Approx. 11,000 operations with the insertion of a knee arthroplasty (knee prosthesis) are performed annually DK. The durability and quality of the treatment are assessed with prosthesis survival, that expresses the proportion of prostheses that are still functional after a given number of years (eg the 10-year prosthesis survival for all types of knee prostheses in DK is approximately 94%).~Unicompartmental implants are increasingly used, so that only the worn part of the knee is replaced. Especially for osteoarthritis between the patella and the femur, a patellofemoral prosthesis (PFA - patellofemoral alloplasty) can be inserted, which is much smaller than the traditional full prosthesis (TKA - total knee arthroplasty).~PFA operations are controversial. A recently published Danish study (double-blind RCT) comparing TKA and PFA has shown that PFA patients achieve greater satisfaction, better knee function and greater quality of life than TKA patients. A recent study has also demonstrated that the cost of a PFA procedure is less than that of a TKA. As a paradox to this clear RCT finding, all national implant registers (Sweden, England, New Zealand, Denmark, etc.) show a significantly poorer prosthesis survival for PFA compared with TKA.~It is important for the future treatment of patients with severe osteoarthritis between the patella and femur to understand the cause of the discrepancy between RCT and registry results. The discrepancy gives rise to a number of questions regarding. indications, techniques, competences, postoperative regimens etc.~The divergence between the RCT and registry studies can only be clarified by a study that 1) examines the influence of preperative factors (patient history, physical findings, radiology etc.) on outcome, and that 2) attempts a causal analysis for each reoperation. The investigators intend to do this though a cohort study including all cases of patellofemoral arthroplasty performed in Denmark from January 1, 2008 until December 31, 2015.~The purpose is to determine preoperative risk factors for revision after PFA and to provide a detailed description of indications for revision after PFA.
The purpose of the retrospective cohort study is 1) to determine preoperative risk factors for revision af patellofemoral arthroplasty, and 2) to provide a detailed description of indications for revision after patellofemoral arthroplasty. All patients operated with patellofemoral arthroplasty in Denmark from January 1, 2008 to December 31, 2015, will be included in the cohort.