Failure Modes

Abstract

This chapter discusses the failure of orthopaedic devices, the causes of such failure and methods of predicting various modes of failure. The primary modes of failure are mechanical, chemical and the adverse biological response to implants. Orthopaedic devices may fracture or deform under loads applied in their use. A prediction of the ability of a device to withstand expected loading in the hostile envioronment in which they are used may be developed from a knowledge of the mechanical and corrosive properties of the materials used and a knowledge of the loading to which they are subjected. Knowing these, stress analysis may be applied, usually the ,,Finite Element“ method, to determine the stresses and strains throughout the body of the device being evaluated. These stresses and strains are then compared to ,,safe“ values by varous criteria. Wear is the primary mechanical failure mode either by wear through of the part or by the adverse biological response to the products of wear. The various forms of wear are discuseed with the means for minimizing such wear. The dominant mode of wear type failure in incongruent knee repacement bearings is actually fatigue of the articular surface due to excessive contact stresses. Stress analysis of typical incongruent (fixed) and congruent (mobile) knee replacment bearings indicates that the fixed bearing knee bearings analysed are grossly overstressed. Only the mobile bearing device that was analysed has acceptable contact stress values. Mechanical testing confirmed this analysis showing much higher wear for the fixed bearing devices with wear increasing with increasing stress.