Fatigue of cemented hip replacements under torsional loads

Fatigue resistance of hip replacement prostheses is becoming ever more impo rtant as the operation is carried out on younger and more active patients. Torsional loading of the implant, which occurs especially during activities , such as rising from a chair or climbing stairs, is implicated in the fail ure process, To examine fatigue failure of the implant-bone fixation, which is made using polymethylmethacrylate cement, an experimental model was des igned and 16 specimens tested at torsional moments of 50 Nm and 80 Nm to bo th 1 and 2 million cycles. The numbers and lengths of cracks initiated unde r cyclic loading were quantified using dye penetrant to highlight the crack s and a profile projector to magnify them. The majority of cracks initiated from the PMMA/metal and PMMA/bone interfaces, more often than from pores i n the PMMA. A bimaterial fracture mechanics analysis confirmed that the int erfaces are too weak to sustain in vivo levels of cyclic loading. It is pro posed that, under torsional loading, fatigue failure of PMMA fixated implan ts originates from pores located on the interfaces.