Targeted bone remodeling involves BMU steering as well as activation

Microdamage removal is an important function of bone remodeling. Experiments have repeatedly shown that remodeling of cortical bone by Basic Multicellular Units (BMUs) is initiated in response to microdamage, and this has become known as “targeted remodeling”. This paper considers the possibility that microdamage is not only able to activate new BMUs, but may also attract or “steer” existing BMUs as they continue to tunnel through the bone matrix. An initial analysis of the relationship of between mean microcrack length and BMU resorption space density in cortical bone indicates that BMUs have an effective area about 40 times greater than their actual cross-section. Interpreting this as evidence that the osteoclasts in a tunneling BMU are able to sense and steer toward microdamage, a model is developed for “BMU steering” based on the hypothesis that osteoclasts are guided not only in the principal stress direction, as proposed by Burger et al. (Burger, E.H., Klein-Nulend, J., Smit, T.H. Strain-derived canalicular fluid flow regulates osteoclast activity in a remodelling osteon–a proposal. J. Biomech 36 (2003) 1453–1459), but also toward microdamage, depending on its proximity.