The mechanical stability of various noncemented tibial components.

Four different porous-coated tibial prosthetic implants were tested for their mechanical stability following implantation into paired cadaver tibias: Porous Coated Anatomic (PCA), Miller-Galante, Whiteside, and Anatomic Modular Knee (AMK). Following implantation the test sample was loaded eccentrically over the medial tibial plateau at 40 cycles per minute for 300,000 cycles, using an MTS machine. This represents the number of steps taken by the average person in a six- to 12-month period. Load varied sinusoidally from 5 to 115 kg. Subsidence and lift-off of the tibial plate from its bony bed was recorded for each implant, using linear variable differential transformers. The greatest degree of mechanical stability was observed with the AMK design, which is secured to the bone bed using four peripherally placed 6.5-mm cancellous screws. No motion in excess of 100 micron was observed. The central-stemmed Whiteside design and the cortical screw fixated Miller-Galante designs provided slightly less stability, each demonstrating approximately 200 micron of lift-off on the unweighted side. The PCA design exhibited the greatest amount of micromotion with subsidence and lift-off displacements of 500 micron (0.5 mm).