Effect of test frequency on the in vitro fatigue life of acrylic bone cement
Introduction
Almost invariably, during most normal daily activities, the acrylic bone cement mantle in a cemented arthroplasty is subjected to irregular load–time histories. Furthermore, examination of cement mantles retrieved post-mortem from patients with cemented hip arthroplasties revealed fatigue cracks, even in those prostheses that were deemed “pain-free” [1]. Given these facts, it is not surprising that, in the in vitro evaluation of a bone cement material, determination of its fatigue performance is recognized to be a cornerstone. Over the years, research and development efforts in this field reflect this situation, spawning, in the process, a huge volume of literature [2], [3], [4], [5], [6], [7] that has one key characteristic relevant to the present study. This is that in the myriad reports on the effect of an assortment of variables on the fatigue performance of a large number of cement formulations, certain variables have been extensively investigated—notably, mixing method [3], [8] and reinforcing fillers [9]—but others have attracted very little systematic study. Among the latter variables is test frequency, f. To date, only two reports have appeared on this topic [10], [11], both of them providing limited results. Information on the effect of f on a cement's fatigue performance is important from two perspectives. First, from a practical standpoint, knowledge of this effect would help in selecting the value of f to be used in tests aimed at expeditiously screening candidate cement formulations. Second, from a fundamentals standpoint, the impact of f on a cement's fatigue performance could provide insight into the fatigue mechanisms that are operational in the material.
The objective of the present study was to test the hypothesis that f does not have a statistically significant effect on the in vitro fatigue life of an acrylic bone cement. For this purpose, fatigue tests were conducted, in uniaxial constant-amplitude fully reversed tension-compression loading, on 12 sets of specimens fabricated from three commercially available cements (representing three different values of viscosities), two different mixing methods, and two values of f. All these features were carefully selected to reflect the spectrum of those that are relevant to cemented arthroplasties. The cements used were Orthoset®1 (manufactured for Wright Medical Technology, Inc., Arlington, TN, USA) [OS1], Surgical Simplex®P (Stryker Howmedica Osteonics, Rutherford, NJ, USA) [SSP], and Orthoset®3 (Wright Medical) [OS3], which are classified as “high-” [12], “medium-” [13] and “low-viscosity” [12] formulations, respectively. All of these types of cements, as well as the mixing methods used—hand mixing and vacuum mixing—are employed clinically [14], [15]. One of the two values of f used——is within the range of typical gait cycle frequencies [16]. It is realized that the other f value used——is unlikely to be seen in vivo, but it was selected because, it was argued, an investigation of frequency effect is best carried out over at least a decade of increase of f.
Section snippets
Materials and methods
All cement constituents were stored at ambient laboratory conditions (temperature: 21±1°C; relative humidity: 66.5±2.5%) prior to being mixed. A polymeric spatula was used to mix the cement constituents in a mixing bowl, either under ambient pressure (hand-mixing method) or in a commercially available vacuum chamber (Summit Medical Vacuum Mixing Bowl 550; Summit Medical Ltd., Bourton-on-the-Water, Gloucestershire, UK), with the evacuation pressure being (vacuum-mixing method), at
Results and discussion
All the Nf data are presented in Table 1 and Fig. 2. Sample calculations for the estimation of the Weibull parameters for one set of these Nf results, to yield No, Na, b, and NWM, are presented in Table 2, Fig. 3, Fig. 4. The values of all the Weibull parameters for all 12 test sets are presented in Table 3 and Fig. 5.
Two trends from these results are recognized but are remarked upon only briefly because they are outside the ambit of the study's stated objective. First, regardless of the
Conclusion
The main conclusion of this study is that the results obtained provide support for the working hypothesis, that is, test frequency (over the range used) does not exert a statistically significant effect on the fatigue life of the cements tested.
Acknowledgements
The authors are very grateful to Wright Medical Technology for providing supplies of the Orthoset®1 and Orthoset®3 cements and performance of some of the fatigue tests, and to Mr. Andy Hardison, Department of Biomedical Engineering, The University of Memphis, for performing the particle size analysis of the cement powders.
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2015, Journal of the Mechanical Behavior of Biomedical MaterialsCitation Excerpt :Harper and Bonfield (2000), for example, applied tension–tension fatigue to examine the fatigue properties of various bone cements and reported that “the cements that perform best clinically gave the highest results”. Lewis et al. (2003), in contrast, applied fully reversed tension–compression, considering this mode to provide a better model of a material׳s fatigue behaviour according to Dowling (2007). The selection of a particular stress test, however, depends on the application of the bone cement.
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