The Amount of Periosteal Expansion Required to Maintain Bone Strength During Aging Increases Over Time and Depends on the Set of Acquired Adult Bone Traits

INTRODUCTION: One goal of reducing fracture risk is to maintain bone stiffness and strength with aging despite a net loss in bone mass. It is well recognized that age-related bone loss is often accompanied by continued periosteal apposition [1]. Because variation in the amount of periosteal expansion may be a major contributor to fracture risk [2], having a better understanding of the factors contributing to the interindividual variation in this biological process will benefit efforts to reduce fracture incidence. Many factors contributing to the variation in periosteal expansion have been identified, but a critical, largely neglected factor is external bone size. Prior theoretical work [3] did not incorporate the inter-individual variation in acquired trait sets exhibited by human long bones [4], and thus it was unclear how the amount of periosteal apposition should vary among individuals to maintain strength with aging. We hypothesized that because human long bones show a pattern of acquired trait sets (Fig 1), the amount of periosteal expansion required to maintain strength during aging will depend on external bone size. To test our hypothesis, we developed a new, more general, mathematical model that predicts the periosteal expansion rate required to maintain strength based on the inter-individual variation in adult bone morphology and tissue-quality and the endocortical bone loss and tissuemodulus degradation that occur with aging. We also determined how the periosteal expansion rate must change over time to maintain stiffness.