Effects of tibial components on load transfer in the upper tibia.

Knee specimens were loaded in combinations of compression, anteroposterior force, rotational torque, and varus-valgus moment. The strains on the outer surface of the tibia at four levels were determined from strain gauges. Tibial components of the central peg type were cemented in place and the strains again measured for the same loading combinations, thus obtaining comparisons with the normal. Strain readings were also taken around the central pegs. The variables of the tibial components were all-plastic versus a metal tray; short, medium, and long post; and complete versus incomplete upper surface coverage. For varus-valgus or anteroposterior loading, there were high strains due to bending around the proximal part of the plastic stem compared with the metal stem; the metal stem carried a much higher bending moment than did the plastic stem. In direct loading, there was a rim of low-stressed bone around the proximal perimeter for incomplete contact, and a metal stem longer than the standard, transmitted some load directly to the cortex; a plastic stem carried only small axial loads, probably because of its low axial stiffness. Metal backing is not without disadvantages; notably, some load by-passing, cost, and comparatively difficult removal.