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We are not aware of any information on in vivo clinical results at a minimum of five years after posterior cruciate-substituting total knee arthroplasties performed with a highly cross-linked polyethylene insert. The purpose of the study was to evaluate whether the clinical and radiographic results of posterior cruciate-substituting total knee prostheses, including the prevalences of fracture of the polyethylene post, failure of the locking mechanism of the tibial polyethylene insert, and osteolysis, would be similar between patients treated with conventional polyethylene and those treated with highly cross-linked polyethylene.Three hundred and eight patients with a mean age of 60.3 years (range, twenty-two to sixty-five years) received a posterior cruciate-substituting total knee prosthesis with a conventional polyethylene tibial insert in one knee and the same prosthesis with a highly cross-linked polyethylene tibial insert in the contralateral knee. Twenty patients were men and 288 were women. The mean duration of follow-up was 5.9 years (range, five to 6.8 years). At each follow-up visit, the patients were assessed radiographically and clinically with the rating system of the Knee Society; the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC); and the University of California, Los Angeles (UCLA) activity score.The two groups did not differ significantly (p > 0.05) with regard to the mean postoperative Knee Society total knee scores (93.5 compared with 93.7 points), function scores (82.9 compared with 82.7 points), and pain scores (46.7 compared with 47.1 points); WOMAC scores (16 compared with 15 points); range of motion (129.7° compared with 130.1°); or patient satisfaction assessed with a visual analog scale (7.6 compared with 7.9 points). The mean UCLA activity score was 6.4 points (p > 0.05). There were no significant differences between the two groups with regard to radiographic results. No knee in either group had a fracture of the tibial polyethylene post or failure of the locking mechanism of the tibial polyethylene insert. No knee in either group had osteolysis. One knee (0.3%), treated with highly cross-linked polyethylene, was revised because of infection.The data suggest that clinical and radiographic findings at five years after posterior cruciate-substituting total knee arthroplasty were the same for the patients treated with highly cross-linked polyethylene and those treated with conventional polyethylene.Therapeutic Level II. See Instructions for Authors for a complete description of levels of evidence.