Fatigue failure in the cement mantle of an artificial hip joint

The cement mantle of an artificial hip joint was retrieved, largely intact, during a revision operation, and subjected to detailed failure analysis. The results reveal a number of features which are important to our understanding of the failure of bone cement in situ and its consequences for prosthesis loosening. Microscopic examination showed clear evidence of fatigue cracking in the mantle prior to removal. This took the form of worn areas in certain regions of the fracture surfaces, which elsewhere showed evidence of rapid, brittle fracture. The mantle contained two large defects which had been introduced during insertion; fatigue was shown to have originated both from these defects and from the proximal surface. Results from a finite element analysis were used, together with the techniques of fracture mechanics, in an attempt to explain the magnitude and direction of fatigue cracking. Fracture mechanics calculations, though subject to some uncertainty in this case, indicate that the local stress intensity in the region of the principal defect would have been sufficient to exceed the threshold for fatigue crack propagation in this material. This approach demonstrates the advantages of this 'defect-tolerance' analysis.