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We observed a complication of posterior stabilized total knee arthroplasty involving hypertrophy of tissue proximal to the patella associated with pain during active knee extension from 90° of flexion. The purpose of this paper was to describe synovial entrapment and to determine if design features of the prosthesis predispose patients to the complication.Between April 1990 and June 1999, we performed 459 consecutive posterior stabilized primary total knee arthroplasties using three prosthetic designs with different femoral intercondylar geometries. We identified twenty-six patients (twenty-seven knees) in whom arthroscopic débridement of the knee or open arthrotomy with débridement of the knee had been subsequently performed because of a diagnosis of synovial entrapment. We reviewed the records of these patients to identify the knee components that had been used and the symptoms and conditions that necessitated additional treatment.Symptoms (grating, crepitation, and pain with active knee extension from 90°) necessitating subsequent débridement occurred in 13.5% (nineteen) of 141 knees treated with the Anatomic Modular Knee-Congruency implant, 3.8% (eight) of 212 treated with the Anatomic Modular Knee-Posterior Stabilized implant, and none of the 106 treated with the Press Fit Condylar Sigma-Posterior Stabilized implant. All patients had difficulty rising from a chair and climbing stairs; however, none had symptoms when standing or walking. No patient had a patellar clunk. The symptoms occurred at a mean of seven months after the arthroplasty in the patients with an Anatomic Modular Knee-Congruency implant and at a mean of twenty months after the arthroplasty in those with an Anatomic Modular Knee-Posterior Stabilized implant. Débridement of the frond-like hypertrophic synovial tissue at the distal aspect of the quadriceps tendon alleviated symptoms in all patients. No nodules were identified during the arthroscopy.Synovial entrapment is characterized by hypertrophic synovial tissue at the superior pole of the patella. Use of a posterior stabilized femoral component with a proximally positioned or wide femoral box is more likely to result in this complication.