Human iliac crest cancellous bone elastic modulus and hardness differ with bone formation rate per bone surface but not by existence of prevalent vertebral fracture

Cortical and trabecular mechanical properties in the femoral neck vary differently with changes in bone mineral density

Graphical Abstract.

The PCK2-glycolysis axis assists three-dimensional-stiffness maintaining stem cell osteogenesis

Understanding mechanisms underlying the heterogeneity of multipotent stem cells offers invaluable insights into biogenesis and tissue development. Extracellular matrix (ECM) stiffness has been acknowledged as a crucial factor regulating stem cell fate. However, how cells sense stiffness cues and adapt their metabolism activity is still unknown. Here we report the novel role of mitochondrial phosphoenolpyruvate carboxykinase (PCK2) in enhancing osteogenesis in 3D ECM via glycolysis. We experimentally mimicked the physical characteristics of 3D trabeculae network of normal and osteoporotic bone with different microstructure and stiffness, observing that PCK2 promotes osteogenesis in 3D ECM with tunable stiffness in vitro and in vivo. Mechanistically, PCK2 enhances the rate-limiting metabolic enzyme pallet isoform phosphofructokinase (PFKP) in 3D ECM, and further activates AKT/extracellular signal-regulated kinase 1/2 (ERK1/2) cascades, which directly regulates osteogenic differentiation of MSCs. Collectively, our findings implicate an intricate crosstalk between cell mechanics and metabolism, and provide new perspectives for strategies of osteoporosis.

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As the key rate-limiting enzyme of gluconeogenesis, PCK2 manipulates osteogenesis in stiff and soft ECM in vitro and in vivo.

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PCK2 regulates osteogenic capacity of BMMSCs in 3D ECM with different stiffness, via modulating glycolysis and regulating PFKP-AKT/ERK signaling pathways.

Effects of Osteoporosis on Bone Morphometry and Material Properties of Individual Human Trabeculae in the Femoral Head

Osteoporosis is the most common bone disease and is conventionally classified as a decrease of total bone mass. Current diagnosis of osteoporosis is based on clinical risk factors and dual energy X‐ray absorptiometry (DEXA) scans, but changes in bone quantity (bone mass) and quality (trabecular structure, material properties, and tissue composition) are not distinguished. Yet, osteoporosis is known to cause a deterioration of the trabecular network, which might be related to changes at the tissue scale—the material properties. The goal of the current study was to use a previously established test method to perform a thorough characterization of the material properties of individual human trabeculae from femoral heads in cyclic tensile tests in a close to physiologic, wet environment. A previously developed rheological model was used to extract elastic, viscous, and plastic aspects of material behavior. Bone morphometry and tissue mineralization were determined with a density calibrated micro‐computed tomography (μCT) set‐up. Osteoporotic trabeculae neither showed a significantly changed material or mechanical behavior nor changes in tissue mineralization, compared with age‐matched healthy controls. However, donors with osteopenia indicated significantly reduced apparent yield strain and elastic work with respect to osteoporosis, suggesting possible initial differences at disease onset. Bone morphometry indicated a lower bone volume to total volume for osteoporotic donors, caused by a smaller trabecular number and a larger trabecular separation. A correlation of age with tissue properties and bone morphometry revealed a similar behavior as in osteoporotic bone. In the range studied, age does affect morphometry but not material properties, except for moderately increased tissue strength in healthy donors and moderately increased hardening exponent in osteoporotic donors. Taken together, the distinct changes of trabecular bone quality in the femoral head caused by osteoporosis and aging could not be linked to suspected relevant changes in material properties or tissue mineralization. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Multicenter Pilot Study to Assess a Biphasic Calcium Phosphate Implant for Functional and Aesthetic Septorhinoplasty

Importance: A validated biomaterial would have several medical advantages in septorhinoplasties requiring a large-volume graft such as avoiding donor site morbidity, making ambulatory surgery possible, and reducing surgical costs. Objective: To assess the safety and efficacy of a ceramic to treat saddle and crooked noses. The main endpoint was the biocompatibility of the implant. The secondary endpoint was its functional and aesthetic efficacy. Design, Setting, and Participants: The nasal septum (NASEPT) study is a pilot multicenter noncomparative prospective phase IIa clinical trial. The biomaterial tested was a biphasic calcium phosphate implant composed of 75% hydroxyapatite and 25% beta tri calcium phosphate. This versatile material can be used to replace septal skeleton when it is absent or nonusable. We included 25 patients with a multifractured osseous and cartilaginous framework after several traumas or surgeries. The implant placement technique was identical to an extracorporeal septoplasty through the external approach. Main Outcomes and Measures: The primary endpoint was the occurrence of expected adverse and severe adverse events. The secondary endpoints were clinical functional and aesthetic results and histological microscopic modifications. Results: Any extrusion, infection, pain, and epistaxis were observed. All implants were placed in a sagittal, straight, and solid position without extralobular depression. Comparisons between pre- and postoperative symptoms showed that nasal comfort (p < 10^-4) and quality of life (p < 10^-4) were dramatically improved in all patients. The nasolabial angle (p = 0.047) and the columellar projection (p = 0.024) were improved after surgery. Histological data showed little submucosal inflammation at 6 months with well-differentiated epithelium. The mean follow-up was 23 months: three patients underwent revision surgery for functional or aesthetic details and four implants were removed (16%) owing to a foreign body reaction between 17 and 74 months. Conclusion and Relevance: The NASEPT implant meets functional and aesthetic requirements in complex septorhinoplasties but its long-term biocompatibility needs to be improved. It could potentially avoid donor site morbidity.

"Tree to Bone": Lignin/Polycaprolactone Nanofibers for Hydroxyapatite Biomineralization

Bone contains an organic matrix composed of aligned collagen fibers embedded with nanosized inorganic hydroxyapatite (HAp). Many efforts are being made to mimic the natural mineralization process and create artificial bone scaffolds that show elaborate morphologies, excellent mechanical properties, and vital biological functions. This study reports a newly discovered function of lignin mediating the formation of human bone-like HAp. Lignin is the second most abundant organic material in nature, and it exhibits many attractive properties for medical applications, such as high durability, stability, antioxidant and antibacterial activities, and biocompatibility. Numerous phenolic and aliphatic hydroxyl moieties exist in the side chains of lignin, which donate adequate reactive sites for chelation with Ca²⁺ and the subsequent nucleation of HAp through coprecipitation of Ca²⁺ and PO₄^3-. The growth of HAp crystals was facilitated by simple incubation of the electrospun lignin/polycaprolactone (PCL) matrix in a simulated body fluid. Multiple analyses revealed that HAp crystals were structurally and mechanically similar to the native bone. Furthermore, the mineralized lignin/PCL nanofibrous films facilitated efficient adhesion and proliferation of osteoblasts by directing filopodial extension. Our results underpin the expectations for this lignin-based biomaterial in future biointerfaces and hard-tissue engineering.

Assessing bone quality through mechanical properties in postmenopausal trabecular bone

BACKGROUND

The inner structure of trabecular bone is a result of structural optimization provided by remodeling processes. Changes in hormonal status related to menopause cause bone tissue loss and micro-architectural deterioration with a consequent susceptibility to fracture. Accumulation of micro-damage in bone, as a function of the rate of production and rate of repair, underlies the development of stress fractures, increasing fragility associated to age and osteoporosis, especially in transmenopausal women.

PATIENTS AND METHODS

Quasi-static and nano-dynamic mechanical characterization were undertaken in trabecular bone from femoral neck biopsies of postmenopausal women. AFM (Atomic Force Microscopy) complementary studies were performed to determine nano-roughness (SRa) and the fibrils width of collagen. Nanoindentations were used to quantify transmenopausal changes in intrinsic mechanical properties of trabecular bone: hardness (Hi), modulus of Young (Ei), complex modulus (E*), tan delta (δ), storage modulus (E') and loss modulus (E").

RESULTS

As result of the quasi-static measurements, 0.149 (0.036) GPa and 2.95 (0.73) GPa of Hi and Ei were obtained, respectively. As result of the nano-dynamic measurements, 17.94 (3.15), 0.62 (0.10), 13.79 (3.21 and 6.39 (1.28) GPa of E*, tan (δ), E' and E" were achieved, respectively. 101.07 SRa and 831.28 nm of fibrils width were additionally obtained.

CONCLUSIONS

This study poses a first approach to the measurement of bone quality in postmenopausal trabecular bone by combining quasi-static, nano-DMA analysis and tribology of dentin surface through AFM characterization.

Biomimetic Bone-like Hydroxyapatite by Mineralization on Supramolecular Porous Fiber Networks

Hydroxyapatite (HA), the main inorganic component of bone tissue, is mineralized with collagen fibril scaffolds during bone formation. Inspired by the process, a self-assembled porous network architecture was designed and synthesized by using the 2-ureido-4[1H]-pyrimidone (UPy) modified glycerol molecule UPy-Gly, which was further utilized as a template for biomimetic mineralization. When incubated in simulated body fluid (SBF), the HA nucleus first formed in the holes of the template by the induction of hydroxyls on the surface, grew along the nanofibers, and fused with the template to fabricate hydroxyapatite composites (UPy-Gly/HA). Transmission electron microscopic observation demonstrates that the mineral clusters are accumulated by lamella-like nano hydroxyapatite and the elasticity modulus measured by atomic force microscopy is about 5.5 GPa, which is quite close to the natural cancellous bone tissue of human both in structure and in mechanical properties. The Cell Counting Kit 8 (CCK-8) assay of UPy-Gly and UPy-Gly/HA shows noncytotoxicity to mouse fibroblast L-929 cells. This bioinspired composite will be a promising material for potential use in bone tissue implantation and regeneration engineering.

Intrinsic material property differences in bone tissue from patients suffering low-trauma osteoporotic fractures, compared to matched non-fracturing women

Osteoporotic (low-trauma) fractures are a significant public health problem. Over 50% of women over 50yrs. of age will suffer an osteoporotic fracture in their remaining lifetimes. While current therapies reduce skeletal fracture risk by maintaining or increasing bone density, additional information is needed that includes the intrinsic material strength properties of bone tissue to help develop better treatments, since measurements of bone density account for no more than ~50% of fracture risk. The hypothesis tested here is that postmenopausal women who have sustained osteoporotic fractures have reduced bone quality, as indicated with measures of intrinsic material properties compared to those who have not fractured. Transiliac biopsies (N=120) were collected from fracturing (N=60, Cases) and non-fracturing postmenopausal women (N=60, age- and BMD-matched Controls) to measure intrinsic material properties using the nano-indentation technique. Each biopsy specimen was embedded in epoxy resin and then ground, polished and used for the nano-indentation testing. After calibration, multiple indentations were made using quasi-static (hardness, modulus) and dynamic (storage and loss moduli) testing protocols. Multiple indentations allowed the median and variance to be computed for each type of measurement for each specimen. Cases were found to have significantly lower median values for cortical hardness and indentation modulus. In addition, cases showed significantly less within-specimen variability in cortical modulus, cortical hardness, cortical storage modulus and trabecular hardness, and more within-specimen variability in trabecular loss modulus. Multivariate modeling indicated the presence of significant independent mechanical effects of cortical loss modulus, along with variability of cortical storage modulus, cortical loss modulus, and trabecular hardness. These results suggest mechanical heterogeneity of bone tissue may contribute to fracture resistance. Although the magnitudes of differences in the intrinsic properties were not overwhelming, this is the first comprehensive study to investigate, and compare the intrinsic properties of bone tissue in fracturing and non-fracturing postmenopausal women.

Clinical Evaluation of Bone Strength and Fracture Risk

PURPOSE OF REVIEW

This paper seeks to evaluate and compare recent advances in the clinical assessment of the changes in bone mechanical properties that take place as a result of osteoporosis and other metabolic bone diseases and their treatments.

RECENT FINDINGS

In addition to the standard of DXA-based areal bone mineral density (aBMD), a variety of methods, including imaging-based structural measurements, finite element analysis (FEA)-based techniques, and alternate methods including ultrasound, bone biopsy, reference point indentation, and statistical shape and density modeling, have been developed which allow for reliable prediction of bone strength and fracture risk. These methods have also shown promise in the evaluation of treatment-induced changes in bone mechanical properties. Continued technological advances allowing for increasingly high-resolution imaging with low radiation dose, together with the expanding adoption of DXA-based predictions of bone structure and mechanics, as well as the increasing awareness of the importance of bone material properties in determining whole-bone mechanics, lead us to anticipate substantial future advances in this field.

Tissue-Level Mechanical Properties of Bone Contributing to Fracture Risk

Tissue-level mechanical properties characterize mechanical behavior independently of microscopic porosity. Specifically, quasi-static nanoindentation provides measurements of modulus (stiffness) and hardness (resistance to yielding) of tissue at the length scale of the lamella, while dynamic nanoindentation assesses time-dependent behavior in the form of storage modulus (stiffness), loss modulus (dampening), and loss factor (ratio of the two). While these properties are useful in establishing how a gene, signaling pathway, or disease of interest affects bone tissue, they generally do not vary with aging after skeletal maturation or with osteoporosis. Heterogeneity in tissue-level mechanical properties or in compositional properties may contribute to fracture risk, but a consensus on whether the contribution is negative or positive has not emerged. In vivo indentation of bone tissue is now possible, and the mechanical resistance to microindentation has the potential for improving fracture risk assessment, though determinants are currently unknown.

A Novel Composite PMMA-based Bone Cement with Reduced Potential for Thermal Necrosis

Percutaneous vertebroplasty (VP) and balloon kyphoplasty (BKP) are now widely used to treat patients who suffer painful vertebral compression fractures. In each of these treatments, a bone cement paste is injected into the fractured vertebral body/bodies, and the cement of choice is a poly(methyl methacrylate) (PMMA) bone cement. One drawback of this cement is the very high exothermic temperature, which, it has been suggested, causes thermal necrosis of surrounding tissue. In the present work, we prepared novel composite PMMA bone cement where microcapsules containing a phase change material (paraffin) (PCMc) were mixed with the powder of the cement. A PCM absorbs generated heat and, as such, its presence in the cement may lead to reduction in thermal necrosis. We determined a number of properties of the composite cement. Compared to the values for a control cement (a commercially available PMMA cement used in VP and BKP), each composite cement was found to have significantly lower maximum exothermic temperature, increased setting time, significantly lower compressive strength, significantly lower compressive modulus, comparable biocompatibility, and significantly smaller thermal necrosis zone. Composite cement containing 20% PCMc may be suitable for use in VP and BKP and thus deserves further evaluation.

Micro/Nanostructures and Mechanical Properties of Trabecular Bone in Ovariectomized Rats

Bone mechanical properties encompass both geometric and material factors, while the effects of estrogen deficiency on the material and structural characteristics of bone at micro- to nanoscales are still obscure. We performed a series of combined methodological experiments, including nanoindentation assessment of intrinsic material properties, atomic force microscopy (AFM) characterization of trabecular (Tb) nanostructure, and Tb microarchitecture and 2D BMD. At 15 weeks after surgery, we found significantly less Tb bone mineral density (BMD) at organ (-27%) and at tissue level (-12%), Tb bone volume fraction (-29%), Tb thickness (-14%), and Tb number (-17%) in ovariectomy (OVX) rats than in sham operated (SHAM) rats, while the structure model index (+91%) and Tb separation (+19%) became significantly greater. AFM images showed lower roughness Tb surfaces with loosely packed large nodular structures and less compacted interfibrillar space in OVX than in SHAM. However, no statistically significant changes were in the Tb intrinsic material properties-nanoindentation hardness, elastic modulus, and plastic deformation-nanoindentation depths, and residual areas. Therefore, estrogen deprivation results in a dramatic deterioration in Tb micro/nanoarchitectures, 3D volumetric BMD at both organ and tissue levels, and 2D BMD, but not in the nanomechanical properties of the trabeculae per se.

Nanomechanical mapping of bone tissue regenerated by magnetic scaffolds

Nanoindentation can provide new insights on the maturity stage of regenerating bone. The aim of the present study was the evaluation of the nanomechanical properties of newly-formed bone tissue at 4 weeks from the implantation of permanent magnets and magnetic scaffolds in the trabecular bone of rabbit femoral condyles. Three different groups have been investigated: MAG-A (NdFeB magnet + apatite/collagen scaffold with magnetic nanoparticles directly nucleated on the collagen fibers during scaffold synthesis); MAG-B (NdFeB magnet + apatite/collagen scaffold later infiltrated with magnetic nanoparticles) and MAG (NdFeB magnet). The mechanical properties of different-maturity bone tissues, i.e. newly-formed immature, newly-formed mature and native trabecular bone have been evaluated for the three groups. Contingent correlations between elastic modulus and hardness of immature, mature and native bone have been examined and discussed, as well as the efficacy of the adopted regeneration method in terms of "mechanical gap" between newly-formed and native bone tissue. The results showed that MAG-B group provided regenerated bone tissue with mechanical properties closer to that of native bone compared to MAG-A or MAG groups after 4 weeks from implantation. Further, whereas the mechanical properties of newly-formed immature and mature bone were found to be fairly good correlated, no correlation was detected between immature or mature bone and native bone. The reported results evidence the efficacy of nanoindentation tests for the investigation of the maturity of newly-formed bone not accessible through conventional analyses.

Changes in the mechanical properties and composition of bone during microdamage repair

Under normal conditions, loading activities result in microdamage in the living skeleton, which is repaired by bone remodeling. However, microdamage accumulation can affect the mechanical properties of bone and increase the risk of fracture. This study aimed to determine the effect of microdamage on the mechanical properties and composition of bone. Fourteen male goats aged 28 months were used in the present study. Cortical bone screws were placed in the tibiae to induce microdamage around the implant. The goats were euthanized, and 3 bone segments with the screws in each goat were removed at 0 days, 21 days, 4 months, and 8 months after implantation. The bone segments were used for observing microdamage and bone remodeling, as well as nanoindentation and bone composition, separately. Two regions were measured: the first region (R1), located 1.5 mm from the interface between the screw hole and bone; and the second region (R2), located>1.5 mm from the bone-screw interface. Both diffuse and linear microdamage decreased significantly with increasing time after surgery, with the diffuse microdamage disappearing after 8 months. Thus, screw implantation results in increased bone remodeling either in the proximal or distal cortical bone, which repairs the microdamage. Moreover, bone hardness and elastic modulus decreased with microdamage repair, especially in the proximal bone tissue. Bone composition changed greatly during the production and repair of microdamage, especially for the C (Carbon) and Ca (Calcium) in the R1 region. In conclusion, the presence of mechanical microdamage accelerates bone remodeling either in the proximal or distal cortical bone. The bone hardness and elastic modulus decreased with microdamage repair, with the micromechanical properties being restored on complete repair of the microdamage. Changes in bone composition may contribute to changes in bone mechanical properties.

Reduced tissue-level stiffness and mineralization in osteoporotic cancellous bone

Osteoporosis alters bone mass and composition ultimately increasing the fragility of primarily cancellous skeletal sites; however, effects of osteoporosis on tissue-level mechanical properties of cancellous bone are unknown. Dual-energy X-ray absorptiometry (DXA) scans are the clinical standard for diagnosing osteoporosis though changes in cancellous bone mass and mineralization are difficult to separate using this method. The goal of this study was to investigate possible difference in tissue-level properties with osteoporosis as defined by donor T scores. Spine segments from Caucasian female cadavers (58-92 years) were used. A T score for each donor was calculated from DXA scans to determine osteoporotic status. Tissue-level composition and mechanical properties of vertebrae adjacent to the scan region were measured using nanoindentation and Raman spectroscopy. Based on T scores, six samples were in the Osteoporotic group (58-74 years) and four samples were in the Not Osteoporotic group (65-92 years). The indentation modulus and mineral to matrix ratio (mineral:matrix) were lower in the Osteoporotic group than the Not Osteoporotic group. Mineral:matrix ratio decreased with age (r (2) = 0.35, p = 0.05), and the indentation modulus increased with areal bone mineral density (r (2) = 0.41, p = 0.04). This study is the first to examine cancellous bone composition and mechanical properties from a fracture prone location with osteoporosis. We found differences in tissue composition and mechanical properties with osteoporosis that could contribute to increased fragility in addition to changes in trabecular architecture and bone volume.

Production and repair of implant-induced microdamage in the cortical bone of goats after long-term estrogen deficiency

SUMMARY

By using an ovariectomized goat model, we found that estrogen depletion decreases bone quality and makes it susceptible to screw-induced mechanical microdamage. Both diffuse microdamage and linear cracks accumulated up to 3 weeks after screw implantation, and the microdamage was repaired gradually after 4-8 months.

INTRODUCTION

The aim of this study was to observe the effect of long-term estrogen deficiency on the creation and repair of microdamage in cortical bone adjacent to bone screw.

METHODS

Cortical bone screws were placed in the tibial diaphyses 28 months after ovariectomy (OVX) or sham operation (Sham-Op) in female goats. The goats were euthanized at 0 day, 21 days, 4 months, and 8 months after screw implantation. Microdamage morphology and repair were examined in peri-screw bone using histomorphometric method, and the nanomechanical properties of peri-screw bone were examined with nanoindentation testing.

RESULTS

Tibiae from ovariectomized goats in which screws had been placed had significantly higher levels of diffuse microdamage and significantly more linear cracks than those from sham goats, and the diffuse microdamage was more obvious than linear cracks in the region adjacent to the implant. Both diffuse microdamage and linear cracks accumulated up to day 21 and then gradually repaired at 4 and 8 months after surgery. The trend for bone remodeling in each group was consistent with changes in the level of microdamage. Nanoindentation testing showed that both elastic modulus and hardness in peri-screw bone were significantly decreased in OVX group compared to Sham-Op group. The hardness and elastic modulus also showed a downward trend up to 4 months after screw implantation and then exhibited some recovery after 8 months.

CONCLUSIONS

Estrogen depletion decreases bone quality and makes it vulnerable to screw-induced mechanical damage, which may compromise the initial stability of an orthopedic implant.

Determinants of microdamage in elderly human vertebral trabecular bone

Previous studies have shown that microdamage accumulates in bone as a result of physiological loading and occurs naturally in human trabecular bone. The purpose of this study was to determine the factors associated with pre-existing microdamage in human vertebral trabecular bone, namely age, architecture, hardness, mineral and organic matrix. Trabecular bone cores were collected from human L2 vertebrae (n = 53) from donors 54–95 years of age (22 men and 30 women, 1 unknown) and previous cited parameters were evaluated. Collagen cross-link content (PYD, DPD, PEN and % of collagen) was measured on surrounding trabecular bone. We found that determinants of microdamage were mostly the age of donors, architecture, mineral characteristics and mature enzymatic cross-links. Moreover, linear microcracks were mostly associated with the bone matrix characteristics whereas diffuse damage was associated with architecture. We conclude that linear and diffuse types of microdamage seemed to have different determinants, with age being critical for both types.

Bone properties by nanoindentation in mild and severe osteogenesis imperfecta

BACKGROUND

Osteogenesis imperfecta is a heterogeneous genetic disorder characterized by bone fragility. Previous research suggests that impaired collagen network and abnormal mineralization affect bone tissue properties, however, little data is yet available to describe bone material properties in individuals with this disorder. Bone material properties have not been characterized in individuals with the most common form of osteogenesis imperfecta, type I.

METHODS

Bone tissue elastic modulus and hardness were measured by nanoindentation in eleven osteotomy specimens that were harvested from children with osteogenesis imperfecta during routine surgeries. These properties were compared between osteogenesis imperfecta types I (mild, n=6) and III (severe, n=5), as well as between interstitial and osteonal microstructural regions using linear mixed model analysis.

FINDINGS

Disease severity type had a small but statistically significant effect on modulus (7%, P=0.02) and hardness (8%, P<0.01). Individuals with osteogenesis imperfecta type I had higher modulus and hardness than did those with type III. Overall, mean modulus and hardness values were 13% greater in interstitial lamellar bone regions than in osteonal regions (P<0.001).

INTERPRETATION

The current study presents the first dataset describing bone material properties in individuals with the most common form of osteogenesis imperfecta, i.e., type I. Results indicate that intrinsic bone tissue properties are affected by phenotype. Knowledge of the material properties of bones in osteogenesis imperfecta will contribute to the ability to develop models to assist in predicting fracture risk.

Intrinsic material properties of trabecular bone by nanoindentation testing of biopsies taken from healthy women before and after menopause

Postmenopausal osteoporosis in women is characterized by an increase in bone fragility and risk of fracture. In addition to transmenopausal decline in three-dimensional trabecular bone architecture, changes in intrinsic material properties (local stiffness, damping, and hardness) may contribute to increased bone fragility. In this study, nanoindentation was used to quantify transmenopausal changes in the intrinsic properties of trabecular bone. Paired transilial biopsy specimens were used from a previously reported study in which bone biopsies were obtained from women prior to menopause (premenopausal, age 49.0 ± 1.9) and at 12 months past the last menstrual period (postmenopausal, age 54.6 ± 2.2). Elastic and viscoelastic material properties of the trabecular bone were measured using quasi-static and dynamic nanoindentation techniques, respectively. Paired Student's t tests (n = 15) were performed to assess the significance of the measured intrinsic properties. Trabecular bone microarchitecture is compromised in postmenopausal women, and although this loss is associated with a trend toward reduction in some intrinsic properties (storage modulus), we found no statistically significant changes in bone intrinsic properties between healthy pre- and postmenopausal biopsies in the quasi-static results and frequency-averaged dynamic results.

Do regional modifications in tissue mineral content and microscopic mineralization heterogeneity adapt trabecular bone tracts for habitual bending? Analysis in the context of trabecular architecture of deer calcanei

Calcanei of mature mule deer have the largest mineral content (percent ash) difference between their dorsal 'compression' and plantar 'tension' cortices of any bone that has been studied. The opposing trabecular tracts, which are contiguous with the cortices, might also show important mineral content differences and microscopic mineralization heterogeneity (reflecting increased hemi-osteonal renewal) that optimize mechanical behaviors in tension vs. compression. Support for these hypotheses could reveal a largely unrecognized capacity for phenotypic plasticity - the adaptability of trabecular bone material as a means for differentially enhancing mechanical properties for local strain environments produced by habitual bending. Fifteen skeletally mature and 15 immature deer calcanei were cut transversely into two segments (40% and 50% shaft length), and cores were removed to determine mineral (ash) content from 'tension' and 'compression' trabecular tracts and their adjacent cortices. Seven bones/group were analyzed for differences between tracts in: first, microscopic trabecular bone packets and mineralization heterogeneity (backscattered electron imaging, BSE); and second, trabecular architecture (micro-computed tomography). Among the eight architectural characteristics evaluated [including bone volume fraction (BVF) and structural model index (SMI)]: first, only the 'tension' tract of immature bones showed significantly greater BVF and more negative SMI (i.e. increased honeycomb morphology) than the 'compression' tract of immature bones; and second, the 'compression' tracts of both groups showed significantly greater structural order/alignment than the corresponding 'tension' tracts. Although mineralization heterogeneity differed between the tracts in only the immature group, in both groups the mineral content derived from BSE images was significantly greater (P < 0.01), and bulk mineral (ash) content tended to be greater in the 'compression' tracts (immature 3.6%, P = 0.03; mature 3.1%, P = 0.09). These differences are much less than the approximately 8% greater mineral content of their 'compression' cortices (P < 0.001). Published data, suggesting that these small mineralization differences are not mechanically important in the context of conventional tests, support the probability that architectural modifications primarily adapt the tracts for local demands. However, greater hemi-osteonal packets in the tension trabecular tract of only the mature bones (P = 0.006) might have an important role, and possible synergism with mineralization and/or microarchitecture, in differential toughening at the trabeculum level for tension vs. compression strains.

Mechanical property and tissue mineral density differences among severely suppressed bone turnover (SSBT) patients, osteoporotic patients, and normal subjects

Pathogenesis of atypical fractures in patients on long term bisphosphonate therapy is poorly understood, and the type, the manner in which they occur and the fracture sites are quite different from the usual osteoporotic fractures. We hypothesized that the tissue-level mechanical properties and mean degree of mineralization of the iliac bone would differ among 1) patients with atypical fractures and severely suppressed bone turnover (SSBT) associated with long-term bisphosphonate therapy, 2) age-matched, treatment-naïve osteoporotic patients with vertebral fracture, 3) age-matched normals and 4) young normals. Large differences in tissue-level mechanical properties and/or mineralization among these groups could help explain the underlying mechanism(s) for the occurrence of typical osteoporotic and the atypical femoral shaft fractures. Elastic modulus, contact hardness, plastic deformation resistance, and tissue mineral densities of cortical and trabecular bone regions of 55 iliac bone biopsies--12 SSBT patients (SSBT; aged 49-77), 11 age-matched untreated osteoporotic patients with vertebral fracture (Osteoporotic), 12 age-matched subjects without bone fracture (Age-Matched Normal), and 20 younger subjects without bone fracture (Young Normal)--were measured using nanoindentation and quantitative backscattered electron microscopy. For cortical bone nanoindentation properties, only plastic deformation resistance was different among the groups (p<0.05), with greater resistance to plastic deformation in the SSBT group compared to all other groups. For trabecular bone, all nanoindentation properties and mineral density of the trabecular bone were different among the groups (p<0.05). The SSBT group had greater plastic deformation resistance and harder trabecular bone compared to the other three groups, stiffer bone compared to the Osteoporotic and Young Normal groups, and a trend of higher mineral density compared to the Age-Matched Normal and Osteoporotic groups. Lower heterogeneity of modulus and contact hardness for cortical bone of the SSBT and trabecular bone of the Osteoporotic fracture groups, respectively, compared to the non-fractured groups, may contribute to fracture susceptibility due to lowered ability to prevent crack propagation. We tentatively conclude that, in addition to extremely low bone formation rate, atypical fractures in SSBT and/or long-term bisphosphonate treatment may be associated with greater mean plastic deformation resistance properties and less heterogeneous elastic properties of the bone.

Trabecular packet-level lamellar density patterns differ by fracture status and bone formation rate in white females

Spatial patterns of mineralization for human iliac crest cancellous bone were measured from images obtained by quantitative backscattered electron microscopy. Biopsies collected from vertebral fracture patients and healthy individuals with high or low bone formation rate (BFR(s)) were examined (fracture/low BFR(s): N=12, fracture/high BFR(s): N=10, normal/low BFR(s): N=12, normal/high BFR(s): N=15). 20 by 20 pixel square areas or smaller were sampled from superficial and deep remodeling packets. Mean (Z(mean)) and standard deviation (SD) of mineralization were measured, and coefficients of variation (CV=SD/Z(mean)) were calculated. Fast Fourier transform analysis was used to quantify the distribution of the mineral in the packets. "FFT_ratio" was defined as the ratio magnitude of the principal spatial frequency to the average atomic number density. A higher FFT_ratio occurred in specimens with more pronounced alternating layers of light and dark as visible in the backscattered electron image, which was defined as lamellar patterning. Two-way ANOVA revealed that the coefficients of variation of mineralization for both superficial and deep packets were significantly lower in fracture patients than in normal individuals. However, the interaction between turnover rate and group (fracture/non-fracture) indicated that the difference in packet CV occurred among the low turnover individuals and not among those with high turnover. Mean mineralization levels and CV between deep and superficial packets were highly correlated. Regressions of packet CV of mineralization and FFT_ratio were highly significant (p<0.001) for all packets pooled and for packets divided by group (fracture/normal). However, analyses of packet CV and FFT_ratio by individual were variable (R(2) from 0.00338 to 0.700). Packet-level mineralization variability may be associated with fracture toughness, and fracture patients had less variable packet-level mineralization. The result that the packet CV varied significantly between fracture and non-fracture individuals with low turnover suggests that for low turnover subjects without fracture, high variability in mineralization may have a protective effect. In high turnover patients, the accelerated turnover may prevent the lamellar variability from developing over time. Strong correlations between CV and Z(mean) for both superficial and deep packets imply that newly formed bone is created similarly to older bone within an individual. Fourier transform results show that the mineralization variability found within packets is associated with lamellar patterning. Lamellar structure has been hypothesized to guide microcrack propagation in order to optimize bone strength and toughness. Osteoporotics with fracture had less pronounced lamellation than healthy normals and may be more prone to fracture.

Microstructure and mineral composition of dystrophic calcification associated with the idiopathic inflammatory myopathies

INTRODUCTION

Calcified deposits (CDs) in skin and muscles are common in juvenile dermatomyositis (DM), and less frequent in adult DM. Limited information exists about the microstructure and composition of these deposits, and no information is available on their elemental composition and contents, mineral density (MD) and stiffness. We determined the microstructure, chemical composition, MD and stiffness of CDs obtained from DM patients.

METHODS

Surgically-removed calcinosis specimens were analyzed with fourier transform infrared microspectroscopy in reflectance mode (FTIR-RM) to map their spatial distribution and composition, and with scanning electron microscopy/silicon drift detector energy dispersive X-ray spectrometry (SEM/SDD-EDS) to obtain elemental maps. X-ray diffraction (XRD) identified their mineral structure, X-ray micro-computed tomography (microCT) mapped their internal structure and 3D distribution, quantitative backscattered electron (qBSE) imaging assessed their morphology and MD, nanoindentation measured their stiffness, and polarized light microscopy (PLM) evaluated the organic matrix composition.

RESULTS

Some specimens were composed of continuous carbonate apatite containing small amounts of proteins with a mineral to protein ratio much higher than in bone, and other specimens contained scattered agglomerates of various sizes with similar composition (FTIR-RM). Continuous or fragmented mineralization was present across the entire specimens (microCT). The apatite was much more crystallized than bone and dentin, and closer to enamel (XRD) and its calcium/phosphorous ratios were close to stoichiometric hydroxyapatite (SEM/SDD-EDS). The deposits also contained magnesium and sodium (SEM/SDD-EDS). The MD (qBSE) was closer to enamel than bone and dentin, as was the stiffness (nanoindentation) in the larger dense patches. Large mineralized areas were typically devoid of collagen; however, collagen was noted in some regions within the mineral or margins (PLM). qBSE, FTIR-RM and SEM/SDD-EDS maps suggest that the mineral is deposited first in a fragmented pattern followed by a wave of mineralization that incorporates these particles. Calcinosis masses with shorter duration appeared to have islands of mineralization, whereas longstanding deposits were solidly mineralized.

CONCLUSIONS

The properties of the mineral present in the calcinosis masses are closest to that of enamel, while clearly differing from bone. Calcium and phosphate, normally present in affected tissues, may have precipitated as carbonate apatite due to local loss of mineralization inhibitors.

The use of nanoindentation for characterizing the properties of mineralized hard tissues: state-of-the art review

The use of nanoindentation to determine nanomechanical properties of mineralized tissues has been investigated extensively. A detailed, critical, and comprehensive review of this literature is the subject of the present work. After stating the motivation for the review, a succinct presentation of the challenges, advantages, and disadvantages of the various quasi-static nanoindentation test methods (to obtain elastic modulus, E, and hardness, H) and dynamic test methods (to obtain storage and loss moduli and/or loss/damping factor) is given in the form of a primer. Explicative summaries of literature reports on various intrinsic and extrinsic factors that significantly influence E and H, followed by 15 suggested topics for future research, are included additionally. This review is designed to present a compact guide to the principles of the nanoindentation technique and to emphasize considerations when determining material properties of mineralized tissues.

Fourier transform infrared imaging microspectroscopy and tissue-level mechanical testing reveal intraspecies variation in mouse bone mineral and matrix composition

Fracture susceptibility is heritable and dependent upon bone morphology and quality. However, studies of bone quality are typically overshadowed by emphasis on bone geometry and bone mineral density. Given that differences in mineral and matrix composition exist in a variety of species, we hypothesized that genetic variation in bone quality and tissue-level mechanical properties would also exist within species. Sixteen-week-old female A/J, C57BL/6J (B6), and C3H/HeJ (C3H) inbred mouse femora were analyzed using Fourier transform infrared imaging and tissue-level mechanical testing for variation in mineral composition, mineral maturity, collagen cross-link ratio, and tissue-level mechanical properties. A/J femora had an increased mineral-to-matrix ratio compared to B6. The C3H mineral-to-matrix ratio was intermediate of A/J and B6. C3H femora had reduced acid phosphate and carbonate levels and an increased collagen cross-link ratio compared to A/J and B6. Modulus values paralleled mineral-to-matrix values, with A/J femora being the most stiff, B6 being the least stiff, and C3H having intermediate stiffness. In addition, work-to-failure varied among the strains, with the highly mineralized and brittle A/J femora performing the least amount of work-to-failure. Inbred mice are therefore able to differentially modulate the composition of their bone mineral and the maturity of their bone matrix in conjunction with tissue-level mechanical properties. These results suggest that specific combinations of bone quality and morphological traits are genetically regulated such that mechanically functional bones can be constructed in different ways.

Identification of material parameters based on Mohr-Coulomb failure criterion for bisphosphonate treated canine vertebral cancellous bone

Nanoindentation has been widely used to study bone tissue mechanical properties. The common method and equations for analyzing nanoindentation, developed by Oliver and Pharr, are based on the assumption that the material is linearly elastic. In the present study, we adjusted the constraint of linearly elastic behavior and use nonlinear finite element analysis to determine the change in cancellous bone material properties caused by bisphosphonate treatment, based on an isotropic form of the Mohr-Coulomb failure model. Thirty-three canine lumbar vertebrae were used in this study. The dogs were treated daily for 1 year with oral doses of alendronate, risedronate, or saline vehicle at doses consistent, on a mg/kg basis, to those used clinically for the treatment of post-menopausal osteoporosis. Two sets of elastic modulus and hardness values were calculated for each specimen using the Continuous Stiffness Measurement (CSM) method (E(CSM) and H(CSM)) from the loading segment and the Oliver-Pharr method (E(O-P) and H(O-P)) from the unloading segment, respectively. Young's modulus (E(FE)), cohesion (c), and friction angle (phi) were identified using a finite element model for each nanoindentation. The bone material properties were compared among groups and between methods for property identification. Bisphosphonate treatment had a significant effect on several of the material parameters. In particular, Oliver-Pharr hardness was larger for both the risedronate- and alendronate-treated groups compared to vehicle and the Mohr-Coulomb cohesion was larger for the risedronate-treated compared to vehicle. This result suggests that bisphosphonate treatment increases the hardness and shear strength of bone tissue. Shear strength was linearly predicted by modulus and hardness measured by the Oliver-Pharr method (r(2)=0.99). These results show that bisphosphonate-induced changes in Mohr-Coulomb material properties, including tissue shear cohesive strength, can be accurately calculated from Oliver-Pharr measurements of Young's modulus and hardness.

Cancellous bone lamellae strongly affect microcrack propagation and apparent mechanical properties: separation of patients with osteoporotic fracture from normal controls using a 2D nonlinear finite element method (biomechanical stereology)

Biomechanical stereology is proposed as a two-dimensional (2D) finite element (FE) method to estimate the ability of bone tissue to sustain damage and to separate patients with osteoporotic fracture from normal controls. Briefly, 2D nonlinear compact tension FE models were created from quantitative back scattered electron images taken of iliac crest bone specimens collected from the individuals with or without osteoporotic fracture history. The effects of bone mineral microstructure on predicted bone fracture toughness and microcrack propagation were examined. The 2D FE models were used as surrogates for the real bone tissues. The calculated microcrack propagation results and bone mechanical properties were examined as surrogates for measurements from mechanical testing of actual specimens. The results for the 2D FE simulation separated patients with osteoporotic fracture from normal controls even though only the variability in tissue mineral microstructure was used to build the models. The models were deliberately created to ignore all differences in mean mineralization. Hence, the current results support the following hypotheses: (1) that material heterogeneity is important to the separation of patients with osteoporotic fracture from normal controls; and (2) that 2D nonlinear finite element modeling can produce surrogate mechanical parameters that separate patients with fracture from normal controls.