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A lifetime of successive bone remodeling events leads to trabeculae which are composed of a patchwork of bone structural units (BSUs) called hemi-osteons or trabecular packets. Traditionally, only intact surface BSUs have been studied, which are those that have been created most recently. Accordingly, the complex changes in the size and distribution of BSU throughout the trabeculae have been overlooked. In this study, the BSUs within the trabeculae of the second lumbar vertebrae were manually traced, using ImageJ software, in osteopontin immunostained sections of eight young women (aged 19-38 yr) and eight older women (aged 69-96 yr). A series of BSU profile properties including area, width, length, and perimeter were quantified, along with properties of each trabecular profile such as the number of BSU and cement line length. The relationships between these properties and age, as well as selected trabecular microstructural properties assessed with microcomputed tomography, and bone strength assessed on the neighboring third lumbar vertebrae, were investigated. The median BSU profile length and perimeter decreased with age, while the median BSU profile area and width was unchanged. Moreover, age was associated with an increase in the number of BSU profiles and cement line length per trabecular profile area. However, changes in BSU profile geometry, the number of BSU profiles, and the cement line length per trabecular profile were strongly correlated with trabecular bone volume fraction, structure model index, and bone strength. Further research is needed to understand how these changes in BSU properties affect the mechanical and failure properties of trabecular bone., Competing Interests: The authors have no conflicts to disclose., (© The Author(s) 2024. Published by Oxford University Press on behalf of the American Society for Bone and Mineral Research.)

Bone tissue is continuously remodeled. In trabecular bone, each remodeling transaction forms a microscopic bone structural unit (BSU), also known as a hemiosteon or a trabecular packet, which is bonded to existing tissue by osteopontin-rich cement lines. The size and shape of the BSUs are determined by the size and shape of the resorption cavity, and whether the cavity is potentially over- or under-filled by the subsequent bone formation. The present study focuses on the recently formed trabecular BSUs, and how their 2D size and shape changes with age and trabecular microstructure. The study was performed using osteopontin-immunostained frontal sections of L2 vertebrae from 8 young (aged 18.5-37.6 years) and 8 old (aged 69.1-96.4 years) control females, which underwent microcomputed tomography (μCT) imaging prior to sectioning. The contour of 4230 BSU profiles (181-385 per vertebra) within 1024 trabecular profiles were outlined, and their 2D width, length, area, and shape were assessed. Of these BSUs, 22 (0.5%) were generated by modeling-based bone formation (i.e. without prior resorption), while 99.5% were generated by remodeling-based bone formation (i.e. with prior resorption). The distributions of BSU profile width, length, and area were significantly smaller in the old versus young females (p < 0.005), and the median profile width, length, and area were negative correlated with age (p < 0.018). Importantly, these BSU profile size parameters were more strongly correlated with trabecular bone volume (BV/TV, p < 0.002) and structure model index (SMI, p < 0.008) assessed by μCT, than age. Moreover, the 2D BSU size parameters were positively correlated to the area of the individual trabecular profiles (p < 0.0001), which were significantly smaller in the old versus young females (p < 0.024). The BSU shape parameters (aspect ratio, circularity, and solidity) were not correlated with age, BV/TV, or SMI. Collectively, the study supports the notion that not only the BSU profile width, but also its length and area, are more influenced by the age-related bone loss and shift from plates to rods (SMI), than age itself. This implies that BSU profile size is mainly driven by changes in the trabecular microstructure, which affect the size of the resorption cavity that the BSU refills., (Copyright © 2022. Published by Elsevier Inc.)