From 1981 through 1991, 3,032 primary total knee arthroplasties were performed using the Insall-Burstein Posterior Stabilized Condylar Prosthesis (IB-I, IB-II, and IB-II modified) (Zimmer, Warsaw, IN). Fifteen posterior dislocations occurred: 4 with the IB-I system occurring 2 or more years after surgery, 10 with the IB-II system (8 occurring 6 months after surgery and 2 occurring 2-3 years after surgery), and 1 with the IB-II modified system occurring 9 months after surgery. Statistically significant differences for the rate of dislocation between both the IB-I and IB-II modified arthroplasties versus the IB-II arthroplasties were found (P < .001). In an attempt to identify a cause for these dislocations, the authors retrospectively assessed the 15 dislocated cases with respect to sex, age, weight, height, preoperative and postoperative Hospital for Special Surgery scores, preoperative and postoperative alignment, preoperative versus postoperative reconstruction dimensions, patellar thickness and height, and postoperative flexion and compared the results with those patients who did not experience dislocation. Possible etiologies and mechanisms of dislocation were sought. There were no significant differences between the control and study groups for any variable assessed, with the exception of postoperative flexion, which averaged 118 degrees for the study group and 105 degrees for the control group (P < .001). Conservative management was successful in 11 cases. In September 1988 the IB-II system was introduced; modification of the tibial insert was made in January 1990.(ABSTRACT TRUNCATED AT 250 WORDS)

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