Temporal evolution of skeletal regenerated tissue: what can mechanical investigation add to biological?

The objective here was to experimentally characterize the temporal evolution of the structural and mechanical properties of large volume immature regenerated tissues. We studied these evolving tissues from their genesis in controlled mechanical conditions. We developed an animal model based on the periosteal properties leading to unloaded regenerated skeletal tissue. To characterize the temporal evolution of mechanical properties, we carried out indentation tests coupled with macroscopic examinations and histological studies. This combined methodology yielded a range of information on osteogenesis at different scales: macroscopic by simple observation, mesoscopic by indentation test and microscopic by histological study. Results allowed us to identify different periods, providing a link between biological changes and material property evolution in bone tissue regeneration. The regenerated tissue evolves from a viscous, homogeneous, soft material to a heterogeneous stiffer material endowed with a lower viscosity. From a biological point of view, cell organization progresses from a proliferated cell clot to a mature structure closer to that of the bone. During the first 7 days, mechanical and biological results revealed the same evolution: first, the regenerated tissue grew, then, differentiated into an osteochondral tissue and finally calcification began. While our biological results confirm those of other studies, our mechanical results provide the first experimental mechanical characterization by reduced Young's modulus of such tissue.