Influence of test specimen fabrication method and cross-section configuration on tension–tension fatigue life of PMMA bone cement

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

“…The analysis of fatigue results using S-N curves has shown that specimen shape and surface production method (specimen type) can significantly alter the measured fatigue life of bone cement, supporting the trends of our previous findings using tension-tension and tension-compression both at a single stress amplitude and using the Weibull functions for data analysis [7,9]. The effect of specimen shape on fatigue behaviour can be influenced by the material, thus two different cement formulations have been investigated.…”

“…According to Euler buckling calculations, if entire length between the grips was at the gauge cross section, compression stress levels of at least 31 or 35 MPa are required to produce buckling for the rectangular and circular specimens, respectively, however, the presence of the sample shoulders lead to a substantially higher Euler buckling load. The number of cycles to failure were recorded, with run-out set at 5 million load cycles [7][8][9]. The specimens that were found after testing to have pores with a major diameter of 1mm or greater in the gauge section were excluded and replaced [22,23].…”

“…If the linearity of the S-N curve, as assumed by authors such as Murphy and Prendergast [12] who examined a range of stress amplitudes between 13 and 25 MPa using zero-to-tension loading at 10 Hz, can continue to lower amplitudes than those examined in the current study (say down to 7 MPa that represents the average stress encountered in vivo), then the in vitro fatigue life can be estimated at these amplitudes, which will be rather difficult to measure experimentally by direct testing due to being time consuming when using as low test frequency as 2-5 Hz and typically at 10MPa 5million load cycle runouts occur [8]. It is to be considered also, the mode of stress (tensioncompression or tension-only) as demonstrated earlier [9] is important in identifying the highest stress amplitudes that are appropriate to include in testing and establishing more accurate S-N curves to predict the fatigue life at the in vivo stress levels.…”

“…A review paper by Lewis [4] examined the "intrinsic" and "extrinsic" factors that can affect the measured fatigue performance of bone cement, concluded that there are "only a few areas of agreement" and "many areas of disagreement". The effects of specimen preparation variables (cross section shape and surface production technique) on fatigue behaviour at a specific constant-amplitude stress have been examined in previous studies considering the influence of specimen shape in tension-compression only [5], surface preparation method in tension-tension only [6] or the effects of both configuration and fabrication methods in tension-compression [7,8] and tension-tension loading [8,9]. Due to testing at only one stress amplitude, the fatigue data has been analysed using Weibull relationships in most of these studies [5,[7][8][9] and only one study [8] has compared two testing methods: tension-only of rectangular moulded specimens at a single stress level versus tension-compression at multiple stress levels of circular machined specimens analysing the results using Weibull and Wöhler approaches, respectively.…”

“…However, as both machined and moulded surfaces and rectangular and circular cross-sections have been used it is important to examine the influence of specimen preparation method on more than one cement and one stress level [7,9]. This requires the comparison of the fatigue performance of different specimen types at various stress amplitudes and examining whether testing specimen variables, along with changing the stress amplitude, can affect the fatigue behaviour of different specimens and cements and their relative ranking.…”