoral component.A 58-year-old man met with a road traffic accident 10 months after a mobile-bearing unicompartmental knee arthroplasty (UKA, Oxford UKA; Biomet, Swindon, United Kingdom). His car was struck from the side at an intersection. He could walk on his legs; however, he complained a bruise on the anterior knee. The outcome of the arthroplasty was good with radiological evidence of well-fixed implants before the accident. However, he developed pain and hemarthrosis of the knee joint 1 month after the accident, which subsequently required multiple aspirations of blood (30-50 ml) at an interval of 3-12 days. The range of motion was normal (0/140°), but the patient complained of anteromedial knee pain. Physical examination showed no instability. Plain radiograph revealed no signs of loosening such as radiolucent lines or osteolysis (Figure 1). A small bony gap anterior to the femoral component and a small posterior overhanging were noted in the lateral view of x-ray, however, there was no change compared with the immediate post operative radiographs. All laboratory data, including bleeding and coagulation times, C-reactive protein, ESR were within normal limits. Diagnostic arthroscopy demonstrated loosening of the femoral component and a stabl