[Mechanical loading and biomechanical problems of total joint surface replacement of the tibial plateau]

Recent reports of fractures of the tibial component of total knee prostheses have raised the question of their long term endurance. The mechanical load for the tibial plateau consists of the two forces deriving from the condyles of the femur. As the collateral and crucial ligaments always allow a little play for the femoral and tibial components, the location of the load of the condyles on the tibial plateau is either on the medial or lateral side depending on minor disturbances during gait. For a tibial plateau with only little medullary anchorage this load pattern means an alternating induction of compression and tensile stresses for the underlying bone. This alternating stress pattern is discussed as a possible mechanism of aseptic loosening of such tibial components. For two cases of such aseptic loosening and consecutive fracture of the tibial plateau of a metal backed unconstrained sledge knee prosthesis the radiologic findings are demonstrated and correlated to the biomechanical mechanisms described. It is demonstrated that the structural endurance of the tibial plateau does not withstand the mechanical stresses of such a situation of aseptic loosening. In these cases further loosening or fracture of the tibial plateau results.