INTERGRANULAR CORROSION-FATIGUE FAILURE OF COBALT-ALLOY FEMORAL STEMS- A FAILURE ANALYSIS OF 2 IMPLANTS

Two modular hip implants with a cobalt-alloy head and a cobalt-alloy s tem were retrieved after a fracture had occurred in the neck region of the femoral component, eighty-five and seventy months after implantat ion. Both implants failed less than one millimeter distal to the taper junction between the head and the stem (outside of the taper). The-fr acture surfaces of the implant were investigated with the use of scann ing electron microscopy, to determine the nature of the failure proces s. The fractures occurred at the grain boundaries of the microstructur e and appeared to be the result of three factors: porosity at the grai n boundaries; intergranular corrosive attack, initiated both at the he ad-neck taper and at the free surface; and cyclic fatigue-loading of t he stem. The corrosive attack of the free surface was initiated, in pa rt, by the egression of surface grains and by the ingression of fluid into the intergranular regions. Sectioned surfaces showed extensive in tergranular corrosive attack in the prosthetic neck localized in the r egion of the head-neck taper junction and penetrating deeply into the microstructure.