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

Mechanical fatigue of bone cement leading to damage accumulation is implicated in the loosening of cemented hip components. Even though cracks have been identified in autopsy-retrieved mantles, damage accumulation by continuous growth and increase in number of microcracks has not yet been demonstrated experimentally. To determine just how damage accumulation occurs in the cement layer of a hip replacement, a physical model of the joint was used in an experimental study. The model regenerates the stress pattern found in the cement layers whilst at the same time allowing visualisation of microcrack initiation and growth. In this way the gradual process of damage accumulation 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 flexural loading allowed by the model, the majority of cracks come from pores in the bulk cement and not from the interfaces. Furthermore, the lateral and medial 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 on in the loading history and that it is continuously increasing with load in the form of crack initiation and crack propagation. The results highlight the importance of replicating the loading and restraint conditions of clinical cement mantles when endeavouring to accurately model the damage accumulation process.