Collagen and mineral deposition in rabbit cortical bone during maturation and growth: effects on tissue properties

A persistent mineralization process in alveolar bone throughout the postnatal growth stage in rats

OBJECTIVE

Alveolar bone quality is essential for the maxillofacial integrity and function, and depends on alveolar bone mineralization. This study aims to investigate the in vivo changes in alveolar bone mineralization, from the perspective of mineral deposition and crystal transition in postnatal rats.

DESIGN

Nine postnatal time points of Wistar rats, ranging from day 1 to 56, were set to obtain the maxillary alveolar bone samples. Each time point consisted of ninety rats, with 45 females and 45 males. Macromorphology of alveolar bone was reconducted by Micro-Computed Tomography and the mineral content was quantified via Thermogravimetric analysis, Scanning Electron Microscope, High-Resolution Transmission Electron Microscopy and vibrational spectroscopy. Furthermore, the crystallinity and composition were characterized by vibrational spectroscopy, X-ray Diffraction, X-ray Photoelectron Spectroscopy and Selected Area Electron Diffraction.

RESULTS

The progressive increase of mineral deposition was accompanied by substantial growth in alveolar bone mass and volume in postnatal rats. Whereas the mineral percentage initially decreased and then increased, reaching a nadir on postnatal day 14 (P14) when tooth eruption was first observed. Besides, localized mineralization was initiated by the formation of amorphous precursors and then converted into mineral crystals, while there was no statistically significant change in the average crystallinity of the bone during growth.

CONCLUSION

Mineralization of alveolar bone is ongoing throughout the early growth in postnatal rats. Mineral deposition increases with age, whereas the crystallinity remains stable within a certain range. Besides, the mineral percentage reaches its lowest point on P14, which may be attributed to tooth eruption.

Immature porcine cortical bone mechanical properties and composition change with maturation and displacement rate

Computational models of mature bone have been used to predict fracture; however, analogous study of immature diaphyseal fracture has not been conducted due to sparse experimental mechanical data. A model of immature bone fracture may be used to aid in the differentiation of accidental and non-accidental trauma fractures in young, newly ambulatory children (0-3 years). The objective of this study was to characterize the evolution of tissue-level mechanical behavior, composition, and microstructure of maturing cortical porcine bone with uniaxial tension, Raman spectroscopy, and light microscopy as a function of maturation. We asked: 1) How do the monotonic uniaxial tensile properties change with maturation and displacement rate; 2) How does the composition and microstructure change with maturation; and 3) Is there a correlation between composition and tensile properties with maturation? Elastic modulus (p < 0.001), fracture stress (p < 0.001), and energy absorption (p < 0.014) increased as a function of maturation at the quasistatic rate by 110%, 86%, and 96%, respectively. Fracture stress also increased by 90% with maturation at the faster rate (p = 0.001). Fracture stress increased as a function of increasing displacement rate by 28% (newborn p = 0.048; 1-month p = 0.004; 3-month p= < 0.001), and fracture strain decreased by 68% with increasing displacement rate (newborn p = 0.002; 1-month p = 0.036; 3-month p < 0.001). Carbonate-to-phosphate ratio was positively linearly related to elastic modulus, and fracture stress was positively related to carbonate-to-phosphate ratio and matrix maturation ratio. The results of this study support that immature bone is strain-rate dependent and becomes more brittle at faster rates, contributing to the foundation upon which a computational model can be built to evaluate immature bone fracture.

Effect of calcium carbonate nanoparticles, silver nanoparticles and advanced platelet-rich fibrin for enhancing bone healing in a rabbit model

This study aimed to evaluate the efficacy of calcium carbonate nanoparticles (CCNPs) to induce new bone formation in a critical size segmental bone defect in rabbit's radius when used alone, combined with silver nanoparticles (AgNPs) as a paste, or as a composite containing CCNPs, AgNPs, and advanced platelet-rich fibrin (A-PRF). Thirty-six adult apparently healthy male New Zealand White rabbits aging from 5 to 6 months and weighting 3.5 ± 0.5 kg were used. The animals were divided into four groups; control group, CCNPs group, CCNPs/AgNPs paste group, and CCNPs/AgNPs/A-PRF composite group. The animals were investigated at 4, 8, and 12 weeks post-implantation in which the healing was evaluated using computed tomographic (CT) and histopathological evaluation. The results revealed that CCNPs/AgNPs paste and CCNPs/AgNPs/A-PRF composite has a superior effect regarding the amount and the quality of the newly formed bone compared to the control and the CCNPs alone. In conclusion, addition of AgNPs and/or A-PRF to CCNPs has reduced its resorption rate and improved its osteogenic and osteoinductive properties.

The Hydration State of Bone Tissue Affects Contrast in Neutron Tomographic Images

Neutron tomography has emerged as a promising imaging technique for specific applications in bone research. Neutrons have a strong interaction with hydrogen, which is abundant in biological tissues, and they can penetrate through dense materials such as metallic implants. However, in addition to long imaging times, two factors have led to challenges in running in situ mechanical characterization experiments on bone tissue using neutron tomography: 1) the high water content in specimens reduces the visibility of internal trabecular structures; 2) the mechanical properties of bone are dependent on the hydration state of the tissue, with drying being reported to cause increased stiffness and brittleness. This study investigates the possibility of improving image quality in terms of neutron transmission and contrast between material phases by drying and rehydrating in heavy water. Rat tibiae and trabecular bovine bone plugs were imaged with neutron tomography at different hydration states and mechanical testing of the bone plugs was carried out to assess effects of drying and rehydration on the mechanical properties of bone. From analysis of image histograms, it was found that drying reduced the contrast between bone and soft tissue, but the contrast was restored with rehydration. Contrast-to-noise ratios and line profiles revealed that the contrast between bone tissue and background was reduced with increasing rehydration duration but remained sufficient for identifying internal structures as long as no free liquid was present inside the specimen. The mechanical analysis indicated that the proposed fluid exchange protocol had no adverse effects on the mechanical properties.

Dual modality neutron and x-ray tomography for enhanced image analysis of the bone-metal interface

The bone tissue formed at the contact interface with metallic implants, particularly its 3D microstructure, plays a pivotal role for the structural integrity of implant fixation. X-ray tomography is the classical imaging technique used for accessing microstructural information from bone tissue. However, neutron tomography has shown promise for visualising the immediate bone-metal implant interface, something which is highly challenging with x-rays due to large differences in attenuation between metal and biological tissue causing image artefacts. To highlight and explore the complementary nature of neutron and x-ray tomography, proximal rat tibiae with titanium-based implants were imaged with both modalities. The two techniques were compared in terms of visualisation of different material phases and by comparing the properties of the individual images, such as the contrast-to-noise ratio. After superimposing the images using a dedicated image registration algorithm, the complementarity was further investigated via analysis of the dual modality histogram, joining the neutron and x-ray data. From these joint histograms, peaks with well-defined grey value intervals corresponding to the different material phases observed in the specimens were identified and compared. The results highlight differences in how neutrons and x-rays interact with biological tissues and metallic implants, as well as the benefits of combining both modalities. Future refinement of the joint histogram analysis could improve the segmentation of structures and tissues, and yield novel information about specimen-specific properties such as moisture content.

Bone remodeling and cyclical loading in maxillae of New Zealand white rabbits (Oryctolagus cuniculus)

Mammalian feeding behaviors are altered when mechanically challenging (e.g., tough, stiff) foods require large bite forces or prolonged mastication. Bony responses to high bite forces are well-documented for the mammalian skull, but osteogenesis due to cyclical loading, caused by repetitive chewing, is more poorly understood. Previous studies demonstrate that cyclical loading results in greater bone formation in the rabbit masticatory apparatus and in substantial Haversian remodeling in primate postcrania. Here we assess the relationship between cyclical loading and remodeling in the rabbit maxilla. Twenty male New Zealand white rabbits (Oryctolagus cuniculus) were raised on either an overuse or control diet (10 per group) for 48 weeks, beginning at weaning onset. The control group was raised on a diet of rabbit pellets (E = 29 MPa, R = 1031 J/m² ), whereas the overuse group ate rabbit pellets and hay, which has high stiffness (E = 3336 MPa) and toughness (R = 2760 J/m² ) properties. Hay requires greater chewing investment (475 chews/g) and longer chewing durations (568 s/g) than pellets (161 chews/g and 173 s/g), therefore causing cyclical loading of the jaws. Remodeling was measured as osteon population density (OPD), percent Haversian bone (%HAV), and osteon cross-sectional area (On.Ar). The only significant difference found was greater On.Ar in the alveolar region of the maxilla (p < 0.001) in the overuse group. The hypothesis that cyclical loading engenders Haversian remodeling in the developing maxilla is not supported. The continuation of modeling throughout the experimental duration may negate the need for remodeling as newly laid bone tends to be more compliant and resistant to crack propagation.

Multimodal Evaluation of the Spatiotemporal Variations of Periprosthetic Bone Properties

Titanium implants are widely used in dental and orthopedic surgeries. However, implant failures still occur because of a lack of implant stability. The biomechanical properties of bone tissue located around the implant need to be assessed to better understand the osseointegration phenomena and anticipate implant failure. The aim of this study was to explore the spatiotemporal variation of the microscopic elastic properties of newly formed bone tissue close to an implant. Eight coin-shaped Ti6Al4V implants were inserted into rabbit tibiae for 7 and 13 weeks using an in vivo model allowing the distinction between mature and newly formed bone in a standardized configuration. Nanoindentation and micro-Brillouin scattering measurements were carried out in similar locations to measure the indentation modulus and the wave velocity, from which relative variations of bone mass density were extracted. The indentation modulus, the wave velocity and mass density were found to be higher (1) in newly formed bone tissue located close to the implant surface, compared to mature cortical bone tissue, and (2) after longer healing time, consistently with an increased mineralization. Within the bone chamber, the spatial distribution of elastic properties was more heterogeneous for shorter healing durations. After 7 weeks of healing, bone tissue in the bone chamber close to the implant surface was 12.3% denser than bone tissue further away. Bone tissue close to the chamber edge was 16.8% denser than in its center. These results suggest a bone spreading pathway along tissue maturation, which is confirmed by histology and consistent with contact osteogenesis phenomena.

Unfavorable Contribution of a Tissue-Engineering Cartilage Graft to Osteochondral Defect Repair in Young Rabbits

A stem cell-based tissue-engineering approach is a promising strategy for treatment of cartilage defects. However, there are conflicting data in the feasibility of using this approach in young recipients. A young rabbit model with an average age of 7.7 months old was used to evaluate the effect of a tissue-engineering approach on the treatment of osteochondral defects. Following in vitro evaluation of proliferation and chondrogenic capacity of infrapatellar fat pad-derived stem cells (IPFSCs) after expansion on either tissue culture plastic (TCP) or decellularized extracellular matrix (dECM), a premature tissue construct engineered from pretreated IPFSCs was used to repair osteochondral defects in young rabbits. We found that dECM expanded IPFSCs exhibited higher proliferation and chondrogenic differentiation compared to TCP expanded cells in both pellet and tissue construct culture systems. Six weeks after creation of bilateral osteochondral defects in the femoral trochlear groove of rabbits, the Empty group (left untreated) had the best cartilage resurfacing with the highest score in Modified O’Driscoll Scale (MODS) than the other groups; however, this score had no significant difference compared to that of 15-week samples, indicating that young rabbits stop growing cartilage once they reach 9 months old. Interestingly, implantation of premature tissue constructs from both dECM and TCP groups exhibited significantly improved cartilage repair at 15 weeks compared to those at six weeks (about 9 months old), indicating that a tissue-engineering approach is able to repair adult cartilage defects. We also found that implanted pre-labeled cells in premature tissue constructs were undetectable in resurfaced cartilage at both time points. This study suggests that young rabbits (less than 9 months old) might respond differently to the classical tissue-engineering approach that is considered as a potential treatment for cartilage defects in adult rabbits.

Multiscale Characterization of Embryonic Long Bone Mineralization in Mice

Long bone mineralization occurs through endochondral ossification, where a cartilage template mineralizes into bone-like tissue with a hierarchical organization from the whole bone-scale down to sub-nano scale. Whereas this process has been extensively studied at the larger length scales, it remains unexplored at some of the smaller length scales. In this study, the changes in morphology, composition, and structure during embryonic mineralization of murine humeri are investigated using a range of high-resolution synchrotron-based imaging techniques at several length scales. With micro- and nanometer spatial resolution, the deposition of elements and the shaping of mineral platelets are followed. Rapid mineralization of the humeri occurs over approximately four days, where mineral to matrix ratio and calcium content in the most mineralized zone reach adult values shortly before birth. Interestingly, zinc is consistently found to be localized at the sites of ongoing new mineralization. The mineral platelets in the most recently mineralized regions are thicker, longer, narrower, and less aligned compared to those further into the mineralized region. In summary, this study demonstrates a specific spatial distribution of zinc, with highest concentration where new mineral is being deposited and that the newly formed mineral platelets undergo slight reshaping and reorganization during embryonic development.

Comparison of small-angle neutron and X-ray scattering for studying cortical bone nanostructure

In this study, we present a combined small-angle neutron and X-ray scattering (SANS and SAXS) study of the nanoscale structure of cortical bone specimens from three different species. The variation of the scattering cross section of elements across the periodic table is very different for neutrons and X-rays. For X-rays, it is proportional to the electron density while for neutrons it varies irregularly with the atomic number. Hence, combining the two techniques on the same specimens allows for a more detailed interpretation of the scattering patterns as compared to a single-contrast experiment. The current study was performed on bovine, porcine and ovine specimens, obtained in two perpendicular directions with respect to the main axis of the bone (longitudinal and radial) in order to maximise the understanding of the nanostructural organisation. The specimens were also imaged with high resolution micro-computed tomography (micro-CT), yielding tissue mineral density and microstructural orientation as reference. We show that the SANS and SAXS patterns from the same specimen are effectively identical, suggesting that these bone specimens can be approximated as a two-component composite material. Hence, the observed small-angle scattering results mainly from the mineral-collagen contrast, apart from minor features associated with the internal collagen structure.

Quantitative µMRI and PLM study of rabbit humeral and femoral head cartilage at sub-10 µm resolutions

This study aimed to establish the baseline characteristics in humeral and femoral cartilage in rabbit, using quantitative magnetic resonance imaging (MRI) relaxation times (T2, T1ρ, and T1) at 9.75 and 70-82 µm pixel resolutions, and quantitative polarized light microscopy (PLM) measures (retardation, angle) at 1.0 and 4.0 µm pixel resolutions. Five intact (i.e., unopened) shoulder joints (the scapula and humeral heads) and three femoral heads of the hip joints from five healthy rabbits were imaged in MRI at 70-82 µm resolution. Thirteen cartilage-bone specimens were harvested from these joints and imaged in µMRI at 9.75 µm resolution. Subsequently, quantitative PLM study of these specimens enabled the examination of the fibril orientation and organization in both intact joints and individual specimens. Quantitative MRI relaxation data and PLM fibril structural data show distinct features in tissue properties at different depths of cartilage, different in individual histological zones. The thicknesses of the histological zones in µMRI and PLM were successfully obtained. This is the first correlated and quantitative MRI and PLM study of rabbit cartilage at sub-10 µm resolutions, which benefits future investigation of osteoarthritis using the rabbit model. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1052-1062, 2020.

Mineralization of cortical bone during maturation and growth in rabbits

INTRODUCTION

The composite nature of bone as a material governs its structure and mechanical behavior. How the collagenous matrix mineralizes, in terms of both mineral deposition and structure of the mineral crystals, is highly interesting when trying to elucidate the complex structural changes that occur during bone growth and maturation. We have previously looked at mineral deposition and structural evolution of the collagenous matrix, linking both to changes in mechanics. The purpose of this study was to provide specific information on changes in crystal size and organization as a function of growth and maturation.

MATERIALS AND METHODS

Using micro-computed tomography (µCT) and micro-focused scanning small-angle X-ray scattering (SAXS) we investigated cortical bone in two orthogonal directions relative to the long axis of the humeri of New Zealand White rabbits spanning from new-born to 6-months of age. We also investigated the changes with tissue age by looking at radial profiles of osteonal structures in the 6-months old rabbits. The findings were compared to our previous compositional, structural and mechanical data on the same sample cohort.

RESULTS

µCT showed a continuous mineral deposition up until 3-months of age, whilst the SAXS data showed an increase in both crystal thickness and degree of orientation up until 6-months of age. The osteonal profiles showed no statistically significant changes in crystal thickness.

CONCLUSIONS

Comparison to previously collected mechanical data suggests that changes are not only explained by amount of mineral in the tissue but also by the crystal dimensions.

Internal architecture of the mandibular condyle of rabbits is related to dietary resistance during growth

Although there is considerable evidence that bone responds to the loading environment in which it develops, few analyses have examined phenotypic plasticity or bone functional adaptation in the masticatory apparatus. Prior work suggests that masticatory morphology is sensitive to differences in food mechanical properties during development; however, the importance of the timing/duration of loading and variation in naturalistic diets is less clear. Here, we examined microstructural and macrostructural differences in the mandibular condyle in four groups of white rabbits (Oryctolagus cuniculus) raised for a year on diets that varied in mechanical properties and timing of the introduction of mechanically challenging foods, simulating seasonal variation in diet. We employed sliding semilandmarks to locate multiple volumes of interest deep to the mandibular condyle articular surface, and compared bone volume fraction, trabecular thickness and spacing, and condylar size/shape among experimental groups. The results reveal a shared pattern of bony architecture across the articular surface of all treatment groups, while also demonstrating significant among-group differences. Rabbits raised on mechanically challenging diets have significantly increased bone volume fraction relative to controls fed a less challenging diet. The post-weaning timing of the introduction of mechanically challenging foods also influences architectural properties, suggesting that bone plasticity can extend well into adulthood and that bony responses to changes in loading may be rapid. These findings demonstrate that bony architecture of the mandibular condyle in rabbits responds to variation in mechanical loading during an organism's lifetime and has the potential to track dietary variation within and among species.

Development of CaCO3 microsphere-based composite hydrogel for dual delivery of growth factor and Ca to enhance bone regeneration

Injectable scaffolds have attracted much attention because of their minimum surgical invasiveness. However, limited osteogenic induction property and low mechanical properties hampered their application in bone tissue engineering. CaCO3 microspheres, which possess osteoinductivity, rough surfaces and specific binding sites for BMP-2, were first fabricated; after BMP-2 uploading, microspheres were further entrapped in fibrin-glue hydrogel. CaCO3 microspheres were co-functionalized with casein and heparin. To obtain a high encapsulation of heparin and thus BMP-2 uploading, along with controlled release and simultaneous maintenance of the presence of vaterite which had osteogenic induction property, fabrication parameters were optimized and microspheres were characterized using XRD, FITR and SEM. The formed CaCO3 had a microsphere morphology of ∼1 μm. Both vaterite and calcite phases were present and the relative amount of calcite phase increased with the amount of heparin. Sample 25 mM_4-1Hep with the highest loading amount of heparin was selected as carrier for BMP-2 and BMP-2 loaded CaCO3 microspheres were further entrapped in fibrin-glue hydrogel (FC-B). For the as-prepared composite hydrogel, mechanical properties were characterized and the presence of CaCO3 significantly elevated the tensile strength; controlled release of BMP-2 was sustained until day 21. Based on ALP activity, alizarin red staining and RT-PCR, in vitro osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) was found to be significantly enhanced under induction of FC-B. Rabbit tibia bone defect model was applied to evaluate its in vivo performance. After implantation for 4 weeks, presence of composite hydrogel was observed in defects. After 8 weeks, bone defects of FC-B group were nearly completely healed. Using the fact that autologous scaffolds can be derived based on fibrin-glue hydrogel, the well-designed BMP-2 loaded fibrin-glue composite hydrogel demonstrated good potential in bone tissue engineering.

The different distribution of enzymatic collagen cross-links found in adult and children bone result in different mechanical behavior of collagen

Enzymatic collagen cross-linking has been shown to play an important role in the macroscopic elastic and plastic deformation of bone across ages. However, its direct contribution to collagen fibril deformation is unknown. The aim of this study is to determine how covalent intermolecular connections from enzymatic collagen cross-links contribute to collagen fibril elastic and plastic deformation of adults and children's bone matrix. We used ex vivo data previously obtained from biochemical analysis of children and adults bone samples (n = 14; n = 8, respectively) to create 22 sample-specific computational models of cross-linked collagen fibrils. By simulating a tensile test for each fibril, we computed the modulus of elasticity (E), ultimate tensile and yield stress (σ_u and σ_y), and elastic, plastic and total work (W_e, W_p and W_tot) for each collagen fibril. We present a novel difference between children and adult bone in the deformation of the collagen phase and suggest a link between collagen fibril scale and macroscale for elastic behavior in children bone under the influence of immature enzymatic cross-links. We show a parametric linear correlation between W_e and immature enzymatic collagen cross-links at the collagen fibril scale in the children population that is similar to the one we found at the macroscale in our previous study. Finally, we suggest the key role of covalent intermolecular connections to stiffness parameters (e.g. elastic modulus and W_e) in children's collagen fibril and to toughness parameters in adult's collagen fibril, respectively.

Betwixt and Between: Intracranial Perspective on Zygomatic Arch Plasticity and Function in Mammals

The zygomatic arch is morphologically complex, providing a key interface between the viscerocranium and neurocranium. It also serves as an attachment site for masticatory muscles, thereby linking it to the feeding apparatus. Though morphological variation related to differential loading is well known for many craniomandibular elements, the adaptive osteogenic response of the zygomatic arch remains to be investigated. Here, experimental data are presented that address the naturalistic influence of masticatory loading on the postweaning development of the zygoma and other cranial elements. Given the similarity of bone-strain levels among the zygoma and maxillomandibular elements, a rabbit and pig model were used to test the hypothesis that variation in cortical bone formation and biomineralization along the zygomatic arch and masticatory structures are linked to increased stresses. It was also hypothesized that neurocranial structures would be minimally affected by varying loads. Rabbits and pigs were raised for 48 weeks and 8 weeks, respectively. In both experimental models, CT analyses indicated that elevated masticatory loading did not induce differences in cortical bone thickness of the zygomatic arch, though biomineralization was positively affected. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements. Varying osteogenic responses in the arch suggests that skeletal adaptation, and corresponding variation in performance, may reside differentially at one level of bony architecture. Thus, it is possible that phenotypic diversity in the mammalian zygoma is due more singularly to natural selection (vs. plasticity). These findings underscore the complexity of the zygomatic arch and, more generally, determinants of skull form. Anat Rec, 299:1646-1660, 2016. © 2016 Wiley Periodicals, Inc.

In situ examination of osteoblast biomineralization on sulfonated polystyrene-modified substrates using Fourier transform infrared microspectroscopy

Osteoporosis is a skeletal disorder that is characterized by the loss of bone mineral density (BMD) resulting in increased risk of fracture. However, it has been shown that BMD is not the only indicator of fracture risk, as the strength of bone depends on a number of factors, including bone mass, architecture and material properties. Physiological mineral deposition requires the formation of a properly developed extracellular matrix (ECM), which recruits calcium and phosphate ions into the synthesis of apatite crystals. Temporal and spatial compositional and structural changes of biological apatite greatly depend on the properties of the crystals initially formed. As such, Fourier-transform infrared microspectroscopy (FTIRM) is capable of examining adaptive remodeling by providing compositional information such as the level of mineralization and carbonate substitution, as well as quality and perfection of the mineral phase. The objective of this study was to evaluate the in vitro mineralization development of MC3T3-E1 murine calvarial preosteoblasts cultured on different substrata by comparing FTIRM measurements from two subclones (mineralizing subclone 4 and nonmineralizing subclone 24) maintained in culture for up to 21 days. The results showed that modulation of the substrate surface using a thin coating of sulfonated polystyrene (SPS) provided favorable conditions for the development of a mineralizable ECM and that the mineral formed by the osteoblasts was similar to that of fully mineralized bone tissue. Specifically, the mineralizing subclone produced significantly more mineral phosphate when cultured on SPS-coated substrates for 21 days, compared to the same culture on bare substrates. In contrast, the level of mineralization in nonmineralizing subclone was low on both SPS-coated and uncoated substrates. The mineralizing subclone also produced comparable amounts of collagen on both substrates; however, mineralization was significantly higher in the SPS culture. The nonmineralizing subclone produced comparable amounts of collagen on day 1 but much less on day 21. Collagen maturity ratio increased in the mineralizing subclone from day 1 to day 21, but remained unchanged in the nonmineralizing subclone. These results suggest that SPS-treatment of the substrate surface may alter collagen remodeling; however, other factors may also influence osteoblast mineralization in the long term.

Effects of cyclic compression on the mechanical properties and calcification process of immature chick bone tissue in culture

Contribution of mechanical loading to tissue growth during both the development and post-natal maturation is of a particular interest, as its understanding would be important to strategies in bone tissue engineering and regenerative medicine. The present study has been performed to investigate how immature bone responds to mechanical loading using an ex vivo culture system. A slice of the tibia, with the thickness of 3 mm, was obtained from 0-day-old chick. For the ex vivo culture experiment in conjunction with cyclic compressive loading, we developed a custom-made, bioreactor system where both the load and the deformation applied to the specimen was recorded. Cyclic compression, with an amplitude of 0.3 N corresponding to 1 to 2% compressive strain, was applied to immature bone specimen during a 3-day culture period at an overall loading rate 3–4 cycles/min, in the presence of β-glycerol phosphate and dexamethasone in culture medium. The stress-strain relationship was obtained at the beginning and the end of the culture experiment. In addition, analyses for alkaline phosphate release, cell viability and tissue calcification were also performed. It was exhibited that elastic moduli of bone slices were significantly elevated at the end of the 3-day culture in the presence of cyclic compression, which was a similar phenomenon to significant elevation of the elastic moduli of bone tissue by the maturation from 0-day old to 3-day old. By contrast, no significant changes in the moduli were observed in the absence of cyclic compression or in deactivated, cell-free samples. The increases in the moduli were coincided with the increase in calcified area in the bone samples. It was confirmed that immature bone can respond to compressive loading in vitro and demonstrate the growth of bone matrix, similar to natural, in vivo maturation. The elevation of the elastic moduli was attributable to the increased calcified area and the realignment of collagen fibers parallel to the loading direction. The ex vivo loading system established here can be further applied to study responses to mechanical loading in osteogenesis as well as callus maturation for better understanding of factors to consider in successful bone regeneration with mechanical factors.

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We examined how immature bone responds to cyclic compressive loading, using a newly established tissue loading system.

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Loading culture of 0-day-old chick tibia slice for 3 days significantly increased elastic moduli and calcified area.

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The moduli and calcified area after 3-day loading culture were comparable with those from 3-day-old chick tibia.

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No increases in the moduli or in calcified area were observed in non-loaded samples and deactivated samples.

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It was suggested that cyclic compressive loading is an essential factor of normal bone growth in ex vivo bone culture.

Intracranial and hierarchical perspective on dietary plasticity in mammals

The effect of dietary properties on craniofacial form has been the focus of numerous functional studies, with increasingly more work dedicated to the importance of phenotypic plasticity. As bone is a dynamic tissue, morphological variation related to differential loading is well established for many masticatory structures. However, the adaptive osteogenic response of several cranial sites across multiple levels of bony organization remains to be investigated. Here, rabbits were obtained at weaning and raised for 48 weeks until adulthood in order to address the naturalistic influence of altered loading on the long-term development of masticatory and non-masticatory elements. Longitudinal data from micro-computed tomography (μCT) scans were used to test the hypothesis that variation in cortical bone formation and biomineralization in masticatory structures is linked to increased stresses during oral processing of mechanically challenging foods. It was also hypothesized that similar parameters for neurocranial structures would be minimally affected by varying loads as this area is characterized by low strains during mastication and reduced hard-tissue mechanosensitivity. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements, though biomineralization trends of masticatory structures did not mirror macroscale findings. Varying osteogenic responses in masticatory elements suggest that physiological adaptation, and corresponding variation in skeletal performance, may reside differentially at one level of bony architecture, potentially affecting the accuracy of behavioral and in silico reconstructions. Together, these findings underscore the complexity of bone adaptation and highlight functional and developmental variation in determinants of skull form.

Assessment of a polyelectrolyte multilayer film coating loaded with BMP-2 on titanium and PEEK implants in the rabbit femoral condyle

The aim of this study was to evaluate the osseointegration of titanium implants (Ti-6Al-4V, noted here TA6V) and poly(etheretherketone) PEEK implants induced by a BMP-2-delivering surface coating made of polyelectrolyte multilayer films. The in vitro bioactivity of the polyelectrolyte film-coated implants was assessed using the alkaline phosphatase assay. BMP-2-coated TA6V and PEEK implants with a total dose of 9.3μg of BMP-2 were inserted into the femoral condyles of New Zealand white rabbits and compared to uncoated implants. Rabbits were sacrificed 4 and 8weeks after implantation. Histomorphometric analyses on TA6V and PEEK implants and microcomputed tomography on PEEK implants revealed that the bone-to-implant contact and bone area around the implants were significantly lower for the BMP-2-coated implants than for the bare implants. This was confirmed by scanning electron microscopy imaging. This difference was more pronounced at 4weeks in comparison to the 8-week time point. However, bone growth inside the hexagonal upper hollow cavity of the screws was higher in the case of the BMP-2 coated implants. Overall, this study shows that a high dose of BMP-2 leads to localized and temporary bone impairment, and that the dose of BMP-2 delivered at the surface of an implant needs to be carefully optimized.

STATEMENT OF SIGNIFICANCE

The presentation of growth factors from material surfaces currently presents significant challenges in academia, clinics and industry. Applying osteoinductive factors to different types of implants, made of metals or polymers, may improve bone repair in difficult situations. Here, we show the effects of an osteoinductive coating made of polyelectrolyte multilayer films on two widely used materials, titanium TA6V alloys and PEEK implants, which were implanted in the rabbit femoral condyle. We show that a too high dose of BMP-2 delivered from the screw surface has a negative short-term effect on bone regeneration in close vicinity of the screw surface. In contrast, bone formation was increased at early times in the empty spaces around the screw. These results highlight the need for future dose-dependence studies on bone formation in response to osteoinductive coatings.

Implant stability and bone remodeling up to 84 days of implantation with an initial static strain. An in vivo and theoretical investigation

Objectives

When implants are inserted, the initial implant stability is dependent on the mechanical stability. To increase the initial stability, it was hypothesized that bone condensation implants will enhance the mechanical stability initially and that the moderately rough surface will further contribute to the secondary stability by enhanced osseointegration. It was further hypothesized that as the healing progresses the difference in removal torque will diminish. In addition, a 3D model was developed to simulate the interfacial shear strength. This was converted to a theoretical removal torque that was compared to the removal torque obtained in vivo.

Material and methods

Condensation implants, inducing bone strains of 0.015, were installed into the left tibia of 24 rabbits. Non‐condensation implants were installed into the right tibia. All implants had a moderately rough surface. The implants had an implantation time of 7, 28, or 84 days before the removal torque was measured. The interfacial shear strength at different healing time was estimated by the means of finite element method.

Results

At 7 days of healing, the condensation implant had an increased removal torque compared to the non‐bone‐condensation implant. At 28 and 84 days of healing, there was no difference in removal torque. The simulated interfacial shear strength ratios of bone condensation implants at different implantation time were in line with the in vivo data.

Conclusions

Moderately rough implants that initially induce bone strain during installation have increased stability during the early healing period. In addition, the finite element method may be used to evaluate differences in interlocking capacity.

Tissue reaction and material biodegradation of a calcium sulfate/apatite biphasic bone substitute in rat muscle

Background/Objective

A biphasic ceramic bone substitute consisting of calcium sulfate and hydroxyapatite has been reported to give good clinical outcome regarding bone regeneration and may serve as a carrier for antibiotics in the treatment of bone infections. Often, the overlying muscle is in direct contact with the synthetic graft. The dissolving bone substitute induces inflammation, which may be harmful to the surrounding soft and muscle tissue. The aim of the present study was to evaluate the surrounding soft tissue reaction and the biodegradation of the biphasic bone substitute.

Methods

Rods (3 mm × 6 mm) were cast and implanted in the rat abdominal rectus muscle. The rods were either soaked or not soaked in autologous bone marrow before insertion to induce bone formation. Thirty-two rats underwent bilateral operation. After 6 weeks and 12 weeks, the bone substitute material and the surrounding muscle were harvested. The right rod was evaluated by histology to study tissue reaction and the left rod was analysed with micro-computed tomography and scanning electron microscopy to study bone substitute degradation.

Results

The muscle tissue around the material was similar at 6 weeks and 12 weeks, with or without prior treatment with bone marrow. The remaining material showed close contact with the muscle, and blood vessels penetrated the material in both groups. Wide bundles of collagen were embedded around the apatite particles, more at the 12-week time point. No bone formation was found, either at 6 weeks or 12 weeks, and scanning electron microscopy showed that the calcium sulfate phase was resorbed after 6 weeks with the calcium phosphate phase remaining intact. Micro-computed tomography showed significantly more hydroxyapatite at 6 weeks than after 12 weeks.

Conclusion

Calcium sulfate hydroxyapatite bone substitute can be used as a carrier for antibiotics or other drugs, without adverse reaction due to the fast resorption of the calcium sulfate. No bone formation was seen despite treating the bone substitute with autologous bone marrow.

Ratio between mature and immature enzymatic cross-links correlates with post-yield cortical bone behavior: An insight into greenstick fractures of the child fibula

As a determinant of skeletal fragility, the organic matrix is responsible for the post-yield and creep behavior of bone and for its toughness, while the mineral apatite acts on stiffness. Specific to the fibula and ulna in children, greenstick fractures show a plastic in vivo mechanical behavior before bone fracture. During growth, the immature form of collagen enzymatic cross-links gradually decreases, to be replaced by the mature form until adolescence, subsequently remaining constant throughout adult life. However, the link between the cortical bone organic matrix and greenstick fractures in children remains to be explored. Here, we sought to determine: 1) whether plastic bending fractures can occur in vitro, by testing cortical bone samples from children's fibula and 2) whether the post-yield behavior (ωp plastic energy) of cortical bone before fracture is related to total quantity of the collagen matrix, or to the quantity of mature and immature enzymatic cross-links and the quantity of non-enzymatic cross-links. We used a two-step approach; first, a 3-point microbending device tested 22 fibula machined bone samples from 7 children and 3 elderly adults until fracture. Second, biochemical analysis by HPLC was performed on the sample fragments. When pooling two groups of donors, children and elderly adults, results show a rank correlation between total energy dissipated before fracture and age and a linear correlation between plastic energy dissipated before fracture and ratio of immature/mature cross-links. A collagen matrix with more immature cross-links (i.e. a higher immature/mature cross-link ratio) is more likely to plastically deform before fracture. We conclude that this ratio in the sub-nanostructure of the organic matrix in cortical bone from the fibula may go some way towards explaining the variance in post-yield behavior. From a clinical point of view, therefore, our results provide a potential explanation of the presence of greenstick fractures in children.

Strontium-containing apatite/polylactide composites enhance bone formation in osteopenic rabbits

Strontium (Sr) has been shown to favor bone formation and is used clinically to treat osteoporosis. We have previously reported that Sr addition in apatite/polylactide composites could enhance the BMP-induced bone formation around implants at ectopic site in healthy animals. In this study we aimed to investigate the effectiveness of Sr addition on the local bone formation in osteoporosis. Apatite/polylactide composite granules with different Sr content were loaded with equal amount of rhBMP-2 and implanted intramuscularly in healthy rabbits (Con) and rabbits that received bilateral ovariectomy and daily injection of glucocorticoid (OP) for 12 weeks. The potential effect of Sr on the final volume of BMP-induced bone in both groups was investigated histologically and histomorphometrically. The de novo bone formed in OP implants was significantly less than in Con group when the implants contained no Sr, indicating that the BMP-induced osteogenesis was impaired in OP animals. Sr substitution as low as 0.5 mol% in apatite increased the bone volume in OP implants to levels comparable to that in the Con group, indicating a positive effect of Sr addition on the local bone formation in OP animals. In addition, more adipose tissue formed in parallel with the appearance of cartilage tissue in OP implants, suggesting that the differentiation potential of stem cell in OP animals may have shifted towards adipogenesis and chondrogenesis. From these results, we conclude that the use of Sr addition to enhance the bone growth surrounding implants in osteoporosis merits further study.

STATEMENT OF SIGNIFICANCE

The impaired bone healing capacity of osteoporotic patients might result in poor osteointegration and surgical failure in case implants are placed. In this study we aimed to enhance the bone formation around implants under such scenario by adding strontium as the stimulus. Different from other studies, the samples were loaded with rhBMP-2 and implanted at an ectopic site (spinal muscles of New Zealand rabbits) to exclude the influence of conductive bone repair. The results showed that the addition of strontium could enhance the BMP-2-induced bone formation on implants in osteopenic rabbits to levels comparable to that in healthy rabbits. Secondarily, we observed more adipose tissue and cartilage tissue in osteopenic implants, suggesting the role of adipogenesis and chondrogenesis in osteopenia/osteoporosis.

The decrease in silicon concentration of the connective tissues with age in rats is a marker of connective tissue turnover

Silicon may be important for bone and connective tissue health. Higher concentrations of silicon are suggested to be associated with bone and the connective tissues, compared with the non-connective soft tissues. Moreover, in connective tissues it has been suggested that silicon levels may decrease with age based upon analyses of human aorta. These claims, however, have not been tested under controlled conditions. Here connective and non-connective tissues were collected and analysed for silicon levels from female Sprague-Dawley rats of different ages (namely, 3, 5, 8, 12, 26 and 43 weeks; n=8-10 per age group), all maintained on the same feed source and drinking water, and kept in the same environment from weaning to adulthood. Tissues (696 samples) were digested in nitric acid and analysed by inductively coupled plasma optical emission spectrometry for total silicon content. Fasting serum samples were also collected, diluted and analysed for silicon. Higher concentrations of silicon (up to 50-fold) were found associated with bone and the connective tissues compared with the non-connective tissues. Although total silicon content increased with age in all tissues, the highest connective tissue silicon concentrations (up to 9.98 μg/g wet weight) were found in young weanling rats, decreasing thereafter with age (by 2-6 fold). Fasting serum silicon concentrations reflected the pattern of connective tissue silicon concentrations and, both measures, when compared to collagen data from a prior experiment in Sprague-Dawley rats, mirrored type I collagen turnover with age. Our findings confirm the link between silicon and connective tissues and would imply that young growing rats have proportionally higher requirements for dietary silicon than mature adults, for bone and connective tissue development, although this was not formally investigated here. However, estimation of total body silicon content suggested that actual Si requirements may be substantially lower than previously estimated which could explain why absolute silicon deficiency is difficult to achieve but, when it is achieved in young growing animals, it results in stunted growth and abnormal development of bone and other connective tissues.

Simulation of the mechanical interlocking capacity of a rough bone implant surface during healing

Background

When an implant is inserted in the bone the healing process starts to osseointegrate the implant by creating new bone that interlocks with the implant. Biomechanical interlocking capacity is commonly evaluated in in vivo experiments. It would be beneficial to find a numerical method to evaluate the interlocking capacity of different surface structures with bone. In the present study, the theoretical interlocking capacity of three different surfaces after different healing times was evaluated by the means of explicit finite element analysis.

Methods

The surface topographies of the three surfaces were measured with interferometry and were used to construct a 3D bone-implant model. The implant was subjected to a displacement until failure of the bone-to-implant interface and the maximum force represents the interlocking capacity.

Results

The simulated ratios (test/control) seem to agree with the in vivo ratios of Halldin et al. for longer healing times. However the absolute removal torque values are underestimated and do not reach the biomechanical performance found in the study by Halldin et al. which might be a result of unknown mechanical properties of the interface.

Conclusion

Finite element analysis is a promising method that might be used prior to an in vivo study to compare the load bearing capacity of the bone-to-implant interface of two surface topographies at longer healing times.

Frequency-Dependence of Mechanically Stimulated Osteoblastic Calcification in Tissue-Engineered Bone In Vitro

The effect of mechanical stimulation on osteogenesis remains controversial, especially with respect to the loading frequency that maximizes osteogenesis. Mechanical stimulation at an optimized frequency may be beneficial for the bone tissue regeneration to promote osteoblastic calcification. The objective of this study was to investigate the frequency-dependent effect of mechanical loading on osteoblastic calcification in the tissue-engineered bones in vitro. Tissue-engineered bones were constructed by seeding rat osteoblasts into a type I collagen sponge scaffold at a cell density of 1600 or 24,000 cells/mm(3). Sinusoidal compressive deformation at the peak of 0.2% was applied to the tissue-engineered bones at 0.2, 2, 10, 20, 40, and 60 Hz for 3 min/day for 14 consecutive days. Optically-monitored calcium content started to increase on days 5-7 and reached the highest value at 2 Hz on day 14; however, no increase was observed at 0.2 Hz and in the control. Ash content measured after the mechanical stimulation also showed the highest at 2 Hz despite the differences in cell seeding density. It was concluded that mechanical stimulation at 2 Hz showed the highest promotional effect for osteogenesis in vitro among the frequencies selected in this study.

Relative fixation strength of rabbit subscapularis repair is comparable to human supraspinatus repair at time 0

BACKGROUND

Recent evidence suggests that the rabbit subscapularis tendon may be anatomically, biomechanically, and histologically suitable to study rotator cuff pathology and repair. However, biomechanical comparisons of rotator cuff repairs in this model have not been evaluated and compared to those in human cadaveric specimens.

QUESTIONS/PURPOSES

We quantified the biomechanical properties of the repaired rabbit subscapularis tendon after (1) single-row, (2) double-row, and (3) transosseous-equivalent rotator cuff repair techniques and compared the ratios of repairs to previously published data for human repairs.

METHODS

Tensile testing was performed on 21 New Zealand White rabbit subscapularis tendon-humerus complexes for single-row repair, double-row repair, and transosseous-equivalent repair (n = 7 for each group). Video digitizing software was used to quantify deformation. Load elongation data were then used to quantify structural properties. We compared the ratios of rotator cuff repairs for the rabbit data to data from human supraspinatus repair studies previously performed in our laboratory. For our primary end points (linear stiffness, yield load, ultimate load, and energy absorbed to failure), with the numbers available, our statistical power to detect a clinically important difference (defined as 15%) was 85%.

RESULTS

The ratios of single-row/double-row repair were 0.72, 0.73, 0.71, and 0.66 for human supraspinatus and 0.77, 0.74, 0.79, and 0.89 for rabbit subscapularis repair for linear stiffness, yield load, ultimate load, and energy absorbed to failure, respectively. The ratios of double-row/transosseous-equivalent repair were 1.0, 0.86, 0.70, and 0.41 for human supraspinatus and 1.22, 0.85, 0.76, and 0.60 for rabbit subscapularis for linear stiffness, yield load, ultimate load, and energy absorbed to failure, respectively. There were no differences comparing rabbit to human repair ratios for any parameter (p > 0.09 for all comparisons).

CONCLUSIONS

Subscapularis repairs in the rabbit at Time 0 result in comparable ratios to human supraspinatus repairs.

CLINICAL RELEVANCE

The biomechanical similarities between the different types of rotator cuff repair in the rabbit subscapularis and human supraspinatus at Time 0 provide more evidence that the rabbit subscapularis may be an appropriate model to study rotator cuff repairs.

Short-term exercise-induced improvements in bone properties are for the most part not maintained during aging in hamsters

Physical exercise during growth affects composition, structure and mechanical properties of bone. In this study we investigated whether the beneficial effects of exercise during the early growth phase have long-lasting effects or not. Female Syrian golden hamsters (total n=152) were used in this study. Half of the hamsters had access to running wheels during their rapid growth phase (from 1 to 3months of age). The hamsters were sacrificed at the ages of 1, 3, 12, and 15months. The diaphysis of the mineralized humerus was analyzed with microCT and subjected to three-point-bending mechanical testing. The trabecular bone in the tibial metaphysis was also analyzed with microCT. The collagen matrix of the humerus bone was studied by tensile testing after decalcification. The weight of the hamsters as well as the length of the bone and the volumetric bone mineral density (BMDvol) of the humerus was higher in the running group at the early age (3months). Moreover, the mineralized bone showed improved mechanical properties in humerus and had greater trabecular thickness in the subchondral bone of tibia in the runners. However, by the age of 12 and 15months, these differences were equalized with the sedentary group. The tensile strength and Young's modulus of decalcified humerus were higher in the runners at early stage, indicating a stronger collagen network. In tibial metaphysis, trabecular thickness was significantly higher for the runners in the old age groups (12 and 15months). Our study demonstrates that physical exercise during growth improves either directly or indirectly through weight gain bone properties of the hamsters. However, the beneficial effects were for the most part not maintained during aging.

Teaching an old jaw new tricks: Diet-induced plasticity in a model organism, from weaning to adulthood

Many organisms exhibit a decrease in the ability to modify their phenotypes in response to shifts in environmental conditions as they mature. Such age-dependent plasticity has important implications in a variety of evolutionary and ecological contexts, particularly with respect to understanding adaptive responses to heterogeneous environments. In this study we used experimental diet manipulation to examine the life-history trajectory of plasticity in the feeding complex of a model organism, the white rabbit (Oryctolagus cuniculus). We demonstrate that, contrary to expectations derived from previous cross-sectional studies of skeletal plasticity, the jaws of weanlings and young adults exhibit similar increases in relative bone cross-sectional areas in response to the introduction of mechanically challenging foods into their diets. Furthermore, we present evidence that sensitivity to loading patterns persists well into adulthood in some regions of the masticatory apparatus in rabbits, indicating that there is an extended window of opportunity to respond to changes in dietary properties during an animal's life span. We conclude that certain aspects of the facial skeleton of rabbits, and perhaps mammals in general, are sensitive to environmental stimuli long after skeletal maturity is achieved, highlighting the importance of plasticity as a source of adaptive variation at later life-history stages.

The influence of follistatin on mechanical properties of bone tissue in growing mice with overexpression of follistatin

Mechanical competence of bones is mainly associated with tissue quality that depends on proper bone metabolism processes. An imbalance in the regulation of bone metabolism leads to pathological changes in bone tissue leading to susceptibility to bone fractures and bone deterioration processes. Bone metabolism is regulated to a large extent by the members of the transforming growth factor-β superfamily, i.e., activins and bone morphogenetic proteins. However, their function is regulated by a single-chain protein called follistatin (FS). The aim of this study was to test the hypothesis that overexpression of FS in growing mice results in impairments in bone morphology and mechanical properties. Moreover, we wanted to investigate how geometrical, structural and material properties of bone tissue change with age. The experiment was performed on growing C57BL/6 TgNK14-mFst/6J mice, overexpressing FS (F mice) versus C57BL/6J mice used as controls (C mice). To establish how overexpression of FS influences bone tissue quality, we studied mice femurs to determine geometrical, structural and material properties of the skeleton. To determine mechanical resistance of bone tissue, femurs were loaded to failure in a three-point bending test. Obtained results indicated that overexpression of FS negatively influences bone metabolism. It was found that mutation results with a significant decrease of all measured biomechanical strength variables in F mice in comparison to C mice. Overexpression of FS leads to decreased quality of skeleton, increasing susceptibility to bone fractures.

Age-related changes in organization and content of the collagen matrix in rabbit cortical bone

The organization and composition of the collagen matrix of cortical bone changes as the bone matures due to growth and mechanical loading. We aimed to investigate the composition and organization of the collagen matrix in rabbit cortical bone during maturation using Fourier transform infrared (FTIR) microspectroscopy and polarized light microscopy (PLM). FTIR and PLM findings were compared to biochemical analysis from an earlier study. Mid-diaphyseal samples from left femora of female New Zealand White rabbits were used. The animal age ranged from newborn to 18-month old (5 age groups, n = 10 per group). The bones had earlier been decalcified and evaluated with biochemistry. In this study, collagen content, orientation, collagen cross-linking and spatial heterogeneity of all parameters was evaluated. Similar results were obtained when collagen content was evaluated with FTIR and PLM compared to the collagen content assessed with BA. Collagen content, orientation and collagen maturity increased significantly until the age of 3 months and remained similar thereafter. Simultaneously, spatial heterogeneity of the measured parameters decreased. Based on these findings, it seems that the collagen matrix of rabbit bone attains its mature state around 3 months of age, which is before the overall skeletal maturity is reached.

Analysis of plastic deformation in cortical bone after insertion of coated and non-coated self-tapping orthopaedic screws

Insertion of internal fracture fixation devices, such as screws, mechanically weakens the bone. Diamond-like carbon has outstanding tribology properties which may decrease the amount of damage in tissue. The purpose of this study was to investigate methods for quantification of cortical bone damage after orthopaedic bone screw insertion and to evaluate the effect of surface modification on tissue damage. In total, 48 stainless steel screws were inserted into cadaver bones. Half of the screws were coated with a smooth amorphous diamond coating. Geometrical data of the bones was determined by peripheral quantitative computed tomography. Thin sections of the bone samples were prepared after screw insertion, and histomorphometric evaluation of damage was performed on images obtained using light microscopy. Micro-computed tomography and scanning electron microscopy were also used to examine tissue damage. A positive correlation was found between tissue damage and the geometric properties of the bone. The age of the cadaver significantly affected the bone mineral density, as well as the damage perimeter and diameter of the screw hole. However, the expected positive effect of surface modification was probably obscured by large variations in the results and, thus, statistically significant differences were not found in this study. This can be explained by natural variability in bone tissue, which also made automated image analysis difficult.

Comparison between infrared and Raman spectroscopic analysis of maturing rabbit cortical bone

The molecular composition of the organic and inorganic matrices of bone undergoes alterations during maturation. The aim of this study was to compare Fourier transform infrared (FT-IR) and near-infrared (NIR) Raman microspectroscopy techniques for characterization of the composition of growing and developing bone from young to skeletally mature rabbits. Moreover, the specificity and differences of the techniques for determining bone composition were clarified. The humeri of female New Zealand White rabbits, with age range from young to skeletally mature animals (four age groups, n = 7 per group), were studied. Spectral peak areas, intensities, and ratios related to organic and inorganic matrices of bone were analyzed and compared between the age groups and between FT-IR and Raman microspectroscopic techniques. Specifically, the degree of mineralization, type-B carbonate substitution, crystallinity of hydroxyapatite (HA), mineral content, and collagen maturity were examined. Significant changes during maturation were observed in various compositional parameters with one or both techniques. Overall, the compositional parameters calculated from the Raman spectra correlated with analogous parameters calculated from the IR spectra. Collagen cross-linking (XLR), as determined through peak fitting and directly from the IR spectra, were highly correlated. The mineral/matrix ratio in the Raman spectra was evaluated with multiple different peaks representing the organic matrix. The results showed high correlation with each other. After comparison with the bone mineral density (BMD) values from micro-computed tomography (micro-CT) imaging measurements and crystal size from XRD measurements, it is suggested that Raman microspectroscopy is more sensitive than FT-IR microspectroscopy for the inorganic matrix of the bone. In the literature, similar spectroscopic parameters obtained with FT-IR and NIR Raman microspectroscopic techniques are often compared. According to the present results, however, caution is required when performing this kind of comparison.

Spatial-temporal mapping of bone structural and elastic properties in a sheep model following osteotomy

The course of bone healing in animal models is conventionally monitored by morphologic approaches, which do not allow the determination of the material properties of the tissues involved. Mechanical characterization techniques are either dedicated to the macroscopic evaluation of the entire organ or to the microscopic evaluation of the tissue matrix. The latter provides insight to regionally specific alterations at the tissue level in the course of healing. In this study, quantitative scanning acoustic microscopy was used at 50 MHz to investigate microstructural and elastic alterations of mineralized callus and cortical tissue after transverse osteotomy in sheep tibiae. Analyses were performed after 2, 3, 6 and 9 weeks of consolidation with stabilization by either a rigid or a semi-rigid external fixator. Increased stiffness and decreased porosity were observed in the callus tissue over the course of the healing process, which was dependent on the fixator type. In the adjacent cortical tissue, stiffness decreased during the first 3 weeks. Cortical porosity increased over time but the time-dependence was different between the two fixator types. The changes of stiffness of cortical and callus tissues were measured with respect to the distance to the periosteal cortex-callus boundary. Stiffness of cortex and callus tissue smoothly decreased as a function of the distance from the inner cortical region. The data obtained in this study can help to understand the processes involved in tissue maturation during endogenous bone healing.

Rabbit cortical bone tissue increases its elastic stiffness but becomes less viscoelastic with age

Bone is dynamic tissue undergoing changes in its composition, structure and functional properties during growth. It has been proposed that especially changes in the collagen phase of bone are responsible for making the bone more fragile, and potentially less viscoelastic with age. Hence, robust methods to measure viscoelasticitiy are needed. This study aimed to characterize the development of the elastic and viscoelastic mechanical properties of rabbit bone during maturation and growth, as assessed by nanoindentation. The humeri from female New Zealand white rabbits of varying age (newborn, 11 days, 4 weeks, 3 and 6 months old, n=8 per group) were investigated. Mid-diaphyseal cortical bone samples were cut, dehydrated, embedded and polished. Nanoindentation probing, semi-dynamic testing with a frequency of 20 Hz and creep with a dwell time of 60 s were performed under load control to quantify the elastic and the time-dependent viscoelastic mechanical properties of bone. The elastic moduli were evaluated with all three methods and the viscoelastic parameters were assessed using the phase-shift and the creep time constant. The elastic stiffness of bone increased significantly with each consecutive age group, from 11 days to 6 months of age, based on the reduced modulus from the indentation probing, the storage modulus from the semi-dynamic test, and the first elastic parameter from the creep test. These elastic parameters correlated significantly (R=0.88-0.94, p<0.01). The values of viscoelastic parameters, the phase-shift and time creep constant, decreased significantly with age. The viscous properties determined by the creep and the semi-dynamic testing correlated significantly (R=0.90, p<0.01), however, no correlation was found between the phase-shift and the creep time constant. Additionally, the present results showed specific associations with tissue composition, as measured with Fourier Transform Infrared spectroscopy of the same samples. In summary, the present results reveal significant changes in material properties of rabbit cortical bone with age. The elastic modulus of bone tissue increased by approximately 60%, whereas the viscoelastic parameters decreased by 10% to 25% during the first 6 months of the rabbit's life. Together, this indicates significant structural and functional maturation of the bone matrix during growth of the rabbit.

Cluster analysis of infrared spectra of rabbit cortical bone samples during maturation and growth

Bone consists of an organic and an inorganic matrix. During development, bone undergoes changes in its composition and structure. In this study we apply three different cluster analysis algorithms [K-means (KM), fuzzy C-means (FCM) and hierarchical clustering (HCA)], and discriminant analysis (DA) on infrared spectroscopic data from developing cortical bone with the aim of comparing their ability to correctly classify the samples into different age groups. Cortical bone samples from the mid-diaphysis of the humerus of New Zealand white rabbits from three different maturation stages (newborn (NB), immature (11 days-1 month old), mature (3-6 months old)) were used. Three clusters were obtained by KM, FCM and HCA methods on different spectral regions (amide I, phosphate and carbonate). The newborn samples were well separated (71-100% correct classifications) from the other age groups by all bone components. The mature samples (3-6 months old) were well separated (100%) from those of other age groups by the carbonate spectral region, while by the phosphate and amide I regions some samples were assigned to another group (43-71% correct classifications). The greatest variance in the results for all algorithms was observed in the amide I region. In general, FCM clustering performed better than the other methods, and the overall error was lower. The discriminate analysis results showed that by combining the clustering results from all three spectral regions, the ability to predict the correct age group for all samples increased (from 29-86% to 77-91%). This study is the first to compare several clustering methods on infrared spectra of bone. Fuzzy C-means clustering performed best, and its ability to study the degree of memberships of samples to each cluster might be beneficial in future studies of medical diagnostics.