Observations on two types of resorption cavities in human lamellar cortical bone

Generating Virtual Bone Scans for the Purpose of Investigating the Effects of Cortical Microstructure

Evaluating the contribution of microstructure to overall bone strength is tricky since it is difficult to control changes to pore structure in human or animal samples. We developed an open-source program that can generate three-dimensional (3D) models of micron-scale cortical bone. These models can be highly customized with a wide array of variable input parameters to allow for generation of samples similar to micro-computed topography scans of cortical bone or with specific geometric features. The program can generate samples with specific desired porosities and minor deviations in pore diameter from human samples: 1.67% (±4.90) using literature values, and 1.36% (±2.39) with optimized values. When coupled with finite element analysis, this open-source program could be a useful tool for evaluating stress distributions caused by microstructural changes.

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.

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.

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.

Relation between cross-sectional bone geometry and double zonal osteon frequency and morphology

OBJECTIVES

While double-zonal osteons (DZ) are characterized by a hyper-mineralized ring inside their lamellae, recent findings suggest that this ring is also defined by a change in the collagen fibers' orientation. Collagen and minerals are essential components to the maintenance of adequate bone strength and their alteration can modify the mechanical properties of the bone tissue. Consequently, the aim of this study is to explore the effect of past loads, as estimated from cross-sectional geometric properties, on the formation of DZ osteons compared to type I (common) osteons.

MATERIALS AND METHODS

The sample consists of paired humerus and femur midshaft sections (n = 23) of Eurocanadian settlers from the historical St. Matthew cemetery, Quebec City (1771-1860). Histomorphometric variables included in this study are osteon density for DZ and type I osteons (DZD; OPD), osteon area (DZOn.Ar; On. Ar), Haversian canal area (DZH.Ar; H.Ar), and the area within the hypermineralized ring (HR. Ar). Loading history is estimated from cross-sectional properties including the following variable: cortical and total area (CA, TA), maximum and minimum second moment of area (I_max , I_min ) and polar moment of area (J).

RESULTS

When the humerus and femur of the same individuals are compared, the femur has a higher OPD, DZD, and relative DZD (DZD/OPD). DZ osteons have a smaller area and Haversian canal area compared to type I osteons. The area within the hypermineralized ring in DZ is higher than the Haversian canal area of the type I osteons. Correlations between the residual scores of the regression of histomorphometric variables and cross-sectional properties of the humerus on the femur were not significant.

DISCUSSION

Based on the analysis of the entire cross-section, the lack of correlation between variations in cross-sectional properties and remodeling combined with the significant differences between humeri and femura suggests that the creation of DZ or type I osteons in the bone tissue might be due to a bone specific response, possibly related to differences in bone tissue age that needs to be further investigated. Definitive conclusion regarding biomechanical loads still seem to be premature as regional variations associated with mechanical properties remain to be explored.

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.

Brief communication: Test of a method to identify double-zonal osteon in polarized light microscopy

OBJECTIVES

Double-zonal osteons (DZ) have been of interest in paleopathological research because they might be linked to physiological pathology. DZ are thought to be evidence of arrested osteon formation with a brief but abrupt increase in mineralization of lamellae occurring during bone remodeling. Originally identified from microradiographs as hypermineralized rings, recent studies have identified DZ from linear polarized light microscopy (PLM). However, PLM does not guarantee the adequate detection of DZ since PLM captures bone birefringence and not hyper-mineralization. Scanning electron microscopy with backscatter electrons (BSE-SEM) allows observation of DZ by detecting differences in mineralization. The purpose of this study is to investigate whether DZ, as identified by BSE-SEM, can indeed be identified with PLM.

MATERIALS AND METHODS

The sample consists of an archaeological collection of adult midshaft femurs (n = 30) from St. Matthew cemetery, Quebec City (1771-1860). DZ were identified and counted independently with PLM and BSE-SEM for the same sections. Results from both methods were compared.

RESULTS

Chi-square test shows that there was no significant difference between the two methods (p = 0.404). No significant bias was found on Bland-Altman analysis and Cohen's kappa shows a substantial agreement between the two methods (Κ = 0.66). PLM shows a good accuracy (sensitivity 79%, specificity 99.4%) and reliability (Positive Predictive Value: 86.71%; Negative Predictive Value: 99.45%).

DISCUSSION

These findings indicate that the two methods are interchangeable. PLM, using our proposed protocol, is reliable to accurately identify DZ. We discuss how PLM and BSE-SEM that measure different features of the bone tissue can converge on the identification of DZ.

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

Intracortical bone remodeling normally ensures maintenance of the cortical bone matrix and strength, but during aging, this remodeling generates excessive porosity. The mechanism behind the age-induced cortical porosity is poorly understood and addressed in the present study. This study consists of a histomorphometric analysis of sections of iliac bone specimens from 35 women (age 16-78 years). First, the study shows that the age-induced cortical porosity reflects an increased pore size rather than an increased pore density. Second, it establishes a novel histomorphometric classification of the pores, which is based on the characteristics of the remodeling sites to which each pore is associated. It takes into consideration (i) the stage of the remodeling event at the level where the pore is sectioned, (ii) whether the event corresponds with the generation of a new pore through penetrative tunneling (type 1 pores) or with remodeling of an existing pore (type 2 pores), and (iii) in the latter case, whether or not the new remodeling event leads to the coalescence of pores. Of note, the advantage of this classification is to relate porosity with its generation mechanism. Third, it demonstrates that aging and porosity are correlated with: a shift from type 1 to type 2 pores, reflecting that the remodeling of existing pores is higher; an accumulation of eroded type 2 pores, reflecting an extended resorption-reversal phase; and a coalescence of these eroded type 2 pores into enlarged coalescing type 2 cavities. Collectively, this study supports the notion, that age-related increase in cortical porosity is the result of intracortical remodeling sites upon existing pores, with an extended reversal-resorption phase (eroded type 2 pores) that may likely result in a delayed or absent initiation of the subsequent bone formation. © 2017 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals Inc.

Coupling of Bone Resorption and Formation in Real Time: New Knowledge Gained From Human Haversian BMUs

It is well known that bone remodeling starts with a resorption event and ends with bone formation. However, what happens in between and how resorption and formation are coupled remains mostly unknown. Remodeling is achieved by so-called basic multicellular units (BMUs), which are local teams of osteoclasts, osteoblasts, and reversal cells recently proven identical with osteoprogenitors. Their organization within a BMU cannot be appropriately analyzed in common histology. The originality of the present study is to capture the events ranging from initiation of resorption to onset of formation as a functional continuum. It was based on the position of specific cell markers in longitudinal sections of Haversian BMUs generating new canals through human long bones. It showed that initial resorption at the tip of the canal is followed by a period where newly recruited reversal/osteoprogenitor cells and osteoclasts alternate, thus revealing the existence of a mixed "reversal-resorption" phase. Three-dimensional reconstructions obtained from serial sections indicated that initial resorption is mainly involved in elongating the canal and the additional resorption events in widening it. Canal diameter measurements show that the latter contribute the most to overall resorption. Of note, the density of osteoprogenitors continuously grew along the "reversal/resorption" surface, reaching at least 39 cells/mm on initiation of bone formation. This value was independent of the length of the reversal/resorption surface. These observations strongly suggest that bone formation is initiated only above a threshold cell density, that the length of the reversal/resorption period depends on how fast osteoprogenitor recruitment reaches this threshold, and thus that the slower the rate of osteoprogenitor recruitment, the more bone is degraded. They lead to a model where the newly recognized reversal/resorption phase plays a central role in the mechanism linking osteoprogenitor recruitment and the resorption-formation switch. © 2017 American Society for Bone and Mineral Research.

Cortical Bone Porosity: What Is It, Why Is It Important, and How Can We Detect It?

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.

Three-dimensional reconstruction of Haversian systems in human cortical bone using synchrotron radiation-based micro-CT: morphology and quantification of branching and transverse connections across age

This study uses synchrotron radiation-based micro-computed tomography (CT) scans to reconstruct three-dimensional networks of Haversian systems in human cortical bone in order to observe and analyse interconnectivity of Haversian systems and the development of total Haversian networks across different ages. A better knowledge of how Haversian systems interact with each other is essential to improve understanding of remodeling mechanisms and bone maintenance; however, previous methodological approaches (e.g. serial sections) did not reveal enough detail to follow the specific morphology of Haversian branching, for example. Accordingly, the aim of the present study was to identify the morphological diversity of branching patterns and transverse connections, and to understand how they change with age. Two types of branching morphologies were identified: lateral branching, resulting in small osteon branches bifurcating off of larger Haversian canals; and dichotomous branching, the formation of two new osteonal branches from one. The reconstructions in this study also suggest that Haversian systems frequently target previously existing systems as a path for their course, resulting in a cross-sectional morphology frequently referred to as 'type II osteons'. Transverse connections were diverse in their course from linear to oblique to curvy. Quantitative assessment of age-related trends indicates that while in younger human individuals transverse connections were most common, in older individuals more evidence of connections resulting from Haversian systems growing inside previously existing systems was found. Despite these changes in morphological characteristics, a relatively constant degree of overall interconnectivity is maintained throughout life. Altogether, the present study reveals important details about Haversian systems and their relation to each other that can be used towards a better understanding of cortical bone remodeling as well as a more accurate interpretation of morphological variants of osteons in cross-sectional microscopy. Permitting visibility of reversal lines, synchrotron radiation-based micro-CT is a valuable tool for the reconstruction of Haversian systems, and future analyses have the potential to further improve understanding of various important aspects of bone growth, maintenance and health.

Magnetic resonance imaging assessed cortical porosity is highly correlated with μCT porosity

Cortical bone is typically regarded as "MR invisible" with conventional clinical magnetic resonance imaging (MRI) pulse sequences. However, recent studies have demonstrated that free water in the microscopic pores of cortical bone has a short T2* but a relatively long T2, and may be detectable with conventional clinical spin echo (SE) or fast spin echo (FSE) sequences. In this study we describe the use of a conventional two-dimensional (2D) FSE sequence to assess cortical bone microstructure and measure cortical porosity using a clinical 3T scanner. Twelve cadaveric human cortical bone samples were studied with MRI and microcomputed tomography (μCT) (downsampled to the same spatial resolution). Preliminary results show that FSE-determined porosity is highly correlated (R(2)=0.83; P<0.0001) with μCT porosity. Bland-Altman analysis suggested a good agreement between FSE and μCT with tight limit of agreement at around 3%. There is also a small bias of -2% for the FSE data, which suggested that the FSE approach slightly underestimated μCT porosity. The results demonstrate that cortical porosity can be directly assessed using conventional clinical FSE sequences. The clinical feasibility of this approach was also demonstrated on six healthy volunteers using 2D FSE sequences as well as 2D ultrashort echo time (UTE) sequences with a minimal echo time (TE) of 8μs, which provide high contrast imaging of cortical bone in vivo.

The long bone deformity of osteogenesis imperfecta III: analysis of structural changes carried out with scanning electron microscopic morphometry

The wedges of the mid-diaphyseal osteotomies carried out to correct the femoral and/or tibial native deformity in type III osteogenesis imperfecta (OI III) were used to study the remodeling patterns and lamellar organization at the level of the major deformity. Histology and scanning electron microscopy (SEM) morphology showed abnormal cortical remodeling characterized by the failure to form a cylinder of compact bone with a regular marrow canal. Atypical, flattened, and large resorption lacunae with a wide resorption front on one side and systems of parallel lamellae on the opposite side were observed, resembling those formerly reported as drifting osteons. SEM morphometry documented a higher percentage of nonossified vascular/resorption area (44.3 %) in OI than in controls (13.6 %), a lower density of secondary osteons, and lower values for the parameters expressing the individual osteon size. The mean osteon total area, the mean central canal area, and the mean osteon bone area of two selected, randomized populations of secondary osteons were significantly higher (p < 0.001, p = 0.028, and p < 0.001, respectively) in control bones than in OI. The mean ossified matrix area was not significantly different, but the mean secondary osteon number and mean density were higher in controls (both p < 0.001). Osteon wedges were carried out to correct the native deformity of OI III and morphologic analysis suggested that the abnormal remodeling pattern (with "drifting osteons") may result from the altered load and tensile stresses on the deformed tubular bones.

Are Distributions of Secondary Osteon Variants Useful for Interpreting Load History in Mammalian Bones?

BACKGROUND/AIMS

In cortical bone, basic multicellular units (BMUs) produce secondary osteons that mediate adaptations, including variations in their population densities and cross-sectional areas. Additional important BMU-related adaptations might include atypical secondary osteon morphologies (zoned, connected, drifting, elongated, multiple canal). These variants often reflect osteonal branching that enhances toughness by increasing interfacial (cement line) complexity. If these characteristics correlate with strain mode/magnitude-related parameters of habitual loading, then BMUs might produce adaptive differences in unexpected ways.

METHODS

We carried out examinations in bones loaded in habitual torsion (horse metacarpals) or bending: sheep, deer, elk, and horse calcanei, and horse radii. Atypical osteons were quantified in backscattered images from anterior, posterior, medial, and lateral cortices. Correlations were determined between atypical osteon densities, densities of all secondary osteons, and associations with habitual strain mode/magnitude or transcortical location.

RESULTS

Osteon variants were not consistently associated with 'tension', 'compression', or neutral axis ('shear') regions, even when considering densities or all secondary osteons, or only osteon variants associated with relatively increased interfacial complexity. Similarly, marrow- and strain-magnitude-related associations were not consistent.

CONCLUSION

These data do not support the hypothesis that spatial variations in these osteon variants are useful for inferring a habitual bending or torsional load strain history.

Three-dimensional microcomputed tomography imaging of basic multicellular unit-related resorption spaces in human cortical bone

This study employed microcomputed tomography (micro-CT) as a novel means for visualizing the morphology and quantifying the range (length) of basic multicellular unit (BMU)-related resorption spaces in human cortical bone. We tested the hypotheses that the density and range of spaces vary with age and sex. The sample included 82 human (18-92 years) anterior femoral midshaft samples. The morphology of the spaces (n = 99) was varied, including unidirectional, bidirectional, branched, and even highly clustered forms. The density of resorption spaces was negatively correlated with age for the combined sexes and females, with Spearman's rho values of -0.355 (P < 0.001) and -0.522 (P = 0.002), respectively. The density of spaces did not differ significantly between the sexes (P = 0.735). Mean range +/- SD for the combined sexes, females, and males was 2,706 +/- 1,177, 2,681 +/- 1,247, and 2,718 +/- 1,150 microm, respectively. Numerical simulation of the effect of the 7,000 microm scan field of view suggested that the actual mean range of the spaces for the pooled sample was actually on the order of 3,770 microm. Range did not correlate significantly with age for the combined sexes (P = 0.587) or females (P = 0.345) and males (P = 0.896) considered separately and was not significantly different (P = 0.883) between the sexes. These results suggest that the range of BMUs is not affected by age. The age-dependent decrease in resorption space density for the females and pooled sexes was most likely a consequence of cortical rarefaction, leading to difficulty detecting resorption spaces with micro-CT, rather than a decrease in overall remodeling activity.

Bone's mechanostat: a 2003 update

The still-evolving mechanostat hypothesis for bones inserts tissue-level realities into the former knowledge gap between bone's organ-level and cell-level realities. It concerns load-bearing bones in postnatal free-living bony vertebrates, physiologic bone loading, and how bones adapt their strength to the mechanical loads on them. Voluntary mechanical usage determines most of the postnatal strength of healthy bones in ways that minimize nontraumatic fractures and create a bone-strength safety factor. The mechanostat hypothesis predicts 32 things that occur, including the gross anatomical bone abnormalities in osteogenesis imperfecta; it distinguishes postnatal situations from baseline conditions at birth; it distinguishes bones that carry typical voluntary loads from bones that have other chief functions; and it distinguishes traumatic from nontraumatic fractures. It provides functional definitions of mechanical bone competence, bone quality, osteopenias, and osteoporoses. It includes permissive hormonal and other effects on bones, a marrow mediator mechanism, some limitations of clinical densitometry, a cause of bone "mass" plateaus during treatment, an "adaptational lag" in some children, and some vibration effects on bones. The mechanostat hypothesis may have analogs in nonosseous skeletal organs as well.

Quantitative 3D analysis of the canal network in cortical bone by micro-computed tomography

Cortical bone is perforated by an interconnected network of porous canals that facilitate the distribution of neurovascular structures throughout the cortex. This network is an integral component of cortical microstructure and, therefore, undergoes continual change throughout life as the cortex is remodeled. To date, the investigation of cortical microstructure, including the canal network, has largely been limited to the two-dimensional (2D) realm due to methodological hurdles. Thanks to continuing improvements in scan resolution, micro-computed tomography (muCT) is the first nondestructive imaging technology capable of resolving cortical canals. Like its application to trabecular bone, muCT provides an efficient means of quantifying aspects of 3D architecture of the canal network. Our aim here is to introduce the use of muCT for this application by providing examples, discussing some of the parameters that can be acquired, and relating these to research applications. Although several parameters developed for the analysis of trabecular microstructure are suitable for the analysis of cortical porosity, the algorithm used to estimate connectivity is not. We adapt existing algorithms based on skeletonization for this task. We believe that 3D analysis of the dimensions and architecture of the canal network will provide novel information relevant to many aspects of bone biology. For example, parameters related to the size, spacing, and volume of the canals may be particularly useful for investigation of the mechanical properties of bone. Alternatively, parameters describing the 3D architecture of the canal network, such as connectivity between the canals, may provide a means of evaluating cumulative remodeling related change.

The pathophysiology of bone loss

Aging is associated with a decline in cancellous and cortical bone mass and with a deterioration of microarchitecture in both skeletal compartments. These changes are more marked in women than men and are exaggerated in patients with fracture. With the insight gained from histomorphometry, we are beginning to understand the cellular mechanisms that underlie these changes. We recognize that deterioration in microarchitecture contributes to fracture risk, independently of bone mass. Techniques to assess bone microarchitecture noninvasively in a clinical setting are currently under development; it is likely that advances in this area will improve our ability to identify and manage patients with osteoporosis in the not too distant future.

Cancellous bone remodeling occurs in specialized compartments lined by cells expressing osteoblastic markers

We describe a sinus, referred to as a bone remodeling compartment (BRC), which is intimately associated with cancellous bone remodeling. The compartment is lined on its marrow side by flattened cells and on its osseous side by the remodeling bone surface, resembling a roof of flattened cells covering the bone surface. The flat marrow lining cells are in continuity with the bone lining cells at the margins of the BRC. We examined a large number of diagnostic bone biopsy specimens received during recent years in the department. Furthermore, 10 patients (8 women and 2 men, median age 56 [40-69] years) with the high turnover disease of primary hyperparathyroidism who were treated with parathyroidectomy and followed for 3 years were included in the histomorphometric study. Bone samples for the immuno-enzyme staining were obtained from an amputated extremity of child. The total cancellous bone surface covered by BRC decreases by 50% (p < 0.05) following normalization of turnover and is paralleled by a similar 50% decrease in remodeling surface (p < 0.05). The entire eroded surface and two-thirds of the osteoid surface are covered by a BRC. BRC-covered uncompleted walls are 30% (p < 0.05) thinner than those without a BRC. This indicates that the BRC is invariably associated with the early phases of bone remodeling, that is, bone resorption, whereas it closes during the late part of bone formation. Immuno-enzyme staining shows that the flat marrow lining cells are positive for alkaline phosphatase, osteocalcin, and osteonectin, suggesting that they are bone cells. The first step in cancellous bone remodeling is thought to be the lining cells digesting the unmineralized matrix membrane followed by their disappearance and the arrival of the bone multicellular unit (BMU). We suggest that the lining cell barrier persists during bone remodeling; that the old lining cells become the marrow lining cells, allowing bone resorption and bone formation to proceed under a common roof of lining cells; that, at the end of bone formation, new bone lining cells derived from the flattened osteoblasts replace the marrow lining cells thereby closing the BRC; and that the two layers of lining cells eventually becomes a single layer. The integrity of the osteocyte-lining cell system is reestablished by the new generation of lining cells. The BRC most likely serves multiple purposes, including efficient exchange of matrix constituents and minerals, routing, monitoring, or modulating bone cell recruitment, and possibly the anatomical basis for the coupling of bone remodeling.

From Wolff's law to the Utah paradigm: insights about bone physiology and its clinical applications

Efforts to understand our anatomy and physiology can involve four often overlapping phases. We study what occurs, then how, then ask why, and then seek clinical applications. In that regard, in 1960 views, bone's effector cells (osteoblasts and osteoclasts) worked chiefly to maintain homeostasis under the control of nonmechanical agents, and that physiology had little to do with anatomy, biomechanics, tissue-level things, muscle, and other clinical applications. But it seems later-discovered tissue-level mechanisms and functions (including biomechanical ones, plus muscle) are the true key players in bone physiology, and homeostasis ranks below the mechanical functions. Adding that information to earlier views led to the Utah paradigm of skeletal physiology that combines varied anatomical, clinical, pathological, and basic science evidence and ideas. While it explains in a general way how strong muscles make strong bones and chronically weak muscles make weak ones, and while many anatomists know about the physiology that fact depends on, poor interdisciplinary communication left people in many other specialties unaware of it and its applications. Those applications concern 1.) healing of fractures, osteotomies, and arthrodeses; 2.) criteria that distinguish mechanically competent from incompetent bones; 3.) design criteria that should let load-bearing implants endure; 4.) how to increase bone strength during growth, and how to maintain it afterwards on earth and in microgravity situations in space; 5.) how and why healthy women only lose bone next to marrow during menopause; 6.) why normal bone functions can cause osteopenias; 7.) why whole-bone strength and bone health are different matters; 8.) why falls can cause metaphyseal and diaphyseal fractures of the radius in children, but mainly metaphyseal fractures of that bone in aged adults; 9.) which methods could best evaluate whole-bone strength, "osteopenias" and "osteoporoses"; 10.) and why most "osteoporoses" should not have bone-genetic causes and some could have extraosseous genetic causes. Clinical specialties that currently require this information include orthopaedics, endocrinology, radiology, rheumatology, pediatrics, neurology, nutrition, dentistry, and physical, space and sports medicine. Basic science specialties include absorptiometry, anatomy, anthropology, biochemistry, biomechanics, biophysics, genetics, histology, pathology, pharmacology, and cell and molecular biology. This article reviews our present general understanding of this new bone physiology and some of its clinical applications and implications. It must leave to other times, places, and people the resolution of questions about that new physiology, and to understand the many devils that should lie in its details. (Thompson D'Arcy, 1917).

Estimation of volume referent bone turnover in the otic capsule after sequential point labeling

Using fluorochrome labeling and a newly validated method for bone turnover estimation, we determined absolute values for canine perilabyrinthine bone remodeling. The overall capsular bone turnover was found to be 2.1% per year, compared to 13.9% per year for the neighboring cranial bones and 7.4% per year for the humerus compacta. This gross 2.1% per year conceals a vast range, from 0.13% per year for the innermost perilymphatic zone, through a centrifugal increment toward 8% to 10% per year in the periphery. The underlying individual bone remodeling units exhibit a similar centrifugal pattern in numerical density and size. These findings indicate an inhibition of remodeling, seemingly emanating from the perilymphatic spaces, and affecting both the activation of osteoclasts and the extent of resorption by the osteoclasts. These values satisfactorily explain the preservation of such fetal remnants as the globuli ossei, the interglobular spaces, and the skein bone. In humans, local ineffective inhibition of bone resorption may play a role in the initiation of otosclerosis.

Cortical remodeling following suppression of endogenous estrogen with analogs of gonadotrophin releasing hormone

The effects of estrogen suppression on osteonal remodeling in young women was investigated using transiliac biopsies (eight paired biopsies + four single pre; three single post biopsies) taken before and after treatment for endometriosis (6 months) with analogs of gonadotrophin releasing hormone (GnRH). Estrogen withdrawal increased the proportion of Haversian canals with an eroded surface (106%, p = 0.047), a double label (238%, p = 0.004), osteoid (71%, p = 0.002), and alkaline phosphatase (ALP) 116%, p = 0.043) but not those showing tartrate-resistant acid phosphatase (TRAP) activity (p = 0.25) or a single label (p = 0.30). Estrogen withdrawal increased TRAP activity in individual osteoclasts in canals with diameters greater than 50 microns (p = 0.0089) and also the number of osteons with diameters over 250 microns (p = 0.049). ALP activity in individual osteoblasts was increased but not significantly following treatment (p = 0.051). Wall thickness was significantly correlated with osteon diameter (p < 0.001). In a separate group of patients (four pairs + one post biopsy) on concurrent treatment with tibolone, there was no significant increase in the osteon density, cortical porosity, median canal diameter, or the markers of bone formation and resorption. Enzyme activities and numbers of active canals were also not increased with the concurrent treatment, but there was still an increase in the osteon diameter. As previously shown for cancellous bone, estrogen withdrawal increased cortical bone turnover. We have now shown that resorption depth within Haversian systems was also increased with treatment. The enhanced TRAP activity in individual osteoclasts supports the concept that osteoclasts are more active following estrogen withdrawal in agreement with theoretical arguments advanced previously. Understanding the cellular and biochemical mechanisms responsible for increased depth of osteoclast resorption when estrogen is withdrawn may allow the development of new strategies for preventing postmenopausal bone loss.

Evidence for interrupted bone resorption in human iliac cancellous bone

Bone resorption and formation are coupled both in time and space and may occur simultaneously in the same remodeling unit. A number of studies have shown that the formative phase of the remodeling sequence may undergo temporary interruptions prior to completion and it is possible that bone resorption may be subject to similar interruptions. We have investigated this hypothesis by studying the distribution of eroded depth in resorption cavities in human cancellous bone. Eroded depth was assessed in iliac crest cancellous bone from 41 normal healthy subjects using a cubic spline curve fitting technique. The distribution of mean eroded depths was skewed to the right. Comparison of the observed distribution with an expected distribution, which was calculated from previously published data and assumes resorption begins rapidly and slows as it approaches completion, showed a significantly greater proportion of shallower cavities than expected (p<0001). Similarly, comparison of observed and uniform distributions, which assumes a constant rate of resorption throughout the erosion period, also showed a significantly greater proportion of smaller cavities (p<0.01). In subjects aged less than 39 years, there were fewer small cavities than in those aged 40-59 years. In addition, there was some evidence that females of 40-59 years had a proportionately greater number of smaller cavities than males; however, there were no differences in other age groups. Our results demonstrate a significantly greater proportion of smaller resorption cavities than would be expected from current models of bone remodeling and are consistent with the hypothesis that resorption undergoes temporary interruptions and/or permanent arrest during the process of bone remodeling.

Hypermineralized lamellae below the bone surface: a quantitative microradiographic study

Hypermineralized lamellae similar to interstitial resting lines were identified by microradiography beneath haversian, endocortical, and trabecular resting surfaces in 42 subjects aged 18-96 years. In cross-sectioned osteons, this hypermineralized lamella appeared as a circle showing the same high microdensity as the inner margin of the haversian canal. Bone tissue separating this circle and the margin was more mineralized than the peripheral lamellae of the osteon. In the tibia, 13.7 +/- 0.9% (Mean +/- SE) of Haversian canals exhibited a hypermineralized circle, localized at a distance of 20 +/- 0.4 microns from the canal wall. The scalloped haversian canals, different from osteoclastic resorption cavities, represented 9.9 +/- 0.7% of the haversian canals. There was a significant correlation between both types of haversian canals. Therefore, hypermineralized lamellae can appear at the end of bone apposition, whatever the bone surface. Their high mineral content may secondarily lead to an increased brittleness of the most superficial lamellae and even to their disaggregation.

Short-term oral phosphate and bone remodeling in beagles

Attempts to increase bone volume in osteoporotic patients are still in the experimental stage. The coherence therapy, proposed by Frost, suggests that the activated bone units can remodel bone matrix in tandem. The cells (i.e., osteoclast and osteoblast which compose the remodeling units) are manipulated through specific medications timed to each of their duration of actions. The current study was to examine the effect of withdrawal of oral phosphate on bone in ovariectomized dogs. The present report demonstrates the capability of short-term oral phosphate to activate bone remodeling in the ovariectomized animal model. Results from biochemical and histomorphometric analyses confirm that remodeling units are activated following the release of parathyroid hormone. This transient scenario inflicts a shift of mineral density distribution in cancellous bone matrix of the iliac crest. Nevertheless, the bone remodeling units appear to be synchronized with each other and thus their resorptive and formative phases should be amenable to further pharmacological manipulation.

Distribution of bone remodeling units in the otic capsule of the rabbit. A semiquantitative morphometric study

Distribution of bone remodeling units (BRU) in relation to the perilymphatic space was studied in undecalcified temporal bones from adult rabbits labeled in vivo with bone-seeking fluorochromes. Based on recordings of focal bone formation, relative densities of BRUs inside concentric tissue zones around the inner ear spaces were estimated. Zonal densities of BRUs were found to decline towards the perilymphatic space, lending further support to the existence of a local inner ear mechanism in control of capsular bone tissue dynamics. The possible nature of this mechanism is considered briefly with special reference to inner ear electromechanic activity.

Longitudinal microradioscopic comparisons on endosteal and juxtaendosteal bone loss in premenopausal and postmenopausal women, and in those with end-stage renal disease

Endosteal bone resorption is the principal mechanism of bone loss in involutional osteoporosis and in renal osteodystrophy. In the cortical bone it is often accompanied by juxtaendostal bone resorption. Using fine-detail radiographs and x6 magnified viewing, longitudinal radiographic observations and measurements were made on these two forms of bone resorption in the metacarpals II, III, and IV in three groups of women: (1) premenopausal, (2) postmenopausal, and (3) patients with end-stage renal disease. Bone loss was found to be negligible in the premenopausal women, but in postmenopausal and renal patients both endosteal and juxtaendosteal bone resorption were frequently demonstrable. It is suggested that when a base-line fine-detail hand radiograph is obtained at the time of the menopause, follow-up radiographs may permit detection of relatively early endosteal and juxtaendosteal bone loss by comparing the respective areas in metacarpals with those of the original radiograph. Since the methodology does not require expensive equipment, has a low intraobserver error and is simple to perform, it may deserve to be further evaluated in studies aimed at developing a simple and inexpensive approach as a screening method for early detection of postmenopausal osteoporosis.

Radiographically detectable intracortical porosity. The dimensions and frequencies of its components in hand bones of normal men and women

Since the measurement of intracortical resorptive spaces by histologic methods is difficult and very few data are available in normal humans, we have measured their lengths and widths and calculated the intracortical porosity in metacarpals and phalanges of 79 normal women and 69 normal men, using fine-detail radiographs of the hands and a computerized semi-automatic image analysis system (Zeiss MOP-3), this being the first study of this kind. Several methodological problems were solved satisfactorily, and the results of this study could serve as a data bank for further investigations concerned with intracortical resorption. Significant differences were found between age and sex versus several intracortical resorptive parameters; also significant correlations were found with age in some cases. Normal intracortical porosity was found to be about three times greater in the proximal phalanges than in the metacarpals. It is concluded that this methodology could be used for further studies of intracortical resorption in osteoporosis and other metabolic bone diseases.

Microradiographic and histomorphometric indices of mandibles for diagnosis of osteopenia

Mandibular autopsy specimens of 100 Danish subjects without known systemic diseases were obtained. Microradiograms of 100-micrometer-thick undemineralized vertical buccolingual grounds cross-sections and these sections, stained with basic fuchsin, were used. Microradiographic and histologic readings of bone turnover foci on Haversian canal and endosteal-trabecular surfaces were compared and combined. The analyses show the following. 1) number/area Haversian canals with bone formation and resorption foci and total of Haversian canals are reproducible and representative measures for bone turnover, while estimation of % bone turnover surfaces on periosteal and endosteal-trabecular surfaces in biopsies of mandibles has no diagnostic value. 2) Percentage of bone mass and mean cortical width in standard locality indicate the level in group of mandibles. 3) the cortical variables of bone turnover are sex independent; number/area Haversian canals with resorption foci is age independent; number/area Haversian canals with bone formation foci, total of Haversian canals and percentage of subendosteal (Haversian canals + marrow spaces) are increased with age.

A hypothetical mechanism for the stimulation of osteonal remodelling by fatigue damage

This paper presents a theory describing mechanisms by which repetitive stress initiates remodelling in compact bone. The theory is based on the observation that the lamellar structure of osteons arrests and traps microcracks produced by cyclic loading. Debonding of an osteon by a crack may produce changes in the Haversian canal wall adjacent to the crack which initiate a new secondary osteon. The repair of damaged areas by secondary osteons prevents the accumulation of microdamage due to repetitive loading and protects compact bone from fatigue failure.

Differences in intracortical bone remodeling in three aboriginal American populations: possible dietary factors

Cross sections of intracortical bone revealed differences among three early American populations--Eskimo, Arikara, and Pueblo--in the frequency of a form of remodeling called, here, type II. This remodeling appears to occur exclusively within the walls of haversian canals of well-mineralized osteons. The populations are known to have differed in their nutrition. The Eskimo, with a high-protein diet, exhibited the most frequent type II remodeling, whereas the Pueblo, with a low-protein diet, showed the least. Type II remodeling probably reflects the physiological state of the group. Variation in frequency of classic intracortical bone remodeling was not apparent.

Histomorphometric analysis of normal bone from the iliac crest

Bone specimens from a standardized area of the iliac crest were obtained at autopsy in 105 individuals after sudden unexpected death and at biopsy in 30 living volunteers. Seven micron thick sections of the plastic embedded undecalcified material were produced and stained. The amount of cancellous and cortical bone and the parameters attached to description of bone remodelling were determined by point counting and simple measurements, in order to establish normal mean values and range in a Danish population. A decrease in the amount of bone, trabecular as well as cortical, was found with increasing age in both males and females. The extent, volume and width of osteoid seams and the osteoclastic resorption were found to be age-independent. The osteoclastic resorption in cortical bone in males decreased, however, with increasing age. The reported values are of the greatest importance for the use of histomorphometric analysis of bone biopsies as a diagnostic tool in metabolic bone diseases. It is stressed, however, that supplementary information of the dynamic aspects of the bone remodelling would be necessary for the understanding of the pathogenesis of bone changes. Such information might be obtained by using tetracyline double labelling.

Three-dimensional studies on resorption spaces and developing osteons

Resorption spaces and their continuations as developing osteons were traced in serial cross sections from decalcified long bones of dogs, baboons and a man, and from a human rib. Processes of formation of osteons and transverse (Volkmann's) canals can be inferred from three-dimensional studies. Deposits of new osseous tissue begin to line the walls of the spaces soon after termination of resorption. The first deposits are osteoid, usually stained very darkly by the silver nitrate procedure utilized, but a lighter osteoid zone adjacent to the canals occurs frequently. Osteoid linings continue to be produced as lamellar bone forms around them; the large canals of immature osteons usually narrow very gradually. Frequently they terminate both proximally and distally as resorption spaces, indicating that osteons often advance in opposite directions as they develop. Osteoclasts of resorption spaces tunnel preferentially into highly mineralized bone, and usually do not use previously existing canals as templates for their advance. Osteons evidently originate by localized resorption of one side of the wall of an existing vascular channel in bone, with subsequent orientation of the resorption front along the axis of the shaft. Advancing resorption spaces also apparently stimulate the formation of numerous additional transverse canal connections to neighboring longitudinal canals. Serial tracing and silver nitrate differential staining combine to reveal many of the processes of bone remodeling at work, and facilitate quantitative treatment of the data. Further uses in studies of bone tissue and associated cells are recommended.

The actions of parathyroid hormone on bone: relation to bone remodeling and turnover, calcium homeostasis, and metabolic bone disease. Part I of IV parts: mechanisms of calcium transfer between blood and bone and their cellular basis: morphological and kinetic approaches to bone turnover

The supracellular organization of living bone enables the study of isolated cellular and subcellular systems to be related to the study of the whole organism. Bone is formed by osteoblasts in successive stages, separated in both time and space, of matrix formation and primary mineralization. Osteoblasts are joined by tight junctions and largely cover the osteoid seam which separates them from mineralized bone. Secondary mineralization is not completed for several months and is not regulated by the osteoblast. Bone is resorbed by osteoclasts which simultaneously accomplish mineral dissolution and matrix digestion. Active osteoblasts occupy about 5% of the free bone surface, osteoid seams with less active osteoblasts about 10%, active osteoclasts about 0.5%, and Howship's lacunae at which bone remodeling is either quiescent or arrested about 5%. The remaining 80% of the free bone surface is covered by a leaky envelope of thin flattened cells, termed surface osteocytes. Some osteoblasts become permanently buried in the bone as deep osteocytes, around which a specialized and metabolically active perilacunar bone is formed. This bone is less highly mineralized and can temporarily lose or gain calcium in accordance with homeostatic needs. Deep osteocytes maintain contact with each other and with the surface osteocytes, their cell processes within canaliculi being joined by gap junctions. Remodeling of cortical bone proceeds with the excavation by osteoclasts of a longitudinal tunnel which is refilled by osteoblasts to form a new osteon. The anatomically discrete longitudinally oriented structure consisting of a cutting cone of osteoclasts in front and a closing cone of osteoblasts behind is termed a cortical remodeling unit. The events of centrifugal resorption and centripetal formation which occur in a single cross section is termed a cortical remodeling cycle. Normally each new cycle is slightly out of phase with its predecessor. The quantities which characterize cortical remodeling are the birth rate of new remodeling cycles or activation frequency (mu), and the durations of the resorptive period (sigma r), the quiescent interval (sigma q) and the formation period (sigma f). The average distances traveled by the osteoclast and osteoblast are indicated respectively by the mean cement line diameter and mean wall thickness of completed osteons. These quantities show little interindividual variation. Because of this constancy the magnitude of bone turnover (the bone formation rate) is almost entirely a function of mu, the activation frequency of new remodeling cycles. Variations in the velocity of advance of osteoclasts (the linear resorption rate) or of osteoblasts (the appositional rate) alter inversely both the extent of surface engaged in resorption or formation and the time taken to replace a particular moiety of bone, but in a steady state do not influence the rate of turnover of the skeleton as a whole...

Aging effects on osteon remodeling

Analysis of partial cross sections of 101 human tibiae indicated that osteon remodeling in the outer cortex is affected by age. The frequency of resorption spaces remained constant throughout life suggesting no loss of osteoclast function with age. However, the frequency of both forming osteons and osteons which were structurally complete but not completely mineralized increased with age. This suggests that protein matrix synthesis by osteoblasts slows with age and that initial mineralization, possibly mediated by osteroblasts, and final mineralization, possibly mediated by osteocytes, becomes increasingly deficient with increasing age. The frequency of osteons which have dense (sclerotic) inner lamellae decreases with age. This supports a hypothesis that such lamellae are functional, perhaps representing a specialized, labile, mineral phase and that osteons having this feature become less frequent as part of the general degenerative changes associated with aging.