Understanding Age-Induced Cortical Porosity in Women: The Accumulation and Coalescence of Eroded Cavities Upon Existing Intracortical Canals Is the Main Contributor

Preclinical Rodent Models for Human Bone Disease, Including a Focus on Cortical Bone

Preclinical models (typically ovariectomized rats and genetically altered mice) have underpinned much of what we know about skeletal biology. They have been pivotal for developing therapies for osteoporosis and monogenic skeletal conditions, including osteogenesis imperfecta, achondroplasia, hypophosphatasia, and craniodysplasias. Further therapeutic advances, particularly to improve cortical strength, require improved understanding and more rigorous use and reporting. We describe here how trabecular and cortical bone structure develop, are maintained, and degenerate with aging in mice, rats, and humans, and how cortical bone structure is changed in some preclinical models of endocrine conditions (eg, postmenopausal osteoporosis, chronic kidney disease, hyperparathyroidism, diabetes). We provide examples of preclinical models used to identify and test current therapies for osteoporosis, and discuss common concerns raised when comparing rodent preclinical models to the human skeleton. We focus especially on cortical bone, because it differs between small and larger mammals in its organizational structure. We discuss mechanisms common to mouse and human controlling cortical bone strength and structure, including recent examples revealing genetic contributors to cortical porosity and osteocyte network configurations during growth, maturity, and aging. We conclude with guidelines for clear reporting on mouse models with a goal for better consistency in the use and interpretation of these models.

How osteons form: A quantitative hypothesis-testing analysis of cortical pore filling and wall asymmetry

Osteon morphology provides valuable information about the interplay between different processes involved in bone remodelling. The correct quantitative interpretation of these morphological features is challenging due to the complexity of interactions between osteoblast behaviour, and the evolving geometry of cortical pores during pore closing. We present a combined experimental and mathematical modelling study to provide insights into bone formation mechanisms during cortical bone remodelling based on histological cross-sections of quiescent human osteons and hypothesis-testing analyses. We introduce wall thickness asymmetry as a measure of the local asymmetry of bone formation within an osteon and examine the frequency distribution of wall thickness asymmetry in cortical osteons from human iliac crest bone samples from women 16-78 years old. Our measurements show that most osteons possess some degree of asymmetry, and that the average degree of osteon asymmetry in cortical bone evolves with age. We then propose a comprehensive mathematical model of cortical pore filling that includes osteoblast secretory activity, osteoblast elimination, osteoblast embedment as osteocytes, and osteoblast crowding and redistribution along the bone surface. The mathematical model is first calibrated to symmetric osteon data, and then used to test three mechanisms of asymmetric wall formation against osteon data: (i) delays in the onset of infilling around the cement line; (ii) heterogeneous osteoblastogenesis around the bone perimeter; and (iii) heterogeneous osteoblast secretory rate around the bone perimeter. Our results suggest that wall thickness asymmetry due to off-centred Haversian pores within osteons, and that nonuniform lamellar thicknesses within osteons are important morphological features that can indicate the prevalence of specific asymmetry-generating mechanisms. This has significant implications for the study of disruptions of bone formation as it could indicate what biological bone formation processes may become disrupted with age or disease.

Intracortical remodelling increases in highly loaded bone after exercise cessation

Resorption within cortices of long bones removes excess mass and damaged tissue and increases during periods of reduced mechanical loading. Returning to high-intensity exercise may place bones at risk of failure due to increased porosity caused by bone resorption. We used point-projection X-ray microscopy images of bone slices from highly loaded (metacarpal, tibia) and minimally loaded (rib) bones from 12 racehorses, 6 that died during a period of high-intensity exercise and 6 that had a period of intense exercise followed by at least 35 days of rest prior to death, and measured intracortical canal cross-sectional area (Ca.Ar) and number (N.Ca) to infer remodelling activity across sites and exercise groups. Large canals that are the consequence of bone resorption (Ca.Ar >0.04 mm2 ) were 1.4× to 18.7× greater in number and area in the third metacarpal bone from rested than exercised animals (p = 0.005-0.008), but were similar in number and area in ribs from rested and exercised animals (p = 0.575-0.688). An intermediate relationship was present in the tibia, and when large canals and smaller canals that result from partial bony infilling (Ca.Ar >0.002 mm2 ) were considered together. The mechanostat may override targeted remodelling during periods of high mechanical load by enhancing bone formation, reducing resorption and suppressing turnover. Both systems may work synergistically in rest periods to remove excess and damaged tissue.

MarShie: a clearing protocol for 3D analysis of single cells throughout the bone marrow at subcellular resolution

Analyzing immune cell interactions in the bone marrow is vital for understanding hematopoiesis and bone homeostasis. Three-dimensional analysis of the complete, intact bone marrow within the cortex of whole long bones remains a challenge, especially at subcellular resolution. We present a method that stabilizes the marrow and provides subcellular resolution of fluorescent signals throughout the murine femur, enabling identification and spatial characterization of hematopoietic and stromal cell subsets. By combining a pre-processing algorithm for stripe artifact removal with a machine-learning approach, we demonstrate reliable cell segmentation down to the deepest bone marrow regions. This reveals age-related changes in the marrow. It highlights the interaction between CX3CR1+ cells and the vascular system in homeostasis, in contrast to other myeloid cell types, and reveals their spatial characteristics after injury. The broad applicability of this method will contribute to a better understanding of bone marrow biology.

Understanding basic multicellular unit activity in cortical bone through 3D morphological analysis: New methods to define zones of the remodeling space

The activity of basic multicellular units (BMU) in cortical bone is classically described as a sequential order of events- resorption, reversal and formation. This simplified portrayal of the remodeling process is pervasive despite the reported variability in remodeling space morphology. These variations may reflect meaningful nuances in BMU activity but methods to quantify 3D remodeling space morphology within the context of the cellular activity are currently lacking. This study developed new techniques to define zones of BMU activity based on the 3D morphology of remodeling spaces in rabbit cortical bone and integrated morphological data with the BMU longitudinal erosion rate (LER) to elucidate the spatial-temporal coordination of BMUs and estimate mineral apposition rate (MAR). The tibiae of New Zealand white rabbits (n = 5) were imaged in vivo using synchrotron radiation and two weeks later ex vivo with desktop microCT. The in vivo and ex vivo datasets were co-registered, and 27 remodeling spaces were identified at both timepoints. A radial profile representing the 3D morphology was the platform for partitioning the remodeling spaces into resorption, reversal and formation zones. Manual, automated and semi-automated partitioning approaches were compared, and the zone-segmentations were used to calculate the length, change in radius and slope of each zone. The manual approach most accurately defined the zones of idealized remodeling spaces with known dimensions (relative error = 0.9-9.2 %) while the semi-automated method reliably defined the zones in rabbit remodeling spaces (ICC = 0.85-1.00). Combining LER and the manually derived zone dimensions indicated that a BMU passes through a cross-section in approximately 18.8 days with resorption, reversal and formation taking 4.1, 2.2, and 12.5 days, respectively. MAR estimated by the 3D analysis was not significantly different than that determined with classic histomorphometry (p = 0.48). These techniques have the potential to assess dynamic parameters of bone resorption and formation, eliminate the need for fluorochrome labeling and provide a more comprehensive perspective of the remodeling process.

Evaluation of cortical bone remodeling in canines treated with daily and weekly administrations of teriparatide by establishing AI-driven morphometric analyses and GIS-based spatial mapping

Larger animal models with a well-developed Haversian system, as observed in humans, are ideal to analyze cortical bone remodeling in pharmacological studies of anti-osteoporosis drugs, although they have some limitations in controlling individual variability in size, weight, age, and number. This study aimed to morphometrically analyze cortical bone remodeling focusing on Haversian canals in dogs using four regimens of TPTD with daily and weekly administrations at lower and higher weekly doses (4.9 μg/kg/week and 19.8 μg/kg/week, respectively) for 9 months. A micro-computed tomography-based analysis showed no significant differences among regimen groups. By establishing artificial intelligence (AI)-driven morphometric analyses and geographical information system (GIS)-based spatial mapping of Haversian canals that does not require confocal microscopy but is possible with more commonly used wide field microscopes, we successfully observed significant morphometric distinctions among regimens applied even in dogs. Our analytical results suggested that the daily higher regimen specifically increased the number of eroded pores creating spaces between existing canals, thus stimulating cortical bone remodeling.

Osteoprogenitor recruitment and differentiation during intracortical bone remodeling of adolescent humans

BACKGROUND

Recruitment and proliferation of osteoprogenitors during the reversal-resorption phase, and their differentiation into mature bone-forming osteoblasts is crucial for initiation of bone formation during bone remodeling. This study investigates the osteoprogenitors' gradual recruitment, proliferation, and differentiation into bone-forming osteoblasts within intracortical remodeling events of healthy adolescent humans.

METHODS

The study was conducted on cortical bone specimens from 11 adolescent human controls - patients undergoing surgery due to coxa valga. The osteoprogenitor recruitment route and differentiation into osteoblasts were backtracked using immunostainings and in situ hybridizations with osteoblastic markers (CD271/NGFR, osterix/SP7, COL3A1 and COL1A1). The osteoblastic cell populations were defined based on the pore surfaces, and their proliferation index (Ki67), density and number/circumference were estimated in multiplex-immunofluorescence (Ki67, TRAcP, CD34) stained sections.

RESULTS

During the reversal-resorption phase, osteoclasts are intermixed with (COL3A1+NFGR+) osteoblastic reversal cells, which are considered to be osteoprogenitors of (COL1A1+SP7+) bone-forming osteoblasts. Initiation of bone formation requires a critical density of these osteoprogenitors (43 ± 9 cells/mm), which is reached though proliferation (4.4 ± 0.5 % proliferative) and even more so through recruitment of osteoprogenitors, but challenged by the ongoing expansion of the canal circumference. These osteoprogenitors most likely originate from osteoblastic bone lining cells and mainly lumen osteoprogenitors, which expand their population though proliferation (4.6 ± 0.3 %) and vascular recruitment. These lumen osteoprogenitors resemble canopy cells above trabecular remodeling sites, and like canopy cells they extend above bone-forming osteoblasts where they may rejuvenate the osteoblast population during bone formation.

CONCLUSION

Initiation of bone formation during intracortical remodeling requires a critical density of osteoprogenitors on eroded surfaces, which is reached though proliferation and recruitment of local osteoprogenitors: bone lining cells and lumen osteoprogenitors.

A microfluidic organ-on-a-chip: into the next decade of bone tissue engineering applied in dentistry

A comprehensive understanding of the complex physiological and pathological processes associated with alveolar bones, their responses to different therapeutics strategies, and cell interactions with biomaterial becomes necessary in precisely treating patients with severe progressive periodontitis, as a bone-related issue in dentistry. However, existing monolayer cell culture or pre-clinical models have been unable to mimic the complex physiological, pathological and regeneration processes in the bone microenvironment in response to different therapeutic strategies. In this point, 'organ-on-a-chip' (OOAC) technology, specifically 'alveolar-bone-on-a-chip', is expected to resolve the problems by better imitating infection site microenvironment and microphysiology within the oral tissues. The OOAC technology is assessed in this study toward better approaches in disease modeling and better therapeutics strategy for bone tissue engineering applied in dentistry.

Trabecular bone deterioration in a postmenopausal female suffering multiple spontaneous vertebral fractures due to a delayed denosumab injection - A post-treatment re-initiation bone biopsy-based case study

Background

Denosumab, is a potent anti-resorptive that, increases bone mineral density, and reduces fracture risk in osteoporotic patients. However, several case studies have reported multiple vertebral fractures in patients discontinuing denosumab.

Case presentation

This case report describes a 64-year-old female with postmenopausal osteoporosis treated with denosumab, who had her 11th injection delayed by 4 months. The patient suffered eight spontaneous vertebral fractures. After consent, an iliac crest bone biopsy was obtained following re-initiation of the denosumab treatment and analyzed by micro-computed tomography and histomorphometry.

Results

micro-computed tomography analysis revealed a low trabecular bone volume of 10 %, a low trabecular thickness of 97 μm, a low trabecular spacing of 546 μm, a high trabecular number of 1.8/mm, and a high structure model index of 2.2, suggesting trabecular thinning and loss of trabecular plates. Histomorphometric trabecular bone analysis revealed an eroded perimeter per bone perimeter of 33 % and an osteoid perimeter per bone perimeter of 62 %. Importantly, 88 % of the osteoid perimeter was immediately above an eroded-scalloped cement line with no sign of mineralization, and often with no clear bone-forming osteoblasts on the surface. Moreover, only 5 % of the bone perimeter was mineralizing, reflecting that only 8 % of the osteoid perimeter underwent mineralization, resulting in a mineralization lag time of 545 days. Taken together, this indicates limited bone formation and delayed mineralization.

Conclusion

We present a case report of multiple vertebral fractures after denosumab discontinuation with histomorphometric evidence that denosumab discontinuation leads to extensive trabecular bone resorption followed by a limited bone formation and delayed mineralization if the denosumab treatment is reinitiated. This highlights the importance of developing optimal discontinuation strategies for patients that are to discontinue treatment.

Transitory Activation and Improved Transition from Erosion to Formation within Intracortical Bone Remodeling in Hypoparathyroid Patients Treated with rhPTH(1-84)

In hypoparathyroidism, lack of parathyroid hormone (PTH) leads to low calcium levels and decreased bone remodeling. Treatment with recombinant human PTH (rhPTH) may normalize bone turnover. This study aimed to investigate whether rhPTH(1-84) continued to activate intracortical bone remodeling after 30 months and promoted the transition from erosion to formation and whether this effect was transitory when rhPTH(1-84) was discontinued. Cortical histomorphometry was performed on 60 bone biopsies from patients (aged 31 to 78 years) with chronic hypoparathyroidism randomized to either 100 μg rhPTH(1-84) a day (n = 21) (PTH) or similar placebo (n = 21) (PLB) for 6 months as add-on to conventional therapy. This was followed by an open-label extension, where patients extended their rhPTH(1-84) (PTH) (n = 5), continued conventional treatment (CON) (n = 5), or withdrew from rhPTH(1-84) and resumed conventional therapy (PTHw) for an additional 24 months (n = 8). Bone biopsies were collected at months 6 (n = 42) and 30 (n = 18). After 6 and 30 months, the overall cortical microarchitecture (cortical porosity, thickness, pore density, and mean pore diameter) in the PTH group did not differ from that of the PLB/CON and PTHw groups. Still, the PTH group had a significantly and persistently higher percentage of pores undergoing remodeling than the PLB/CON groups. A significantly higher percentage of these pores was undergoing bone formation in the PTH compared with the PLB/CON groups, whereas the percentage of pores with erosion only was not different. This resulted in a shift in the ratio between formative and eroded pores, reflecting a faster transition from erosion to formation in the PTH-treated patients. In the rhPTH(1-84) withdrawal group PTHw, the latter effects of PTH were completely reversed in comparison to those of the PLB/CON groups. In conclusion, rhPTH(1-84) replacement therapy in hypoparathyroidism patients promotes intracortical remodeling and its transition from erosion to formation without affecting the overall cortical microstructure. The effect persists for at least 30 months and is reversible when treatment is withdrawn. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Daily administration of parathyroid hormone slows the progression of basic multicellular units in the cortical bone of the rabbit distal tibia

Basic Multicellular Units (BMUs) conduct bone remodeling, a critical process of tissue turnover which, if imbalanced, can lead to disease, including osteoporosis. Parathyroid hormone (PTH 1-34; Teriparatide) is an osteoanabolic treatment for osteoporosis; however, it elevates the rate of intra-cortical remodeling (activation frequency) leading, at least transiently, to increased porosity. The purpose of this study was to test the hypothesis that PTH not only increases the rate at which cortical BMUs are initiated but also increases their progression (Longitudinal Erosion Rate; LER). Two groups (n = 7 each) of six-month old female New Zealand white rabbits were both administered 30 μg/kg of PTH once daily for a period of two weeks to induce remodeling. Their distal right tibiae were then imaged in vivo by in-line phase contrast micro-CT at the Canadian Light Source synchrotron. Over the following two weeks the first group (PTH) received continued daily PTH while the second withdrawal group (PTHW) was administrated 0.9 % saline. At four weeks all animals were euthanized, their distal tibiae were imaged by conventional micro-CT ex vivo and histomorphometry was performed. Matching micro-CT datasets (in vivo and ex vivo) were co-registered in 3D and LER was measured from 612 BMUs. Counter to our hypothesis, mean LER was lower (p < 0.001) in the PTH group (30.19 ± 3.01 μm/day) versus the PTHW group (37.20 ± 2.77 μm/day). Despite the difference in LER, osteonal mineral apposition rate (On.MAR) did not differ between groups indicating the anabolic effect of PTH was sustained after withdrawal. The slowing of BMU progression by PTH warrants further investigation; slowed resorption combined with elevated bone formation rate, may play an important role in how PTH enhances coupling between resorption and formation within the BMU. Finally, the prolonged anabolic response following withdrawal may have utility in terms of optimizing clinical dosing regimens.

Local coordination between intracortical bone remodeling and vascular development in human juvenile bone

Although failure to establish a vascular network has been associated with many skeletal disorders, little is known about what drives development of vasculature in the intracortical bone compartments. Here, we show that intracortical bone resorption events are coordinated with development of the vasculature. We investigated the prevalence of vascular structures at different remodeling stages as well as their 3D organization using proximal femoral cortical bone from 5 girls and 6 boys (aged 6-15 years). A 2D analysis revealed that non-quiescent intracortical pores contained more vascular structures than quiescent pores (p < 0.0001). Type 2 pores, i.e., remodeling of existing pores, had a higher density of vascular structures than type 1 pores, i.e., de novo created pores (p < 0.05). Furthermore, pores at the eroded-formative remodeling stage, had more vascular structures than pores at any other remodeling stage (p < 0.05). A 3D reconstruction of an intracortical remodeling event showed that osteoclasts in the advancing tip of the cutting cone as well as preosteoclasts in the lumen expressed vascular endothelial growth factor-A (VEGFA), while VEGFA-receptors 1 and 2 mainly were expressed in endothelial cells in the adjacent vasculature. Consequently, we propose that the progression of the vascular network in intracortical remodeling events is driven by osteoclasts expressing VEGFA. Moreover, the vasculature is continuously reconfigured according to the demands of the remodeling events at the surrounding bone surfaces.

Increased Bone Volume by Ixazomib in Multiple Myeloma: 3-Month Results from an Open Label Phase 2 Study

Multiple myeloma (MM) is an incurable bone marrow cancer characterized by the development of osteolytic lesions due to the myeloma-induced increase in osteoclastogenesis and decrease in osteoblastic activity. The standard treatment of MM often involves proteasome inhibitors (PIs), which can also have a beneficial off-target bone anabolic effect. However, long-term treatment with PIs is unadvised due to their high side-effect burden and inconvenient route of administration. Ixazomib is a new-generation, oral PI that is generally well tolerated; however, its bone effect remains unknown. Here, we describe the 3-month results of a single-center phase II clinical trial investigating the effect of ixazomib treatment on bone formation and bone microstructure. Thirty patients with MM in stable disease not receiving antimyeloma treatment for ≥3 months and presenting ≥2 osteolytic lesions received monthly ixazomib treatment cycles. Serum and plasma samples were collected at baseline and monthly thereafter. Sodium ¹⁸ F-Fluoride positron emission tomography (NaF-PET) whole-body scans and trephine iliac crest bone biopsies were collected before and after three treatment cycles. The serum levels of bone remodeling biomarkers suggested an early ixazomib-induced decrease in bone resorption. NaF-PET scans indicated unchanged bone formation ratios; however, histological analyses of bone biopsies revealed a significant increase in bone volume per total volume after treatment. Further analyses of bone biopsies showed unchanged osteoclast number and COLL1A1^High -expressing osteoblasts on bone surfaces. Next, we analyzed the superficial bone structural units (BSUs), which represent each recent microscopic bone remodeling event. Osteopontin staining revealed that following treatment, significantly more BSUs were enlarged (>200,000 μm² ), and the distribution frequency of their shape was significantly different from baseline. Overall, our data suggest that ixazomib induces overflow remodeling-based bone formation by decreasing the level of bone resorption and promoting longer bone formation events, making it a potentially valuable candidate for future maintenance treatment. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Pathology or expected morphology? Investigating patterns of cortical porosity and trabecularization during infancy and early childhood

Increased cortical porosity is associated with a heightened risk of skeletal fragility due to bone loss and structural decay in adults. However, few studies have examined the etiology of cortical porosity in infants and children. This study examines whether age-related changes in femoral growth and locomotor development influence femoral midshaft cortical porosity in a sample of 48 individuals (fetal to 3.99 years) from the 10th-13th century cemetery of St. Étienne de Toulouse, France. Histological sections were prepared and imaged using light microscopy. Midshaft geometric variables such as total area, cortical area, and pore area were calculated using BoneJ. Increased porosity and cortical trabecularization were found to be significantly associated with age, being almost exclusively present in individuals aged 0.5-1.99 years. At approximately 6 months of age infants typically begin engaging in regular femoral loading and experience an acceleration in growth. The observed increase in midshaft porosity and trabecularization, therefore, likely results from the reorganization and redistribution of cortical bone, stimulated by increased growth velocity and the onset of weight-bearing activities. The reduction in cortical porosity and trabecularization in individuals aged 2.0-3.99 years indicates that children are approaching some sort of homeostasis as growth velocity slows and their femora adapt to consistent loading. Understanding what expected skeletal development looks like is necessary when conducting bioarcheological studies and this study provides evidence for a pattern of transient midshaft porosity during infancy and early childhood.

The influence of intra-cortical microstructure on the contrast in ultrasound images of the cortex of long bones: A 2D simulation study

Decreased thickness of the bone cortex due to bone loss in the course of ageing and osteoporosis is associated with reduced bone strength. Cortical thickness measurement from ultrasound images was recently demonstrated in young adults. This requires the identification of both the outer (periosteum) and inner (endosteum) surfaces of the bone cortex. However, with bone loss, the cortical porosity and the size of the vascular pores increase resulting in enhanced ultrasound scattering which may prevent the detection of the endosteum. The aim of this work was to study the influence of cortical bone microstructure variables, such as porosity and pore size, on the contrast of the endosteum in ultrasound images. We wanted to estimate the range of these variables for which ultrasound imaging of the endosteum is feasible. We generated synthetic data using a two-dimensional time-domain code to simulate the propagation of elastodynamic waves. A synthetic aperture imaging sequence with an array transducer operating at a center frequency of 2.5 MHz was used. The numerical simulations were conducted for 105 cortical microstructures obtained from high resolution X-ray computed tomography images of ex vivo bone samples with a porosity ranging from 2% to 24 %. Images were reconstructed using a delay-and-sum (DAS) algorithm with optimized f-number, correction of refraction at the periosteum, and sample-specific wave-speed. We observed a range variation of 18 dB of endosteum contrast in our data set depending on the bone microstructure. We found that as porosity increases, speckle intensity inside the bone cortex increases whereas the intensity of the signal from the endosteum decreases. Also, a microstructure with large pores (diameter >250 μm) was associated with poor endosteum visibility, compared with a microstructure with equal porosity but a more narrow distribution of pore sizes. These findings suggest that ultrasound imaging of the bone cortex with a probe operating at a central frequency of 2.5 MHz using refraction-corrected DAS is capable of detecting the endosteum of a cortex with moderate porosity (less than about 10%) if the largest pores remain smaller than about 200 μm.

Long-term Care Facility and its Elderly Chronic Diseases in Jishou: Insights into Underdeveloped Area of China

Insight into the current status of long-term care facilities (LTCFs) and chronic diseases in underdeveloped areas in China is scant. Using a census method to survey older adults ≥60 years old (154 older adult residents) in all LTCFs in Jishou area. The International Resident Assessment Instrument for Long-Term Care Facilities (interRAI-LTCF) was used to collect information and analyze the current status of chronic diseases among older adult residents. There were 62 187 residents ≥60 years old in Jishou area. According to the survey, there were only 154 older adult residents living in the LTCFs of Jishou, with a ratio was 0.3% of all older adult residents, which was much lower than China's 3.0%. Of respondents (109 older adult residents), the prevalence of chronic diseases was 70.6%. The prevalence increases with age, as well as in the female is higher than in the male. The top 3 chronic diseases were hypertension (41.3%), bone and joint disease (12.8%) and cerebrovascular disease (12.8%). It was found that different age groups and genders lead to differences in the prevalence and the order of chronic diseases. The prevalence of older adults with impaired balance ability, sleep disturbance and swallowing disturbance was higher than that of normal older adults. The results of the one-way analysis of variance showed that the age and balance ability of the older adults with chronic diseases were statistically significant (P < .05). In addition, the prevalence of chronic diseases in the LTCFs older adult was higher than the home care (HC) older adults in Jishou. The age and the prevalence of chronic diseases of LTCFs in older adults with professional nursing staff were higher than in those without. This study provides a theoretical basis for the healthcare distribution, prevention and treatment of chronic diseases in underdeveloped areas. The undeveloped area has lower LTCFs staying rate and chronic disease prevalence of older adults than relatively developed areas. More public health attention and capital investment are needed to increase the LTCFs number and strengthen disease testing and screening. As well as, the proportion of professional nursing staff and specialist doctors in LTCFs and improve the quality of care and medical treatment for older adults.

2D size of trabecular bone structure units (BSU) correlate more strongly with 3D architectural parameters than age in human vertebrae

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.

Alendronate prolongs the reversal-resorption phase in human cortical bone remodeling

Despite their ability to reduce fracture-risk and increase Bone Mineral Density (BMD) in osteoporotic women, bisphosphonates are reported to reduce formation of new bone. Reduced bone formation has been suggested to lead to accumulation of microfractures and contribute to rare side effects in cortical bone such as atypical femur fractures. However, most studies are limited to trabecular bone. In this study, the cortical bone remodeling in human iliac bone specimens of 65 non-treated and 24 alendronate-treated osteoporotic women was investigated using a new histomorphometric classification of intracortical pores. The study showed that only 12.4 ± 11% of the cortical pore area reflected quiescent pores/osteons in alendronate-treated patients versus 8.5 ± 5% in placebo, highlighting that new cortical remodeling events remain to be activated. The percent and size of eroded pores (events in resorption-reversal phase) remained unchanged, but their contribution to total pore area was 1.4-fold higher in alendronate versus placebo treated patients (66 ± 22% vs 48 ± 22%, p < 0.001). On the other hand, the mixed eroded-formative pores (events with mixed resorption-reversal-formation phases) was 2-fold lower in alendronate versus placebo treated patients (19 ± 14% vs 39 ± 23% of total pore area, p < 0.001), and formative pores (event in formation phase) was 2.2-fold lower in alendronate versus placebo treated patients (2.1 ± 2.4% vs 4.6 ± 3.6%, p < 0.01), and their contribution to total pore area was 2.4-fold lower (1.3 ± 2.1% vs 3.1 ± 4.4%, p < 0.05). Importantly, these differences between alendronate and placebo treated patients were significant in patients after 3 years of treatment, not after 2 years of treatment. Collectively, the results support that cortical remodeling events activated during alendronate treatment has a prolonged reversal-resorption phase with a delayed transition to formation, becoming increasingly evident after 3-years of treatment. A potential contributor to atypical femur fractures associated with long-term bisphosphonate treatment.

Impaired bone fracture healing in type 2 diabetes is caused by defective functions of skeletal progenitor cells

The mechanisms of obesity and type 2 diabetes (T2D)-associated impaired fracture healing are poorly studied. In a murine model of T2D reflecting both hyperinsulinemia induced by high fat diet (HFD) and insulinopenia induced by treatment with streptozotocin (STZ), we examined bone healing in a tibia cortical bone defect. A delayed bone healing was observed during hyperinsulinemia as newly formed bone was reduced by – 28.4±7.7% and was associated with accumulation of marrow adipocytes at the defect site +124.06±38.71%, and increased density of SCA1+ (+74.99± 29.19%) but not Runx2 +osteoprogenitor cells. We also observed increased in reactive oxygen species production (+101.82± 33.05%), senescence gene signature (≈106.66± 34.03%) and LAMIN B1 - senescent cell density (+225.18± 43.15%), suggesting accelerated senescence phenotype. During insulinopenia, a more pronounced delayed bone healing was observed with decreased newly formed bone to -34.9± 6.2% which was inversely correlated with glucose levels (R 2=0.48, p&lt;0.004) and callus adipose tissue area (R 2=0.3711, p&lt;0.01). Finally, to investigate the relevance to human physiology, we observed that sera from obese and T2D subjects had disease state-specific inhibitory effects on osteoblast related gene signatures in human bone marrow stromal cells which resulted in inhibition of osteoblast and enhanced adipocyte differentiation. Our data demonstrate that T2D exerts negative effects on bone healing through inhibition of osteoblast differentiation of skeletal stem cells and induction of accelerated bone senescence and that the hyperglycaemia per se and not just insulin levels is detrimental for bone healing.

The combination of aging and chronic kidney disease leads to an exacerbated cortical porosity phenotype

PURPOSE

Chronic kidney disease (CKD) and aging are each independently associated with higher fracture risk. Although CKD is highly prevalent in the aging population, the interaction between these two conditions with respect to bone structure and mechanics is not well understood. The purpose of this study was to examine cortical porosity and mechanical properties in skeletally mature young and aging mice with CKD.

METHODS

CKD was induced by feeding 16-week and 78-week male mice 0.2% adenine (AD) for six weeks followed by two weeks of maintenance on a control diet for a total study duration of eight weeks of CKD; control (CON) animals of each age were fed a standard diet. Serum biochemistries, μCT imaging, and mechanical properties via four-point bending were assessed at the endpoint.

RESULTS

Phosphorus, parathyroid hormone, and blood urea nitrogen were elevated in both ages of AD mice compared to age-matched CON; aging AD mice had PTH and BUN values higher than all other groups. Femoral cortical porosity was more than four-fold higher in aging AD mice compared to young AD mice and more than two-fold higher compared to age-matched controls. Structural and estimated material mechanical properties were both lower in aging mice, but there were no significant interactions between AD treatment and age.

CONCLUSION

These data show an interaction between CKD and aging that produces a more severe biochemical and cortical bone phenotype. This highlights the importance of studying mechanisms and potential interventions in both young and aged animals to translate to a broader spectrum of CKD patients.

Pore-Size Distribution and Frequency-Dependent Attenuation in Human Cortical Tibia Bone Discriminate Fragility Fractures in Postmenopausal Women With Low Bone Mineral Density

Osteoporosis is a disorder of bone remodeling leading to reduced bone mass, structural deterioration, and increased bone fragility. The established diagnosis is based on the measurement of areal bone mineral density by dual‐energy X‐ray absorptiometry (DXA), which poorly captures individual bone loss and structural decay. Enlarged cortical pores in the tibia have been proposed to indicate structural deterioration and reduced bone strength in the hip. Here, we report for the first time the in vivo assessment of the cortical pore‐size distribution together with frequency‐dependent attenuation at the anteromedial tibia midshaft by means of a novel ultrasonic cortical backscatter (CortBS) technology. We hypothesized that the CortBS parameters are associated with the occurrence of fragility fractures in postmenopausal women (n = 55). The discrimination performance was compared with those of DXA and high‐resolution peripheral computed tomography (HR‐pQCT). The results suggest a superior discrimination performance of CortBS (area under the receiver operating characteristic curve [AUC]: 0.69 ≤ AUC ≤ 0.75) compared with DXA (0.54 ≤ AUC ≤ 0.55) and a similar performance compared with HR‐pQCT (0.66 ≤ AUC ≤ 0.73). CortBS is the first quantitative bone imaging modality that can quantify microstructural tissue deteriorations in cortical bone, which occur during normal aging and the development of osteoporosis. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Mechanical stimuli-mediated modulation of bone cell function-implications for bone remodeling and angiogenesis

Bone remodeling, expressed as bone formation and turnover, is a complex and dynamic process closely related to its form and function. Different events, such as development, aging, and function, play a critical role in bone remodeling and metabolism. The ability of the bone to adapt to new loads and forces has been well known and has proven useful in orthopedics and insightful for research in bone and cell biology. Mechanical stimulation is one of the most important drivers of bone metabolism. Interestingly, different types of forces will have specific consequences in bone remodeling, and their beneficial effects can be traced using different biomarkers. In this narrative review, we summarize the major mediators and events in bone remodeling, focusing on the effects of mechanical stimulation on bone metabolism, cell populations, and ultimately, bone health.

Physiological Loading-Induced Interstitial Fluid Dynamics in Osteon of Osteogenesis Imperfecta Bone

Osteogenesis imperfecta (OI), also known as "brittle bone disease," is a genetic bone disorder. OI bones experience frequent fractures. Surgical procedures are usually followed by clinicians in the management of OI. It has been observed physical activity is equally beneficial in reducing OI bone fractures in both children and adults as mechanical stimulation improves bone mass and strength. Loading-induced mechanical strain and interstitial fluid flow stimulate bone remodeling activities. Several studies have characterized strain environment in OI bones, whereas very few studies attempted to characterize the interstitial fluid flow. OI significantly affects bone micro-architecture. Thus, this study anticipates that canalicular fluid flow reduces in OI bone in comparison to the healthy bone in response to physiological loading due to altered poromechanical properties. This work attempts to understand the canalicular fluid distribution in single osteon models of OI and healthy bone. A poromechanical model of osteon is developed to compute pore-pressure and interstitial fluid flow as a function of gait loading pattern reported for OI and healthy subjects. Fluid distribution patterns are compared at different time-points of the stance phase of the gait cycle. It is observed that fluid flow significantly reduces in OI bone. Additionally, flow is more static than dynamic in OI osteon in comparison to healthy subjects. This work attempts to identify the plausible explanation behind the diminished mechanotransduction capability of OI bone. This work may further be extended for designing better biomechanical therapies to enhance the fluid flow in order to improve osteogenic activities in OI bone.

Aging delays epimorphic regeneration in mice

Epimorphic regeneration is a multi-tissue regeneration process where amputation does not lead to scarring, but blastema formation and patterned morphogenesis for which cell plasticity and concerted cell-cell interactions are pivotal. Tissue regeneration declines with aging, yet if and how aging impairs epimorphic regeneration is unknown. Here we show for the first time that aging derails the spatiotemporal regulation of epimorphic regeneration in mammals, first, by exacerbating tissue histolysis and delaying wound closure, and second, by impairing blastema differentiation and skeletal regrowth. Surprisingly, aging did not limit stem cell availability in the blastema, but reduced osteoblast-dependent bone formation. Our data suggest that aging delays regeneration not by stem cell exhaustion, but functional defects of differentiated cells that may be driven by an aged wound environment and alterations in the spatiotemporal regulation of regeneration events. Our findings emphasize the importance of accurate timing of signaling events for regeneration, and highlight the need for carefully timed interventions in regenerative medicine.

Disturbance of osteonal bone remodeling and high tensile stresses on the lateral cortex in atypical femoral fracture after long-term treatment with Risedronate and Alfacalcidol for osteoporosis

An 83 year-old Japanese woman complained of left lateral thigh pain following a low-energy fall 4 months prior to admission. She had been treated for osteoporosis with Risedronate and Alfacalcidol for the previous five years. She was diagnosed with an atypical femoral fracture (AFF) according to the American Society for Bone and Mineral Research (ASBMR) Task Force revised criteria. Radiographs revealed cortical thickening and a transverse radiolucent fracture line in the lateral cortex of the shaft. MRI showed a high intensity signal on the T2WI image 1 cm long in the lateral cortex. The patient had normal levels of bone resorption and formation biomarkers except for low 25(OH) Vitamin D. Double fluorescent labeling was done preoperatively.

Due to significant bowing, a corrective osteotomy and intramedullary nailing were performed, and the resected bone wedge was analyzed by bone histomorphometry. Three ground sections of the lateral cortex at the fracture site showed many and large pores, with or without tetracycline labeling. Histomorphometric assessment was done on intracortical pores, classified by a novel criteria, only to assess size of the pores to know prolonged osteoclastic activity and its characteristics of inner surfaces to assess whether bone formation has been occurring or not in labeling period in remodeling cycle, and coalition of multi-pores. Increased size with widespread variation of pores suggested prolonged osteoclastic activity in the reversal/resorptive phase. Bone labeling showed lamellar bone on the endocortical surface.

We hypothesize that the case had developed from a regional disturbance of osteonal remodeling in the lateral cortex, in which accumulated microcracks might have initiated a resorption process resulting in resorption cavities, i.e., pores, which became larger due to prolonged activity of secondary osteoclasts. Various sized pores could form lamellar bone, still forming at the time of biopsy, some had formed lamellar bone, but stopped to form before labeling and not to start to form at all, probably due to incomplete coupling. Endocortical lamellar bone might had started to resorbed to smooth off endocortical surface, followed by formation of lamellar bone. The endocortical bone formation was assessed and its formation period is about 2.7 years.

A finite element analysis using preoperative CT data revealed high tensile stresses on the lateral aspect of the femur. Histomorphometric results suggest that there might be more pores in the tensile area than the compressive area. These findings may subsequently connect accumulation of microcracks, an increase of size and number of pores and coalition and subsequent fracture in the lateral cortex.

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The lateral cortex of the fracture site of atypical femoral fracture was assessed by bone histomorphometry and FEA.

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Many enlarged pores may suggest a prolonged resorptive phase, resulting in excessive resorption by secondary osteoclasts.

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There is large variation in size of pores, which is much more than that of osteons, normally observed.

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Pores were classified as types with/without label, and with/without parallel lamellae to inner surface of the pores.

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More pores in size and number were observed in the lateral cortex under tensile force than compressive force by FEA.

Estimation of Cortical Bone Microstructure From Ultrasound Backscatter

Multichannel pulse-echo ultrasound using linear arrays and single-channel data acquisition systems opens new perspectives for the evaluation of cortical bone. In combination with spectral backscatter analysis, it can provide quantitative information about cortical microstructural properties. We present a numerical study, based on the finite-difference time-domain method, to estimate the backscatter cross section of randomly distributed circular pores in a bone matrix. A model that predicts the backscatter coefficient using arbitrary pore diameter distributions was derived. In an ex vivo study on 19 human tibia bones (six males, 13 females, 83.7 ± 8.4 years), multidirectional ultrasound backscatter measurements were performed using an ultrasound scanner equipped with a 6-MHz 128-element linear array with sweep motor control. A normalized depth-dependent spectral analysis was performed to derive backscatter and attenuation coefficients. Site-matched reference values of tissue acoustic impedance Z , cortical thickness (Ct.Th), pore density (Ct.Po.Dn), porosity (Ct.Po), and characteristic parameters of the pore diameter (Ct.Po.Dm) distribution were obtained from 100-MHz scanning-acoustic microscopy images. Proximal femur areal bone mineral density (aBMD), stiffness S , and ultimate force Fu from the same donors were available from a previous study. All pore structure and material properties could be predicted using linear combinations of backscatter parameters with a median to high accuracy (0.28 ≤ adjusted R² ≤ 0.59). The combination of cortical thickness and backscatter parameter provided similar or better prediction accuracies than aBMD. For the first time, a method for the noninvasive assessment of the pore diameter distribution in cortical bone by ultrasound is proposed. The combined assessment of cortical thickness, sound velocity, and pore size distribution in a mobile, nonionizing measurement system could have a major impact on preventing osteoporotic fractures.

Modelling cell guidance and curvature control in evolving biological tissues

Tissue geometry is an important influence on the evolution of many biological tissues. The local curvature of an evolving tissue induces tissue crowding or spreading, which leads to differential tissue growth rates, and to changes in cellular tension, which can influence cell behaviour. Here, we investigate how directed cell motion interacts with curvature control in evolving biological tissues. Directed cell motion is involved in the generation of angled tissue growth and anisotropic tissue material properties, such as tissue fibre orientation. We develop a new cell-based mathematical model of tissue growth that includes both curvature control and cell guidance mechanisms to investigate their interplay. The model is based on conservation principles applied to the density of tissue synthesising cells at or near the tissue's moving boundary. The resulting mathematical model is a partial differential equation for cell density on a moving boundary, which is solved numerically using a hybrid front-tracking method called the cell-based particle method. The inclusion of directed cell motion allows us to model new types of biological growth, where tangential cell motion is important for the evolution of the interface, or for the generation of anisotropic tissue properties. We illustrate such situations by applying the model to simulate both the resorption and infilling components of the bone remodelling process, and to simulate root hair growth. We also provide user-friendly MATLAB code to implement the algorithms.

Frequency-dependent analysis of ultrasound apparent absorption coefficient in multiple scattering porous media: application to cortical bone

The effect of matrix viscoelastic absorption on frequency-dependent attenuation in porous structures mimicking simplified cortical bone is addressed in this numerical study. An apparent absorption is defined to quantify the difference between total attenuation (resulting from both absorption and scattering) and attenuation exclusively due to scattering. A power-law model is then used to describe the frequency-dependent apparent absorption as a function of pore diameter and density. The frequency response of the porous structures to a Gaussian pulse is studied to determine the frequency range over which the system can be considered linear. The results show that for low scattering regimes (normalized frequency [Formula: see text]0.80), the system and its apparent absorption can be considered linear. Hence, the total attenuation coefficient results from the summation of scattering and absorption coefficients. However, for highly scattering regimes, the system can no longer be considered linear, as the apparent absorption vs. frequency deviates from a linear trend. As the pore density increases, the apparent absorption coefficient increases as well.

Re-thinking the bone remodeling cycle mechanism and the origin of bone loss

Proper bone remodeling necessarily requires that osteoblasts reconstruct the bone that osteoclasts have resorbed. However, the cellular events connecting resorption to reconstruction have remained poorly known. The consequence is a fragmentary understanding of the remodeling cycle where only the resorption and formation steps are taken into account. New tools have recently made possible to elucidate how resorption shifts to formation, thereby allowing to comprehend the remodeling cycle as a whole. This new knowledge is reviewed herein. It shows how teams of osteoclasts and osteoblast lineage cells are progressively established and how they are subjected therein to reciprocal interactions. Contrary to the common view, osteoclasts and osteoprogenitors are intermingled on the eroded surfaces. The analysis of the resorption and cell population dynamics shows that osteoprogenitor cell expansion and resorption proceed as an integrated mechanism; that a threshold cell density of osteoprogenitors on the eroded surface is mandatory for onset of bone formation; that the cell initiating osteoprogenitor cell expansion is the osteoclast; and that the osteoclast therefore triggers putative osteoprogenitor reservoirs positioned at proximity of the eroded bone surface (bone lining cells, canopy cells, pericytes). The interplay between magnitude of resorption and rate of cell expansion governs how soon bone reconstruction is initiated and may determine uncoupling and permanent bone loss if a threshold cell density is not reached. The clinical perspectives opened by these findings are discussed.

Cortical Bone Porosity in Rabbit Models of Osteoporosis

Cortical bone porosity is intimately linked with remodeling, is of growing clinical interest, and is increasingly accessible by imaging. Thus, the potential of animal models of osteoporosis (OP) to provide a platform for studying how porosity develops and responds to interventions is tremendous. To date, rabbit models of OP have largely focused on trabecular microarchitecture or bone density; some such as ovariectomy (OVX) have uncertain efficacy and cortical porosity has not been extensively reported. Our primary objective was to characterize tibial cortical porosity in rabbit-based models of OP, including OVX, glucocorticoids (GC), and OVX + GC relative to controls (SHAM). We sought to: (i) test the hypothesis that intracortical remodeling is elevated in these models; (ii) contrast cortical remodeling and porosity in these models with that induced by parathyroid hormone (1-34; PTH); and (iii) contrast trabecular morphology in the proximal tibia across all groups. Evidence that an increase in cortical porosity occurred in all groups was observed, although this was the least robust for GC. Histomorphometric measures supported the hypothesis that remodeling rate was elevated in all groups and also revealed evidence of uncoupling of bone resorption and formation in the GC and OVX + GC groups. For trabecular bone, a pattern of loss was observed for OVX, GC, and OVX + GC groups, whereas the opposite was observed for PTH. Change in trabecular number best explained these patterns. Taken together, the findings indicated rabbit models provide a viable and varied platform for the study of OP and associated changes in cortical remodeling and porosity. Intriguingly, the evidence revealed differing effects on the cortical and trabecular envelopes for the PTH model. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

Cortical thinning and accumulation of large cortical pores in the tibia reflect local structural deterioration of the femoral neck

INTRODUCTION

Cortical bone thinning and a rarefaction of the trabecular architecture represent possible causes of increased femoral neck (FN) fracture risk. Due to X-ray exposure limits, the bone microstructure is rarely measurable in the FN of subjects but can be assessed at the tibia. Here, we studied whether changes of the tibial cortical microstructure, which were previously reported to be associated with femur strength, are also associated with structural deteriorations of the femoral neck.

METHODS

The cortical and trabecular architectures in the FN of 19 humans were analyzed ex vivo on 3D microcomputed tomography images with 30.3 μm voxel size. Cortical thickness (Ct.Th_tibia), porosity (Ct.Po_tibia) and pore size distribution in the tibiae of the same subjects were measured using scanning acoustic microscopy (12 μm pixel size). Femur strength during sideways falls was simulated with homogenized voxel finite element models.

RESULTS

Femur strength was associated with the total (vBMD_tot; R² = 0.23, p < 0.01) and trabecular (vBMD_trab; R² = 0.26, p < 0.01) volumetric bone mineral density (vBMD), with the cortical thickness (Ct.Th_FN; R² = 0.29, p < 0.001) and with the trabecular bone volume fraction (Tb.BV/TV_FN; R² = 0.34, p < 0.001), separation (Tb.Sp_FN; R² = 0.25, p < 0.01) and number (Tb.N_FN; R² = 0.32, p < 0.001) of the femoral neck. Moreover, smaller Ct.Th_tibia was associated with smaller Ct.Th_FN (R² = 0.31, p < 0.05), lower Tb.BV/TV_FN (R² = 0.29, p < 0.05), higher Tb.Sp_FN (R² = 0.33, p < 0.05) and lower Tb.N_FN (R² = 0.42, p < 0.01). A higher prevalence of pores with diameter > 100 μm in tibial cortical bone (relCt.Po_100μm-tibia) indicated higher Tb.Sp_FN (R² = 0.36, p < 0.01) and lower Tb.N_FN (R² = 0.45, p < 0.01).

CONCLUSION

Bone resorption and structural decline of the femoral neck may be identified in vivo by measuring cortical bone thickness and large pores in the tibia.

Femur strength predictions by nonlinear homogenized voxel finite element models reflect the microarchitecture of the femoral neck

In the human femoral neck, the contribution of the cortical and trabecular architecture to mechanical strength is known to depend on the load direction. In this work, we investigate if QCT-derived homogenized voxel finite element (hvFE) simulations of varying hip loading conditions can be used to study the architecture of the femoral neck. The strength of 19 pairs of human femora was measured ex vivo using nonlinear hvFE models derived from high-resolution peripheral QCT scans (voxel size: 30.3 µm). Standing and side-backwards falling loads were modeled. Quasi-static mechanical tests were performed on 20 bones for comparison. Associations of femur strength with volumetric bone mineral density (vBMD) or microstructural parameters of the femoral neck obtained from high-resolution QCT were compared between mechanical tests and simulations and between standing and falling loads. Proximal femur strength predictions by hvFE models were positively associated with the vBMD of the femoral neck (R² > 0.61, p < 0.001), as well as with its cortical thickness (R² > 0.27, p < 0.001), trabecular bone volume fraction (R² = 0.42, p < 0.001) and with the first two principal components of the femoral neck architecture (R² > 0.38, p < 0.001). Associations between femur strength and femoral neck microarchitecture were stronger for one-legged standing than for side-backwards falling. For both loading directions, associations between structural parameters and femur strength from hvFE models were in good agreement with those from mechanical tests. This suggests that hvFE models can reflect the load-direction-specific contribution of the femoral neck microarchitecture to femur strength.

Antiresorptive and anabolic agents in the prevention and reversal of bone fragility

Bone volume, microstructure and its material composition are maintained by bone remodelling, a cellular activity carried out by bone multicellular units (BMUs). BMUs are focally transient teams of osteoclasts and osteoblasts that respectively resorb a volume of old bone and then deposit an equal volume of new bone at the same location. Around the time of menopause, bone remodelling becomes unbalanced and rapid, and an increased number of BMUs deposit less bone than they resorb, resulting in bone loss, a reduction in bone volume and microstructural deterioration. Cortices become porous and thin, and trabeculae become thin, perforated and disconnected, causing bone fragility. Antiresorptive agents reduce fracture risk by reducing the rate of bone remodelling so that fewer BMUs are available to remodel bone. Bone fragility is not abolished by these drugs because existing microstructural deterioration is not reversed, unsuppressed remodelling continues producing microstructural deterioration and unremodelled bone that becomes more mineralized can become brittle. Anabolic agents reduce fracture risk by stimulating new bone formation, which partly restores bone volume and microstructure. To guide fracture prevention, this Review provides an overview of the structural basis of bone fragility, the mechanisms of remodelling and how anabolic and antiresorptive agents target remodelling defects.

The generation of enlarged eroded pores upon existing intracortical canals is a major contributor to endocortical trabecularization

The gradual conversion of cortical bone into trabecular bone on the endocortical surface contributes substantially to thinning of the cortical bone. The purpose of the present study was to characterize the intracortical canals (3D) and pores (2D) in human fibular bone, to identify the intracortical remodeling events leading to this endocortical trabecularization. The analysis was conducted in fibular diaphyseal bone specimens obtained from 20 patients (6 women and 14 men, age range 41-75 years). μCT revealed that endosteal bone had a higher cortical porosity (p< 0.05) and canals with a larger diameter (p< 0.05) than periosteal bone, while the canal spacing and number were similar in the endosteal and periosteal half. Histological analysis showed that the endosteal half versus the periosteal half: (i) had a higher likelihood of being non-quiescent type 2 pores (i.e. remodeling of existing pores) than other pore types (OR = 1.6, p< 0.01); (ii) that the non-quiescent type 2 pores contributed to a higher porosity (p< 0.001); and that (iii) amongst these pores especially eroded type 2 pores contributed to the elevated cortical porosity (p< 0.001). In conclusion, we propose that endocortical trabecularization results from the accumulation of eroded cavities upon existing intracortical canals, favored by delayed initiation of bone formation.

The age-related decrease in material properties of BALB/c mouse long bones involves alterations to the extracellular matrix

One possibility for the disproportionate increase in fracture risk with aging relative to the decrease in bone mass is an accumulation of changes to the bone matrix which deleteriously affect fracture resistance. In order to effectively develop new targets for osteoporosis, a preclinical model of the age-related loss in fracture resistance needs to be established beyond known age-related decreases in bone mineral density and bone volume fraction. To that end, we examined long bones of male and female BALB/c mice at 6-mo. and 20-mo. of age and assessed whether material and matrix properties of cortical bone significantly differed between the age groups. The second moment of area of the diaphysis (minimum and maximum principals for femur and radius, respectively) as measured by ex vivo micro-computed tomography (μCT) was higher at 20-mo. than at 6-mo. for both males and females, but ultimate moment as measured by three-point bending tests did not decrease with age. Cortical thickness was lower with age for males, but higher for old females. Partially accounting for differences in structure, material estimates of yield, ultimate stress, and toughness (left femur) were 12.6%, 11.1%, and 40.9% lower, respectively, with age for both sexes. The ability of the cortical bone to resist crack growth (right femur) was also 18.1% less for the old than for the young adult mice. These decreases in material properties were not due to changes in intracortical porosity as pore number decreased with age. Rather, age-related alterations in the matrix were observed for both sexes: enzymatic and non-enzymatic crosslinks by high performance liquid chromatography increased (femur), volume fraction of bound water by 1H-nuclear magnetic resonance relaxometry decreased (femur), cortical tissue mineral density by μCT increased (femur and radius), and an Amide I sub-peak ratio I1670/I1640 by Raman spectroscopy increased (tibia). Overall, there are multiple matrix changes to potentially target that could prevent the age-related decrease in fracture resistance observed in BALB/c mouse.

Network architecture strongly influences the fluid flow pattern through the lacunocanalicular network in human osteons

A popular hypothesis explains the mechanosensitivity of bone due to osteocytes sensing the load-induced flow of interstitial fluid squeezed through the lacunocanalicular network (LCN). However, the way in which the intricate structure of the LCN influences fluid flow through the network is largely unexplored. We therefore aimed to quantify fluid flow through real LCNs from human osteons using a combination of experimental and computational techniques. Bone samples were stained with rhodamine to image the LCN with 3D confocal microscopy. Image analysis was then performed to convert image stacks into mathematical network structures, in order to estimate the intrinsic permeability of the osteons as well as the load-induced fluid flow using hydraulic circuit theory. Fluid flow was studied in both ordinary osteons with a rather homogeneous LCN as well as a frequent subtype of osteons-so-called osteon-in-osteons-which are characterized by a ring-like zone of low network connectivity between the inner and the outer parts of these osteons. We analyzed 8 ordinary osteons and 9 osteon-in-osteons from the femur midshaft of a 57-year-old woman without any known disease. While the intrinsic permeability was 2.7 times smaller in osteon-in-osteons compared to ordinary osteons, the load-induced fluid velocity was 2.3 times higher. This increased fluid velocity in osteon-in-osteons can be explained by the longer path length, needed to cross the osteon from the cement line to the Haversian canal, including more fluid-filled lacunae and canaliculi. This explanation was corroborated by the observation that a purely structural parameter-the mean path length to the Haversian canal-is an excellent predictor for the average fluid flow velocity. We conclude that osteon-in-osteons may be particularly significant contributors to the mechanosensitivity of cortical bone, due to the higher fluid flow in this type of osteons.

Adding Marrow Adiposity and Cortical Porosity to Femoral Neck Areal Bone Mineral Density Improves the Discrimination of Women With Nonvertebral Fractures From Controls

Advancing age is accompanied by a reduction in bone formation and remodeling imbalance, which produces microstructural deterioration. This may be partly caused by a diversion of mesenchymal cells towards adipocytes rather than osteoblast lineage cells. We hypothesized that microstructural deterioration would be associated with an increased marrow adiposity, and each of these traits would be independently associated with nonvertebral fractures and improve discrimination of women with fractures from controls over that achieved by femoral neck (FN) areal bone mineral density (aBMD) alone. The marrow adiposity and bone microstructure were quantified from HR-pQCT images of the distal tibia and distal radius in 77 women aged 40 to 70 years with a recent nonvertebral fracture and 226 controls in Melbourne, Australia. Marrow fat measurement from HR-pQCT images was validated using direct histologic measurement as the gold standard, at the distal radius of 15 sheep, with an agreement (R² = 0.86, p < 0.0001). Each SD higher distal tibia marrow adiposity was associated with 0.33 SD higher cortical porosity, and 0.60 SD fewer, 0.24 SD thinner, and 0.72 SD more-separated trabeculae (all p < 0.05). Adjusted for age and FN aBMD, odds ratios (ORs) (95% CI) for fracture per SD higher marrow adiposity and cortical porosity were OR, 3.39 (95% CI, 2.14 to 5.38) and OR, 1.79 (95% CI, 1.14 to 2.80), respectively. Discrimination of women with fracture from controls improved when cortical porosity was added to FN aBMD and age (area under the receiver-operating characteristic curve [AUC] 0.778 versus 0.751, p = 0.006) or marrow adiposity was added to FN aBMD and age (AUC 0.825 versus 0.751, p = 0.002). The model including FN aBMD, age, cortical porosity, trabecular thickness, and marrow adiposity had an AUC = 0.888. Results were similar for the distal radius. Whether marrow adiposity and cortical porosity indices improve the identification of women at risk for fractures requires validation in prospective studies. © 2019 American Society for Bone and Mineral Research.

The role of EGFR signaling in age-related osteoporosis in mouse cortical bone

So far, there has been no effective cure for osteoporotic cortical bone, the most significant change in long bone structure during aging and the main cause of bone fragility fractures, because its underlying molecular and cellular mechanisms remain largely unknown. We used 3- and 15-mo-old mice as well as 15-mo-old mice treated with vehicle and gefitinib to evaluate structural, cellular, and molecular changes in cortical bone. We found that the senescence of osteoprogenitors was increased, whereas the expression of phosphorylated epidermal growth factor receptor (EGFR) on the endosteal surface of cortical bone down-regulated in middle-aged 15-mo-old mice compared with young 3-mo-old mice. Further decreasing EGFR signaling by gefitinib treatment in middle-aged mice resulted in promoted senescence of osteoprogenitors and accelerated cortical bone degeneration. Moreover, inhibiting EGFR signaling suppressed the expression of enhancer of zeste homolog 2 (Ezh2), the repressor of cell senescence-inducer genes, through ERK1/2 pathway, thereby promoting senescence in osteoprogenitors. Down-regulated EGFR signaling plays a physiologically significant role during aging by reducing Ezh2 expression, leading to the senescence of osteoprogenitors and the decline in bone formation on the endosteal surface of cortical bone.-Liu, G., Xie, Y., Su, J., Qin, H., Wu, H., Li, K., Yu, B., Zhang, X. The role of EGFR signaling in age-related osteoporosis in mouse cortical bone.

Regulatory mechanisms and clinical manifestations of musculoskeletal aging

Aging is the strongest risk factor for degenerative bone and joint diseases. Clinical therapies for age-related musculoskeletal disorders face significant challenges as their pathogenic mechanisms remain largely unclear. This review article focuses on the recent advances in the understanding of regulatory mechanisms of musculoskeletal aging and their clinical relevance. We begin with the prevalence and socioeconomic impacts of major age-related musculoskeletal disorders such as sarcopenia, osteoporosis, osteoarthritis, and degenerative tendinopathy. The current understanding of responsible biological mechanisms involved in general aging is then summarized. Proposed molecular, cellular, and biomechanical mechanisms relevant to the clinical manifestations of aging in the musculoskeletal system are discussed in detail, with a focus on the disorders affecting muscle, bone, articular cartilage, and tendon. Although musculoskeletal aging processes share many common pathways with the aging of other body systems, unique molecular and cellular mechanisms may be involved in the aging processes of musculoskeletal tissues. Advancements in the understanding of regulatory mechanisms of musculoskeletal aging may promote the development of novel treatments for age-related musculoskeletal disorders. Finally, future research directions for major musculoskeletal tissues including functional interaction between the tissues and their clinical relevance to age-related musculoskeletal disorders are highlighted in the Future Prospects section. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1475-1488, 2019.

External Bone Size Is a Key Determinant of Strength-Decline Trajectories of Aging Male Radii

Given prior work showing associations between remodeling and external bone size, we tested the hypothesis that wide bones would show a greater negative correlation between whole-bone strength and age compared with narrow bones. Cadaveric male radii (n = 37 pairs, 18 to 89 years old) were evaluated biomechanically, and samples were sorted into narrow and wide subgroups using height-adjusted robustness (total area/bone length). Strength was 54% greater (p < 0.0001) in wide compared with narrow radii for young adults (<40 years old). However, the greater strength of young-adult wide radii was not observed for older wide radii, as the wide (R² = 0.565, p = 0.001), but not narrow (R² = 0.0004, p = 0.944) subgroup showed a significant negative correlation between strength and age. Significant positive correlations between age and robustness (R² = 0.269, p = 0.048), cortical area (Ct.Ar; R² = 0.356, p = 0.019), and the mineral/matrix ratio (MMR; R² = 0.293, p = 0.037) were observed for narrow, but not wide radii (robustness: R² = 0.015, p = 0.217; Ct.Ar: R² = 0.095, p = 0.245; MMR: R² = 0.086, p = 0.271). Porosity increased with age for the narrow (R² = 0.556, p = 0.001) and wide (R² = 0.321, p = 0.022) subgroups. The wide subgroup (p < 0.0001) showed a significantly greater elevation of a new measure called the Cortical Pore Score, which quantifies the cumulative effect of pore size and location, indicating that porosity had a more deleterious effect on strength for wide compared with narrow radii. Thus, the divergent strength-age regressions implied that narrow radii maintained a low strength with aging by increasing external size and mineral content to mechanically offset increases in porosity. In contrast, the significant negative strength-age correlation for wide radii implied that the deleterious effect of greater porosity further from the centroid was not offset by changes in outer bone size or mineral content. Thus, the low strength of elderly male radii arose through different biomechanical mechanisms. Consideration of different strength-age regressions (trajectories) may inform clinical decisions on how best to treat individuals to reduce fracture risk. © 2019 American Society for Bone and Mineral Research.

Large cortical bone pores in the tibia are associated with proximal femur strength

Alterations of structure and density of cortical bone are associated with fragility fractures and can be assessed in vivo in humans at the tibia. Bone remodeling deficits in aging women have been recently linked to an increase in size of cortical pores. In this ex vivo study, we characterized the cortical microarchitecture of 19 tibiae from human donors (aged 69 to 94 years) to address, whether this can reflect impairments of the mechanical competence of the proximal femur, i.e., a major fracture site in osteoporosis. Scanning acoustic microscopy (12 μm pixel size) provided reference microstructural measurements at the left tibia, while the bone vBMD at this site was obtained using microcomputed tomography (microCT). The areal bone mineral density of both left and right femoral necks (aBMD_neck) was measured by dual‐energy X‐ray absorptiometry (DXA), while homogenized nonlinear finite element models based on high-resolution peripheral quantitative computed tomography provided hip stiffness and strength for one-legged standing and sideways falling loads. Hip strength was associated with aBMD_neck (r = 0.74 to 0.78), with tibial cortical thickness (r = 0.81) and with measurements of the tibial cross-sectional geometry (r = 0.48 to 0.73) of the same leg. Tibial vBMD was associated with hip strength only for standing loads (r = 0.59 to 0.65). Cortical porosity (Ct.Po) of the tibia was not associated with any of the femoral parameters. However, the proportion of Ct.Po attributable to large pores (diameter > 100 μm) was associated with hip strength in both standing (r = -0.61) and falling (r = 0.48) conditions. When added to aBMD_neck, the prevalence of large pores could explain up to 17% of the femur ultimate force. In conclusion, microstructural characteristics of the tibia reflect hip strength as well as femoral DXA, but it remains to be tested whether such properties can be measured in vivo.

Innervation is higher above Bone Remodeling Surfaces and in Cortical Pores in Human Bone: Lessons from patients with primary hyperparathyroidism

Mounting evidence from animal studies suggests a role of the nervous system in bone physiology. However, little is known about the nerve fiber localization to human bone compartments and bone surface events. This study reveals the density and distribution of nerves in human bone and the association of nerve profiles to bone remodeling events and vascular structures in iliac crest biopsies isolated from patients diagnosed with primary hyperparathyroidism (PHPT). Bone sections were sequentially double-immunostained for tyrosine hydroxylase (TH), a marker for sympathetic nerves, followed by protein gene product 9.5 (PGP9.5), a pan-neuronal marker, or double-immunostained for either PGP9.5 or TH in combination with CD34, an endothelial marker. In the bone marrow, the nerve profile density was significantly higher above remodeling surfaces as compared to quiescent bone surfaces. Ninety-five percentages of all nerve profiles were associated with vascular structures with the highest association to capillaries and arterioles. Moreover, vasculature with innervation was denser above bone remodeling surfaces. Finally, the nerve profiles density was 5-fold higher in the intracortical pores compared to bone marrow and periosteum. In conclusion, the study shows an anatomical link between innervation and bone remodeling in human bone.

Intracortical Bone Mechanics Are Related to Pore Morphology and Remodeling in Human Bone

During aging and in osteoporosis, cortical bone becomes more porous, making it more fragile and susceptible to fractures. The aim of this study was to investigate the intracortical compression- induced strain energy distribution, and determine whether intracortical pores associated with high strain energy density (SED) in the surrounding bone matrix have a different morphology and distribution, as well as different remodeling characteristics than matrix with normal SED. Fibular diaphyseal specimens from 20 patients undergoing a jaw reconstruction (age range 41 to 75 years; 14 men and 6 women) were studied. Bone specimens were µCT-scanned, plastic embedded, and sectioned for histology. Three-dimensional microfinite element models of each specimen were tested in compression, and the SED of the bone immediately surrounding the intracortical pores was calculated within a plane of interest corresponding to the histological sections. The SED of a pore, relative to the distribution of the SED of all pores in each specimen, was used to classify pores as either a high or normal SED pore. Pores with high SED were larger, less circular, and were located closer to the endosteal surface of the cortex than normal SED pores (p < 0.001). Histological analysis of the remodeling events generating the pores revealed that the high SED pores compared with normal SED pores had 13.3-fold higher odds of being an erosive (70%) or formative (7%) pore versus a quiescent pore (p < 0.001), 5.9-fold higher odds of resulting from remodeling upon existing pores (type 2 pore) versus remodeling generating new pores (type 1 pore) (p < 0.001), and 3.2-fold higher odds of being a coalescing type 2 pore versus a noncoalescing type 2 pore (p < 0.001). Overall, the study demonstrates a strong relationship between cortical bone mechanics and pore morphology, distribution, and remodeling characteristics in human fibular bone. © 2018 American Society for Bone and Mineral Research.

Understanding age-induced cortical porosity in women: Is a negative BMU balance in quiescent osteons a major contributor?

Cortical bone is remodeled by intracortical basic multicellular units (BMUs), whose end result can be observed as quiescent osteons in histological sections. These osteons offer a unique opportunity to investigate the BMU balance between the magnitude of bone resorption and subsequent bone formation at the BMU level. Our main objective was to investigate whether the latter parameters change between defined categories of osteons and with age, and to which extend these changes contribute to age-induced cortical porosity. Cortices of iliac bone specimens from 35 women (aged 16-78 years) with a higher porosity with age were investigated. A total of 3084 quiescent osteons reflecting 75% of the intracortical pores were histological examined. The osteons diameter, pore diameter, wall thickness, prevalence and contribution to the porosity were highly variable, but unchanged with age. Next, the osteons were categorized according to whether they reflected the remodeling of existing canals (type 2Q osteons) or the generation of new canals (type 1Q osteons). Type 2Q osteons versus type 1Q osteons: (i) had more frequently a pore diameter > 75 μm (7.4 vs. 1.3%; p < 0.001); (ii) had a larger mean pore diameter (40 ± 10 vs. 25 ± 4 μm; p < 0.001), osteon diameter (120 ± 21 vs. 94 ± 21 μm; p < 0.001) and wall thickness (40 ± 10 vs. 35 ± 9; p < 0.05); (iii) had a larger contribution to the cortical porosity (29 ± 18 vs. 8 ± 8%; p < 0.001); (iv) were more prevalent (44 ± 10 vs. 31 ± 11%; p < 0.001); and (v) were more prevalent with age. Collectively, this study demonstrates that quiescent osteons with age more frequently result from remodeling of existing canals, which in some cases had a more negative BMU balance. Still, the osteons showed no overall age-related change in their pore diameter i.e. BMU balance. In contrast to conventional wisdom, these data show that non-quiescent pores, not pores of quiescent osteons, were the main contributor to a higher cortical porosity.

The Influence of Cortical Porosity on the Strength of Bone During Growth and Advancing Age

PURPOSE OF REVIEW

Bone densitometry provides a two-dimensional projected areal apparent bone mineral density that fails to capture the heterogeneity of bone's material composition and macro-, micro-, and nano-structures critical to its material and structural strength. Assessment of the structural basis of bone fragility has focused largely on trabecular bone based on the common occurrence of fragility fractures at sites with substantial amounts of trabecular bone. This review focuses on the contribution of cortical bone to bone fragility throughout life.

RECENT FINDINGS

Accurately differentiating cortical and trabecular bone loss has important implications in quantifying bone fragility as these compartments have differing effects on bone strength. Recent advances in imaging methodology have improved distinction of these two compartments by (i) recognition of a cortico-trabecular transitional zone and (ii) quantifying bone microstructure in a region of interest that is a percentage of bone length rather than a fixed point. Additionally, non-invasive three-dimensional imaging methods allow more accurate quantification of changes in the cortical, trabecular, and cortico-trabecular compartments during growth, aging, disease, and treatment. Over 75% of the skeleton is assembled as cortical bone. Of all fragility fractures, ~ 80% are appendicular and involve regions rich in cortical bone and ~ 70% of all age-related appendicular bone loss is cortical and is mainly due to unbalanced intracortical remodeling which increases cortical porosity. The failure to achieve the optimal peak bone microstructure during growth due to disease and the deterioration in cortical and trabecular bone produced by bone loss compromise bone strength. The loss of strength produced by microstructural deterioration is disproportionate to the bone loss producing this deterioration. The reason for this is that the loss of strength increases as a 7th power function of the rise in cortical porosity and a 3rd power function of the fall in trabecular density (Schaffler and Burr in J Biomech. 21(1):13-6, 1988), hence the need to quantify bone microstructure.