Synchrotron Nano-Mechanical Imaging Techniques to Understand How Altered Bone Quality Increases Fracture Risk in Secondary Osteoporosis

Abstract

Bone diseases such as osteoporosis and rickets cause significant reduction in both bone quantity and quality, leading to mechanical abnormalities at the organ level. While the reduction of bone quantity can be assessed using existing clinical tools like DXA and pQCT, there is little quantitative knowledge of how altered bone quality in diseased bone increases fracture risk. For example, patients suffering from glucocorticoid induced osteoporosis (GIOP) have a higher fracture risk at similar bone mineral density (BMD) level compared with controls. This indicates that bone quantity, as currently assessed by BMD, is neither the overriding nor sole factor involved in bone fracture risk. There is a clear need for high-resolution diagnostic techniques to close the gap between onset of fracture relevant changes in bone quality and clinical diagnosis. Here, a functional imaging technique (in-situ synchrotron X-ray imaging with micromechanics) was developed to measure alterations in fibrillar deformation mechanisms in GIOP. During applied loading, percentage shifts in Bragg peak positions arising from the meridional collagen stagger, measured from the small angle X-ray scattering (SAXS) patterns, give fibrillar level strain as a function of applied stress in real time. A mouse model of Cushing syndrome, with a N-ethyl-N-nitrosourea (ENU)-induced corticotrophin releasing hormone promoter mutation, that developed secondary osteoporosis was used. The fibril modulus, maximum fibril strain and fibril-to-tissue strain ratio were determined. A significant reduction of fibril modulus and enhancement of maximum fibril strain was found in GIOP mice. A significantly larger fibril strain/tissue strain ratio was also found in GIOP mice compared to wildtype mice, as well as a significantly increased randomness of fibrillar orientation indicative of a lowered mechanical competence at the bone matrix level. These results demonstrate the ability of synchrotron-based in-situ X-ray nanomechanical imaging to identify functional alterations, at the nanoscale, in bone quality in clinically important metabolic bone diseases.