Fatigue of the cement/bone interface: the surface texture of bone and loosening.

Loosening is recognized as one of the primary sources of total hip replacement (THR) failure. In this study the influence of the bone surface texture on loosening of the cement/bone interface was studied. Model cemented hip replacements were prepared and subjected to cyclic loads that induced pure shear fatigue of the cement/bone interface. The femoral canals were textured with the use of specific cutting tools to achieve a desired surface topography. Loosening of the implant with cyclic loading was characterized in terms of the initial migration (Region I), steady-state loosening (Region II), and unstable loosening (Region III). Results from the experiments showed that the initial migration and rate of steady-state loosening were dependent upon the bone surface topography. The apparent fatigue strength ranged from 0.8 to 5.1 MPa, and denotes the cyclic shear stress required for loosening of 1 mm within 10 million cycles. Regardless of the bone surface topography the ratio of apparent fatigue strength and ultimate shear strength of the interfaces was approximately 0.24. In general, the apparent fatigue strength increased proportional to the average surface roughness of the femoral canal and the corresponding volume available for cement interdigitation. In addition, there was a strong correlation between the normalized initial migration and the apparent fatigue strength (i.e., specimens with the highest initial migration exhibited the lowest fatigue strength).