Mixed-mode failure response of the cement–bone interface

Mechanical failure of the cement—bone interface can contribute to clinical loosening of cemented total hip replacements. The conditions that cause loosening are poorly understood, in part, due to a lack of information on the mechanical behavior of the cement—bone interface. The purpose of this study was to determine the mechanical behavior of the cement—bone interface due to mixed-mode (combined tension and shear) loading and to develop a failure model for the cement—bone interface. Laboratory tests of machined cement—bone test specimens were performed with mixed-mode loading conditions (loading angles of 22.5°, 45°, and 67.5°) to determine the mechanical response in the pre-yield and post-yield state. After accounting for the quantity of interdigitated bone as a covariate, the mixed-mode data were combined with previous tension (0°) and shear data (90°) to develop a failure model for the cement—bone interface. The strength of the interface was positively correlated with the quantity of interdigitated bone (r² = 0.70, 0.53, 0.49, for 22.5°, 45°, and 67.5°, respectively). There was a significant increase in failure strength (P < 0.001) with increasing mixed-mode angle. When all data were incorporated into an elliptical failure criterion, the average error between the actual and predicted strength was 33%. These results can now be incorporated into constitutive models of the cement—bone interface to determine the initiation and progression of interface failure in cemented total hip replacements.