On the histologic measurement of osteonal BMU activation frequency

When cortical bone is remodeled by osteonal basic multicellular units (BMUs), activation frequency is defined as the number of new BMUs created per unit volume of cortex per unit time. When osteonal remodeling is histomorphometrically analyzed, a different definition of activation frequency is usually used; it is equivalent to the number of previously activated BMUs reaching the plane of the section per unit area per unit time. Using simple geometric analysis, it is shown that the latter histologic measurement is equal to the former definition multiplied by the mean distance that a BMU tunnels before it ceases its activity.