Читать книгу Biological Mechanisms of Tooth Movement - Группа авторов - Страница 33

Baumrind (1969) explored the assumption that orthodontic forces bend the alveolar bone. He measured changes in PDL cell dimensions, metabolic activity, and fiber synthesis with the help of radioisotopes (tritiated thymine, uridine, and proline). While discussing the findings of his research, he highlighted a conceptual flaw in Schwarz’s pressure–tension hypothesis. He described the PDL as a continuous hydrostatic system, which, in accordance with Pascal’s law, dictates that force applied to this system is distributed equally to all regions of the PDL. He emphasized that the presence of fibers in the PDL does not modify the operation of this law, because of the concomitant existence of a continuous body of liquefied ground substance. He recognized that only part of the PDL, where differential pressures exist, as mentioned in the pressure–tension hypothesis, can be developed, is actually solid, i.e., bone, tooth, and discrete solid fractures of the PDL.

Kingsley (1881) and Farrar (1888) were the first to be credited for proposing the concept of bone bending as being an integral part of OTM. Farrar wrote in favor of this hypothesis: “Teeth move by one of two kinds of tissue changes in the alveolus … by the reduction of the alveolus through what is called absorption on one side of the tooth, followed by the growth of new supporting tissue on the other and by bending of the alveolar bone.” Kingsley and Farrar increased the force levels to such an extent that visible bending of alveolar bone could be observed, but several authors who followed this approach complained that their patients had experienced alveolar fractures. Because of this problem, and the influence of Oppenheim and his lectures in the Angle School, the pressure–tension hypothesis was uplifted, while the bone‐bending hypothesis was abandoned. Baumrind criticized 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.)