Interrelationships of trabecular bone electrical and dielectric properties with mechanical characteristics and density are poorly known. While electrical stimulation is used for healing fractures, better understanding of these relations has clinical importance. Furthermore, earlier studies have suggested that bone electrical and dielectric properties depend on the bone density and could, therefore, be used to predict bone strength. To clarify these issues, volumetric bone mineral density (BMDvol), electrical and dielectric as well as mechanical properties were determined from 40 cylindrical plugs of bovine trabecular bone. Phase angle, relative permittivity, loss factor and conductivity of wet bovine trabecular bone were correlated with Young's modulus, yield stress, ultimate strength, resilience and BMDvol. The reproducibility of in vitro electrical and dielectric measurements was excellent (standardized coefficient of variation less than 1%, for all parameters), especially at frequencies higher than 1 kHz. Correlations of electrical and dielectric parameters with the bone mechanical properties or density were frequency-dependent. The relative permittivity showed the strongest linear correlations with mechanical parameters (r > 0.547, p < 0.01, n = 40, at 50 kHz) and with BMDvol (r = 0.866, p < 0.01, n = 40, at 50 kHz). In general, linear correlations between relative permittivity and mechanical properties or BMDvol were highest at frequencies over 6 kHz. In addition, a significant site-dependent variation of electrical and dielectric characteristics, mechanical properties and BMDvol was revealed in bovine femur (p < 0.05, Kruskall-Wallis H-test). Based on the present results, we conclude that the measurement of electrical and dielectric properties provides quantitative information that is related to bone quantity and quality.

In trabecular bone, the interrelationships of electrical and dielectric properties with mechanical characteristics are poorly known. Information on these relations is crucial for evaluation of the diagnostic potential of impedance techniques. In this study, electrical and dielectric properties, i.e. permittivity, conductivity, phase angle, loss factor, specific impedance and dissipation factor of human trabecular bone samples (n=26, harvested from the distal femur and proximal tibia) were characterized in a wide frequency range (50 Hz-5 MHz). Mechanical properties, i.e. Young's modulus, ultimate strength, yield stress, yield strain and resilience of the samples (n=20) were determined by using destructive compressive testing. Subsequently, measurements of electrical and dielectric properties were repeated after mechanical testing. The measurements were also repeated for the control samples (n=6) that were not mechanically tested. Electrical, dielectric or mechanical properties showed no significant differences between the intact femoral and tibial samples. The electrical and dielectric parameters as well as the linear correlations between the dielectric and electrical parameters with mechanical parameters were strongly frequency dependent. At the frequency of 1.2 MHz, the relative permittivity showed the strongest linear correlations with the Young's modulus (r=0.71, p<0.01, n=20) and ultimate strength (r=0.73, p<0.01, n=20). Permittivity and dissipation factor showed statistically significant changes after mechanical testing. Our results suggest that the measurements of low frequency electrical and dielectric properties may provide information on the mechanical status of trabecular bone and, possibly, may even help to diagnose bone microdamage. In the future, these measurement techniques may be further developed for use during open surgery, such as bone grafting or total hip replacement surgery.

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Microstructural changes, such as reduction of trabecular thickness and number, are characteristic signs of osteoporosis leading to diminished bone strength. Electrical and dielectric parameters might provide diagnostically valuable information on trabecular bone microstructure not extractable from bone mineral density measurements. In this study, structural properties of human trabecular bone samples (n=26) harvested from the distal femur and proximal tibia were investigated using the computed microtomography (microCT) technique. Quantitative parameters, e.g. structural model index (SMI) or trabecular bone volume fraction (BV/TV), were calculated. In addition, the samples were examined electrically over a wide frequency range (50 Hz-5 MHz) using a two-electrode impedance spectroscopy set-up. Relative permittivity, loss factor, conductivity, phase angle, specific impedance and dissipation factor were determined. Significant linear correlations were obtained between the dissipation factor and BV/TV or SMI (|r| 0.70, p<0.01, n=26). Principal component analyses, conducted on electrical and structural parameters, revealed that the high frequency principal component of the dissipation factor was significantly related to SMI (r=0.72, p<0.01, n=26). The linear combination of high and low frequency relative permittivity predicted 73% of the variation in BV/TV. To conclude, electrical and dielectric parameters of trabecular bone, especially relative permittivity and dissipation factor, were significantly and specifically related to a trabecular microstructure as characterized with microCT. The data gathered in this study constitute a useful basis for theoretical and experimental work towards the development of impedance spectroscopy techniques for detection of bone quality in vitro or in special cases of open surgery.