Mixed-mode failure strength of implant-cement interface specimens with varying surface roughness.

Aseptic loosening at the implant-cement interface is a well-documented cause of failure in joint arthroplasty. Traditionally, the strength of the implant-cement interface is determined using uni-axial normal and shear loading tests. However, during functional loading, the implant fixation sites are loaded under more complex stress conditions. For this purpose, the strength of the implant-cement interface under mixed-mode tensile and shear loading conditions was determined in this study using interface specimens with varying interface roughness. For the lowest roughness value analyzed (R(a)=0.89 μm), the interface strength was 0.40-1.95 MPa at loading angles varying between pure tension and shear, whereas this was 4.90-9.90 MPa for the highest roughness value (R(a)=2.76 μm). The interface strength during pure shear (1.95-9.90 MPa) was substantially higher than during pure tension (0.58-6.67 MPa). Polynomial regression was used to fit a second-order interpolation function through the experimental interface strength data (R²=0.85; p<0.001), relating the interface strength (S [MPa]) to the interface loading angle (α [degrees]) and interface roughness (R(a) [μm]): S(α,R(a))=0.891R²(a)+0.001α²-0.189R(a)-0.064α-0.060. Finally, an interface failure criterion was derived from the interface strength measurements, describing the risk of failure at the implant-cement interface when subjected to a certain tensile and shear stress using only the interface strength in pure tensile and shear direction. The findings presented in this paper can be used in numerical models to simulate loosening at the implant-cement interface.