Effect of test specimen cross-sectional shape on the in vitro fatigue life of acrylic bone cement

Constant-amplitude uniaxial tension-compression fatigue tests were conducted on specimens fabricated from 12 sets of acrylic bone cements, covering cement formulations with three different viscosities (so-called "high-", "medium-" and "low-viscosity" varieties), two different methods of mixing the cement constituents (so-called "hand-" and "vacuum-mixed" methods) and two test specimen shapes (rectangular-cross-sectioned or "flat" and circular-cross-sectioned or "round"). The test results-namely, the number of fatigue stress cycles, N(f)-were analyzed using the linearized transformation of the three-parameter Weibull relationship, allowing the values of the Weibull mean, N(WM), to be determined for each set. Values ranged from 14,300 to 1,284,331 for the round specimen sets and from 2898 to 72,960 for the flat specimen sets. Statistical analysis of the ln N(f) data, together with an examination of the N(WM) values, showed that, for any combination of cement formulation and mixing method, round specimens had significantly longer fatigue lives compared to flat ones. These results are explained in terms of two factors. The first is the smaller surface area of the waisted zone in the round specimens compared to that in the flat specimens (nominal value of 157mm(2) versus nominal value of 185mm(2)), leading to the possibility of fewer crack initiation sites on the round specimens compared to the flat ones. Secondly, it is postulated that the crystallinity of the round specimens was higher than that of the flat ones, a consequence of the significantly lower measured residual liquid monomer contents of the former compared to the latter (3.40+/-1.28wt%/wt compared to 3.81+/-1.48wt%/wt). The significance of the present finding is that it indicates that, for a set of bone cement formulation and experimental conditions, discriminating fatigue test results are more likely to be obtained if flat, rather than round, test specimens are used.