From Synchrotron Radiation to the most recent breakthroughs in Clinical Regenerative Dentistry.

In recent years there has been an increasing interest in a novel approach to evaluate human bone biopsy specimens by means of synchrotron micro-tomography (SCT). Using SCT, bone regeneration subsequent to grafting hosting sites with different types of biomaterials (with or without stem cells seeding) is recently explored. Evaluation of the amount of bone formed is usually based on histomorphological data obtained from one or several histological sections; however, conventional histological evaluation and corresponding histomorphometric measurements provide only 2-D information with the consequent risk that the selected sections do not properly represent the entire bone biopsy specimen. Furthermore, if the regenerative potential of neighboring tissues with different morphology (alveolar process, unmineralized extracellular matrix involvement, regenerated vessels, etc.) on a defect or space to regenerate is not clearly verified or unknown, 3-D analyzing methods like high resolution SCT are indicated to explore the dynamic and spatial distribution of regenerative phenomena in such complex anatomic structures. Traditionally, absorption imaging with SCT in medical applications is conducted with almost no distance between sample and detector. Homogeneous materials with a low attenuation coefficient (like collagen, unmineralized extracellular matrix, vessels, nerves, etc.) or heterogeneous materials with a narrow range of attenuation coefficients (like the case of heterologous bone scaffolds or for graded mineralized bone - such as that created by a regeneration process deriving from engrafted stem cells) produce insufficient contrast for absorption imaging. For such materials, the imaging quality can be enhanced through the use of phase contrast tomography (PCT) with an increased distance between sample and detector. In addition, whereas PCT is based on a single distance between the detector and the sample, holotomography (HT) involves imaging at several distances, then combining the phase shift information to generate 3D reconstructions. HT is helpful when the material of interest has very small variations in attenuation coefficients, which lead to unsatisfactory imaging results even with phase contrast techniques. In the present lecture the most recent breakthrougs in Clinical Regenerative Dentistry will be shown, demonstrating the unique capabilities of the SCT in offering not only an advanced characterization of different biomaterials (to understand the mechanism of their biological behavior as bone substitute) but also to investigate the growth kinetics of regenerated bone in different dental implants retrieved from humans. Implant survival, bone regeneration, graft resorption, neo-vascularization and morphometric parameters (including anisotropy and connectivity index of the structures) were evaluated by microCT and HT at different times from implantation or grafting in human bone defects. These innovative techniques allowed not only the visualization and quantification of mineralized tissues, but showed also the eventual presence and distribution of vascularization. This is of paramount importance and demonstrates that X-Ray phase tomography and holotomography appear to be important ways to investigate the cellular events involved in bone regeneration and represent promising tools for future clinical investigations of the cranio-facial tissues.