Fatigue of the bone/cement interface and loosening of total joint replacements

The failure of cemented total joint replacements is often attributed to loosening of the implant in response to fatigue of the interface between the bone and cement. In the present study loosening of model cemented joint replacement specimens was examined under cyclic loading and the fatigue strength of the bone/cement interface was characterized. In addition, the potential for improving the fatigue strength by introducing a surgical bone surface preparation was assessed by texturing the bone to achieve different degrees of cement interdigitation. All specimens were subjected to cyclic shear loads up to 10 million cycles and the extent of loosening at the bone/cement and prosthesis/cement interfaces was quantified using Digital Image Correlation (DIC). Results showed that the displacement history exhibited three distinct regions of response and that fatigue failure occurred as a result of loosening along the bone/cement interface. The apparent fatigue strength of the engineered bone/cement interfaces corresponding to 40 years of function ranged from 0.8 to 4.4 MPa; the fatigue strength increased with the surface roughness of bone and degree of cement interdigitation. Regardless of the bone surface topography the ratio of fatigue strength and ultimate shear strength of the interfaces was approximately 0.2. Results of the evaluation suggest that the fatigue strength of the bone/cement interface in cemented total joint replacements can be estimated from simple quasi-static shear tests.