The role of biology in the orthodontic practice (Page 6)

The sections discussed earlier in this article revealed that tissue remodelling which facilitates orthodontic tooth movement is performed by various cell types. Some of these cells are local, such as fibroblasts and bone surface lining cells. Other cells are migratory, like macrophages and lymphocytes, but evidently play a crucial role in modulating the effect of mechanical forces on paradental cells. Thus, an optimal orthodontic force is capable of evoking an inflammatory response in paradental tissues, leading to remodelling of these tissues and tooth movement in a desirable direction.

In an effort to provide a rationale for the use of magnets in orthodontics, in 1998 Blechman proposed that static magnets generate electromagnetic fields which stimulate bone formation in PDL tension sites, thereby reducing tooth mobility, pain and discomfort. He stated that in routine orthodontic treatment, bone formation lags behind resorption, causing widening of the periodontal space and increased tooth mobility. Re-examination of histologic section of cat jaws after 7 and 14 days of combined orthodontic force/electric stimulation (Davidovitch et al., 1980) supported Blechman’s proposition that exogenous electric signals increase the amount of new bone formation in PDL tension sites. These observations suggest that an optimal orthodontic force is one accompanied by an additional signal, such as an electric current, which accelerates the rate of alveolar bone formation.

Experimenting with avian long bones in vivo, in search of the features of an optimal force for evoking osteogenic reactions, Lanyon and Rubin (1984) observed that the most efficient force was dynamic (intermittent) rather than static (continuous). A short duration of between 5 and 10 minutes a day was adequate time to stimulate a potent periosteal and endosteal osteogenic reactions. The force magnitude was found to be of importance, defined as optimal being in the range of 2,000-4,000 microstrain. However, this magnitude could be much lower, provided the frequency of force application was increased. The target cells in these experiments were postulated to be the osteocytes, and the molecules responsible for bone’s “strain memory” were proposed to be glycosaminoglycans in the mineralized ECM.

An orthodontic equivalent of these experiments was performed in rats be Gibson, King, and Keeling (1992). They reported that only one hour of force application was sufficient to cause maxillary molars to move mesially for 2 weeks. However, to achieve this movement required the extraction of the mandibular molars, to prevent occlusal contacts. Unfortunately, this approach is not a viable option in human orthodontic practice.

Page 1     Introduction

Page 2     Tissue remodelling and orthodontic tooth movement

Page 3     The age factor

Page 4     The effects of pre-existing medical conditions and the development of complications

Page 5     The etiology of tooth resorption

Page 6     The biological nature of an optimal orthodontic force

Page 7     How to move teeth without resorbing their roots

Page 8     Summary