In vivo kinematic data indicate that unicondylar or edge loading occurs during normal activities in well-aligned and malpositioned knee replacements. Using a validated explicit finite element model of a knee replacement, the effects of eccentric loading of a total knee replacement are simulated. Only minor variations were observed in the kinematics with a medial offset of the vertical load of as much as 15 mm (representing a medial:lateral loading ratio of 86:14), although the polyethylene stresses did increase by approximately 3 MPa throughout the stance phase of gait. There was a significant change in the kinematics and stresses when unicondylar loading occurred (95:5 medial:lateral loading ratio). Even for the unicondylar load case, contact always was maintained within the lateral compartment. This raises the question whether lift-off often observed in fluoroscopy studies really occurs. The model predicted regions of plastic deformation that closely resemble those observed in retrieved specimens of catastrophic wear. The explicit finite element model offers considerable insight into the kinematics and stresses generated by total knee replacement during different and varied loading conditions that occur during normal usage
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