Solid state bone behaviour

Prediction of mechanical properties of human trabecular bone by electrical measurements

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.

The effect of gamma-irradiation on the temperature dependence of D.C. electrical conductivity of dry bone

The effect of gamma-irradiation with doses from 10 to 500 kGy on the electrical conductivity (g) of dry bone was studied. Temperature measurement of electrical conductivity were made from 393 to 533 K. The dependence obtained indicates the increase in g with temperature. An increase in irradiation dose resulted in a decreased g value for each dose up to temperature 462 K. Temperature 462 K was interpreted as the temperature of collagen melting point in dry bone. Above 462 K, g values were dose independent. A dose of 500 kGy shifted the melting point to lower temperature. In addition, the activation energy for the charge conduction process was calculated. Obtained values for electrical conductivity and activation energy were typical for dielectrics and indicated degradation of the organic component of bone.