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

Abstract

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.

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

“…Specimens were tested in stress control and at 3Hz under a flow of saline at 37 °C, to model the physiological environment and control the specimen temperature. Testing used a servo hydraulic testing machine (MTS -858 Mini Bionix ® II) and subjected to either tension-tension loading (max stress = 20 MPa; R = 0.1) as reported in Sheafi and Tanner [36] and used by other authors including Harper et al [12] and Tanner et al [38] or fully reversed tension-compression [37]. While 2-20 MPa is above physiological stress levels, it typically leads to fatigue in less than 100,000 load cycles, allowing specimens to be tested in a sensible time [12,38].…”

“…In two of our previous [35,36] the differences in fatigue lives were attributed to variations in specimen production method and resultant surface morphology, leading to changes in the stress concentration factors and also considered the role of cement composition with limited discussion of crack propagation mechanisms. In our subsequent study [37], the specimen variables were examined further in a range of stress amplitudes, reporting a particular influence of the stress amplitude used in describing the fatigue longevity of different cements, without considering in depth the crack initiation and growth mechanisms. In the current study, the changes in absorbed energy per loading cycle are compared for different specimen types and the results have shown interesting findings regarding the effect of specimen type which are also controlled by cement composition.…”

“…Therefore, there are many reports in the literature on in vitro determination of fatigue properties of bone cement, with the results typically presented as number of cycles to fracture (Nf) at specified applied fatigue stress level(s) usually in the form of S-N or Wöhler curves. These results are influenced by many factors [22], such as composition of the cement [12,23,30], specimen configuration [20,21,35,36], type of applied stress [35,36], and specimen surface finish [37]. Additionally, there have been a number of literature reports on the in vitro determination of fatigue crack propagation (FCP) behaviour of bone cement [4,8,14,24,28,29,40].…”