i.e., convex surfaces, showed elevated osteoclastic activity (Figure 2.19). Shapiro, Roeber, and Klempner (1979) suggested that application of piezoelectric charges as pulses of force to the teeth can accelerate the osteogenic response. Based on their experiment in one patient, with application of 8 ounces of force, they reported increase in the rate of movement of maxillary second molar with less pain perception than that of the control teeth.
Figure 2.21 Transverse section, 6 μm thick, of a 1‐year‐old female cat’s mandible, after a 7‐day exposure to sham electrodes (control). Shown is the buccal periosteum of the second premolar opposite the sham cathode, stained immunohistochemically for cAMP. B, Alveolar bone. The bone surface lining cells are flat, and most stain lightly for cAMP.
It has been proposed by Davidovitch et al. (1980a, b) that a physical relationship exists between mechanical and electrical perturbation of bone. Their experiments in female cats with administration of exogenous electrical currents in conjunction with orthodontic forces demonstrated enhanced cellular activities in the PDL and alveolar bone, as well as rapid tooth movement (Figures ). Taken together, these findings led to the suggestion that bioelectric responses (piezoelectricity and streaming potentials) propagated by bone bending incident to orthodontic force application, might act as pivotal cellular first messengers.
Borgens (1984) investigated this phenomenon in bone fracture sites by inducing electric current for healing purposes. His experiments did not disclose any correlation with what had been proposed as piezoelectric effects and showed that the dispersion of current as it enters the lesion is unpredictable. He attributed this finding to the complexity of distribution of mineralized and nonmineralized matrices. However, he observed generation of endogenous ionic currents evoked in intact and damaged mouse bones, and classified these currents as stress‐generated potentials or streaming potentials, rather than piezoelectric currents. In contrast to piezoelectric spikes, the streaming potentials were having long decay periods. This finding led him to hypothesize that the mechanically stressed bone cells themselves, not the matrix, are the source of the electric current. His hypothesis received support from Pollack et al. (1984), who proposed a mechanism by which force‐evoked electric potentials may reach the surface of bone cells. Accordingly, an electric double layer surrounds bone, where electric charges flow in coordination with stress‐related fluid flow. These stress‐generated potentials may affect the charge of cell membranes, and of macromolecules present in the neighborhood. Davidovitch et al. (1980a, b) suggested that piezoelectric potentials result from distortion of fixed structures of the periodontium, such as collagen, hydroxyapatite, or bone cells’ surface. In hydrated tissues, however, streaming potentials (the electrokinetic effects that arise when an electrical double layer overlying a charged surface is displaced) predominate as the interstitial fluid moves. They further reported that mechanical perturbations of the order of about 1 min/day are apparently sufficient to cause an osteogenic response (Figure 2.24), perhaps due to matrix proteoglycan related strain memory.
Figure 2.22 Transverse section, 6 μm thick, of a 1‐year‐old female cat’s mandible (the same animal as shown in Figure 2.21), after exposure for 7 days to a constant application of a 20 μA direct current to the gingival mucosa, noninvasively. Shown are the tissues near the stainless‐steel cathode, stained immunohistochemically for cAMP. B, Alveolar bone. Compared with the cells shown in Figure 2.21, the bone surface lining cells near the cathode are larger and more darkly stained for cAMP.
Figure 2.23 Constant direct current, 20 μA, noninvasively, to the gingival and oral mucosa labial to the left maxillary canine in a cat. The right canine (control) received the same electrodes, but without electrical current. Both canines were moved distally by an 80 g tipping force. The right canine, which had been subjected only to mechanical force, moved distally a smaller distance than the left canine, which had been administered a combination of mechanical force and electrical current.
Figure 2.24 The number of alveolar bone osteoblasts bordering the PDL (±SEM) near cat maxillary canines, intensely stained for cAMP or cGMP following an electric stimulation. Cells were counted along a 0.1 mm surface opposite each electrode. Open circles, Control sites. Solid circles, Electrically treated sites. (a) Osteoblasts near cathode stained for cAMP. (b) Osteoblasts near anode, stained for cAMP. (c) Osteoblasts near cathode, stained for cGMP. (d) Osteoblasts near anode, stained for cGMP. Time periods when differences in number of cells intensely stained for cAMP and cGMP between electrically treated and control sites were not significant: cAMP near the cathode at day 3; cGMP at days 3 and 7 near the cathode. At the other time periods near the cathode and at all time periods near the anode, the differences between the treated and control sites were statistically significant (P < 0.01). It should be noted that all the bone surface cells are labeled as “osteoblasts,” although it is quite possible that some cells near the anode are stimulated by the electric currents to resorb bone rather than to participate in the synthesis and mineralization of new bone matrix. However, the authors did not detect typical osteoclastic lacunae at the alveolar socket wall near the anode and thus defined all bone surface cells as osteoblasts.
(Source: Davidovitch, 1980a. Reproduced with permission of Elsevier.)
The major criticism faced by both the piezoelectric as well as the streaming potentials theories was due to the highly conductive nature of the vascular system, periosteum, and tissue fluids. They often tend to drain away the change in potential difference. Researchers failed to establish whether the generated strain‐induced potentials were actual players in cellular remodelling or irrelevant by‐products of bone deformation. McDonald (1993) argued against the role of electrical signals in bone remodeling by stating that “surface potential
