Date of Award
Mitchell B. Schaffler
Bone, Mirodamage, Diffuse damage, self-repair, Mineral, collagen
Bone microdamage is a result of fatigue, creep or ‘wear and tear’ caused by physiological activities and largely contributes to bone fragility. Bone, unlike engineered materials, has the ability to repair the microscopic cracking or microdamage through targeted, osteoclast-mediated bone remodeling. This capability is crucial for preservation of its structural integrity and quality; failure of the skeleton to effectively repair microdamage leads to accumulation of damage, which is one of the main contributors to bone fragility. Linear microcracks (50-100 µm) and diffuse damage (Dif.Dx) (sub-micron) are the two types of microdamage. Recent studies show that Dif.Dx repairs without bone remodeling, pointing to a direct healing mechanism for these small cracks. These discoveries raise questions about the mechanism(s) by which localized matrix damage in bone undergoes self-repair and whether of overall mechanical integrity is completely restored. Thus it is reasonable to hypothesize that small (“diffuse”) crack damage repairs principally through a physicochemical mechanism that restores integrity of the bone mineral and the tissue mechanical properties, with no significant change in the organic component of the bone matrix. We first tried to understand the nature of mineral and collagen changes in diffuse damage and repaired tissue. Then we tested the hypothesis that the repair of Dif.Dx regions in bone, as assessed functionally by recovery of mechanical properties, can occur ex-vivo in the absence of viable osteocytes. Then we tested whether mineral deposition is an essential component of repair of diffuse damage in vivo, in order to gain insight into the importance of physicochemical mineral deposition in repair of diffuse damage
Raman microspectroscopy studies showed no difference in mineral/matrix, carbonate/phosphate ratio and crystallinity among diffuse damage, control and repaired diffuse damage tissue or survival groups. Raman microspectroscopy studies revealed a weak shoulder at 945-950 1/cm in diffuse damage area as an amorphous or crystalographically disordered. This peak is chemically calibrated and has been reported. Small Angle X-ray scattering (SAXS) showed no change in mineral thickness between diffuse damage area compare to control. However crystal thickness in survival group increased by 2 Å compare to control group. SAXS data showed decrease in degree of crystals alignment (more isotropic) in Dif.Dx area compare to control. However, in repaired diffuse damage area bone mineral crystal became more anisotropic. Increase in bone mineral thickness and degree of bone mineral alignment in repaired diffuse damage tissue are in consistent with non-classical crystallization pathway.
The results of ex-vivo and in-vivo remineralization studies support the concept that the spontaneous recovery of mechanical and microstructural alterations caused by Dif.Dx in bone requires deposition of Ca and Pi, but does not require the direct action of osteocytes. This repair process appears to resemble physicochemical remineralization like that occurring in enamel. Nevertheless, osteocytes and their lacunar-canalicular system likely play an essential indirect role in healing of these submicron-sized cracks in bone in vivo, by maintaining the ionic bone tissue fluid required for chemical repair of bone mineral and perfusing it through the matrix to access damage sites.
We examined collagen fibril architecture and orientation in bone containing diffuse damage induced in vivo, using a combination of SEM and SHG imaging approaches. Scanning electron microscopy studies revealed changes in tissue collagen fibrillar structure in diffuse damage regions compared to non-loaded control bone. Surprisingly, collagen fibril bundles in diffuse damaged bone were on average ~ 10% greater in diameter than those in non-loaded control bone. In addition, there was a substantial increase in the number of large diameter (550-650nm) fibril bundles; fibril bundles of this size were not seen in control bone. Importantly, ruptured or torn collagen fibrils were not seen in Dif.Dx bone.
Recovery of tissue stiffness mediated by remineralization may involve multiple pathways. Our results in consistent with non-classical crystallization pathways that involves the arrangement of primary nanoparticles into an iso-oriented crystal via oriented attachment. Non-classical crystallization pathway is always particle facilitated and in consistent with mesoscopic transformation process, where oriented attachment of smaller building blocks is required to achieve a uniform orientation in the bulk phase.
Mehraban Alvandi, Leila, "Diffuse Damage Repair Mechanism in Bone" (2020). CUNY Academic Works.