the application of excessive forces for bone bending by describing them as “practical excesses of Kingsley and Farrar rather than theoretical misconceptions they had.” He revived the legacy of bone bending by basing it on Hook’s law (any solid body subjected to a load within its elastic limit will, if maintained in a static position, deform to a degree proportional to the magnitude of the applied force), the physical law of elasticity, which is fundamental to solid‐state mechanics, and stated that “alveolar bone does indeed deflect under mechanical loading and these can be produced by forces lower than those required to produce consequential changes in the PDL width.”
Proposing the bone‐bending hypothesis, Baumrind (1969) stated that “when orthodontic appliances are placed, forces delivered to the tooth are transmitted to all the tissues in the region of force application. In accordance with universally operating physical laws, each of the three types of structure in the area (tooth, PDL, and bone) is deformed. The amount of deformation produced in each material by a given force is a function of the elastic properties of that material. The elastic properties of the tooth itself have not been studied. Of the other two materials, I contend that the bone deforms far more readily than PDL.” When bone is held under mechanical forces, the remodeling and reorganization process is accelerated not only in the lamina dura, but also on the surface of every trabeculum within the corpus of the bone. The force/stress directed to the teeth will be dissipated by the development of stress lines in the deflected bone and becomes a major stimulus for altered biological activity, which in turn brings about adaptive changes. Baumrind claimed that his proposed hypothesis was complying with the basic rules of Wolff ’s law, as outlined by D’Arcy Thompson (1917), that strains are induced in bone by deflective forces within the elastic limit, and the tissue turnover and renewal are active so that bone can reorganize to accommodate the applied stress. In accordance with this theory, he could explain the phenomena behind
Relative slowness of en‐mass movements, and relative rapidity in the alignment of crowded anterior teeth.
The rapidity in which teeth can be moved into an extraction site.
The appearance of an axis of rotation beyond the apex of the incisors. The logic of the pressure–tension hypothesis makes it mandatory to have the axis between the apex and the alveolar crest.
The relative rapidity of tooth movement in children.
Baumrind also challenged the existence of the fluid dynamic hypothesis of Bien by stating that “there is simply no objective evidence for theories which postulate ‘squeezing out’ of tissue fluids from the PDL on the ‘pressure’ side. In any event, the PDL is a continuous system, so that if fluid were to be ‘squeezed out’ in one region it would have to be ‘squeezed out’ in all regions.”
The apposition and resorption of bone in response to its bending by orthodontic forces is evidently an attractive hypothesis, but it seems to contradict the current orthopedic dogma (Melsen, 1999), which states that “any mechanical compression stimulates bone formation and tension stimulates resorption.” Epker and Frost (1965) described the change in shape of the alveolar bone circumference resulting from stretching of the PDL fibers. This fiber stretching decreases the radius of the alveolar wall, i.e., bending of bone in the tension zone, where apposition of bone takes place. They attributed this response to a regional acceleratory phenomenon (RAP). Frost (1983) demonstrated that regional noxious stimuli of sufficient magnitude result in markedly accelerated reorganizing activity of osseous and soft tissues. It is a burst of localized remodeling process, which speeds up the healing potential, especially following the surgical wounding of cortical bone. Accordingly, any regional noxious stimulus of sufficient magnitude can evoke RAP. The extent of the affected region and intensity of the response vary directly with the magnitude and nature of the stimulus.
Experimenting with dog mandibles in vitro and in vivo, Zengo et al. (1973) demonstrated that orthodontic canine tipping bends the alveolar bone, creating on it concave and convex surfaces identical to those generated in bent long bones. In areas of PDL tension, the interfacing bone surface assumes a concave configuration in which the molecules are compressed, whereas in zones of compressed PDL the adjacent alveolar bone surface becomes convex (Figure 2.19). Hence, there is no contradiction between the response of alveolar bone and other parts of the skeleton to mechanical loading. The confusion in this regard has resulted from the usage of the same descriptions for different tissues. While orthodontic “tension” refers to the PDL, an orthopedist may declare the area as being under “compression,” because the bone near the stretched PDL has become concave.
Figure 2.19 Behavior of bone during orthodontic tooth movement. The net force, compression, and tension applied by the “leading” edge of the tooth deforms the alveolar bone convexly toward the root. At the “trailing” edge, the periodontal fibers distort the alveolar bone, producing concavity toward the root. Areas that have been described as characterized by osteoblastic activity were electronegative and, conversely, areas of positivity of electrical neutrality were observed in regions characterized by osteoclasia.
(Source: Zengo et al., 1973. Reproduced with permission of Elsevier.)
Bioelectric signals in orthodontic tooth movement
In 1957, Fukada and Yasuda published the results of their systematic investigation on dry specimens cut from human and bovine long bones, in an article titled “On the piezoelectric effect of bone,” which credited them with the discovery of the existence of piezoelectricity in bone. They demonstrated that dry bone under proper load application generates surface charges, called piezoelectric currents. They established that the piezoelectric effect appears only when shearing force is applied to the collagen fibers in the bone, which are highly oriented, to make them slip past each other. There exist two types of piezoelectric effects – positive and negative. The former is due to strains generated within the crystal lattice of a material, leading to the production of a potential difference across the faces of that crystal and the latter, when an electric charge is passed across a molecule or crystal and leads to an inherent strain within that molecule (Isaacs, 1987). Both effects involve the organic molecules of collagen and the inorganic crystals of hydroxyapatite (McDonald, 1993). Bassett and Becker (1962) extended that research and discovered that the charges emanating from the bone surface at the time of bending are proportional to the internal strains engendered by the bending. They also showed that the polarization sign always depended upon the type of stress – there was a positive sign where there is tension and a negative sign where there is compression. These experiments were further developed by Shamos et al. (1963) and Shamos and Lavine (1964), who reported finding this phenomenon in a number of different bones, in different anatomical sites and species. They suggested that local electric fields resulting from these surface changes influence the deposition of ions and polarizable molecules.
The first observations of the piezoelectric phenomenon in wet and living bone was made by Bassett (1968), and this finding has contributed to the working hypothesis that piezoelectricity leads to a physical explanation of Wolff ’s law. Following this discovery, the universal existence of piezoelectricity in biological tissues was demonstrated by Fukada and Hara (1969) through their experiments on trachea, aorta, intestines, ligaments, and venous vessels. Marino and Becker (1975) reported on the piezoelectric
