Microdamage accumulation in the cement layer of hip replacements under flexural loading

Mechanical fatigue of bone cement leading to damage accumulation is implica ted in the loosening of cemented hip components. Even though cracks have be en identified in autopsy-retrieved mantles, damage accumulation by continuo us growth and increase in number of microcracks has not yet been demonstrat ed experimentally. To determine just how damage accumulation occurs in the cement layer of a hip replacement, a physical model of the joint was used i n an experimental study. The model regenerates the stress pattern found in the cement layers whilst at the same time allowing visualisation of microcr ack initiation and growth, In this way the gradual process of damage accumu lation can be determined. Six specimens were tested to 5 million cycles and a total of 1373 cracks were observed. It was found that, under the flexura l loading allowed by the model, the majority of cracks come from pores in t he bulk cement and not from the interfaces. Furthermore, the lateral and me dial sides have statistically different damage accumulation behaviours, and pre-load cracks significantly accelerate the damage accumulation process. The experimental results confirm that damage accumulation commences early o n in the loading history and that it is continuously increasing with load i n the form of crack initiation and crack propagation. The results highlight the importance of replicating the loading and restraint conditions of clin ical cement mantles when endeavouring to accurately model the damage accumu lation process. (C) 1999 Elsevier Science Ltd, All rights reserved.