INITIAL IN-VITRO STABILITY OF THE TIBIAL COMPONENT IN A CANINE MODEL OF CEMENTLESS TOTAL KNEE REPLACEMENT

The tibial component of a canine cementless total knee replacement mod el was used to determine the degree to which pegs and screws contribut ed to the initial in vitro stability of the device. Three implant desi gns were investigated: (1) a four-peg implant in which cortical bone s crews passed through the pegs, (2) the four-peg implant without adjuva nt screw fixation, and (3) a flat implant with screws placed in the sa me positions as in the first design. For measuring the interface motio n, the tibial component and proximal tibia were modeled as rigid bodie s and an experimental method was developed which permitted all six deg rees of freedom of the motion between these two objects to be determin ed. In tests performed to validate this methodological approach, the p otential confounding influences of tibial deformation and differential amounts of tibial deformation with the use of screws or pegs were sho wn to be minimal, supporting the use of the rigid-body method. In gene ral, the areas of greatest motion were at the periphery of the bone-im plant interface, regardless of whether or not screws or pegs were used . The components secured with screws had up to five-fold reductions in interface motion compared to components which had pegs but lacked scr ew fixation. Components with pegs and screws and components with screw s only had the same amount of interface motion. Thus, in the presence of screw fixation, the addition of pegs did not increase the stability of the tibial component.