Experimental and finite element comparison of various fixation designs in combined loads

The short- and long-term successes of tibial cementless implants depend on the initial fixation stability often provided by posts and screws. In this work, a metallic plate was fixed to a polyurethane block with either two bo ne screws, two smooth-surfaced posts, or two novel smooth-surfaced posts wi th adjustable inclinations. For this last case, inclinations of 0, 1.5, and 3 deg were considered following insertion. A load of 1031 N was eccentrica lly applied on the plate at an angle of similar to 14 deg, which resulted i n a 1000 N axial compressive force and a 250 N shear force. The response wa s measured under static and repetitive loading up to 4000 cycles at 1 Hz. T he measured results demonstrate subsidence under load, lift-off on the unlo aded side, and horizontal translation of the plate specially at the loaded side. Fatigue loading increased the displacements, primarily during the fir st 100 cycles. Comparison of various fixation systems indicated that the pl ate with screw fixation was the stiffest with the least subsidence and lift off. The increase in post inclination from 0 to 3 deg stiffened the plate b y diminishing the liftoff. All fixation systems demonstrated deterioration under repetitive loads. In general, the finite element predictions of the e xperimental fixation systems were in agreement with measurements. The finit e element analyses showed that porous coated posts (modeled with nonlinear interface friction with and without coupling) generated slightly less resis tance to liftoff than smooth-surfaced posts. In the presence of porous coat ed posts, Coulomb friction greatly overestimated the rigidity by reducing t he liftoff and subsidence to levels even smaller than those predicted for t he design with screw fixation. The sequence of combined load application al so influenced the predicted response. Finally, the finite element model inc orporating measured interface friction and pull-out responses can be used f or the analysis of cementless total joint replacement systems during the po st-operation period.