Posterior-stabilized (PS) prostheses have been used extensively in total knee arthroplasty (TKA), with excellent long-term results. The key feature of these prostheses is the femoral cam and tibial post mechanism that limits posterior displacement and produces femoral rollback. Although articular-surface polyethylene wear of the tibial component has not been a significant clinical problem, tibial post wear has been reported. In distinction to chronic post wear, little information exists about catastrophic post failure. We present the case of a 56-year-old woman who presented 63 months after TKA with a PS prosthesis with acute fracture of the polyethylene post. The evaluation and treatment of this patient, including the previously unreported use of computed tomography arthrography to diagnose this rare problem, is reviewed.