Function Of Mineral Content Research Articles

Estimation of the in-plane ultimate stress of lamellar tissue as a function of bone mineral density and osteocyte lacunae porosity

Background and ObjectiveDetailed finite element models based on medical images (μ-CT) are commonly used to analyze the mechanical behavior of bone at microscale. In order to simulate the tissue failure onset, isotropic failure criteria of lamellar tissue are often used, despite its non-isotropic and heterogeneous nature. The main goal of the present work is to estimate the in-plane ultimate stress of lamellar bone, considering the influence of mineral content and the porosity due to the osteocyte lacunae concentration. MethodsTo this aim, a representative volume cell of lamellar tissue is modeled numerically, including: (1) non-isotropic elastic properties of tissue as a function of the bone mineral density and (2) explicit modeling of the osteocyte lacunae, considering the range of porosity content, size and orientation of ellipsoid-shaped lacunae. Firstly, the element size for the finite element models have been defined from a preliminary convergence analysis. Bounds on the ultimate stress of non-porous lamellar tissue are estimated for two values of bone mineral density, considering the results of tensile and compressive tests of wet osteons from the literature. Subsequently, the ultimate stress of lamellar tissue considering several values of micro-porosity is addressed. ResultsResults obtained in this work show that the strength of lamellar bone decreases exponentially with the increase of lacunae porosity concentration. Ultimate stress of non-porous tissue (p=0%) increases with high mineral content, reaching a value of S¯transc=355.40±39.80 MPa for compression in the transversal direction of the fiber bundles, being BMD=1.246g/cm3. The mean value for the longitudinal to transverse strength ratio evaluated for porosity p=0%,1% and 5% and a mineral content BMD=1.2g/cm3, is 2.47:1 for tension and 1.55:1 for compression. These values are in agreement with literature. ConclusionsOsteocyte lacunae act as stress concentrators, acting as potential stimulus for the bone regeneration process. A novel micromechanical model for the in-plane ultimate stress of lamellar tissue as a function of mineral content and lacunae concentration is presented. Additional considerations about the intralamellar shear stress evolution are also given.

Mineralized Microgels via Electrohydrodynamic Atomization: Optimization and In Vitro Model for Dentin-Pulp Complex.

There is growing interest in the use of micro-sized hydrogels, including bioactive signals, as efficient platforms for tissue regeneration because they are able to mimic cell niche structure and selected functionalities. Herein, it is proposed to optimize bioactive composite microgels via electrohydrodynamic atomization (EHDA) to regenerate the dentin-pulp complex. The addition of disodium phosphate (Na2HPO4) salts as mineral precursors triggered an in situ reaction with divalent ions in solution, thus promoting the encapsulation of different amounts of apatite-like phases. Morphological analysis via image analysis of optical images confirmed a narrow distribution of perfectly rounded particles, with an average diameter ranging from 223 ± 18 μm to 502 ± 64 μm as a function of mineral content and process parameters used. FTIR, TEM, and EDAX analyses confirmed the formation of calcium phosphates with a characteristic Ca/P ratio close to 1.67 and a needle-like crystal shape. In vitro studies-using dental pulp stem cells (DPSCs) in crown sections of natural teeth slices-showed an increase in cell viability until 14 days, recording a decay of proliferation at 21 days, independent on the mineral amount, suggesting that differentiation is started, as confirmed by the increase of ALP activity at 14 days. In this view, mineralized microgels could be successfully used to support in vitro osteogenesis, working as an interesting model to study dental tissue regeneration.

Optical changes of dentin in the near-IR as a function of mineral content.

The optical properties of human dentin can change markedly due to aging, friction from opposing teeth, and acute trauma, resulting in the formation of transparent or sclerotic dentin with increased mineral density. The objective of this study was to determine the optical attenuation coefficient of human dentin tissues with different mineral densities in the near-infrared (NIR) spectral regions from 1300-2200 nm using NIR transillumination and optical coherence tomography (OCT). N=50 dentin samples of varying opacities were obtained by sectioning whole extracted teeth into ~ 150 μm transverse sections at the cemento-enamel junction or the apical root. Transillumination images were acquired with a NIR camera and attenuation measurements were acquired at various NIR wavelengths using a NIR sensitive photodiode. Samples were imaged with transverse microradiography (gold standard) in order to determine the mineral density of each sample.

Variability of the mechanical properties of bone, and its evolutionary consequences

The relative variabilities (coefficient of variation (CV)) of 10 different mechanical properties of compact bone were determined from 2166 measurements. All measures of variability were made on a minimum of four specimens from any bone. Three pre-yield properties had a CV of about 12%. Six post-yield properties had CVs varying from 24 to 46%. Pre-yield properties increase as a function of mineral content, whereas post-yield properties decrease. These differences give insight into mechanical phenomena occurring at different stages during loading. Furthermore, the fact that some properties are more tightly determined than others has implications for the optimum values set by natural selection. This assertion is made more rigorous using a simple mathematical model for the evolutionarily optimal allocation in a trade-off where one property is imprecisely determined. It is argued that in general the optimum will be biased in favour of the more tightly determined properties than would be the case if all properties had the same CV.

Variability of the mechanical properties of bone

The relative variabilities (coefficients of variation; CV) of ten different mechanical properties of compact bone were determined from 2166 measurements. The unit of measurement was the individual bone, and all measures of variability were made on a minimum of four specimens from any bone. Three ‘pre-yield’ properties had a CV of about 12%. Six post-yield properties had CVs varying from 24% to 46%. Pre-yield properties increase as a function of mineral content, post-yield properties decrease. These differences give insight into mechanical phenomena occurring at different stages during loading. Furthermore, the fact that some properties are more tightly determined than others has implications for the optimum values set by natural selection. In general the optimum will be biased in favour of the more tightly determined properties than would be the case if all properties had the same CV.

Stone fragility in shock wave lithotripsy can be predicted in vitro.

To investigate the effect of stone size, chemical structure, radiologic appearance, stone weight and stone mineral density on in vitro stone fragility. A total of 216 stones obtained by open surgery were stratified according to their size, chemical structure (X-ray crystallography), radiologic appearance, mineral content and density (dual photon absorptiometry) and weight. Stone fragility was measured by the number of shock waves needed to completely fragment the stones in a phantom model by Dornier. Stone weight increased according to stone size and mineral density varied in relation to chemical composition. The radiologic appearance was not predictive of the chemical content. The most significant variable to predict the number of shock waves needed for full fragmentation was the stone weight. The stone weight could be formulated as a function of mineral content and the equation had statistical significance (p = 0.000). The necessary number of shock waves for complete disintegration also could be estimated by using stone weight, stone size and mineral density. This equation was also statistically significant (p = 0.000). Stone weight which is a function of stone mineral content seems to be the single most important parameter to predict stone fragility in vitro. Stone weight can be estimated by using the stone mineral content. Prediction of the necessary number of shock waves for full fragmentation seems possible and is formulated into an equation that proved to be statistically significant in vitro. In vivo application of this estimation awaits further research.

Stress relaxation in native and EDTA-treated bone as a function of mineral content

The relaxation Young's modulus, ε( t), of bovine femoral bone was measured as a function of mineral content. Five different specimens with different mineral contents were prepared by the EDTA treatment. The relaxation curves for specimens of different mineral contents were superimposable upon one another by shifting along the log t axis as well as the log ε( t) axis. A well-specified master curve of stress relaxation was obtained from a set of relaxation curves for samples with different mineral contents. The mineral content dependence of vertical shift factors, along the log ε( t) axis, accorded well with the mineral content dependence of Young's modulus itself, the accordance indicating the plausibility of the vertical shifting procedure. The stress relaxation in bone has been reported to be related to the viscoelastic properties of the collagen matrix. It is considered that the reinforcement of the matrix around the mineral, by the mineral as fillers, increases the average modulus of the matrix and lengthens the characteristic time of the relaxation. This consideration explains the existence of a shift factor along the log t axis and its mineral content dependence. As the structures of bone specimens treated by EDTA are expected to be different from that of normal bone, the conclusion drawn from this experiment cannot be immediately applied to normal bone. It can be, however, concluded that the size of the mineral and also the interface area between mineral and collagen matrix play an important role in the viscoelastic properties of normal bone and the bone speciment treated by EDTA.

Structural studies of the mineral phase of calcifying cartilage

The calcified cartilage of the epiphyseal growth plate of young calves has been studied by x-ray diffraction. Fourier transform infrared spectroscopy, magic angle 31P nuclear magnetic resonance spectroscopy, and chemical composition. The powdered tissue was separated by density centrifugation as a function of mineral content and thus qualitatively of the age of the calcium-phosphorus mineral phase. The individual density centrifugation fractions were examined separately. X-ray diffraction of the samples, especially of the lowest density fractions, revealed very poorly crystalline apatite. Fourier transform infrared spectroscopy and 31P nuclear magnetic resonance spectroscopy revealed the presence of significant amounts of nonapatitic phosphate ions. The concentration of such nonapatitic phosphates increases during the early stages of mineralization but then decreases as the mineral content steadily rises until full mineralization is achieved. The total concentration of carbonate ions was found to be much lower in calcified cartilage than in bone from the same organ (scapula). The carbonate ions are located in both A sites (OH-) and B sites (PO4(3-)), with a distribution similar to that found in bone mineral. However, discrepancies between infrared resolution factors of phosphate and carbonate bands are consistent with a heterogeneous distribution of carbonate ions in poorly organized domains of the solid phase of calcium phosphate. These initial studies permit one to characterize the calcium phosphate mineral phase as a very poorly crystalline, immature calcium phosphate apatite, rich in labile nonapatitic phosphate ions, with a low concentration of carbonate ions compared with bone mineral of the same animal, indeed from the bone of the same organ (scapula).