Relationship between fatigue parameters and fatigue crack growth in PMMA bone cement

Poly(methyl methacrylate) (PMMA) bone cement is used to anchor the majority of total joint replacements (TJRs). Many brands of cement are used, both with and without the addition of antibiotics to reduce the risk of infection. The present study involved determination of various parameters in tensile fatigue loading: 1) energy absorbed (U) vs number of loading cycles (N) and creep strain (ε) vs N, during fatigue tests on specimens of an antibiotic-containing cement (SmartSet GHV) and a plain cement (CMW1) and 2) crack length (a) vs fatigue loading cycles (N) and crack growth rate (da/dN) vs Mode I stress intensity factor range (ΔKI), during Fatigue Crack Propagation (FCP) tests. In the fatigue tests, four different sample types (round, machined; round, directly moulded; rectangular, machined, and rectangular, directly moulded) and tension-tension loading were used. In the FCP tests, compact tension specimens under tension-tension loading were used. It was found that there were limited effects of sample type, except at the highest stress levels, but that these two cements had different rates of crack propagation. These differences were reflected in the fracture surfaces with SmartSet GHV showing accumulation of opacifier around the particles and crack progression around the intial beads, while for CMW1 the opacifier was evenly distributed and the cracks went through the initial beads

Effects of specimen variables and stress amplitude on the S-N analysis of two PMMA based bone cements

The fatigue performance of bone cement is influenced by the testing parameters. In previous in vitro fatigue studies, different testing conditions have been used leading to inconsistencies in the findings between the studies, and consequent uncertainties about the effects of testing specimen specifications and stress parameters. This study evaluates the role of specimen variables (namely; specimen cross-section shape, surface production method and cement composition) in a range of in vitro stress amplitudes (±12.5, ±15, ±20, ±30 MPa), using S-N (Wöhler) analysis. The two main findings are: while specimen cross-section configuration and fabrication method (specimen type) played a key role in controlling the fatigue longevity of the same cement, the stress amplitude was seen as the dominant controlling variable to affect the fatigue behaviour of different cements when using the same specimen type. Thus, considering the effect of specimen type, testing at high stress amplitudes should be treated with caution, particularly in tension-compression loading, to ensure fatigue failure occurs due to mechanical rather than thermal effects and thus models the in vivo behaviour