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Why knees fail.

Successful revision knee arthroplasty requires an accurate understanding of the cause of failure. Because a differential diagnosis is necessary, 9 causes of failure are proposed: i) aseptic loosening with or without osteolysis or progression of arthritis in a unicondylar arthroplasty; ii) tibial femoral instability because of collateral ligament instability; iii) patellar complications and malrotation; iv) no diagnosis, the so-called mystery knee; v) structural failure of the implant; vi) sepsis; vii) extensor mechanism rupture; viii) stiffness; and ix) fracture. A revision surgery should correct shortcomings in the original arthroplasty and not simply restore a mechanical situation that has failed.