Polyethylene Luxation in an Oxinium Fixed-bearing Unicompartmental Knee Replacement Leading to Metallosis: A Case Reports for an Early and a Late Presentation.

Introduction: Polyethylene (PE) subluxation is a rare complication after fixed-bearing unicompartmental knee replacement. We present two cases of PE luxation with a rapid onset of metallosis in a unicompartmental knee replacement made of Oxinium, one early presentation 3 months after surgery and the other case 6 years after. Case Report: Case 1: A 84-year-old male underwent a fixed-bearing medial unicompartmental knee replacement in his left knee. Standard surgery was carried out with robotic assistance. Three months after surgery, he felt acute pain in the anterior portion of his knee. Radiographs showed that the PE was luxated anteriorly and a radiopaque "cloud" that blurs the vision of the implants suggesting metallosis. In the revision surgery, abundant blackish liquid and metallic-like debris were deposited in the synovial tissue and capsule around the knee. The tibial and femoral components presented extensive wear areas, suggesting extensive metal-to-metal contact. Both components were removed, and a posterior stabilized total knee prosthesis was placed. Case 2: A 50-year-old male patient underwent a fixed-bearing medial unicompartmental knee replacement. After 6 years, he complained of insidious pain in the posteromedial side of his knee. During the physical examination, significant joint effusion, a loss of extension, and a limited flexion were observed, with flexion reaching only up to 80°. The patient did not report systemic symptoms associated with metallosis, and no signs of systemic involvement were found. Radiographs show similar findings than in case 1, but the radiopaque "cloud" was limited to the knee. In the revision surgery, both components were removed. The tibial and femur components had extensive wear, especially in the posterior-central portion. A posterior stabilized total knee prosthesis was used. Conclusion: Emphasis on careful engagement between the PE and the tibial component is a must, checking that no tissue or cement gets in the way of the anchoring system. Furthermore, prosthesis designs must look for models with a more friendly, engaging system to prevent early presentation. The radiographs assessing the height of the PE - especially in the posterior area - must be carried out. In the case of metallosis and PE luxation, we suggest performing a revision surgery with total knee arthroplasty; however, there is little evidence for a strong recommendation.