Time and space resolved mechanical behavior of regenerating rat calvaria bones via synchrotron X-ray micro-diffraction

In recent years, research in bone surgery has focused on developing strategies to accelerate bone regeneration. The major challenge of these researches is to repair large bone defects. Bone grafting technique is the gold standard to fill and heal these kind of defects. However, the mechanical behavior recovering of the newly formed bone, as the regeneration process takes place, remains still poorly understood. Only few studies actually address bone regeneration microstructure as regard to the mechanical properties induced. Recently, Hoerth and al.1 investigated the effect of different mechanical situations during bone healing of a rat femoral osteotomy model onto the microstructure developed and the mechanical properties induced thanks to nanoindentation. However, mechanical investigations have been carried out exclusively on post-regenerated specimen, i.e. on the completely reconstructed bones, and the related observed spatial-resolved mechanical properties gradient could not be solved.To overcome this lack of understanding, this work is leaning on mechanically characterize the bone regeneration process in the tissue engineering frameworks related to bone substitute isolated implantation (inside a 3-mm diameter surgically created defect). This study particularly highlights the distribution of mechanical stresses induced by the healing process through mechanical state mapping in various regeneration levels thanks to rat calvarias samples harvested at different regeneration stages (2, 4, 6 and 8 weeks after implantation). In situ tensile testing under SAXS/WAXS (Small- and Wide- Angle X-ray Scattering) synchrotron µ-beam (P03 beamline-PETRA III- Hamburg) have been achieved to spatially resolve the strain/stress response of collagen fibers and mineral part (hydroxyapatite - Hap) of bone architecture, respectively. In the light of those results, a complete expertise is carried out onto biological processes occurring at the microscopic level involved in the bone regeneration thanks to their correlation to various microstructural analysis (histological or micro-tomography observations)