DECIPHERING THE SMART MECHANICAL PROPERTIES OF PERIOSTEUM, BONE'S BOUNDING ENVELOPE

Summary Statement The purpose of this experimental imaging study is to determine the Poisson9s ratio of ovine periosteum, using strain mapping data from an imaging study designed to elucidate the mechanical environment of periosteal progenitor cells in situ during stance shift loading. Introduction Periosteum is a composite, so-called “smart” or stimuli responsive material that provides a niche for pluripotent cells that exhibit mechanosensitivity in their proliferative and differentiation behavior. The overarching aim of this research program is to explore, understand, and exploit the mechanical signals that promote cell lineage commitment and de novo bone generation during embryonic development and postnatal healing. Further, our working hypothesis is that periosteum derived progenitor cells are highly sensitive to their local mechanical milieu, which guides their proliferation, motility and differentiation behavior. As a first step toward understand the role of periosteum anisotropy on defining the local mechanical milieu of a given progenitor cell, the objective of the current study is to determine the Poisson9s ratio of ovine periosteum and its sensitivity to near, mid- and long-range strains. Methods The Poisson9s ratio for the ovine periosteum was determined using strain mapping data from an high resolution imaging study designed to elucidate the mechanical environment of periosteal progenitor cells in situ . The Poisson9s ratio of long bone periosteum is given by the relative ratio of strain in the transverse direction to strain in the axial or longitudinal direction. Given high resolution video imaging data, digital image correlation is used to calculate the average strain and Poisson9s ratio between three closest neighbors (nearest neighbor), between 25 points in a 50–150 pixel distance (short range, SR), and between 25 points in a 200–400 pixel distance (long range, LR) of the periosteum during an ex vivo loading setup designed to mimic stance shift loading. Results Short and long range strains vary with spatial location and time during a given gait cycle. Calculations based on nearest neighbor, SR and LR show maximum strain at different time points in the gait cycle, different ranges of strains, as well as a non-uniform strain field that exhibits both spatial and temporal variation. Hence, the Poisson9s ratio is highly dependent on location and time. Follow on studies at lower length scales allowing for subcellular length scale strain measurement are underway to accurately account for the in situ mechanical environment of a given periosteal progenitor cell, e.g. in order to relate its functional loading environment to its biological (proliferation, migration, and differentiation) behavior. Discussion/Conclusion These results underscore the imperative not only to carry out high resolution imaging measurements but also to elucidate the structure-function relationships at smaller length scales, as these are necessary to elucidate both the origins of emergent, advanced material properties of the periosteum as well as mechanically modulation of progenitor cell proliferation, migration and differentiation.