MECHANICAL FAILURE PROCESSES AND MODELS AT THE BONE-CEMENT INTERFACE.

Total joint replacement has become a highly effective treatment for severe degenerative joint disease, but there is still concern for long term viability. Loosening of the implants from their bony connection remains the major problem of concern. The authors hypothesize that mechanical failure is the major factor in implant failure. To examine failure processes, fatigue tests were conducted with 4 point bending bimaterial specimens under cyclic loading, and the loaded interface region observed and filmed through a light microscope. It was concluded that: (1) tensile cracks often formed at the interface; (2) distinct cracks would often not form, but motion between bone and cement and fractured cement could be seen; (3) cracks could form in pure compression, that would remain open upon unloading; and (4) a second tensile crack would often form in the cement and catastrophic failure would then occur by propagation of this second crack, suggesting that the cracked interface was still carrying tensile loads. On the basis of these observations, the authors propose that mechanical degradation of the bone-cement interface occurs by two processes, crack propagation and 'damage'.