Tensile fatigue in bone: are cycles-, or time to failure, or both, important?

In life, bones are subjected to fatigue loading which has different frequen cy and amplitude components, as well as various kinds of loading modes like tension, compression, shear and combinations of them. Considerable variabi lity is observed in fatigue results of bone, which may be caused by these e xperimental variables or by the bone itself. In past studies the effect of magnitude and mode of loading have been examined in standard fatigue streng th (stress vs. cycles to failure) diagrams. The effect of frequency is not clear, but there is clear evidence (from Carter & co-workers) that, at leas t in human bone, tension "fatigue" failure was determined solely by time ra ther than by cycles. We sought to confirm these results in the same and a d ifferent species. We cycled human and bovine bone in tension at two frequen cies: 0.5 and 5 Hz. There was no cycle number effect; the results from the tests at the two frequencies were different if plotted and analysed as a fu nction of cycles to failure, but were not separable if plotted and analysed as a function of time to failure. In this respect bone differs from tendon , in which failure in tension is a function of both cycles and time. (C) 20 01 Academic Press.