Stochastic resonance in osteogenic response to mechanical loading

The Effects of different forms of exercise during the early life on the bone microstructure of ovariectomized mice

Postmenopausal osteoporosis is a type of bone disease with bone loss and deterioration of skeletal function that occurs in women after menopause. Studies have found that early-life exercise can reduce the risk of fractures and decrease the occurrence of osteoporosis, making it a promising approach for preventing and reversing bone loss. In this study, to identify the optimal forms of exercise during early life to optimize bone health and provide suggestions for promoting bone health through exercise training during early life, we conducted different forms of exercise interventions including ladder climbing, treadmill running, combined training, and whole-body vibration (WBV) on adolescent mice for 8 weeks and observed the accumulation of bone mass and arrangement of bone microstructure in adult mice. After removing the ovaries bilaterally, the mice were resting for 22 weeks to simulate the bone loss condition observed in postmenopausal women. We examined the resistance of the bone microstructure to degradation in response to exercise during early life and characterized the specific effects of different forms of exercise on countering bone microstructure deterioration. Our findings demonstrate that early-life exercise exerts long-term beneficial effects on bone health. All four forms of early-life exercise can delay bone loss due to decreased estrogen and improve the bone microstructure to varying degrees, with resistance and vibration training demonstrating superior protective benefits.

Vertical Vibration of Mouse Osteoblasts Promotes Cellular Differentiation and Cell Cycle Progression and Induces Aging In Vitro

BACKGROUND

This study aimed to investigate the effect of the vibration of osteoblasts on the cell cycle, cell differentiation, and aging.

MATERIALS AND METHODS

Primary maxilla osteoblasts harvested from eight-week-old mice were subjected to vibration at 3, 30, and 300 Hz once daily for 30 min; control group, 0 Hz. A cell proliferation assay and Cell-Clock Cell Cycle Assay were performed 24 h after vibration. Osteoblast differentiation assay, aging marker genes, SA-β-Gal activity, and telomere length (qPCR) were assayed two weeks post- vibration once every two days.

RESULTS

Cell proliferation increased significantly at 30 and 300 Hz rather than 0 Hz. Several cells were in the late G₂/M stage of the cell cycle at 30 Hz. The osteoblast differentiation assay was significantly higher at 30 Hz than at 0 Hz. Runx2 mRNA was downregulated at 30 Hz compared to that at 0 Hz, while osteopontin, osteocalcin, and sclerostin mRNA were upregulated. p53/p21, p16, and c-fos were activated at 30 Hz. SA-β-Gal activity increased significantly at 30 or 300 Hz. Telomere length was significantly lower at 30 or 300 Hz.

CONCLUSIONS

The results suggest that providing optimal vibration to osteoblasts promotes cell cycle progression and differentiation and induces cell aging.

Nonlinear dynamics of membrane skeleton in osteocyte

Osteocytes play an important role in mechanosensation and conduction in bone tissue, and the change of mechanical environment can affect the sensitivity of osteocytes to external stimulation. The structure of osteocytes will be changed when they are subjected to vibrations, which influence the mechanosensitivity of osteocytes and alter the regulation of bone remodeling process. As an important mechanotransduction structure in osteocytes, the membrane skeleton greatly affects the mechanosensation and conduction of osteocytes. However, the dynamic responses of membrane skeleton to the vibration and the structural changes of membrane skeleton are unclear. Therefore, we applied a nonlinear dynamics method to explain the time-dependent changes of membrane skeleton. The semi-ellipsoidal reticulate shell structure of membrane skeleton is built based on the experimental observation in our previous work. Then, the nonlinear dynamic equations of membrane skeleton are established according to the theory of plate and shell dynamics, and the displacement-time curves, phase portraits, and Poincaré maps of membrane skeleton structure were obtained. The numeration results show that under the vibration stimulation of 15 Hz, 30 Hz, 60 Hz, and 90 Hz, the membrane skeleton is destroyed after a transient equilibrium position vibration. The vibration of 15 Hz has the most destructive effect on the membrane skeleton, the natural frequency of membrane skeleton may be less than 15 Hz. In addition, the chaos phenomenon occurs to the membrane skeleton during vibration. As a damping factor, the existence of viscosity alleviates the damage of structure. This study can help us to understand the oscillation characteristic of membrane skeleton in osteocyte.

Part 2. Review and meta-analysis of studies on modulation of longitudinal bone growth and growth plate activity: A micro-scale perspective

Macro-scale changes in longitudinal bone growth resulting from mechanical loading were shown in Part 1 of this review to depend on load magnitude, anatomical location, and species. While no significant effect on longitudinal growth was observed by varying frequency and amplitude of cyclic loading, such variations, in addition to loading duration and species, were shown to affect the morphology, viability, and gene and protein expression within the growth plate. Intermittent compression regimens were shown to preserve or increase growth plate height while stimulating increased chondrocyte presence in the hypertrophic zone relative to persistent and static loading regimens. Gene and protein expressions related to matrix synthesis and degradation, as well as regulation of chondrocyte apoptosis were shown to exhibit magnitude-, frequency-, and duration-dependent responses to loading regimen. Chondrocyte viability was shown to be largely preserved within physiological bounds of magnitude, frequency, amplitude, and duration. Persistent static loading was shown to be associated with overall growth plate height in tension only, reducing it in compression, while affecting growth plate zone heights differently across species and encouraging mineralization relative to intermittent cyclic loading. Lateral loading of the growth plate, as well as microfluidic approaches are relatively understudied, and age, anatomical location, and species effects within these approaches are undefined. Understanding the micro-scale effects of varied loading regimes can assist in the development of growth modulation methods and device designs optimized for growth plate viability preservation or mineralization stimulation based on patient age and anatomical location.

Spontaneous cellular vibratory motions of osteocytes are regulated by ATP and spectrin network

Vibration at high frequency has been demonstrated to be anabolic for bone and embedded osteocytes. The response of osteocytes to vibration is frequency-dependent, but the mechanism remains unclear. Our previous computational study using an osteocyte finite element model has predicted a resonance effect involving in the frequency-dependent response of osteocytes to vibration. However, the cellular spontaneous vibratory motion of osteocytes has not been confirmed. In the present study, the cellular vibratory motions (CVM) of osteocytes were recorded by a custom-built digital holographic microscopy and quantitatively analyzed. The roles of ATP and spectrin network in the CVM of osteocytes were studied. Results showed the MLO-Y4 osteocytes displayed dynamic vibratory motions with an amplitude of ~80 nm, which is relied both on the ATP content and spectrin network. Spectrum analysis showed several frequency peaks in CVM of MLO-Y4 osteocytes at 30 Hz, 39 Hz, 83 Hz and 89 Hz. These peak frequencies are close to the commonly used effective frequencies in animal training and in-vitro cell experiments, and show a correlation with the computational predictions of the osteocyte finite element model. These results implicate that osteocytes are dynamic and the cellular dynamic motion is involved in the cellular mechanotransduction of vibration.

Segmental bone replacement via patient-specific, three-dimensional printed bioresorbable graft substitutes and their use as templates for the culture of mesenchymal stem cells under mechanical stimulation at various frequencies

The treatment of large segmental bone defects remains a challenge as infection, delayed union, and nonunion are common postoperative complications. A three-dimensional printed bioresorbable and physiologically load-sustaining graft substitute was developed to mimic native bone tissue for segmental bone repair. Fabricated from polylactic acid, this graft substitute is novel as it is readily customizable to accommodate the particular size and location of the segmental bone of the patient to be replaced. Inspired by the structure of the native bone tissue, the graft substitute exhibits a gradient in porosity and pore size in the radial direction and exhibit mechanical properties similar to those of the native bone tissue. The graft substitute can serve as a template for tissue constructs via seeding with stem cells. The biocompatibility of such templates was tested under in vitro conditions using a dynamic culture of human mesenchymal stem cells. The effects of the mechanical loading of cell-seeded templates under in vitro conditions were assessed via subjecting the tissue constructs to 28 days of daily mechanical stimulation. The frequency of loading was found to have a significant effect on the rate of mineralization, as the alkaline phosphatase activity and calcium deposition were determined to be particularly high at the typical walking frequency of 2 Hz, suggesting that mechanical stimulation plays a significant role in facilitating the healing process of bone defects. Utilization of such patient-specific and biocompatible graft substitutes, coupled with patient's bone marrow cells seeded and exposed to mechanical stimulation of 2 Hz have the potential of reducing significant volumes of cadaveric tissue required, improving long-term graft stability and incorporation, and alleviating financial burdens associated with delayed or failed fusions of long bone defects.

Whole Body Vibration Exposure on Markers of Bone Turnover, Body Composition, and Physical Functioning in Breast Cancer Patients Receiving Aromatase Inhibitor Therapy: A Randomized Controlled Trial

Introduction: Women with breast cancer are often prescribed aromatase inhibitors, which can cause rapid loss of bone mass leading to significant potential for morbidity. Vibration training has been shown to be helpful in reducing bone turnover in postmenopausal women without cancer. Aim: To examine the effect of vibration stimulus on markers of bone turnover in breast cancer patients receiving aromatase inhibitors. Methods: Thirty-one breast cancer survivors undergoing treatment with aromatase inhibitors were randomized to vibration stimulus (n = 14) or usual care control (n = 17). Low-frequency and low-magnitude vibration stimulus (27-32 Hz, 0.3g) was delivered in supervised sessions via standing on a vibration platform for 20 minutes, 3 times per week for 12 weeks. The primary outcome was blood markers of bone resorption (serum N-telopeptide X/creatine) and formation (serum type 1 procollagen N-terminal propeptide; P1NP). Other study outcomes body composition as well as measures of physical functioning. Outcomes were compared between groups using analysis of covariance adjusted for baseline values as well as time on aromatase inhibitors. Outcomes: On average, participants were 61.5 years old and overweight (ie, body mass index = 28.5 kg/m²). Following vibration training, there was no significant difference between groups for bone resorption (adjusted group difference 0.5, P = .929) or formation (adjusted group difference 5.3, P = .286). There were also no changes in any measure of physical functioning body composition. Conclusions: Short-term low-magnitude vibration stimulus does not appear to be useful for reducing markers of bone turnover secondary to aromatase inhibitors in breast cancer patients; nor is it useful in improving physical function or symptoms. However, further investigations with larger samples and higher doses of vibration are warranted. Trial Registration: Australian and New Zealand Clinical Trials Registry (ACTRN12611001094965).

Bone Remodeling under Vibration: A Computational Model of Bone Remodeling Incorporating the Modal Behavior of Bone

Developing precise computational models of bone remodeling can lead to more successful types of orthopedic treatments and deeper understanding of the phenomenon. Empirical evidence has shown that bone adaptation to mechanical loading is frequency dependent and the modal behavior of bone under vibration can play a significant role in remodeling process, particularly in the resonance region. The objective of this study is to develop a bone remodeling algorithm that takes into account the effects of bone vibrational behavior. An extended/modified model is presented based on conventional FE remodeling models. Frequency domain analysis is used to introduce appropriate correction coefficients to incorporate the effect of bone's frequency response into the model. The method is implemented on a bovine bone with known modal/vibration characteristics. The rate and locations of new bone formation depend on the loading frequency and are consistently correlated with the bone modal behavior. The proposed method can successfully integrate the bone vibration conditions and characteristics with the remodeling process. The results obtained support experimental observations in the literature.

In silico dynamic characterization of the femur: Physiological versus mechanical boundary conditions

It is established that bone tissue adapts and responds to mechanical loading. Several studies have suggested an existence of positive influence of vibration on the bone mass maintenance. Thus, some bone regeneration therapies are based on vibration of bone tissue under circumstances of disease to stimulate its formation. Frequency of loading should be properly selected and therefore a correct characterization of the dynamic properties of this tissue may be critical for the success of such orthopedic techniques. On the other hand, many studies implement vibration techniques with in silico models. Numerical results are exclusively dependent on properties of bone tissue, i.e. geometry, density distribution and stiffness, as well as boundary conditions. In the present study, the influence of boundary conditions and material properties on the dynamic characteristics of bone tissue was explored in a human femur. Bone shape and density were directly reconstructed from computer tomographies, whereas natural frequencies and modes of vibration were obtained for different boundary conditions including physiological and mechanical ones. Results of this study show the moderate effect of material properties compared to the much substantial effect of boundary conditions. A factor of 2 in the natural frequency was obtained depending on imposed boundary conditions, highlighting the importance in the selection of appropriate conditions in the analysis of the bone organ.

Random Electromyostimulation Promotes Osteogenesis and the Mechanical Properties of Rat Bones

Exercise is often recommended as a promising non-pharmacologic countermeasure to prevent osteoporosis. However, elderly osteoporotic patients generally have physical fitness difficulties preventing them from performing effective and sustainable exercise. Electromyostimulation should be one effective modality for non-pharmacological prevention of osteoporosis without any voluntary physical movements. However, successful stimulation patterns remain controversial. As suggested by our previous in vitro studies, randomized timing of stimulation could be a candidate to maximize the osteogenic effect of electromyostimulation. In this study, the effects of random stimulation to the quadriceps on osteogenesis in the femurs were investigated using rats, in comparison with a periodic stimulation pattern. In histomorphometric assessments, both stimulation patterns demonstrated increases in bone formation rate either in cortical bone at the midshaft or in trabecular bone at the femoral neck on the stimulated side. However, maximum load and strain energy to failure were enhanced only by the random stimulation, on either the stimulated or non-stimulated side. It is concluded that randomized muscle stimulation has effective osteogenic capability at the stimulation site, similar to periodic stimulation; however, its effectiveness on mechanical properties is expandable to other non-stimulated sites.

Stochastic resonance is a method to improve the biosynthetic response of chondrocytes to mechanical stimulation

Cellular mechanosensitivity is an important factor during the mechanical stimulation of tissue engineered cartilage. While the application of mechanical stimuli improves tissue growth and properties, chondrocytes also rapidly desensitize under prolonged loading thereby limiting its effectiveness. One potential method to mitigate load-induced desensitization is by superimposing noise on the loading waveforms ("stochastic resonance"). Thus, the purpose of this study was to investigate the effects of stochastic resonance on chondrocyte matrix metabolism. Chondrocyte-seeded agarose gels were subjected to dynamic compressive loading, with or without, superimposed vibrations of different amplitudes and frequency bandwidths. Changes in matrix biosynthesis were determined by radioisotope incorporation and subsequent effects on intracellular calcium signaling were evaluated by confocal microscopy. Although dependent on the duration of loading, superimposed vibrations improved cellular sensitivity to mechanical loading by further increasing matrix synthesis between 20-60%. Stochastic resonance also appeared to limit load-induced desensitization by maintaining sensitivity under desensitized loading conditions. While superimposed vibrations had little effect on the magnitude of intracellular calcium signaling, recovery of mechanosensitivity after stimulation was achieved at a faster rate suggesting that less time may be required between successive loading applications. Thus, stochastic resonance appears to be a valuable tool during the mechanical stimulation of cartilage constructs, even when suboptimal stimulation conditions are used.

Effect of mechanical stretch on the proliferation and differentiation of BMSCs from ovariectomized rats

Osteoporosis is characterized by a broken balance between bone formation and bone resorption. Mechanical stress has been considered to be an important factor in bone modeling and remodeling. However, biological responses of stromal cells in osteoporosis to mechanical stimuli remain unknown. To explore the correlation between mechanical stress and osteoblastic differentiation of bone mesenchymal stem cells (BMSCs) in osteoporosis, we built an osteoporosis model in ovariectomized (OVX) rats, and then investigated proliferation, alkaline phosphatase (ALP) activity, and the expression of osteoblastic genes in BMSCs under mechanical stress of 5 and 10% elongation, using the Flexercell Strain system. The proliferation of BMSCs was detected using alamarBlue. The expression of osteoblastic genes was analyzed by real-time quantitative polymerase chain reaction. Protein expression was examined by Western blotting. BMSCs (OVX) and BMSCs (Sham-operated, Sham in short) proliferations were inhibited at 5 and 10% elongation at day 3, compared with the un-stretched group, while BMSCs (OVX) proliferation was slower than BMSCs (Sham). ALP activity increased significantly at 10% elongation in both cells, but it was less active in BMSCs (OVX) than BMSCs (Sham). At days 3 and 7, the mRNA expression of osteoblastic genes was unregulated by mechanical stretch (5 and 10 % elongation); however, osteoblastic gene expression in BMSCs (OVX) was less than that in BMSCs (Sham). The mRNA and protein expression of Runx2 showed similar trends in BMSCs (OVX) under mechanical stretch. These results indicate that the mechanical stretch stimulates osteoblastic differentiation of BMSCs (OVX); however, this differentiation was weaker than that of BMSCs (Sham).

Resonance in the mouse tibia as a predictor of frequencies and locations of loading-induced bone formation

To enhance new bone formation for the treating of patients with osteopenia and osteoporosis, various mechanical loading regimens have been developed. Although a wide spectrum of loading frequencies is proposed in those regimens, a potential linkage between loading frequencies and locations of loading-induced bone formation is not well understood. In this study, we addressed a question: Does mechanical resonance play a role in frequency-dependent bone formation? If so, can the locations of enhanced bone formation be predicted through the modes of vibration? Our hypothesis is that mechanical loads applied at a frequency near the resonant frequencies enhance bone formation, specifically in areas that experience high principal strains. To test the hypothesis, we conducted axial tibia loading using low, medium, or high frequency to the mouse tibia, as well as finite element analysis. The experimental data demonstrated dependence of the maximum bone formation on location and frequency of loading. Samples loaded with the low-frequency waveform exhibited peak enhancement of bone formation in the proximal tibia, while the high-frequency waveform offered the greatest enhancement in the midshaft and distal sections. Furthermore, the observed dependence on loading frequencies was correlated to the principal strains in the first five resonance modes at 8.0-42.9 Hz. Collectively, the results suggest that resonance is a contributor to the frequencies and locations of maximum bone formation. Further investigation of the observed effects of resonance may lead to the prescribing of personalized mechanical loading treatments.

Evidence of interlinks between bioelectromagnetics and biomechanics: from biophysics to medical physics

A vast literature on electromagnetic and mechanical bioeffects at the bone and soft tissue level, as well as at the cellular level (osteoblasts, osteoclasts, keratinocytes, fibroblasts, chondrocytes, nerve cells, endothelial and muscle cells) has been reviewed and analysed in order to show the evident connections between both types of physical energies. Moreover, an intimate link between the two is suggested by transduction phenomena (electromagnetic-acoustic transduction and its reverse) occurring in living matter, as a sound biophysical literature has demonstrated. However, electromagnetic and mechanical signals are not always interchangeable, depending on their respective intensity. Calculations are reported in order to show in which cases (read: for which values of electric field in V/m and of mechanical pressure in Pa) a given electromagnetic or mechanical bioeffect is only due to the directly impinging energy or even to the indirect transductional energy. The relevance of the treated item for the applications of medical physics to regenerative medicine is stressed.

Low-magnitude high-frequency loading via whole body vibration enhances bone-implant osseointegration in ovariectomized rats

Osseointegration is vital to avoid long-time implants loosening after implantation surgery. This study investigated the effect of low-magnitude high-frequency (LMHF) loading via whole body vibration on bone-implant osseointegration in osteoporotic rats, and a comparison was made between LMHF vibration and alendronate on their effects. Thirty rats were ovariectomized to induce osteoporosis, and then treated with LMHF vibration (VIB) or alendronate (ALN) or a control treatment (OVX). Another 10 rats underwent sham operation to establish Sham control group. Prior to treatment, hydroxyapatite (HA)-coated titanium implants were inserted into proximal tibiae bilaterally. Both LMHF vibration and alendronate treatment lasted for 8 weeks. Histomorphometrical assess showed that both group VIB, ALN and Sham significantly increased bone-to-implant contact and peri-implant bone fraction (p < 0.05) when compared with group OVX. Nevertheless the bone-to-implant contact and peri-implant bone fraction of group VIB were inferior to group ALN and Sham (p < 0.05). Biomechanical tests also revealed similar results in maximum push out force and interfacial shear strength. Accordingly, it is concluded that LMHF loading via whole body vibration enhances bone-to-implant osseointegration in ovariectomized rats, but its effectiveness is weaker than alendronate.

Osteogenic effect of high-frequency acceleration on alveolar bone

Mechanical stimulation contributes to the health of alveolar bone, but no therapy using the osteogenic effects of these stimuli to increase alveolar bone formation has been developed. We propose that the application of high-frequency acceleration to teeth in the absence of significant loading is osteogenic. Sprague-Dawley rats were divided among control, sham, and experimental groups. The experimental group underwent localized accelerations at different frequencies for 5 min/day on the occlusal surface of the maxillary right first molar at a very low magnitude of loading (4 µε). Sham rats received a similar load in the absence of acceleration or frequency. The alveolar bone of the maxilla was evaluated by microcomputed tomography (µCT), histology, fluorescence microscopy, scanning electron microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR imaging), and RT-PCR for osteogenic genes. Results demonstrate that application of high-frequency acceleration significantly increased alveolar bone formation. These effects were not restricted to the area of application, and loading could be replaced by frequency and acceleration. These studies propose a simple mechanical therapy that may play a significant role in alveolar bone formation and maintenance.

Design and analysis of a novel mechanical loading machine for dynamic in vivo axial loading

This paper describes the construction of a loading machine for performing in vivo, dynamic mechanical loading of the rodent forearm. The loading machine utilizes a unique type of electromagnetic actuator with no mechanically resistive components (servotube), allowing highly accurate loads to be created. A regression analysis of the force created by the actuator with respect to the input voltage demonstrates high linear correlation (R(2) = 1). When the linear correlation is used to create dynamic loading waveforms in the frequency (0.5-10 Hz) and load (1-50 N) range used for in vivo loading, less than 1% normalized root mean square error (NRMSE) is computed. Larger NRMSE is found at increased frequencies, with 5%-8% occurring at 40 Hz, and reasons are discussed. Amplifiers (strain gauge, linear voltage displacement transducer (LVDT), and load cell) are constructed, calibrated, and integrated, to allow well-resolved dynamic measurements to be recorded at each program cycle. Each of the amplifiers uses an active filter with cutoff frequency at the maximum in vivo loading frequencies (50 Hz) so that electronic noise generated by the servo drive and actuator are reduced. The LVDT and load cell amplifiers allow evaluation of stress-strain relationships to determine if in vivo bone damage is occurring. The strain gauge amplifier allows dynamic force to strain calibrations to occur for animals of different sex, age, and strain. Unique features are integrated into the loading system, including a weightless mode, which allows the limbs of anesthetized animals to be quickly positioned and removed. Although the device is constructed for in vivo axial bone loading, it can be used within constraints, as a general measurement instrument in a laboratory setting.

In vivo assessment of the effect of controlled high- and low-frequency mechanical loading on peri-implant bone healing

The aim of this study was to investigate the effect of controlled high- (HF) and low-frequency (LF) mechanical loading on peri-implant bone healing. Custom-made titanium implants were inserted in both tibiae of 69 adult Wistar rats. For every animal, one implant was loaded by compression through the axis of tibia (test), whereas the other one was unloaded (control). The test implants were randomly distributed among four groups receiving different loading regimes, which were determined by ex vivo calibration. Within the HF (40 Hz) or LF (2 Hz) loading category, the magnitudes were chosen as low- (LM) and high-magnitude (HM), respectively, leading to constant strain rate amplitudes for the two frequency groups. This resulted in the four loading regimes: (i) HF-LM (40 Hz-0.5 N); (ii) HF-HM (40 Hz-1 N); (iii) LF-LM (2 Hz-10 N); and (iv) LF-HM (2 Hz-20 N) loading. Loading was performed five times per week and lasted for one or four weeks. Tissue samples were processed for histology and histomorphometry (bone-to-implant contact, BIC; and peri-implant bone fraction, BF) at the cortical and medullar level. Data were analysed statistically with ANOVA and paired t-tests with the significance level set at 0.05. For the one-week experiments, an increased BF adjacent to the implant surface at the cortical level was exclusively induced by the LF-HM loading regime (2 Hz-20 N). Four weeks of loading resulted in a significant effect on BIC (and not on BF) in case of HF-LM loading (40 Hz-0.5 N) and LF-HM loading (2 Hz-20 N): BIC at the cortical level significantly increased under both loading regimes, whereas BIC at the medullar level was positively influenced only in case of HF-LM loading. Mechanical loading at both HF and LF affects osseointegration and peri-implant BF. Higher loading magnitudes (and accompanying elevated tissue strains) are required under LF loading to provoke a positive peri-implant bone response, compared with HF loading. A sustained period of loading at HF is needed to result in an overall enhanced osseointegration.

Systemic treatment with pulsed electromagnetic fields do not affect bone microarchitecture in osteoporotic rats

Purpose

Pulsed electromagnetic fields (PEMF) are currently used in the treatment of spinal fusions and non-unions. There are indications that PEMF might also be effective in the treatment of osteoporosis. In this study we examined whether whole-body PEMF treatment affects the bone microarchitecture in an osteoporotic rat model.

Methods

Twenty-week-old female rats were ovariectomised (n = 20). Four different PEMF treatment protocols based on previous experimental studies and based on clinically used PEMF signals were examined (2 h/day, 5 days/week). A control group did not receive PEMF. At zero, three and six weeks cancellous and cortical bone architectural changes at the proximal tibia were evaluated using in vivo microCT scanning.

Results

PEMF treatment did not induce any changes in cancellous or cortical bone compared to untreated controls.

Conclusions

Although previous studies have shown strong effects of PEMF in osteoporosis we were unable to demonstrate this in any of the treatment protocols. Using in vivo microCT scanning we were able to identify small bone changes in time. Subtle differences in the experimental set-up might explain the differences in study outcomes in the literature. Since PEMF treatment is safe, future experimental studies on the effect of PEMF on bone can better be performed directly on humans, eliminating the potential translation issues between animals and humans. In this study we found no support for the use of PEMF in the treatment of osteoporosis.

Low-magnitude whole body vibration does not affect bone mass but does affect weight in ovariectomized rats

Mechanical loading has stimulating effects on bone architecture, which can potentially be used as a therapy for osteoporosis. We investigated the skeletal changes in the tibia of ovariectomized rats during treatment with whole body vibration (WBV). Different low-magnitude WBV treatment protocols were tested in a pilot experiment using ovariectomized rats with loading schemes of 2 × 8 min/day, 5 days/week (n = 2 rats per protocol). Bone volume and architecture were evaluated during a 10 week follow-up using in-vivo microcomputed tomography scanning. The loading protocol in which a 45 Hz sine wave was applied at 2 Hz with an acceleration of 0.5g showed an anabolic effect on bone and was therefore further analyzed in two groups of animals (n = 6 each group) with WBV starting directly after or 3 weeks after ovariectomy and compared to a control (non-WBV) group at 0, 3, 6 and 10 weeks' follow-up. In the follow-up experiment the WBV stimulus did not significantly affect trabecular volume fraction or cortical bone volume in any of the treatment groups during the 10 week follow-up. WBV did reduce weight gain that was induced as a consequence of ovariectomy. We could not demonstrate any significant effects of WBV on bone loss as a consequence of ovariectomy in rats; however, the weight gain that normally results after ovariectomy was partly prevented. Treatment with WBV was not able to prevent bone loss during induced osteoporosis.

Stochastic resonance activity influences serum tryptophan metabolism in healthy human subjects

Background

Stochastic resonance therapy (SRT) is used for rehabilitation of patients with various neuropsychiatric diseases. An alteration in tryptophan metabolism along the kynurenine pathway has been identified in the central and peripheral nervous systems in patients with neuroinflammatory and neurodegenerative diseases and during the aging process. This study investigated the effect of SRT as an exercise activity on serum tryptophan metabolites in healthy subjects.

Methods

Serum L-tryptophan, L-kynurenine, kynurenic acid, and anthranilic acid levels were measured one minute before SRT and at one, 5, 15, 30, and 60 minutes after SRT. We found that SRT affected tryptophan metabolism. Serum levels of L-tryptophan, L-kynurenine, and kynurenic acid were significantly reduced for up to 60 minutes after SRT. Anthranilic acid levels were characterized by a moderate, non significant transient decrease for up to 15 minutes, followed by normalization at 60 minutes. Tryptophan metabolite ratios were moderately altered, suggesting activation of metabolism after SRT. Lowering of tryptophan would generally involve activation of tryptophan catabolism and neurotransmitter, protein, and bone biosynthesis. Lowering of kynurenic acid by SRT might be relevant for improving symptoms in patients with neuropsychiatric disorders, such as Parkinson’s disease, Alzheimer’s disease, schizophrenia, and depression, as well as certain pain conditions.

A randomized controlled trial of whole body vibration exposure on markers of bone turnover in postmenopausal women

Purpose. To examine the effects of two doses of low-frequency (12 Hz), low-magnitude (0.3 g), whole body vibration on markers of bone formation and resorption in postmenopausal women. Methods. Women were recruited and randomized into a sham vibration control group, one time per week vibration group (1×/week), or three times per week vibration group (3×/week). Vibration exposure consisted of 20 minutes of intermittent vibration for the 1×/week and 3×/week groups, and sham vibration (<0.1 g) for the control group for eight weeks. Double-blinded primary outcome measures were urine markers of bone resorption: N-telopeptide X normalised to creatinine (NTx/Cr) and bone formation: bone-specific alkaline phosphatase (ALP). Results. Forty-six women (59.8 ± 6.2 years, median 7.3 years since menopause) were enrolled. NTx/Cr was significantly reduced (34.6%) in the 3×/wk vibration group but not in the 1×/wk vibration group compared with sham control (P < .01) group. No effect of time or group allocation was observed on the bone formation marker ALP (P = .27). Conclusion. We have shown for the first time that low-frequency, low-magnitude vibration 3×/week for eight weeks in postmenopausal women results in a significant reduction in NTx/Cr, a marker of bone resorption, when compared with sham vibration exposure.

Evidence for an additional effect of whole-body vibration above resistive exercise alone in preventing bone loss during prolonged bed rest

SUMMARY

The addition of whole-body vibration to high-load resistive exercise may provide a better stimulus for the reduction of bone loss during prolonged bed rest (spaceflight simulation) than high-load resistive exercise alone.

INTRODUCTION

Prior work suggests that the addition of whole-body vibration to high-load resistive exercise (RVE) may be more effective in preventing bone loss in spaceflight and its simulation (bed rest) than resistive exercise alone (RE), though this hypothesis has not been tested in humans.

METHODS

Twenty-four male subjects as part of the 2nd Berlin Bed Rest Study performed RVE (n = 7), RE (n = 8) or no exercise (control, n = 9) during 60-day head-down tilt bed rest. Whole-body, spine and total hip dual X-ray absorptiometry (DXA) measurements as well as peripheral quantitative computed tomography measurements of the tibia were conducted during bed rest and up to 90 days afterwards.

RESULTS

A better retention of bone mass in RVE than RE was seen at the tibial diaphysis and proximal femur (p ≤ 0.024). Compared to control, RVE retained bone mass at the distal tibia and DXA leg sub-region (p ≤ 0.020), but with no significant difference to RE (p ≥ 0.10). RE impacted significantly (p = 0.038) on DXA leg sub-region bone mass only. Calf muscle size was impacted similarly by both RVE and RE. On lumbar spine DXA, whole-body DXA and calcium excretion measures, few differences between the groups were observed.

CONCLUSIONS

Whilst further countermeasure optimisation is required, the results provide evidence that (1) combining whole-body vibration and high-load resistance exercise may be more efficient than high-load resistive exercise alone in preventing bone loss at some skeletal sites during and after prolonged bed rest and (2) the effects of exercise during bed rest impact upon bone recovery up to 3 months afterwards.

Conditional deletion of Pkd1 in osteocytes disrupts skeletal mechanosensing in mice

We investigated whether polycystin-1 is a bone mechanosensor. We conditionally deleted Pkd1 in mature osteoblasts/osteocytes by crossing Dmp1-Cre with Pkd1(flox/m1Bei) mice, in which the m1Bei allele is nonfunctional. We assessed in wild-type and Pkd1-deficient mice the response to mechanical loading in vivo by ulna loading and ex vivo by measuring the response of isolated osteoblasts to fluid shear stress. We found that conditional Pkd1 heterozygotes (Dmp1-Cre;Pkd1(flox/+)) and null mice (Pkd1(Dmp1-cKO)) exhibited a ∼ 40 and ∼ 90% decrease, respectively, in functional Pkd1 transcripts in bone. Femoral bone mineral density (12 vs. 27%), trabecular bone volume (32 vs. 48%), and cortical thickness (6 vs. 17%) were reduced proportionate to the reduction of Pkd1 gene dose, as were mineral apposition rate (MAR) and expression of Runx2-II, Osteocalcin, Dmp1, and Phex. Anabolic load-induced periosteal lamellar MAR (0.58 ± 0.14; Pkd1(Dmp1-cKO) vs. 1.68 ± 0.34 μm/d; control) and increases in Cox-2, c-Jun, Wnt10b, Axin2, and Runx2-II gene expression were significantly attenuated in Pkd1(Dmp1-cKO) mice compared with controls. Application of fluid shear stress to immortalized osteoblasts from Pkd1(null/null) and Pkd1(m1Bei/m1Bei)-derived osteoblasts failed to elicit the increments in cytosolic calcium observed in wild-type controls. These data indicate that polycystin-1 is essential for the anabolic response to skeletal loading in osteoblasts/osteocytes.

Effects of combined whole-body vibration and resistance training on muscular strength and bone metabolism in postmenopausal women

Whole-body vibration (WBV) has been shown to be osteogenic in animal models; however, its application in humans is not clear. The purpose of this study was to examine the effects of an 8-month program involving WBV plus resistance training on bone mineral density (BMD) and bone metabolism in older postmenopausal women. Fifty-five estrogen-deficient postmenopausal women were assigned to a resistance training group (R, n=22), a WBV plus resistance training group (WBVR, n=21), or a control group (CON, n=12). R and WBVR performed upper and lower body resistance exercises 3 days/week at 80% 1 Repetition Maximum (1RM). WBVR received vibration (30-40 Hz, 2-2.8 g) in three different positions preceding the resistance exercises. Daily calcium intake, bone markers (Bone alkaline phosphatase (Bone ALP); C-terminal telopeptide of Type I collagen (CTX), and BMD of the spine, dual femur, forearm, and total body (DXA) were measured at baseline and after the intervention. At baseline, there were no significant group differences in strength, BMD, or bone marker variables. After 8 months of R or WBVR, there were no significant group or time effects in Bone ALP, CTX, or total body, spine, left hip or right trochanter BMD. However, right total hip and right femoral neck BMD significantly (p<0.05) decreased in all groups. A group x time interaction (p<0.05) was detected at radius 33% BMD site, with CON slightly increasing, and WBVR slightly decreasing. R and WBVR significantly (p<0.05) increased 1RM strength for all exercises, while CON generally maintained strength. WBVR had significantly (p<0.05) greater percent increases in muscular strength than R at 4 months for lat pull down, seated row, hip abduction and hip adduction and at 8 months for lat pull down, hip abduction and hip adduction. Bone metabolism in postmenopausal women was not affected by resistance training either with or without WBV. In contrast, the addition of WBV augmented the positive effects of resistance training on muscular strength in these older women.

Polymeric piezoelectric actuator substrate for osteoblast mechanical stimulation

Bone mass distribution and structure are dependent on mechanical stress and adaptive response at cellular and tissue levels. Mechanical stimulation of bone induces new bone formation in vivo and increases the metabolic activity and gene expression of osteoblasts in culture. A wide variety of devices have been tested for mechanical stimulation of cells and tissues in vitro. The aim of this work was to experimentally validate the possibility to use piezoelectric materials as a mean of mechanical stimulation of bone cells, by converse piezoelectric effect. To estimate the magnitude and the distribution of strain, finite numerical models were applied and the results were complemented with the optical tests (Electronic Speckle Pattern Interferometric Process). In this work, osteoblasts were grown on the surface of a piezoelectric material, both in static and dynamic conditions at low frequencies, and total protein, cell viability and nitric oxide measurement comparisons are presented.

Local bone formation due to combined mechanical loading and intermittent hPTH-(1-34) treatment and its correlation to mechanical signal distributions

We evaluated the local response of cortical bone in the rat tibia due to combined treatment with synthetic parathyroid hormone, hPTH-(1-34), and mechanical stimulation by four-point bending. Forty-eight female retired breeder Sprague-Dawley rats were divided into six groups. Mechanically stimulated animals included the following groups: (1) Bend+PTH, (2) Sham+PTH, (3) Bend+Vehicle, (4) Sham+Vehicle. Non-mechanically stimulated animals included a (5) Control group that received neither loading nor injections, and a (6) PTH group that received only hPTH-(1-34) injections. The right limbs of mechanically loaded animals were exposed to a peak force of 50 N for 36 cycles at 2 Hz, three days per week for four weeks, and PTH-treated animals received injections equivalent to 50 microg/kg BW. Fluorochrome labeling was used to measure local formation at 12 sectors about the endocortical periphery. The distributions of endocortical bone formation were compared to the local formation differences between treatment groups and to a variety of potential mechanical stimuli signals. Results indicated that hPTH-(1-34) exerted a potent anabolic effect with near-uniform formation about the endocortical surface, and that localized formation peaks due to bending were further augmented in the presence of hPTH-(1-34) treatment. Correlation of formation patterns to mechanical signal distributions highlighted several candidate signals including the mid-principal stress, the dilatational strain, and the radial gradient of the local radial strain.

Ovariectomy sensitizes rat cortical bone to whole-body vibration

This study was designed to determine the modulatory effect of estrogen on mechanical stimulation in bone. Trabecular and cortical bone compartments of ovariectomized rats exposed to whole-body vibration of different amplitudes were evaluated by peripheral quantitative computed tomographic (pQCT) analysis and histomorphometry and compared to controls not exposed to vibration. Rats underwent whole-body vibration (20 minutes/day, 5 days/week) on a vibration platform for 2 months. The control rats were placed on the platform without vibration for the same time. We divided rats into six groups: a sham control (SHAM); a sham vibrated (SHAM-V) at 30 Hz, 0.6 g; a SHAM-V at 30 Hz, 3g; an ovariectomized control (OVX); an ovariectomized vibrated (OVX-V) at 30 Hz, 0.6 g; and an OVX-V at 30 Hz, 3g. In vivo, pQCT analyses of the tibiae were performed at the start of the experiment and after 4 and 8 weeks. After 8 weeks the tibiae were excised for histomorphometric and for in vitro pQCT analyses. In the SHAM-V group, vibration had no effect upon the different bone parameters. In the OVX-V group, vibration induced a significant increase compared to the OVX group of the cortical and medullary areas (P < 0.01) and of the periosteal (P < 0.01) and endosteal (P < 0.05) perimeters at the 3 g vibration. The strain strength index increased in the OVX-V group significantly (P < 0.01) at the higher vibration. The results showed that low-amplitude, high-frequency whole-body vibration is anabolic to bone in OVX animals. The osteogenic potential is limited to the modeling of the bone cortex and depends on the amplitude of the vibration.

Effect of stochastic resonance on bone loss in osteopenic conditions

We investigate the effect of noise on the remodelling process of the inner spongy part of the trabecular bone. Recently, a new noise-induced phenomenon in bone formation has been reported experimentally. We propose the first conceptual model for this finding, explained by the stochastic resonance effect, and provide a theoretical basis for the development of new countermeasures for bone degeneration in long space flights, which currently has dramatic consequences on return to standard gravity. These results may also be applicable on Earth for patients under osteopenic conditions.

Low-level accelerations applied in the absence of weight bearing can enhance trabecular bone formation

High-frequency whole body vibrations can be osteogenic, but their efficacy appears limited to skeletal segments that are weight bearing and thus subject to the induced load. To determine the anabolic component of this signal, we investigated whether low-level oscillatory displacements, in the absence of weight bearing, are anabolic to skeletal tissue. A loading apparatus, developed to shake specific segments of the murine skeleton without the direct application of deformations to the tissue, was used to subject the left tibia of eight anesthesized adult female C57BL/6J mice to small (0.3 g or 0.6 g) 45 Hz sinusoidal accelerations for 10 min/day, while the right tibia served as an internal control. Video and strain analysis revealed that motions of the apparatus and tibia were well coupled, inducing dynamic cortical deformations of less than three microstrain. After 3 weeks, trabecular metaphyseal bone formation rates and the percentage of mineralizing surfaces (MS/BS) were 88% and 64% greater (p < 0.05) in tibiae accelerated at 0.3 g than in their contralateral controls. At 0.6 g, bone formation rates and mineral apposition rates were 66% and 22% greater (p < 0.05) in accelerated tibiae. Changes in bone morphology were evident only in the epiphysis, where stimulated tibiae displayed significantly greater cortical area (+8%) and thickness (+8%). These results suggest that tiny acceleratory motions--independent of direct loading of the matrix--can influence bone formation and bone morphology. If confirmed by clinical studies, the unique nature of the signal may ultimately facilitate the stimulation of skeletal regions that are prone to osteoporosis even in patients that are suffering from confinement to wheelchairs, bed rest, or space travel.

Whole-body vibration can reduce calciuria induced by high protein intakes and may counteract bone resorption: A preliminary study

Excess protein intake can adversely affect the bone via an increase in calcium excretion, while suitable mechanical loading promotes osteogenesis. We therefore investigated whether vibration exposure could alleviate the bone mineral losses associated with a metabolic acidosis. Ten healthy individuals aged 22 - 29 years (median = 25) underwent three 5-day study periods while monitoring their dietary intake. The study consisted of recording the participants' usual dietary intake for 5 consecutive days. Participants were then randomly divided into two groups, one of which received a protein supplement (2 g x kg(-1) body mass x day(-1); n = 5) and the other whole-body low-magnitude (3.5 g), low-frequency (30 Hz) mechanical vibration (WBV) delivered through a specially designed vibrating plate for 10 min each day (n = 5). Finally, for the third treatment period, all participants consumed the protein supplement added to their normal diet and were exposed to WBV exercise for 10 min per day. Daily urine samples were collected throughout the experimental periods to determine the excretion of calcium, phosphate, titratable acid, urea, and C-telopeptide. As expected, when the participants underwent the high protein intake, there was an increase in urinary excretion rates of calcium (P < 0.001), phosphate (P < 0.003), urea (P < 0.001), titratable acid (P < 0.001), and C-telopeptide (P < 0.05) compared with baseline values. However, high protein intake coupled with vibration stimulation resulted in a significant reduction in urinary calcium (P = 0.006), phosphate excretion (P = 0.021), and C-telopeptide (P < 0.05) compared with protein intake alone, but did not affect titratable acid and urea output. The participants showed no effect of WBV exercise alone on urinary excretion of calcium, phosphate, urea, titratable acid, or C-telopeptide. The results indicate that vibration stimulation can moderate the increase in bone resorption and reduction in bone formation caused by a metabolic acidosis.

Low-amplitude, broad-frequency vibration effects on cortical bone formation in mice

Mechanical loading of the skeleton is necessary to maintain bone structure and strength. Large amplitude strains associated with vigorous activity typically result in the greatest osteogenic response; however, data suggest that low-amplitude, broad-frequency vibration results in new bone formation and may enhance adaptation through a stochastic resonance (SR) phenomenon. That is, random noise may maximally enhance bone formation to a known osteogenic stimulus. The aims of this study were to (1) assess the ability of different vibration signals to enhance cortical bone formation during short- and long-term loading and (2) determine whether vibration could effect SR in bone. Two studies were completed wherein several osteogenic loading waveforms, with or without an additive low-amplitude, broad-frequency (0-50 Hz) vibration signal, were applied to the mouse ulna in axial compression. In study 1, mice were loaded short-term (30 s/day, 2 days) with either a carrier signal alone (1 or 2 N sine waveform), vibration signal alone [0.1 N or 0.3 N root mean square (RMS)] or combined carrier and vibration signal. In study 2, mice were loaded long-term (30 s/day, 3 days/week, 4 weeks) with a carrier signal alone (static or sine waveform), vibration signal alone (0.02 N, 0.04 N, 0.08 N or 0.25 N RMS) or combined carrier and vibration signal. Sequential calcein bone labels were administered at 2 and 4 days and at 4 and 29 days after the first day of loading in study 1 and 2, respectively; bone formation parameters and changes in geometry were measured. Combined application of the carrier and vibration signals in study 1 resulted in significantly greater bone formation than with either signal alone (P < 0.001); however, this increase was independently explained by increased strain levels associated with additive vibration. When load and strain levels were similar across loading groups in study 2, cortical bone formation and changes in geometry were not significantly altered by vibration. Vibration alone did not result in any new bone formation. Our data suggest that low-amplitude, broad-frequency vibration superimposed onto an osteogenic waveform or vibration alone does not enhance cortical bone adaptation at the frequencies, amplitudes and loading periods tested.

Low-magnitude mechanical signals that stimulate bone formation in the ovariectomized rat are dependent on the applied frequency but not on the strain magnitude

There is growing evidence that extremely small mechanical signals, if applied at a sufficiently high frequency, can serve as anabolic signals to bone tissue. To determine if the responsiveness of bone to low-magnitude, high-frequency parameters is modulated by endocrine imbalance, ovariectomized (OVX) Sprague-Dawley rats were subjected to whole body vibrations (WBV, 0.15 g) at 45 Hz (n=6) or 90 Hz (n=6) for 10 min/day, and compared to OVX age-matched controls (n=6). Five additional rats were used, in vivo, to establish the induced bone surface strain magnitudes (and strain rates). Following a 28 d protocol, bone formation rates in the metaphysis of the proximal tibia were 159% greater in 90 Hz rats when compared to age-matched controls, but 45 Hz rats were not significantly different from controls. Bone morphology of 90 Hz rats indicated significantly greater trabecular bone volume (22% and 25%) and thicker trabeculae (11% and 12%) over either controls or 45 Hz rats in the epiphysis of the distal femur, respectively. Despite the enhanced sensitivity of the skeleton towards the 90 Hz signal, the strain magnitudes and strain rates induced by this frequency were significantly lower than during 45 Hz vibration, suggesting that factors other than matrix strain are driving the anabolic response. Ideally, such mechanical signals represent a non-pharmacologic means of controlling bone mass and morphology in spite of systemic pressures for bone resorption.

The Wnt co-receptor LRP5 is essential for skeletal mechanotransduction but not for the anabolic bone response to parathyroid hormone treatment

The cell surface receptor, low-density lipoprotein receptor-related protein 5 (LRP5) is a key regulator of bone mass. Loss-of-function mutations in LRP5 cause the human skeletal disease osteoporosis-pseudoglioma syndrome, an autosomal recessive disorder characterized by severely reduced bone mass and strength. We investigated the role of LRP5 on bone strength using mice engineered with a loss-of-function mutation in the gene. We then tested whether the osteogenic response to mechanical loading was affected by the loss of Lrp5 signaling. Lrp5-null (Lrp5-/-) mice exhibited significantly lower bone mineral density and decreased strength. The osteogenic response to mechanical loading of the ulna was reduced by 88 to 99% in Lrp5-/- mice, yet osteoblast recruitment and/or activation at mechanically strained surfaces was normal. Subsequent experiments demonstrated an inability of Lrp5-/- osteoblasts to synthesize the bone matrix protein osteopontin after a mechanical stimulus. We then tested whether Lrp5-/- mice increased bone formation in response to intermittent parathyroid hormone (PTH), a known anabolic treatment. A 4-week course of intermittent PTH (40 microg/kg/day; 5 days/week) enhanced skeletal mass equally in Lrp5-/- and Lrp5+/+ mice, suggesting that the anabolic effects of PTH do not require Lrp5 signaling. We conclude that Lrp5 is critical for mechanotransduction in osteoblasts. Lrp5 is a mediator of mature osteoblast function following loading. Our data suggest an important component of the skeletal fragility phenotype in individuals affected with osteoporosis-pseudoglioma is inadequate processing of signals derived from mechanical stimulation and that PTH might be an effective treatment for improving bone mass in these patients.

The Effect of Varying Magnitudes of Whole-Body Vibration on Several Skeletal Sites in Mice

It has been reported that whole-body vibration (WBV) is anabolic to trabecular bone in animal models and humans. It is likely that this anabolic response does not occur uniformly throughout the entire body. Two factors that may affect the observed anabolic response are vibration magnitude and skeletal site of interest. In this study, mice were loaded with WBV of varying magnitudes. After five weeks of loading, bone marrow was flushed from tibias in order to quantify osteoprogenitor cells. Staining with alizarin red (an indicator of mineralization) showed a significant decrease in percent stained area in the 0.3 g loaded group compared to the control group and the 1.0 g group. MicroCT analysis was performed at five skeletal sites: the proximal tibial metaphysis, femoral condyles, distal femoral metaphysis, proximal femur, and L5 vertebral body. Increasing magnitudes of WBV were associated with a non-dose-dependent increase in trabecular bone volume (BV/TV) at the proximal tibial metaphysis, although other sites were unresponsive. There were statistically significant increases in BV/TV in the 0.1 g group (32% increase) and 1.0 g group (43% increase) compared to control (p < 0.05). The 0.1 g and 1.0 g groups also had higher BV/TV than the 0.3 g loaded group. If this non-dose-dependent phenomenon is verified by future studies, it suggests that a range of magnitudes should be examined for each application of WBV.

Global gene expression analysis in the bones reveals involvement of several novel genes and pathways in mediating an anabolic response of mechanical loading in mice

To identify the genes and signal pathways responsible for mechanical loading-induced bone formation, we evaluated differential gene expression on a global basis in the tibias of C57BL/6J (B6) mice after four days of four-point bending. We applied mechanical loads to the right tibias of the B6 mice at 9 N, 2 Hz for 36 cycles per day, with the left tibias used as unloaded controls. RNA from the tibias was harvested 24 h after last stimulation and subjected to microarray. Of the 20,280 transcripts hybridized to the array, 346 were differentially expressed in the loaded bones compared to the controls. The validity of the microarray data was established with the increased expression of bone-related genes such as pleiotrophin, osteoglycin, and legumain upon four-point bending and confirmation of increased expression of selected genes by real-time PCR. The list of differentially expressed genes includes genes involved in cell growth, differentiation, adhesion, proteolysis, as well as signaling molecules of receptors for growth factors, integrin, Ephrin B2, endothelin, and adhesion G protein coupled receptor. Pathway analyses suggested that 28 out of the 346 genes exhibited a direct biological association. Among the biological network, fibronectin and pleitrophin function as important signaling molecules in regulating periosteal bone formation and resorption in response to four-point bending. Furthermore, some expressed sequence tags (ESTs) with no prior known function have been identified as potential mediators of mechanotransduction signaling pathways. Further studies on these previously unknown genes will improve our understanding of the molecular pathways and mechanisms involved in bone's response to mechanical stress.

Delayed stimulatory effect of low-intensity shockwaves on human periosteal cells

We investigated the effect of shockwaves on cells explanted from normal human periosteum to study the potential mechanisms of their responses and to determine suitable treatment settings. The cells were subjected to one shockwave treatment with systematic combinations of energy intensities (range, 0.05-0.5 mJ/mm) and number of shocks (range, 500-2000) whereas control cells received no treatment. The immediate effect on cell viability and the long-lasting effect on proliferation, viable cell number at Day 18, and mineralization at Day 35 were assessed. We observed an immediate dose-dependent destructive effect of shockwaves. Energy intensity and number of shocks contributed equally to viability. Total energy dose (intensity x number of shocks) was a better reference for determining the shockwave effect. We also found a long-term stimulatory effect on proliferation, viable cell number, and calcium deposition of human periosteal cells. At the same total energy dose, low-intensity shockwaves with more shocks (0.12 mJ/mm at 1250 shocks) were more favorable for enhancing cellular activities than high-intensity waves with fewer shocks (0.5 mJ/mm at 300 shocks). These findings document some of the biochemical changes of periosteal cells during shockwave treatments.

Bone formation induced by a novel form of mechanical loading on joint tissue

Because of insufficient mechanical loading, exposure to weightlessness in space flight reduces bone mass. In order to maintain bone mass in a weightless condition, we investigated a novel form of mechanical loading--joint loading. Since some part of gravity-induced loading to our skeletal system is absorbed by viscoelastic deformation of joint tissues, we hypothesized that deformation of joint tissues would generate fluid flow in bone and stimulate bone formation in diaphyseal cortical bone. In order to test the hypothesis, we applied directly oscillatory loading to an elbow joint of mice and conducted bone histomorphometry on the diaphysis of ulnae. Using murine femurs ex vivo, streaming potentials were measured to evaluate a fluid flow induced by joint loading. Bone histomorphometry revealed that compared to no loading control, elbow loading increased mineralizing surface, mineral apposition rate, and bone formation rate 3.2-fold, 3.0-fold, and 7.9-fold, respectively. We demonstrated that joint loading generated a streaming potential in a medullar cavity of femurs. The results support a novel mechanism, in which joint loading stimulates effectively bone formation possibly by generating fluid flow, and suggest that a supportive attachment to joints, driven passively or actively, would be useful to maintain bone mass of astronauts during an exposure to weightlessness.

Low amplitude, high frequency strains imposed by electrically stimulated skeletal muscle retards the development of osteopenia in the tibiae of hindlimb suspended rats

The purpose of this study was to determine the extent to which high frequency, low amplitude skeletal muscle contractions, induced using electrical stimulation, could prevent or retard disuse osteopenia. Ten minutes of 30 Hz electrical stimulation was delivered, 5 days a week, during a 4 week rat-hindlimb suspension protocol. Each pulse generated a peak compressive dynamic strain on the tibia of approximately 200 microepsilon. We hypothesized that the electrical stimulation protocol would significantly reduce the loss of tibial bone mineral density compared to the contralateral control tibia that did not receive electrical stimulation. Compared to the contralateral control limb, the tibia of the stimulated limb had significantly higher bone mineral density and enhanced newly formed bone in the tibial diaphysis. The diaphysis, specifically the posterior bone cortex, of the tibia of the limb receiving the stimulation also demonstrated substantially larger mineral-binding fluorochrome biomarker within the osteocyte lacunae and canalicular volumes. Although the protocol did not prevent disuse osteopenia the evidence suggests that it was effective at reducing the extent of the osteopenia. One possibility for this outcome may be the insensitivity of bone to static, compared to dynamic compressive loads. In the present study there was a considerable static component to the compressive loads that accounted for a large component of the peak load generated by the stimulated skeletal muscle. Nevertheless, the results provide impetus for further development of the methods by which muscle contraction-induced loading of bone can be clinically exploited.

Dynamic shear stress in parallel-plate flow chambers

An in vitro model using a parallel-plate fluid flow chamber is supposed to simulate in vivo fluid shear stresses on various cell types exposed to dynamic fluid flow in their physiological environment. The metabolic response of cells in vitro is associated with the wall shear stress. However, parallel-plate flow chambers have not been characterized for dynamic fluid flow experiments. We use a dimensionless ratio h / lambda(v), in determining the exact magnitude of the dynamic wall shear stress, with its oscillating components scaled by a shear factor T. It is shown that, in order to expose cells to predictable levels of dynamic fluid shear stress, two conditions have to be met: (1) h / lambda(v) < 2, where h is the distance between the plates and lambda(v) is the viscous penetration depth; and (2) f(0) < f(c) / m, where the critical frequency f(c) is the upper threshold for this flow regime, m is the highest harmonic mode of the flow, and f(0) is the fundamental frequency of fluid flow.

Osteogenic potentials with joint-loading modality

Osteogenic potentials with a novel joint-loading modality were examined, using mouse ulnae as a model system. Load-induced deformation of rigid bone is known to generate interstitial fluid flow and stimulate osteogenesis. However, in most of the previous studies, loads were applied to cortical bone. In the current study, we addressed the question of whether deformation of the epiphysis underneath the joint would enhance bone formation in the epiphysis and the diaphysis. In order to answer the question, we applied lateral loads to a mouse elbow and conducted a bone histomorphometric analysis, as well as measurements of strains and streaming potentials. Compared to the no-loading control, the histomorphometric results showed that 0.5-N loads, applied to the elbow at 2 Hz for 3 min/day for 3 consecutive days, increased the mineralizing surface (two- to threefold), the rate of mineral apposition (three- to fivefold), and the rate of bone formation (six- to eightfold) in the ulna. Strain measurements indicated that strains of around 30 microstrain, induced with the joint-loading modality, were under the minimum effective strain of around 1000 microstrain, which is considered necessary to achieve strain-driven bone formation. To evaluate the induction of fluid flow with the joint-loading modality, streaming potentials were measured in separate experiments, using mouse femurs ex vivo. We found that the streaming potentials correlated to the magnitude of the load applied to the epiphysis (r(2) = 0.92), as well as the flow speed in the medullary cavity (r(2) = 0.93). Taken together, the findings of the current study support the idea of joint-loading driven osteogenesis, through a mechanism that involves the induction of fluid flow in cortical bone.

Effect of 6-month whole body vibration training on hip density, muscle strength, and postural control in postmenopausal women: a randomized controlled pilot study

High-frequency mechanical strain seems to stimulate bone strength in animals. In this randomized controlled trial, hip BMD was measured in postmenopausal women after a 24-week whole body vibration (WBV) training program. Vibration training significantly increased BMD of the hip. These findings suggest that WBV training might be useful in the prevention of osteoporosis.

INTRODUCTION

High-frequency mechanical strain has been shown to stimulate bone strength in different animal models. However, the effects of vibration exercise on the human skeleton have rarely been studied. Particularly in postmenopausal women-who are most at risk of developing osteoporosis-randomized controlled data on the safety and efficacy of vibration loading are lacking. The aim of this randomized controlled trial was to assess the musculoskeletal effects of high-frequency loading by means of whole body vibration (WBV) in postmenopausal women.

MATERIALS AND METHODS

Seventy volunteers (age, 58-74 years) were randomly assigned to a whole body vibration training group (WBV, n = 25), a resistance training group (RES, n = 22), or a control group (CON, n = 23). The WBV group and the RES group trained three times weekly for 24 weeks. The WBV group performed static and dynamic knee-extensor exercises on a vibration platform (35-40 Hz, 2.28-5.09g), which mechanically loaded the bone and evoked reflexive muscle contractions. The RES group trained knee extensors by dynamic leg press and leg extension exercises, increasing from low (20 RM) to high (8 RM) resistance. The CON group did not participate in any training. Hip bone density was measured using DXA at baseline and after the 6-month intervention. Isometric and dynamic strength were measured by means of a motor-driven dynamometer. Data were analyzed by means of repeated measures ANOVA.

RESULTS

No vibration-related side effects were observed. Vibration training improved isometric and dynamic muscle strength (+15% and + 16%, respectively; p < 0.01) and also significantly increased BMD of the hip (+0.93%, p < 0.05). No changes in hip BMD were observed in women participating in resistance training or age-matched controls (-0.60% and -0.62%, respectively; not significant). Serum markers of bone turnover did not change in any of the groups.

CONCLUSION

These findings suggest that WBV training may be a feasible and effective way to modify well-recognized risk factors for falls and fractures in older women and support the need for further human studies.

Mechanotransduction in the cortical bone is most efficient at loading frequencies of 5-10 Hz

A dose-response relationship has been shown between loading frequency and cortical bone adaptation for frequencies of up to 10 Hz, and is presumed to persist with further increases in frequency. Studies herein aimed to investigate cortical bone adaptation to loading frequencies of 1, 5, 10, 20 and 30 Hz. Two studies were performed in adult C57BL/6 mice using the ulna axial compression-loading model. In the first study, the histomorphometric response of the ulna was studied when loaded for 120 cycles day(-1) for 3 days at one of the five frequencies and one of two load magnitudes (1.5 or 2.0 N). In the second study, the changes in ulna geometry and mechanical properties were studied following loading for 5 min day(-1), 3 days week(-1) for 4 weeks at one of the five frequencies and one of two load magnitudes (1.0 or 1.6 N). Preliminary strain gauge measurements showed that frequency had no effect on mechanical strain per unit load. In study 1, loading frequency significantly influenced bone adaptation when loading at 2.0 N, with loading at 10 Hz resulting in significantly greater adaptation than with loading at other frequencies. In study 2, loading frequency significantly influenced the change in geometry when loading at 1.6 N, with loading at 5, 10 or 30 Hz resulting in significantly greater change than with loading at 1 Hz. Loading at 5 Hz also resulted in significantly greater change than with loading at 20 Hz. No frequency effect was found on any of the mechanical properties at either load. Overall, we found cortical bone adaptation to mechanical loading to increase with increasing loading frequency up to 5-10 Hz and to plateau with frequencies beyond 10 Hz. The mechanism for this nonlinear frequency response is not known; however, based on strain gauge measurements, we do not believe it resulted from dampening associated with high frequency loading through the flexed carpal joint. The obtained findings may relate to the mechanism of mechanotransduction within the bone. This requires further investigation.