Fatigue of cemented hip replacements under torsional loads

Fatigue resistance of hip replacement prostheses is becoming ever more important 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 failure process. To examine fatigue failure of the implant–bone fixation, which is made using polymethylmethacrylate cement, an experimental model was designed and 16 specimens tested at torsional moments of 50 Nm and 80 Nm to both 1 and 2 million cycles. The numbers and lengths of cracks initiated under cyclic loading were quantified using dye penetrant to highlight the cracks 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 in the PMMA. A bimaterial fracture mechanics analysis confirmed that the interfaces are too weak to sustain in vivo levels of cyclic loading. It is proposed that, under torsional loading, fatigue failure of PMMA fixated implants originates from pores located on the interfaces.