that these effects originate in tropocollagen molecules, or in molecules no larger than 50 Å in diameter. However, the hypothesis claiming that piezoelectricity is a major determinant of bone remodeling is weakened by the following:
The generated electric potential is dependent on a strain gradient, and this was not taken into account when the hypothesis was proposed. Bone always experiences nonhomogenous deformation because of its centro‐symmetric nature, and because it can produce electrical polarization proportional to the strain gradient.
The modulus of elasticity (E) of cortical bone under physiologic conditions is frequency dependent. Hence, bone cannot be considered as an elastic‐plastic material.
End‐for‐end rotation of the sample in cantilever bending mode does not change the sign of generated potential as would be expected from classical piezoelectric material.
Proffit (2013) outlined two unusual properties of piezoelectricity, which do not seem to correlate well with OTM:
A quick decay rate, where the electron transfer from one area to another following force application reverts back when the force is removed, which does not or should not happen once orthodontic treatment is over.
Production of an equivalent signal in the opposite direction upon force removal.
Anderson and Erikkson (1968) challenged the piezoelectric hypothesis and reported that although dry collagen is strongly piezoelectric, full hydrated collagen is not, because of the structured water it contains. They argued that bone is a tissue with high symmetry, as the hydroxyapatite it contains is centrosymmetric in nature, and not piezoelectric. Follow‐up experiments conducted by these investigators (1970), using a similar apparatus to the one used by Fukada and Yasuda in 1957, proved that the piezoelectric coefficients varied with the state of hydration of bone, and that the variation decreased as the specimens were dried by evaporation. The loss of piezoelectricity in fully hydrated tendon collagen was explained as attainment of more symmetrical, nonpiezoelectric structure by absorption of water. Instead, the electrical signals generated when stress is applied to fully wet collagen are actually streaming potentials.
The electrokinetic phenomenon known as streaming potential or streaming current was described by many authors, like Glasstone, Overbeek, and Kortum (Gross and Williams, 1982). Anderson and Eriksson (1968) reintroduced the concept against the theoretical faults of the piezoelectric effect, and reported it to be present in bone as it is a porous tissue containing a fluid phase and calcified matrix (which is composed of inorganic (mainly hydroxyapatite) and organic (collagen) contents). Further, the porosity of bone contains membrane‐lined capillary vessels helping in the transportation of nutrition to the inside of bone. There exists a compartmental model for the bone fluid system whereby extracellular fluid in the calcified matrix is separated from membrane‐lined vascular channels. When the bone is mechanically stressed, as part of different loading conditions, or therapeutically induced as in orthodontic treatment, remodeling changes are initiated by the bioelectric potentials generated from inside of the bone. The presence of bioelectric potentials in bone matrix, which also contains extracellular ionic fluid, will generate a charge separation at the matrix–fluid interface. There will be attraction of opposite‐charged molecules at the matrix–fluid interface and repelling of similar charged molecules away from this interface. When the fluid is forced through a bone plug, the ionic charges in the fluid phase near the fluid–matrix interface are carried towards the low pressure end, which is otherwise known as Poiseuille flow. This flow constitutes the streaming current and accumulation of charges set up as an electrical field. This field will result in generation of conduction currents which flow in opposite directions through the bulk of liquid in the porous structure of bone. In steady state the conduction current is equal to the streaming current. The resulting electrostatic potential difference between these two sides of bone plug is known as streaming potential (Figure 2.20) (Walsh and Guzelsu, 1993). Briefly, streaming potential is the electric potential developed between two components by an electrolyte flowing between the solid surfaces. The movement of the electrostatic double layer at the fluid bone interface, created through the net surface charge gained by the bone surface in contact with an ionic fluid, generates this effect (Anderson and Eriksson, 1968). In bone, streaming potentials can be observed in vascular channels, Haversian systems, canaliculi, and microporosities of the structure due to blood flow and interstitial fluid movement. Neutrality is restored following equilibration of ion distribution.
Zeta potential, the common link among different electrokinetic potentials and the one used to allow comparison of different measuring techniques, is defined as average potential difference between the bulk and surface of shear. Surface of shear is an imaginary surface present in the area adjacent to electrically charged bone matrix, where the ions and fluid molecules remain stationary. The role of zeta potential is to separate the movement of ions bound to the solid surface from other ions that show normal viscous behavior under mechanical force application (Lech and Iwaniec, 2010). Zeta potential can be calculated from streaming potential experiments by knowing the applied pressure difference across the sample and generated streaming potentials (Hunter, 1981). Fluid conductivity and fluid viscosity determines the stress‐generated potentials in fluid‐filled bone, and it is possible to calculate the potential generated by the distortion of a fluid by the formula (McDonald, 1993):
Figure 2.20 Results of a typical intact bone‐streaming potential (mV) in pH 7.3, 0.145 M ionic strength buffer (physiologic conditions) versus time at various pressures (kPa). The arrows indicate when an increase in pressure (nitrogen gas) was placed on the sample. Streaming potential magnitude increased with an increase in pressure and a stable streaming potential was obtained. A positive streaming potential versus pressure response corresponds to a negative zeta potential and an exposed organic interface. Streaming potentials were consistently positive throughout all pressure levels in 0.145–0.6 M NaCl.
(Source: Walsh and Guzelsu, 1993. Reproduced with permission of Elsevier.)
where z is the zeta potential; V is the magnitude of the potential; δP is the pressure difference that forces the liquid through the channel; ε is the dielectric constant of the liquid; n is the viscosity of the liquid; σ is the specific conductance.
Scientists working in this field have concluded that piezoelectricity and streaming potentials are two coexisting phenomena and both play an important role in stress information transmission. They combined both phenomena as a bioelectric hypothesis, which states that structural alterations throughout the bone are conducted and/or triggered by ionic charge differences (Masella and Chung, 2008). Stress‐generated electric potentials were first reported in dog mandibles and teeth by Cochran, Pawluk and Bassett (1968). They postulated that these stress‐induced bioelectric potentials play a major role in regulating the orientation and functional demands of bone per se. Zengo et al. (1973) measured the electric potential in mechanically stressed dog alveolar bone during in vivo and in vitro experiments. They have demonstrated that the concave side of orthodontically treated bone is electronegative and favors osteoblastic activity, whereas
