Streaming potential of intact wet bone

The bioelectrical properties of bone tissue

Understanding the bioelectrical properties of bone tissue is key to developing new treatment strategies for bone diseases and injuries, as well as improving the design and fabrication of scaffold implants for bone tissue engineering. The bioelectrical properties of bone tissue can be attributed to the interaction of its various cell lineages (osteocyte, osteoblast and osteoclast) with the surrounding extracellular matrix, in the presence of various biomechanical stimuli arising from routine physical activities; and is best described as a combination and overlap of dielectric, piezoelectric, pyroelectric and ferroelectric properties, together with streaming potential and electro-osmosis. There is close interdependence and interaction of the various electroactive and electrosensitive components of bone tissue, including cell membrane potential, voltage-gated ion channels, intracellular signaling pathways, and cell surface receptors, together with various matrix components such as collagen, hydroxyapatite, proteoglycans and glycosaminoglycans. It is the remarkably complex web of interactive cross-talk between the organic and non-organic components of bone that define its electrophysiological properties, which in turn exerts a profound influence on its metabolism, homeostasis and regeneration in health and disease. This has spurred increasing interest in application of electroactive scaffolds in bone tissue engineering, to recapitulate the natural electrophysiological microenvironment of healthy bone tissue to facilitate bone defect repair.

The effect of low-frequency electromagnetic field on human bone marrow stem/progenitor cell differentiation

Human bone marrow stromal cells (hBMSCs, also known as bone marrow-derived mesenchymal stem cells) are a population of progenitor cells that contain a subset of skeletal stem cells (hSSCs), able to recreate cartilage, bone, stroma that supports hematopoiesis and marrow adipocytes. As such, they have become an important resource in developing strategies for regenerative medicine and tissue engineering due to their self-renewal and differentiation capabilities. The differentiation of SSCs/BMSCs is dependent on exposure to biophysical and biochemical stimuli that favor early and rapid activation of the in vivo tissue repair process. Exposure to exogenous stimuli such as an electromagnetic field (EMF) can promote differentiation of SSCs/BMSCs via ion dynamics and small signaling molecules. The plasma membrane is often considered to be the main target for EMF signals and most results point to an effect on the rate of ion or ligand binding due to a receptor site acting as a modulator of signaling cascades. Ion fluxes are closely involved in differentiation control as stem cells move and grow in specific directions to form tissues and organs. EMF affects numerous biological functions such as gene expression, cell fate, and cell differentiation, but will only induce these effects within a certain range of low frequencies as well as low amplitudes. EMF has been reported to be effective in the enhancement of osteogenesis and chondrogenesis of hSSCs/BMSCs with no documented negative effects. Studies show specific EMF frequencies enhance hSSC/BMSC adherence, proliferation, differentiation, and viability, all of which play a key role in the use of hSSCs/BMSCs for tissue engineering. While many EMF studies report significant enhancement of the differentiation process, results differ depending on the experimental and environmental conditions. Here we review how specific EMF parameters (frequency, intensity, and time of exposure) significantly regulate hSSC/BMSC differentiation in vitro. We discuss optimal conditions and parameters for effective hSSC/BMSC differentiation using EMF treatment in an in vivo setting, and how these can be translated to clinical trials.

Electrical properties of hydroxyapatite

Despite being one of the mostly studied biomaterials for orthopedic, dental, protein purification and stem cell applications, electrical properties of hydroxyapatite has received only limited attention. Since the prediction in 2005 of the possibility of piezo and pyroelectricity in hydroxyapatite several theoretical and experimental works in this field may lead to new understandings of electrical behaviors of calcified tissues in vertebrates. Also, the ability of creating discrete electrostatic domains on nanocrystalline films of hydroxyapatite will open the possibility of understanding how surface charge influences biological interactions. The outlook for future endeavours in this field will be discussed.

Novel in situ normal streaming potential device for characterizing electrostatic properties of confluent cells

The characteristics of transport across confluent cell monolayers may often be attributed to its electrostatic properties. While tangential streaming potential is often used to quantify these electrostatic properties, this method is not effective for transport normal to the apical cell surface where the charge properties along the basolateral sides may be important (i.e., confluent cells with leaky tight junctions). In addition, even when cells have a uniform charge distribution, the shear stress generated by the conventional tangential flow device may dislodge cells from their confluent state. Here we introduce a novel streaming potential measurement device to characterize the normal electrostatic properties of confluent cells. The streaming potential device encompasses a 24 mm cell-seeded Transwell(®) with two AgCl electrodes on either side of the cell-seeded Transwell. Phosphate buffered saline is pressurized transversal to the Transwell and the resultant pressure gradient induces a potential difference. Confluent monolayers of HEK and EA926 cells are used as examples. The corresponding zeta potential of the cell-membrane configuration is calculated using the Helmholtz-Smoluchowski equation and the zeta potential of the confluent cell layer is deconvolved from the overall measurements. For these test models, the zeta potential is consistent with that determined using a commercial dispersed-cell device. This novel streaming potential device provides a simple, easy, and cost-effective methodology to determine the normal zeta potential of confluent cells cultured on Transwell systems while keeping the cells intact. Furthermore, its versatility allows periodic measurements of properties of the same cell culture during transient studies.

Spinal facet joint biomechanics and mechanotransduction in normal, injury and degenerative conditions

The facet joint is a crucial anatomic region of the spine owing to its biomechanical role in facilitating articulation of the vertebrae of the spinal column. It is a diarthrodial joint with opposing articular cartilage surfaces that provide a low friction environment and a ligamentous capsule that encloses the joint space. Together with the disc, the bilateral facet joints transfer loads and guide and constrain motions in the spine due to their geometry and mechanical function. Although a great deal of research has focused on defining the biomechanics of the spine and the form and function of the disc, the facet joint has only recently become the focus of experimental, computational and clinical studies. This mechanical behavior ensures the normal health and function of the spine during physiologic loading but can also lead to its dysfunction when the tissues of the facet joint are altered either by injury, degeneration or as a result of surgical modification of the spine. The anatomical, biomechanical and physiological characteristics of the facet joints in the cervical and lumbar spines have become the focus of increased attention recently with the advent of surgical procedures of the spine, such as disc repair and replacement, which may impact facet responses. Accordingly, this review summarizes the relevant anatomy and biomechanics of the facet joint and the individual tissues that comprise it. In order to better understand the physiological implications of tissue loading in all conditions, a review of mechanotransduction pathways in the cartilage, ligament and bone is also presented ranging from the tissue-level scale to cellular modifications. With this context, experimental studies are summarized as they relate to the most common modifications that alter the biomechanics and health of the spine-injury and degeneration. In addition, many computational and finite element models have been developed that enable more-detailed and specific investigations of the facet joint and its tissues than are provided by experimental approaches and also that expand their utility for the field of biomechanics. These are also reviewed to provide a more complete summary of the current knowledge of facet joint mechanics. Overall, the goal of this review is to present a comprehensive review of the breadth and depth of knowledge regarding the mechanical and adaptive responses of the facet joint and its tissues across a variety of relevant size scales.

A new streaming potential chamber for zeta potential measurements of particulates

A novel streaming potential measurement device has been validated by determining the average electrokinetic (zeta) potential of densely packed particulate such as human erythrocytes and ground bovine cortical bone. The new streaming potential device used in this study is easy to construct in the laboratory, designed to allow dense packing of particles, and determines zeta potentials for a broad range of particle sizes. The streaming potential device consists of four Plexiglas parts: (i) an upper and (ii) a lower chamber, which act as reservoirs for fluid; (iii) a midchamber which connects the upper and lower chambers and holds the sample holder, and (iv) a sample holder. Pressurization of fluid in the top chamber generates a pressure gradient that induces movement of fluid through the stationary sample and into the bottom chamber. Pressure induced flow through the interconnected pores of the densely packed particulate generates a potential difference across the sample that is measured using electrodes housed in the top and bottom chambers. The measured potential difference is then converted to zeta potentials. The advantage of this chamber is its ability to handle densely packed particulates exhibiting a broad distribution of sizes. Dense packing of particulate is achieved by compacting samples at the bottom of the sample holder under centrifugal forces before the device is assembled. This approach allowed us to determine average zeta potentials of densely packed particulate made of soft and hard materials.

Impairment of chondrocyte biosynthetic activity by exposure to 3-tesla high-field magnetic resonance imaging is temporary

The influence of magnetic resonance imaging (MRI) devices at high field strengths on living tissues is unknown. We investigated the effects of a 3-tesla electromagnetic field (EMF) on the biosynthetic activity of bovine articular cartilage. Bovine articular cartilage was obtained from juvenile and adult animals. Whole joints or cartilage explants were subjected to a pulsed 3-tesla EMF; controls were left unexposed. Synthesis of sulfated glycosaminoglycans (sGAGs) was measured by using [³⁵S]sulfate incorporation; mRNA encoding the cartilage markers aggrecan and type II collagen, as well as IL-1β, were analyzed by RT–PCR. Furthermore, effects of the 3-tesla EMF were determined over the course of time directly after exposure (day 0) and at days 3 and 6. In addition, the influence of a 1.5-tesla EMF on cartilage sGAG synthesis was evaluated. Chondrocyte cell death was assessed by staining with Annexin V and TdT-mediated dUTP nick end labelling (TUNEL). Exposure to the EMF resulted in a significant decrease in cartilage macromolecule synthesis. Gene expression of both aggrecan and IL-1β, but not of collagen type II, was reduced in comparison with controls. Staining with Annexin V and TUNEL revealed no evidence of cell death. Interestingly, chondrocytes regained their biosynthetic activity within 3 days after exposure, as shown by proteoglycan synthesis rate and mRNA expression levels. Cartilage samples exposed to a 1.5-tesla EMF remained unaffected. Although MRI devices with a field strength of more than 1.5 T provide a better signal-to-noise ratio and thereby higher spatial resolution, their high field strength impairs the biosynthetic activity of articular chondrocytes in vitro. Although this decrease in biosynthetic activity seems to be transient, articular cartilage exposed to high-energy EMF may become vulnerable to damage.

Effect of pulsed electromagnetic fields on proteoglycan biosynthesis of articular cartilage is age dependent

OBJECTIVE

To investigate the effects of a pulsed electromagnetic field (EMF) on articular cartilage matrix biosynthesis with regard to age and cartilage damage using a matrix depleted cartilage explant model.

METHODS

Cartilage explants were obtained from metacarpophalangeal joints of calves and adult cows. After depletion of the extracellular matrix by trypsin digestion, samples were maintained in serum-free basal medium with and without the addition of interleukin 1beta (IL1beta). Half the samples were subjected to an EMF for 24 minutes daily; the other half were left untreated. Undigested and untreated explants served as negative controls. After 7 days, biosynthesis of matrix macromolecules was assessed by [35S]sulphate incorporation and values were normalised to hydroxyproline content.

RESULTS

The EMF increased matrix macromolecule synthesis in undigested, untreated explants (p<0.009). In matrix depleted samples the EMF had no stimulatory effect on proteoglycan biosynthesis. IL1beta significantly decreased the de novo synthesis of matrix macromolecules (p<0.00004) in young and adult samples, but an EMF partly counteracted this inhibitory effect in cartilage samples from young, but not old animals.

CONCLUSION

EMF promoted matrix macromolecule biosynthesis in intact tissue explants but had no stimulatory effect on damaged articular cartilage. The supressive effects of IL1beta were partially counteracted by EMF exposure, exclusively in cartilage derived from young animals. An EMF has age dependent chondroprotective but not structure modifying properties when cartilage integrity is compromised.

“Whither flows the fluid in bone?” An osteocyte's perspective

Bone represents a porous tissue containing a fluid phase, a solid matrix, and cells. Movement of the fluid phase within the pores or spaces of the solid matrix translates endogenous and exogenous mechanobiological, biochemical and electromechanical signals from the system that is exposed to the dynamic external environment to the cells that have the machinery to remodel the tissue from within. Hence, bone fluid serves as a coupling medium, providing an elegant feedback mechanism for functional adaptation. Until recently relatively little has been known about bone fluid per se or the influences governing the characteristics of its flow. This work is designed to review the current state of this emerging field. The structure of bone, as an environment for fluid flow, is discussed in terms of the properties of the spaces and channel walls through which the fluid flows and the influences on flow under physiological conditions. In particular, the development of the bone cell syncytium and lacunocanalicular system are presented, and pathways for fluid flow are described from the systemic to the organ, tissue, cellular and subcellular levels. Finally, exogenous and endogenous mechanisms for pressure-induced fluid movement through bone, including mechanical loading, vascular derived pressure gradients, and osmotic pressure gradients are discussed. The objective of this review is to survey the current understanding of the means by which fluid flow in bone is regulated, from the level of the skeletal system down to the level of osteocyte, and to provide impetus for future research in this area of signal transduction and coupling. An understanding of this important aspect of bone physiology has profound implications for restoration of function through innovative treatment modalities on Earth and in space, as well as for engineering of biomimetic replacement tissue.

Temporal zeta potential variations of 45S5 bioactive glass immersed in an electrolyte solution

45S5 bioactive glass (BG) is a bioactive material known to bond to bone in vivo through a surface calcium phosphate (Ca-P) layer. The goal of this study was to address the importance of BG surface charge in the bioactive response by examining the relationship between charge variations and the formation of the surface Ca-P layer. The zeta potential of BG in an electrolyte solution (TE) was measured by particle electrophoresis, and the formation of a Ca-P layer was characterized using SEM, EDXA, and FTIR. Si, Ca, and P solution concentrations also were determined. The initial BG surface was negatively charged, and two sign reversals were detected during 3 days of immersion. The first, from negative to positive after 1 day, is attributed to the adsorption of cations at the BG surface, and the second reversal was due to the precipitation of phosphate ions from solution. A strong correlation was found between the formation of a Ca-P layer and BG surface zeta potential variations. The dynamic shift in zeta potential from an initially negative surface to a positively charged surface directly corresponded with the formation of an amorphous Ca-P layer. In addition, when the glass surface matured into a crystalline Ca-P layer, it was associated with a reversal from a positive to a negative surface. Future work will focus on the effects of protein adsorption on BG surface charge and Ca-P layer formation kinetics as well as on cellular response to a changing BG surface.

An ex vivo model to study transport processes and fluid flow in loaded bone

Load-induced fluid flow has been postulated to provide a mechanism for the transmission of mechanical signals (e.g. via shear stresses, enhancement of molecular transport, and/or electrical effects) and the subsequent elicitation of a functional adaptation response (e.g. modeling, remodeling, homeostasis) in bone. Although indirect evidence for such fluid flow phenomena can be found in the literature pertaining to strain generated potentials, actual measurement of fluid displacements in cortical bone is inherently difficult. This problem motivated us to develop and introduce an ex vivo perfusion model for the study of transport processes and fluid flow within bone under controlled mechanical loading conditions. To this end, a closed-loop system of perfusion was established in the explanted forelimb of the adult Swiss alpine sheep. Immediately prior to mechanical loading, a bolus of tracer was introduced intraarterially into the system. Thereafter, the forelimb of the left or right side (randomized) was loaded cyclically, via Schanz screws inserted through the metaphyses, producing a peak compressive strain of 0.2% at the middiaphysis of the anterior metacarpal cortex. In paired experiments with perfusion times totalling 2, 4, 8 and 16 min, the concentration of tracer measured at the middiaphysis of the cortex in cross section was significantly higher in the loaded bone than in the unloaded contralateral control. Fluorometric measurements of procion red concentration in the anterior aspect alone showed an enhancement in transport at early stages of loading (8 cycles, 2 min) but no effect in transport after higher number of cycles or increased perfusion times, respectively. This reflects both the small size of the molecular tracer, which would be expected to be transported rapidly by way of diffusive mechanisms alone, as well as the loading mode to which the anterior aspect was exposed. Thus, using our new model it could be shown that load-induced fluid flow represents a powerful mechanism to enhance molecular transport within the lacunocanalicular system of compact bone tissue. Based on these as well as previous studies, it appears that the degree of this effect is dependent on tracer size as well as the mechanical loading mode to which a given area of tissue is exposed.

Experimental elucidation of mechanical load-induced fluid flow and its potential role in bone metabolism and functional adaptation

Several researchers have developed theories implicating some manifestation of mechanical forces such as stress, strain, and strain energy density for the initiation of cellular processes associated with functional adaptation. The mechanisms underlying dynamic bone growth and repair in response to mechanical stimuli, however, are not fully understood. Load-induced fluid flow has been postulated to provide a mechanism for the transmission of mechanical signals (eg, via shear stresses, enhancement of molecular transport, or electrical effects) and the subsequent elicitation of a functional adaptation response in bone. Although indirect evidence for such fluid flow phenomena can be found in the literature pertaining to strain-generated potentials, experimental studies are inherently difficult. This motivated the authors to develop theoretical as well as ex vivo, in vitro, and in vivo experimental methods for the study of transport processes and fluid flow within bone under well-controlled mechanical loading conditions. By introducing tracer substances such as disulphine blue, procion red, and microperoxidase into the experimental system, transport and fluid flow could be visualized at tissue, cellular, and subcellular levels, respectively. Based on these studies, it could be shown that load-induced fluid flow represents a powerful mechanism to enhance molecular transport within compact bone tissue. Furthermore, the distribution of transport-elucidating tracers is a function of mechanical loading parameters as well as the location within the cross-section of the bone cortex.

Converse piezoelectric effect detected in fresh cow femur bone

A piezoelectric effect has been reported to exist in biological tissues, in particular in dry bone. Since the precision and resolution now obtainable are much greater, we decided to verify the presence of the converse effect (dimensional change under the application of an electric field) in fresh bone samples, by using a very high sensitivity instrument. We took, in varying orientations, five fresh femur cylindrical bone specimens from a cow leg and placed them as a single piece, or as a stack of 10 thin interlayered slices from one specimen to improve sensitivity, in a special microwave double cavity differential dilatometer. The thickness of the specimen was approximately 10mm. The applied field strength for the nonstacked specimen was near 10 kV m-1. Thickness variation was measured along and across the electric field lines. We applied the electric field as a switched polarity square wave. This allows the thermal dilution of specimen warming and possible electrostriction effects, which are insensitive to the direction of the applied field, to be separated from an electromechanical effect which is sensitive to direction. Using coherent signal averaging over approximately 600 cycles to combat instrumental noise we observed nonthermal, nonelectrostrictional thickness variations in all samples. The amplitudes we observed were near 3 pm for the 1 cm nonstacked specimen, and the bone's responses to electric fields ranged from 26 to 38 fm V-1. With response magnitudes approximating those predicted theoretically for the converse piezoelectric effect in bone we conclude that the piezoelectric theory could not be falsified with our experiments.

Demineralized bone matrix as a template for mineral--organic composites

Mineralizing biological tissues are complex bioceramic-biopolymer composites engineered for a variety of functions. The organic and inorganic constituents, morphology, location, orientation, crystallinity and interactions exhibit materials or extremely fine microstructure, unique mechanical and physical properties with high strength and fracture toughness compared to the individual constituents. An understanding of mineralization, ultrastructural organization and interfacial bonding forces in mineralizing biological composite tissues, such as bone, may provide new strategies and techniques for the production of a novel class of man-made organic-ceramic composites. The present study explores the use of the organic matrix remaining after removal of the mineral phase by chelation with EDTA or solubilizing in HCl as a template for mineral deposition and the production of mineral-organic composites. Different pH conditions are employed to alter the inorganic phase which is deposited within the organic matrix. Mechanical testing and ultrastructural evaluations are carried out for characterization.

Zeta potential of bone from particle electrophoresis: solution composition and kinetic effects

The morphology of bone may be influenced by many factors, including electromechanical ones such as electric potentials, electric fields, or zeta potentials. Stress-generated potential studies in bone and particle electrophoresis studies using calcium-deficient hydroxyapatite have shown that the zeta potential depends on the composition of the steeping fluid and steeping time. To better quantify and understand these in situ potential changes in bovine cortical bone, the effects of alterations in calcium, phosphate, and fluoride concentrations in Neuman's Fluid (NF), which simulates in vivo bone extracellular fluid, were investigated using particle electrophoresis. The zeta potential increased in magnitude with increased calcium concentration in NF in as little as 17 min. Increasing phosphate concentration in NF also increased the zeta potential magnitude. These results provide support for a structural model of the bone matrix surface-bone fluid interface, which incorporates the bone surface proper (composed of collagen, mineral, and boundary regions), stationary layer (in which ions, ionic complexes, and proteins may be adsorbed), and bone extracellular fluid. These results, coupled with those of previous studies, indicate that the protein phase probably has an important role in the determination of the physiologic zeta potential; the role of the mineral phase may also be important.

Stress analysis of bone modeling response to rat molar orthodontics

The purpose of this project was to determine if alveolar bone modeling could be associated with altered mechanical environment. Finite element stress analysis of an orthodontically tipped rat molar periodontium was performed. The distributions of mechanical components within the periodontal ligament and cortical bone were compared to the well-documented bone formation and resorption patterns in the alveolus of the tooth. It was concluded that in orthodontically induced bone modeling activity, locations of osteogenesis uniquely coincided with increased tension within the periodontal ligament, while bone resorption areas could be associated with increases in other components (minimum principal and maximum shear stresses, strain energy density, and von Mises) within the bone itself.

The effect of in vitro fluoride ion treatment on the ultrasonic properties of cortical bone

The mechanical properties of composites are influenced, in part, by the volume fraction, orientation, constituent mechanical properties, and interfacial bonding. Cortical bone tissue represents a short-fibered biological composite where the hydroxyapatite phase is embedded in an organic matrix composed of type I collagen and other noncollagenous proteins. Destructive mechanical testing has revealed that fluoride ion treatment significantly lowers the Z-axis tensile and compressive properties of cortical bone through a constituent interfacial debonding mechanism. The present ultrasonic data indicates that fluoride ion treatment significantly alters the longitudinal velocity in the Z-axis as well as the circumferential and radial axes of cortical bone. This suggests that the distribution of constituents and interfacial bonding amongst them may contribute to the anisotropic nature of bone tissue.

Streaming potential measurements at low ionic concentrations reflect bone microstructure

Streaming potentials (SPs) have been proposed as one transduction pathway for mechanically driven bone remodeling. The fluid spaces in which SPs are generated will determine, in part, the structural information that they can provide to bone cells. Streaming potential measurements across cortical bone strips soaked in a range of saline concentrations were used to estimate the mean radii of fluid spaces that contribute to generation of electrokinetic fields. Using a cylindrical pore model, a pore radius of less than 200 A fit SP magnitude as a function of concentration. This pore size was shown to be consistent with estimates obtained from data reported earlier for SP as a function of concentration using a non-specific model, but was smaller than previously reported estimates for pore radius. A pore size in this range indicates that flow either in bone microporosity, or canaliculi that are substantially occluded by cellular material, must generate streaming potentials. Further, the fact that such small pores generate SPs in bone indicates that SPs could provide information regarding local matrix structure to bone cells.

Mechanism of initial attachment of cells derived from human bone to commonly used prosthetic materials during cell culture

The suitability of polymeric biomaterials as surfaces for the attachment and growth of cells has often been investigated in cell culture. In this study the contribution that serum fibronectin (Fn) or vitronectin (Vn) make to the attachment and spreading of cells cultured from explanted human bone (bone-derived cells) during the first 90 min of culture was determined for metallic and ceramic surfaces. The requirement for Fn or Vn for attachment and spreading of bone-derived cells onto stainless steel 316 (SS), titanium (Ti) and alumina (Al2O3) and to polyethyleneterephthalate (PET) was directly tested by selective removal of Fn or Vn from the serum prior to addition to the culture medium. Attachment and spreading of bone-derived cells onto SS, Ti and Al2O3 surfaces were reduced by 73-83% when the cells were seeded in medium containing serum from which the Vn had been removed. Cell attachment and spreading on these surfaces when seeded in medium containing Fn-depleted serum (which contained Vn) were not reduced to the same extent as in the medium containing Vn-depleted serum. The bone-derived cells failed to attach to the surfaces to the same extent when seeded in medium containing serum depleted of both Vn and Fn. Our results show that for human bone-derived cells, the attachment and spreading of cells onto SS, Ti and Al2O3 as well as PET during the first 90 min of a cell culture attachment assay are a function of adsorption of serum Vn onto the surface.

Compressive properties of cortical bone: mineral-organic interfacial bonding

Bone tissue is an anisotropic non-homogeneous composite material composed of inorganic, bone mineral fibres (hydroxyapatite) embedded in an organic matrix (type I collagen and non-collagenous proteins). Factors contributing to the overall mechanical behaviour include constituent volume fraction, mechanical properties, orientation and interfacial bonding interactions. Interfacial bonding between the mineral and organic constituents is based, in part, on electrostatic interactions between negatively charged organic domains and the positively charged mineral surface. Phosphate and fluoride ions have been demonstrated to alter mineral-organic interactions, thereby influencing the mechanical properties of bone in tension. The present study explores the effects of phosphate and fluoride ions on the compressive properties of cortical bone.

Mechanical simulation of the human mandible with and without an endosseous implant

Clinical research has demonstrated that a high remodelling rate for cortical bone exists around a rigid endosseous implant. This phenomenon may be regulated by change in the mechanical environment. 3D finite element models of the human mandible with and without an endosseous implant have been created to investigate the mechanical environment adjacent to the left retromolar area where the ipsilateral implant was located. A bite force of 100 N was applied in the left premolar region. The mechanical environment before and after implantation were computed. The environment was characterized by the following parameters: the principal stresses, dilatational stress, and von Mises stress. The changes in these parameters due to the implantation were calculated. The results showed that the mechanical environment adjacent to the implant changed drastically due to the implant. The major changes in the mechanical parameters occurred adjacent to the bone-implant interface at the bony surface. The changes of the distribution of the mechanical parameters due to implantation were different. Implantation effects were local, and did not alter the overall mechanical environment.

Streaming potentials in healing, remodeling, and intact cortical bone

Electrical fields have been implicated in accelerated bone healing and as a transduction mechanism for mechanically driven bone remodeling. Applied mechanical or electrical stimulation of bone remodeling suggests that this depends on the magnitude, frequency, and duration of the stimulus. The magnitude of endogenous electrical fields, manifest by streaming potentials (SPs) across canine cortical bone, were measured as a function of bending frequency in vivo and then in vitro at healing drill holes and at remodeling (ipsilateral) and normal, intact (contralateral) control sites in canine tibia. SP magnitudes normalized to periosteal strain were smaller for drill holes at 2 and 4 weeks postsurgery relative to either remodeling (P < 0.05 at 10 Hz) or normal intact (P < 0.001 at 10 Hz) controls both in vivo and in vitro. SPs of 12 week drill holes were similar to SPs of remodeling controls and tended to be smaller than SPs of normal intact controls. Mean SP normalized to bone impedance was approximately the same for all sites, suggesting that the smaller SPs during healing and remodeling relate to smaller bone impedance and/or larger porosity. SP as a function of bending frequency for normal sites was similar to that observed previously. SP versus frequency for drill holes and remodeling controls was more variable, probably because of variations in bone microstructure, and displayed a higher frequency content. The observed differences in SP magnitude and frequency response to loading associated with stages of healing indicate that endogenous electrical fields do indeed respond to the structural changes in healing and remodeling and are therefore capable of providing structural feedback information for the repair and remodeling process.

Stress-generated potentials in bone: effects of bone fluid composition and kinetics

It is now generally accepted that stress-generated potentials (SGPs) at low frequencies are due to an electrokinetic phenomenon in the small interior surfaces of bone and are directly proportional to the zeta potential, a property of the poorly characterized bone surface-bone fluid interface. We hypothesized that this interface would be labile and might explain the controversy over whether or not the polarity of SGPs can invert under certain conditions. In this paper, the effects of alterations in the steeping fluid on SGPs for 87 samples from 15 animals were examined in four-point bending for steeping times of < or = 65 h. Calcium and fluoride in distilled-deionized water and constant ionic strength solutions produced concentration-dependent inversions in the SGP sign. A new steady state was reached in approximately 18 h. The effects of the fluoride anion (unlike the calcium cation) apparently were reversible. The results strongly suggest that the zeta potential at the labile bone surface-bone fluid interface can undergo dramatic changes, not only in magnitude but also in sign. The results further suggest that the preparation of bone samples is critical to the understanding of this interface in vivo, and they support the hypothesis that SGPs have a role in bone remodeling.

Ion concentration effects on bone streaming potentials and zeta potentials

Electrical potentials are dependent on the properties of the solid and fluid phases of bone. The solid phase in bone is composed of an organic matrix and inorganic bone mineral fibre, while the fluid phase is separated into compartments associated with the vascular channel system and mineralized matrix. Recently, a piezoelectric and electrokinetic response following mechanical deformation was demonstrated in fully hydrated bone. However, alterations in the fluid phase and the effects on streaming potentials where flow through the sample due to pressure on the fluid phase without prior solid matrix mechanical deformation have not been examined. Streaming potentials in high ionic strength solutions reveal a flow-dependent streaming potential in the absence of mechanical deformation not previously observed in stress-generated potentials. Streaming potentials in high ionic strength sodium chloride solutions (0.75 M) of control and deproteinized samples suggest that organic molecules and ions in the electrical double layer may be susceptible to flow-induced alterations which can modify the streaming potentials generated. Alterations in properties of the fluid phase can modify the streaming and zeta potentials and may play a role in the biofeedback response to bone tissue.

A comparative analysis of streaming potentials in vivo and in vitro

Streaming potentials (SPs) measured in vivo at a specific site on intact cortical bone (canine tibia) have been compared with measurements from the same site in vitro, tested as an excised bone strip soaked in Hank's balanced salt solution. The amplitude of SPs per periosteal strain in vitro was larger in 13 tibias than in vivo (by an average x6.5 at 1 Hz), but values per transcortical strain difference were similar. In vitro, SP magnitudes rose more sharply to an asymptotic value as a function of bending frequency than did in vivo signals, possibly because of a difference in the internal state of canaliculi and/or Haversian systems. Similarly, SP response to step-loading decreased to zero more slowly with time in vitro than in vivo. Difficulties encountered in preliminary measurements due to electrical shunting through electrolyte and soft tissues suggest the need for caution in using both in vivo and in vitro SP measurements to extrapolate to electric field strengths on the cellular level.

Electrokinetic behavior of intact wet bone: compartmental model

Streaming potential experiments were performed on chemically-treated intact wet bone plugs equilibrated in potential-determining ion buffers. Comparison of calculated zeta (zeta) potentials from intact wet bone streaming potentials and bone particle electrophoresis indicates different values. Intact streaming potential experiments, where fluid is forced through the samples, represents flow, primarily through the vascular channel system, and contribution of the organically-lined channels to the electrokinetic zeta potential. Bone particle electrophoresis represents mainly the electrokinetic contribution of exposed mineralized matrix. The organic linings present in the vascular channel system limit potential-determining ions' access to the mineralized matrix. These linings may have an important role in mineral homeostasis and control of ion fluxes between bone compartments.