THE EFFECT OF SCREW FIXATION ON INITIAL RIGIDITY OF TIBIAL KNEE COMPONENTS

Initial rigid fixation is essential for bone ingrowth into a porous-coated prosthesis. In this study, the amount of micromotion between a cut tibial surface and a custom tibial baseplate mounted with various types of screw fixation was measured. Motion transducers were placed at the corners of the baseplate to detect liftoff and subsidence. Central and eccentric loads were then applied. The variables tested included screw number and position, as well as screw length and type of bone purchase. A four-screw peripheral configuration had less micromotion than a three-screw triangulated configuration, and both had less micromotion than a two-screw central condyle configuration. Long screws with cancellous purchase and screws with cortical purchase had less micromotion than short screws with cancellous purchase. These results indicate that important variables which contribute to the initial rigidity of a cementless tibial implant using screw fixation include screw number, configuration, length, and type of bone purchase.