Trystan Casey
Bone bioelectricity is a fascinating and important area of study, with electrical stimulation being used to improve fracture repair, mitigate bone loss, and stimulate bone growth. The electrical properties of bone are also being investigated as a potential diagnostic tool to detect pathological changes in bone composition. The piezoelectric nature of bone has been established, with collagen identified as the generating source in dry bone. Compact bone exhibits a permanent electric polarization, and the orientation of this polarization has been mapped and correlated with developmental events. The electrical conductivity of bone is also being studied, with bone impedance proposed as a measure of osteoporosis. Electrical signal transmission in bone cell networks is influenced by gap junctions, with research suggesting that gap junctional intercellular communication contributes to the regulation of bone cell differentiation. Human bone marrow stromal cells are an important resource in regenerative medicine due to their self-renewal and differentiation capabilities, and exposure to electromagnetic fields can promote their differentiation.
| Characteristics | Values |
|---|---|
| Bone conductivity | 100 kHz |
| Dielectric permittivity | Increases with increasing humidity and decreasing frequency |
| Piezoelectric sensitivity coefficients | Up to 0.7 pC/N |
| Role of gap junctions | Influences intracellular transmission of electrical signals |
| Electrical stimulation | Promotes osteogenesis and fracture repair |
| Electrical measurements | Can be used to detect pathological changes in bone composition, e.g., osteoporosis |
| Bone bioelectricity | Can be used to stimulate bone growth and prevent bone loss |
| Bone marrow stromal cells (hBMSCs) | Can differentiate into skeletal stem cells (hSSCs) with exposure to electromagnetic fields |
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What You'll Learn
Gap junctional intercellular communication (GJIC)
Research has shown that GJIC plays a role in neural function and the development and differentiation of the central nervous system (CNS). For example, in rat neuronal stem cell differentiation, expressions of Cx43 and 32 increased for 72 hours, but the effects decreased after seven days. GJIC has also been found to contribute to the preventive mechanisms of the Chaga mushroom against the inhibition of gap junctional intercellular communication by hydrogen peroxide.
Furthermore, GJIC is involved in the regulation of cell growth and apoptosis. While its role in cell growth regulation has been well studied, its involvement in apoptosis is still unclear. However, some studies suggest that GJIC spreads cell-killing signals initially generated by a single cell that spontaneously initiates apoptosis, leading to increased cell death.
Additionally, GJIC is influenced by the presence of discrete gap junctions in the intracellular transmission of electrical signals in an electrically coupled system of osteocytes and osteoblasts in an osteon. A refined electrical cable model has been formulated to investigate the role of these gap junctions in signal transmission. This model also examines the influence of the ratio q between the membrane's electrical time constant and the characteristic time of pore fluid pressure.
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The role of collagen
Collagen is a key component of bone tissue, and its unique properties play a significant role in the electrical behaviour of bones. The electrical properties of bone have been a subject of research for several decades, and collagen has been identified as the generating source of electrical signals in dry bone.
The dielectric properties of bone arise from the separation of hydrogen bonds between collagen and hydroxyapatite (HA), a mineral component of bone, when an external electrical field is applied. This separation of hydrogen bonds allows for electrical storage in bone. The piezoelectric properties of bone, on the other hand, are a result of the polarization of –CO– and –NH– groups when collagen fibres slide over each other due to mechanical force.
Furthermore, the pyroelectric properties of bone are attributed to the abundance of collagen fibres. When the temperature changes, the triple helical structure of collagen distorts, leading to the polarization of its charged amino acid residues and the subsequent generation of a temporary voltage and electric field. This phenomenon is also observed in dry bone samples, with a pyroelectric coefficient of approximately 0.0036 ± 0.0021 μC/m2K within a temperature range of 25–60°C.
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Electrical stimulation for bone fracture healing
The relationship between physical forces and bone biology has been recognised since the early 1800s. Mechanical forces (compression, distraction, and shear), electrical forces, magnetic forces, and ultrasonic waves have all been found to influence bone growth and healing to some degree.
Electrical stimulation of bone has been proposed as a non-invasive method for enhancing bone healing and treating fracture nonunion. Electrical stimulation techniques have been applied to acute fractures, delayed unions, nonunions, and joint arthrodesis. Electrical stimulation should be seen as an adjunct to, not a replacement for, standard fracture care.
Three techniques for the application of electrical stimulation in fracture healing have been described: direct electrical current, capacitive coupling, and inductive coupling. Direct electrical current techniques are invasive and involve the implantation of one or multiple cathodes into the bone. An anode is usually placed on the skin over the fracture site and a 5 to 100μA current is delivered.
Capacitive coupling involves the non-invasive placement of two cutaneous electrodes on opposite sides of the bone to be stimulated. A power source, typically attached to the patient's cast, is then connected to the electrodes, forming an electrical field within the fracture site. Inductive coupling is formed by placing one or two current-carrying coils on the skin over the fracture site. As the current passes through the coils, an electromagnetic field is produced at right angles to the coil base but within the fracture site. Inductive coupling and capacitive coupling are beneficial treatment options as they are non-invasive, painless, and surgery-free.
Bone growth stimulators are worn to help people with fractures or surgery heal bone. Bone stimulators are not used for routine bone healing but only in situations where there are particular circumstances that make healing less likely. The stimulator emits a pulsed electromagnetic or ultrasonic impulse to the area where bone healing should occur. The goal of a bone stimulator is to activate a series of receptors in the body to encourage a healing response. Essentially, the bone stimulator activates a pathway that releases chemicals within the body. These chemicals are signals inside the body to progress fracture healing. This type of process in the body is called a "cascade" and occurs when one signal stimulates another process to occur, and so on until the healing is complete.
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Electrical measurements as a diagnostic tool
Electrical measurements have proven to be a valuable diagnostic tool in medicine, particularly in the assessment of bone health and the investigation of bone cell networks. One of the most well-known applications of electrical measurements in diagnostics is the use of bone density scans, commonly known as DXA or DEXA scans.
DXA scans, or dual-energy X-ray absorptiometry, are a quick, painless, and non-invasive way to evaluate bone health. These scans use low levels of X-rays to measure bone density and mineral content. The procedure is simple and does not require any special preparation or injections. During the scan, the patient lies on a special X-ray table, and a technician positions their body, sometimes using foam blocks to hold their legs in the proper position. The technician then passes a scanning arm over the body, taking pictures of the bones, typically focusing on the hips, spine, and sometimes other areas like the forearms.
DXA scans are commonly used to screen for osteoporosis, osteopenia, and other conditions that can weaken bones. They help providers evaluate bone changes over time, including age-related bone loss, the impact of treatments or medications that may weaken bones, and the response to treatments for osteoporosis. The scans are particularly recommended for older individuals, those with a family history of osteoporosis, and people who have previously broken a bone.
In addition to bone density scans, electrical measurements have also been explored in the context of bone cell networks and bone regeneration. Research has focused on understanding the role of gap junctions in the transmission of electrical signals within bone cells, specifically osteocytes and osteoblasts. This includes investigating the influence of factors such as the membrane's electrical time constant and the geometry of the osteon. Additionally, the electromagnetic field (EMF) has been studied for its potential to promote the differentiation of bone marrow-derived mesenchymal stem cells (BMSCs) and skeletal stem cells (SSCs), which are important for regenerative medicine and tissue engineering.
Overall, electrical measurements have provided valuable insights into bone health and disease, contributing to the development of diagnostic tools like bone density scans and advancing our understanding of bone cell behaviour, which may lead to future therapeutic interventions.
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The influence of frequency, direction of load, and relative humidity
The electrical and dielectric properties of bone are influenced by several factors, including frequency, direction of load, and relative humidity. These properties are essential in understanding how bones interact with electrical signals and their potential for use in regenerative medicine and tissue engineering.
Frequency plays a crucial role in bone's electrical behaviour. The dielectric permittivity of bone, which determines its capacitance, increases with decreasing frequency and increasing humidity. At lower frequencies, the behaviour of bone becomes more complex, and classical piezoelectric theories may not fully explain the observed electrical phenomena. In the context of bone formation, studies have utilised vibration frequencies ranging from 30 to 90 Hz, with 30 Hz showing positive results in post-menopausal women.
The direction of load also influences bone's electrical properties. Measurements of electrical properties, such as resistivity and specific capacitance, vary depending on the axial, circumferential, or radial direction examined. These directional differences highlight the anisotropic nature of bone's electrical and dielectric properties.
Relative humidity significantly affects bone's electrical behaviour. The resistivity of fully hydrated bone is approximately 100 times greater than that of bone at 98% relative humidity, indicating that at lower humidity levels, larger pores may not be fully filled with fluid. Additionally, the dielectric permittivity of bone increases with increasing humidity, further emphasising the impact of moisture content on bone's electrical properties.
The understanding of bone's electrical properties has implications for bone remodelling, healing, and repair. External electrical stimulation has been explored to aid in bone regeneration, similar to the observed effects of electrical signals in limb regeneration in rats. The piezoelectric nature of bone and the role of collagen contribute to these electrical phenomena.
In summary, the frequency, direction of load, and relative humidity collectively influence the electrical and dielectric properties of bone. These factors are essential considerations in studying bone's interaction with electrical signals and its potential applications in medicine and tissue engineering.
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Frequently asked questions
The piezoelectric theory suggests that bone has piezoelectric properties, meaning it can generate an electric charge in response to applied mechanical stress. This theory has been criticised as ad hoc, but it is supported by some experimental results that don't align with classical piezoelectric theory. The piezoelectric sensitivity coefficients of bone depend on frequency, direction of load, and relative humidity.
Electrical stimulation has been shown to promote osteogenesis and bone growth in cases of nonunion or after spinal fusion. It has also been used to reverse paralysis from spinal cord injury in rats. Electrical measurements may also be used as a diagnostic tool to detect pathological changes in bone composition, such as loss of trabecular structure associated with osteoporosis.
Osteocytes are the most abundant cell type in bone tissue and play a key role in bone electrical stimulation for fracture healing. They can sense mechanical force in the extracellular network via an electrical signal, but their behaviour in electrical environments is not yet fully understood.
