Purpose: The hypothesis of this study is that all-polyethylene (APE) tibial impalnts offer a biomechanical profile similar to metal backed tray (MBT). There are significant financial cost saving implications, in selected patient groups, if APE performs as well MBT. Methods: Using a finite element analysis (FEA) of CAD models provided by DePuy (Leeds), stress distributions were investigated for both an APE and MBT tibial implant. The performance was assessed for cancellous bone at 700MPa (normal) and at 350MPa (less stiff). Plots were recorded along the length of the tibia, showing the loads carried by the bone (cortical and cancellous), the implant interface, cement interface and the stem. Von Mises stress distributions and percentage volumes were used to assess bone resorption and hence potential for failure (fracture). Results: Higher stress shielding (resorption) occurred around the keel of the tray in the MBT. The stiffer MBT tray carried a higher proportion of the load down the stem. MBT stress in cancellous bone is lower than APE, as load is distributed to the cortical rim. APE had no areas of bone resorption (being more flexible resulting in less stress shielding). Conclusions: Higher stresses are produced and more load is transferred to cancellous bone in APE than MBT. MBT reveals greater potential for bone loss around the stem and keel. APE has a marginally favourable strain state in cancellous bone and spreads loads more at the cement interface than MBT. Both exhibit peak cement stresses around the cortical rim, but not to the point that cement failure occurs.
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