Distribution of microcrack lengths in bone in vivo and in vitro.

It is well known that bone contains small cracks; in vivo these microcracks are constantly growing and being repaired. Too rapid crack growth leads to stress fractures or fragility fractures. In vitro, changes occur in this population of microcracks when subjected to cyclic loading up to and including failure. Normally, the only parameters reported from such investigations are the number density of cracks and their average length. In the present work we examined the microcrack population in more detail. We analysed ten different sets of experimental data including in vivo and in vitro microcracks, plus two theoretical simulations. We showed for the first time that the distribution of crack lengths can be described using the two-parameter Weibull equation. The values of the two constants in the equation varied depending on bone type/species and showed consistent trends during in vitro testing. This is the most detailed study to be conducted on microcrack populations in bone; the results will be useful in future studies including the development of theoretical models and computer simulations of bone damage and failure.