The significance of transient changes in trabecular bone remodeling activation

After menopause, bone turnover is increased due to an increase in the activation of bone remodeling basic multicellular units (BMUs). The importance of changes in BMU activation to bone histomorphometry data and bone volume has not received adequate attention by many skeletal researchers. Therefore, the influence of BMU activation on the above parameters was modeled by way of computer simulation, and the results were compared to data from published post-menopausal bone loss studies. Using control theory concepts, the increase in BMU activation after menopause was modeled as a zero-, first-, or underdamped (oscillatory) second-order transient BMU activation response to the step input: the decline in estrogen. A computer simulation was developed to model the influence of these three transient BMU activation responses on quantitative histological surface parameters and bone volume. The transient BMU activation responses doubled the number of active BMUs. All three types of transient BMU activation responses produced a rapid 5% decline in bone volume due to increased remodeling space. Oscillations in bone volume and histologic surface parameters over time, similar in nature to those seen in studies of ovariectomized animals, were predicted by the simulation for the oscillatory activation response examined, underdamped second order. An oscillatory BMU activation response may explain some of the transient events during menopause. The increased coherence of BMUs created by such a response may increase the likelihood of trabecular perforations. the inherent nature of an oscillatory activation response may cause its detection to be overlooked and bone remodeling data to be misinterpreted.(ABSTRACT TRUNCATED AT 250 WORDS)