Electric Bone-conduction Research Articles

Low-Frequency Electrical Conductivity of Trabecular Bone: Insights from In Silico Modeling

Background: The electrical conductivity of trabecular bone at 100 kHz has recently been reported as a good predictor of bone volume fraction. However, quantifying its relationship with free water (or physiological solution) content and the conductivities of its constituents is still difficult. Methods: In this contribution, in silico models inspired by microCT images of trabecular bovine samples were used to build realistic geometries. The finite element method was applied to solve the electrical problem and to robustly fit the conductivity of the constituents to the literature data. The obtained effective electrical conductivity was compared with the Bruggeman three-medium mixture model using a physiological solution, bone marrow and a bone matrix. Results: The values for the physiological solution plus bone marrow (together as one material) and the bone matrix that best captured the bone volume fraction in the two-medium finite element model were σps+bm = 298.4 mS/m and σb = 21.0 mS/m, respectively. Additionally, relatively good results were obtained with the three-medium Bruggeman mixture model, with σbm= 103 mS/m, σb= 21.0 mS/m and σps= 1200 mS/m. Simple linear relationships between the proportions of constituents depending on bone volume fraction were tested. Degree of anisotropy and fractal dimension do not show detectable changes in effective conductivity. Conclusions: These results provided some useful findings for simulation purposes. First, a higher value for the electrical conductivity of bone marrow has to be used in order to obtain similar values to those of experimental published data. Second, anisotropy is not detectable with conductivity measurements for small trabecular samples (5 mm cube). Finally, the simulations presented here showed relatively good fitting of the Bruggeman mixture model, which would potentially account for the free water content and could rescale the model for whole-bone electrical simulations.

Characterization of the electrical conductivity of bone and its correlation to osseous structure

The interaction of osseous tissue with electric fields is an important subject. The electrical stimulation of bone promotes osteogenesis, while bone impedance has been proposed as a measure of osteoporosis, to follow fracture healing, or as a method to improve safety of surgical procedures. However, a deeper understanding of the electrical properties of bone and their relation to the architecture of osseous tissue is required to extend the range of use of electrical measurements to clinical studies. In this paper we apply electrical impedance spectroscopy to study the conductivity of fresh bovine tibia and we correlate the measured conductivities with its structural properties. Impedance was measured using a custom-made cell and a potentiostat. Bone conductivity was determined at 100 kHz, where the phase shift was negligible. A good agreement (R2 = 0.83) was found between the measured conductivity and the bone volume fraction, determined on microCT images. Based on this relationship, an equivalent circuit model was created for bone samples. The results of this ex-vivo study are comparable to previous in-vivo observations reporting bone resistivity as a function of bone density. This information can be used to construct a map of the tissue resistivity directly derived from clinical images.

Characterization of human cancellous and subchondral bone with respect to electro physical properties and bone mineral density by means of impedance spectroscopy

Computational simulation of electrical bone stimulation of the electrical and dielectric parameters of osteoarthritic bone tissue is useful for an exact patient-individual adaptation of the bone models. Therefore, we investigated electrical and dielectric parameters at a frequency of 20Hz of cancellous and subchondral human femoral head bone samples. Furthermore, the mechanical properties and the bone mineral density (BMD) were determined. Finally, these data were compared with the electrical and dielectric parameters. The bone samples were taken from patients with hip osteoarthritis. Electrical conductivity and dielectric permittivity of cancellous bone amounted to 0.043S/m and 8.1⋅106. BMD of the bone samples determined by dual-x-ray-absorptiometry (DXA) and ashing resulted in 193 ± 70mg/cm² and 286 ± 59mg/cm³ respectively. Structural modulus (ES) and ultimate compression strength (σmax) were measured with 227 ± 94N/mm² and 6.5 ± 3.4N/mm². No linear correlation of the electrical and dielectric parameters compared with BMD and mechanical properties of cancellous bone samples was found. Electrical conductivity and dielectric permittivity of subchondral bone resulted in 0.029S/m and 8.97×106.

Tetrapolar Measurement of Electrical Conductivity and Thickness of Articular Cartilage

A tetrapolar method to measure electrical conductivity of cartilage and bone, and to estimate the thickness of articular cartilage attached to bone, was developed. We determined the electrical conductivity of humeral head bovine articular cartilage and subchondral bone from a 1- to 2-year-old steer to be 1.14+/-0.11 S/m (mean+/-sd, n =11) and 0.306+/-0.034 S/m, (mean+/-sd, n =3), respectively. For a 4-year-old cow, articular cartilage and subchondral bone electrical conductivity were 0.88+/-0.08 S/m (mean+/-sd, n =9) and 0.179+/-0.046 S/m (mean+/-sd, n =3), respectively. Measurements on slices of cartilage taken from different distances from the articular surface of the steer did not reveal significant depth-dependence of electrical conductivity. We were able to estimate the thickness of articular cartilage with reasonable precision (<20% error) by injecting current from multiple electrode pairs with different inter-electrode distances. Requirements for the precision of this method to measure cartilage thickness include the presence of a distinct layer of calcified cartilage or bone with a much lower electrical conductivity than that of uncalcified articular cartilage, and the use of inter-electrode distances of the current injecting electrodes that are on the order of the cartilage thickness. These or similar methods present an attractive approach to the non-destructive determination of cartilage thickness, a parameter that is required in order to estimate functional properties of cartilage attached to bone, and evaluate the need for therapeutic interventions in arthritis.

Studies on the temperature dependence of electrical conductivity of solid-state proteins

Studies on the temperature dependence of the electrical conductivity of collagen, keratin and bone have been performed with a constant heating rate, in a temperature range of 295–460 K and an electric field strength of 1.00±0.05 kV/m. The differences observed in thermograms were due to different structures of the biological materials studied. The materials differed in their electrical conductivity and activation energy of charge conduction process. Raising temperature caused the change in the electrical conductivity; however this increase was not continuous. Consecutive heating runs resulted in a reduced electrical conductivity.

Carhart notch effect in otosclerotic ears measured by electric bone-conduction audiometry.

The Carhart notch is an elevation in the middle-frequency bone-conduction threshold of an ear with clinical otosclerosis. The study population consisted of 138 patients with clinical otosclerosis. Conventional air-conduction (AC) and bone-conduction (BC) and electric bone-conduction (EBC, with Audimax 500 audiometer) thresholds were measured 2 weeks before and 7 months after stapes surgery. The EBC thresholds were converted from mA values to dB SPL to compare the results obtained with the different methods. The mean differences in the BC thresholds before and after the operation were 3.2 dB (95% CI 1.9-4.6) at 1 kHz and 7.6 dB (95% CI 6.1-9.1) at 2 kHz. The mean differences in the EBC thresholds were 5.4 dB (95% CI 3.8-6.9) at 1 kHz and 5.3 dB (95% CI 3.4-7.1) at 2 kHz. Thus, both methods showed a distinct Carhart notch effect.

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

The effect of γ-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.

Ototoxicity of aminoglycoside drugs in tuberculosis treatment

The possible ototoxic effect of kanamycin, streptomycin and a standard anti-TB drug combination, used in the treatment of 92 TB patients (7-71 years old), was examined by measuring the highest audible electric bone conduction frequency before and after treatment, using an Audimax 500 audiometer. At the so-called "safe" levels of drug administration it was found that kanamycin was markedly ototoxic, streptomycin very slightly ototoxic and the standard anti-TB drug combination had practically no ototoxic effect. Furthermore, it was found that none of these drugs were gender specific. Lastly, the possible effects of ageing on highest audible bone conduction frequency is discussed.

Tentative age-related norms for high frequency electric bone conduction audiometry

From 172 male and female patients aged 7-71 years old and admitted in one month to a tuberculosis hospital, 150 had their highest audible electric bone conduction (EBC) frequency evaluated, prior to the commencement of any medical treatment. Various problems in establishing high frequency norms are discussed and from the results of this study tentative age-related highest frequency EBC norms are suggested. Possible clinical applications of evaluating the highest audible EBC frequency is suggested.

Carhart Notch and Electric Bone-conduction Audiometry

The Carhart notch is a depression in the bone-conduction audiogram of patients with clinical otosclerosis. The middle frequencies from 0.5 to 2 kHz, which correspond to the resonance frequency of the middle ear, can be substantially improved following successful stapes surgery. Twenty-nine consecutive patients with clinical otosclerosis were tested 2 weeks before and 7 months after stapes surgery with conventional air-conduction (AC) and bone-conduction (BC) audiometry and electric bone-conduction (EBC) audiometry using an Audimax 500 audiometer. The BC thresholds improved at 0.5-2.0 kHz corresponding to the Carhart notch. On average, the EBC thresholds remained more stable, showing no Carhart phenomenon, which supports the concept that EBC stimulation is not entirely equivalent to conventional BC audiometry, but has a different mode of action on the inner ear.

Noise-Induced Low- and High-Frequency Hearing Losses in Finnish Conscripts

Thirty-nine otologically healthy military conscripts were examined at the beginning and at the end of their 1-year service. On entry, they all showed normal findings during clinical otolaryngological examinations. The test battery included both conventional (0.25-8 kHz) and high-frequency "electric bone-conduction" (0.5-20 kHz) audiometry (EBC). The median pure-tone right ear thresholds at the end of service were 5 dB worse over the frequency range of 2-8 kHz compared with the thresholds at the beginning of the service. The difference was statistically highly significant (p = 0.00035). The median left ear pure-tone thresholds at frequencies of 0.25, 2, and 8 kHz were 5 dB worse at the end of the service compared to the thresholds at the beginning of the service. However, the difference was not statistically significant (p = 0.23). The median curves for the high-frequency (EBC) thresholds measured after the service showed worse thresholds over the whole frequency range compared to the median curves measured before the service. The greatest difference was seen in the highest frequencies (15-17 kHz) in both ears. The differences were statistically significant in both ears (p = 0.03 in the right ears; p = 0.01 in the left ears) when threshold values over the whole frequency range were analyzed. Since the otolaryngological history of these conscripts was uneventful during their service, the hearing deteriorations were considered to be caused by shooting practice.

High-frequency audiometry. Masking in electric bone-conduction audiometry.

Recently, the 'electric bone-conduction' (EBC) audiometer (Audimax 500) has been used to measure high-frequency (HF) hearing. With this audiometer stimulation is binaural. No commercial masking method was available. In this study, white noise from a Madsen OB822 audiometer and presented via Sony MDR-V4 dynamic earphones, was used for masking. The masking and cross-hearing effect was measured in 8 unilaterally deaf subjects and the masking procedure was tested with 104 young normal-hearing subjects. The results showed that the EBC signals can be masked with air-conduction signals, and thus, the EBC measurements reflect monaural thresholds. The minimum masking level was 50-60 dB SPL in the HF range. There were no cross-hearing problems in the HF range with the earphones used. At the frequencies 0.5-14 kHz, the better ear's masked EBC thresholds were on the average 2.6 dB (range 0-4.5 dB) poorer, compared with the binaural EBC thresholds, indicating a binaural summation effect.

High-Frequency Air-Conduction and Electric Bone-Conduction Audiometry: Age and Sex Variations

208 subjects representing both sexes and five age groups (15-70 years) were examined to obtain age-related threshold values for high-frequency (HF) electric bone-conduction (EBC) audiometry. The measurements also included conventional pure-tone audiometry and air-conduction (AC) HF (8-18 kHz) audiometry. The measured EBC thresholds were comparable to the values obtained with AC audiometers, and were equal to ISO standards at the frequencies of 0.5-6 kHz. The 15- and 20-year-old groups' EBC thresholds at 8 kHz were equivalent to thresholds of 15-year-old people from a cross-sectional material in Northern Finland. Thresholds deteriorated as a function of age, particularly in the HF range. The males had poorer thresholds than the females, especially in the age groups of 40 and 60 years. This could be attributed mainly to their greater noise exposure. The EBC method is quite practical and reliable for routine clinical measurements, but the dynamic range of the audiometer limits its use to relatively young subjects.

High-Frequency Air-Conduction and Electric Bone-Conduction Audiometry: Comparison of Two Methods

Threshold values for 147 subjects (9-43 years old) were measured with a high-frequency (HF) air-conduction (AC) (Interacoustics AS 10 HF) and an electric bone-conduction (EBC) (Audimax 500) audiometer. In addition, the reproducibility of these methods was studied in another group of 24 subjects. The results confirmed the previous findings of Okstad et al. (1988) that the electric current (i) used as a stimulus in the Audimax 500 audiometer can be converted into decibels with a correction factor of 40 log (i) re 1 mA as Tonndorf & Kurman (1984) have proposed. However, an additive frequency-dependent correction is needed to obtain similar loudness sensation increases with these audiometers. Reproducibility with the EBC audiometer was better than with the AC audiometer, especially in the HF range.

High-frequency audiometry: air- and electric bone-conduction.

Normative values have been obtained for high-frequency air- and electric bone-conduction thresholds in different age groups. Reproducibility with both methods is of the same order of magnitude as with conventional audiometry. The two thresholds can be compared and the air-conduction/electric bone-conduction gap obtained by transforming the electric bone-conduction values to a dB notation by means of a formula containing a frequency-dependent constant.

Dielectric properties of fluid-saturated bone--the effect of variation in conductivity of immersion fluid.

The dielectric permittivity and electrical conductivity of fluid-saturated rat femoral bone were determined as a function of the conductivity of the immersion fluid over a frequency range of 10-100 Hz to 8 MHz. The specimens were equilibrated in various solutions of Hank's balanced salt solution modified to contain various sodium chloride concentrations, with conductivities ranging from 0.13 to 3.55 s/m. The results of these measurements show that the dc conductivity of the tissue is about 1 percent of the conductivity of the immersion solution. Similar conductivity values were predicted from a simple model of the tissue, in which it is assumed that the only significant contribution to the conductivity arises from the capillaries that pass through the tissue. The permittivity of the tissue exhibits a dispersion whose mean relaxation frequency is proportional to the conductivity of the immersion fluid. The permittivity ranges from about 1000 at low frequencies to 10-20 in the high frequency limit. The dielectric data could be fitted to a Cole-Cole function having a distribution parameter of about 0.5-0.6, which suggests the presence of a diffusion-controlled ionic polarization mechanism, although other relaxation processes could also be present.

Effect of Moisture on the Electrical Properties of Bone

Both a‐c (1000 Hz) and d‐c electrical conductivity of human cortical bone are studied after equilibration at various relative humidities. At the same time weight change measurements are carried out as a function of humidity, so as to determine the moisture content of the bone. Both the electrical properties and weight changes show striking hysteresis when plotted vs. relative humidity. However, when electrical properties are plotted as functions of moisture content the hysteresis is eliminated. The hysteresis effects are attributed to capillary condensation in pores. Measurements were made on samples soaked in Ringer's solution as well as samples washed in distilled water, and also with electric field both parallel and perpendicular to the bone axis. The results of these measurements strongly suggest that the large effect of moisture on conductivity is from ionic salt solutions located in pores and channels.

The effect of ultraviolet radiation on the electrical conductivity of human bone.

The electrical conductivity of intact bone, collagen and apatite mineral was determined in the region of moderately high fields. After exposure to ultraviolet (UV) radiation the conductivity of the specimens was redetermined. Following exposure marked decreases in electrical conductivity occurred in all specimens. The possible modes of interaction of UV radiation with bone are discussed. It is suggested that protonic conduction may be an important mode of charge transport in bone.

Temperature dependence of the electrical conductivity of bone.

The D. C. conductivity of human bone is investigated in the region of moderately high fields. A temperature range extending from 30°-60°C is scanned. Results indicate (1) an increase in conductivity with increasing temperature, and (2) the occurrence of a space charge region at relatively low fields. It is pointed out that electronic conduction along with the protonic conduction is responsible for the charge transport in bone.