Bisphosphonate treatment suppresses not only stochastic remodeling but also the targeted repair of microdamage

Two kinds of remodeling, stochastic and targeted, have been proposed based on the observation that microdamage in bone can initiate the remodeling process. Bisphosphonates are known to suppress stochastic bone remodeling. It has been hypothesized that bisphosphonates allow microdamage to accumulate, suggesting that they also suppress targeted remodeling. This study investigated whether suppression of remodeling using bisphosphonates inhibits remodeling targeted to repair microdamage, or whether the suppression of stochastic remodeling alone can account for the observed increase in damage accumulation. Beagle dogs were divided into three groups: control (CNT), risedronate-treated (RIS), and alendronate (ALN)-treated groups. The doses of both bisphosphonates were 6 times higher than the clinical doses. After 1-year treatment, animals were sacrificed and the right 9th rib was assigned to microdamage analysis. There were 3.06 times more associations between cracks and resorption spaces in CNT than expected (P < 0.005), indicating that remodeling normally targets cracks for repair, i.e., cracks can initiate a new remodeling event. However, although there was increased microdamage accumulation in RIS and ALN compared with CNT, fewer cracks than expected were associated with resorption spaces. The observation in RIS and ALN that there were fewer associations between cracks and resorption spaces than expected indicates that both targeted and non-targeted remodeling are suppressed in these groups. These data further suggest that the complete suppression of targeted remodeling could account for the increased microdamage burden.