BONE RECOVERY FOLLOWING MICROINDENTATION

Abstract

Bone microhardness has been successfully correlated with important functional parameters such as mineralisation and stiffness. It provides a means of examining the mechanical competence of bone at a micron scale, averaging the effect of osteonal lamellae but sensitive to variation in mineral content within a bone, and, with careful selection of indentation site, able to obtain material characteristics separate from any effects of porosity. However, the effect of bone’s viscoelasticity on such measurements has been largely ignored. This preliminary study investigates the post-indentation size change of Vickers indentations on wet bone. 4 axial slices of bovine femur were harvested from the same shaft, and polished. Each sample was subjected to 4 sets of 10 Vickers indentations with a load of 50 g and holding period of 15 s. The indentation size was measured immediately after the load was removed, and then again at intervals for a period up to 24 hours after the indentation was made. To avoid dehydration, the bone stood in water during the indentation testing and during measurement, and between each measurement period it was fully immersed in water. Measured hardness significantly decreased with time, by approximately 30% in total. The rate of post-indentation recovery is difficult to analyse since the driving force of residual strain decreases as recovery takes place. However a simple exponential fit to the variation of HV with time in the form of H = H(final).(1−exp(−kt)) + H(initial) suggests that the size of the indentation tends towards a constant size between 5 and 24 hours after indentation. Thus we conclude that care should be taken when making “early” measurements given the rapid rate of change in indentation size. Caution should also be employed when interpreting such data.