There is growing recognition of the role of micro-architecture in osteoporotic bone loss and fragility. This trend has been driven by advances in imaging technology, which have enabled a transition from measures of mass to micro-architecture. Imaging trabecular bone has been a key research focus, but advances in resolution have also enabled the detection of cortical bone micro-architecture, particularly the network of vascular canals, commonly referred to as 'cortical porosity.' This review aims to provide an overview of what this level of porosity is, why it is important, and how it can be characterized by imaging. Moving beyond a 'trabeculocentric' view of bone loss holds the potential to improve diagnosis and monitoring of interventions. Furthermore, cortical porosity is intimately linked to the remodeling process, which underpins bone loss, and thus a larger potential exists to improve our fundamental understanding of bone health through imaging of both humans and animal models.
Bone’s ability to respond to load-related phenomena and repair microdamage is achieved through the remodeling process, which renews bone by activating groups of cells known as basic multicellular units (BMUs). The products of BMUs, secondary osteons, have been extensively studied via classic two-dimensional techniques, which have provided a wealth of information on how histomorphology relates to skeletal structure and function. Remodeling is critical in maintaining healthy bone tissue; however, in osteoporotic bone, imbalanced resorption results in increased bone fragility and fracture. With increasing life expectancy, such degenerative bone diseases are a growing concern. The three-dimensional (3D) morphology of BMUs and their correlation to function, however, are not well-characterized and little is known about the specific mechanisms that initiate and regulate their activity within cortical bone. We believe a key limitation has been the lack of 3D information about BMU morphology and activity. Thus, this paper reviews methodologies for 3D investigation of cortical bone remodeling and, specifically, structures associated with BMU activity (resorption spaces) and the structures they create (secondary osteons), spanning from histology to modern ex vivo imaging modalities, culminating with the growing potential of in vivo imaging. This collection of papers focuses on the theme of “putting the ‘why’ back into bone architecture.” Remodeling is one of two mechanisms “how” bone structure is dynamically modified and thus an improved 3D understanding of this fundamental process is crucial to ultimately understanding the “why.”
Basic Multicellular Units (BMUs) are groups of cells responsible for the coordination of resorption and formation in cortical bone remodeling. Due to a lack of three‐dimensional (3D) methods, relatively little is known regarding BMU spatio‐temporal ‘behavior’. It has, for example, been hypothesized that all remodeling events are targeted towards microdamage and thus BMU ‘steering’ is an essential aspect of this process. A previous 2D study involving Ursus americanus (black bear) bone revealed extensive active remodeling spaces; thus providing a unique platform for the current study. We sought to characterize the 3D morphology of BMUs to assess the extent of steering as indicated by sudden changes in trajectory. Micro‐CT of metacarpal and metatarsal diaphyses (n=4) provided an unprecedented number of BMUs (3,144) for analysis. Results indicated that majority of BMUs were oriented along the longitudinal axes of the diaphysis. Overwhelmingly, BMUs had a roughly linear course with little evidence of sudden changes in course or branching. This suggests that the BMUs observed were largely untargeted or targeting is tightly associated with the initiation rather than progression of BMUs. Grant Funding Source: Supported by the CIHR Training grant in Health Research Using Synchrotron Techniques
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