Measurement of non-linear microcrack accumulation rates in polymethylmethacrylate bone cement under cyclic loading

Damage accumulation in the cement mantle used to fixate bone prosthesis is one failure scenario for joint reconstruction. It can be described as the phenomenon of numerous microcracks initiating and propagating within the material. Microcracks grow in the cement mantle causing it to gradually lose its mechanical integrity, leading to loosening of the prosthesis. In this study microcracking within acrylic bone cement was quantified over the course of a fatigue test. Identification of new cracks and the growth of pre-existing cracks was monitored at intervals during fatigue testing of five specimens at a mean cyclic stress of 7.5 MPa. Given these measurements, an average damage evolution curve was derived for acrylic bone cement. It was observed that the initiation sites for microcracks were the pore perimeters; therefore, the number of microcracks present in a sample is dependent on porosity. Variability was found within the results and the majority of the variability was accounted for by the difference in the porosity of each sample. Results have identified non-linear damage evolution. On a simplified level a power law equation can be used to describe the damage evolution process.