Stability and anchorage considerations for cementless tibial components.

The ability of several idealized tibial anchorage systems to resist distractive relative motion between the substrate and the component when subjected to eccentric loads was assessed using mechanical tests employing a urethane foam as a bone substitute and finite element models. Eccentric loads had to be placed within 30-35% of the edge of the tibial plate for separation to occur between an unanchored plate and the substrate. The finite element model predicted that a 90%/10% medial/lateral load distribution was required to maintain compressive vertical stresses beneath the unanchored plate. An anchorage system with four pegs each 10 mm in diameter and 20 mm long and an anchorage system with four cancellous bone screws each 6.5 mm in diameter were loaded eccentrically, and both systems prevented distraction of greater than 76 microns when mounted on urethane foam. Shorter pegs (10 mm) or a single central peg allowed distractions of 150 microns or greater to occur. These data are relevant to the design and testing of tibial components for total knee arthroplasty.