The role of biology in the orthodontic practice (Page 2)

The actual rate of tooth movement may depend on the rate of bone turnover. The latter was modified pharmacologically in rats undergoing maxillary molar mesial movement, by inducing either hypothyroidism or hyperthyroidism (Verna et al, 2000). In rats with high bone turnover, the rate of tooth movement was increased, while it was reduced in animals with a low turnover. Although all teeth had been moved in the same manner (controlled tipping), the location of the centre of rotation differed, depending on the metabolic state of the bone. Examination of histological sections from the jaws of these rats (Verna et al, 2003) showed that root resorption had occurred in both groups, as well as in the control group, but that it was more pronounced in the low bone turnover group. However, bone metabolism normally demonstrates measurable diurnal fluctuations that may affect the rate of tooth movement. Rats exposed to light for 24 or 12 hours per day for 21 days, and were subjected to orthodontic force only during the light period, presented doubling of the rate of tooth movement and bone remodelling, as compared to animals that received the force during the 12 hours of daily darkness (Miyoshi et al, 2001).

The realization that tissue remodelling in orthodontics in mediated by a variety of cells, including fibroblasts, root and bone surface lining cells, endothelial, epithelial, and nerve cells, as well as different leukocytes, prompted clinical investigators to apply physical and chemical agents, concomitant with orthodontic forces, in order to augment the effect of the mechanical forces. In this vein, Tweedle (1965) used local application of heat to paradental tissues surrounding orthodontically-treated teeth in dogs; Davidovitch et al (1980) used minute electric currents; and Blechman (1998) advocated the use of static magnetic fields. Davidovitch et al placed the electrodes much closer to the cat’s canine, resulting in a significant enhancement of movement. Blechman hypothesized that magnets generate mechanical forces, as well as magnetic fields, and that this combination acts synergistically, causing the teeth to move faster. However, an experiment in rats (Tengku et al, 2000) revealed that magnets do not speed-up the mesial movement of maxillary molars, and actually increase root resorption in the early phases of treatment.

Utilization of chemical agents in attempts to increase the pace of tissue remodelling and tooth movement have been tested in various laboratories and clinics. Yamasaki et al (1984) injected prostaglandin E1 into the gingiva of moving teeth in human subjects, resulting in rapid movement. Systemic application of misoprostol, a PGE1 analog, to rats undergoing tooth movement for 2 wk, increased significantly the pace of movement without enhancing root resorption (Sekhavat et al, 2002). Similar results were reported following intraperitoneal injections of PGE2 in rats (Seifi et al, 2003). Chumbley and Tuncay (1986) administered systemically indomethacin, a prostaglandin synthethase inhibitor; Collins and Sinclair (1988) used local applications of vitamin D, while Engstrom, Granstrom, and Thilander (1988) moved teeth in hypocalcemic, vitamin D-deficient lactating rats. The bone matrix component osteocalcin was injected in rats into the palatal bifurcation of a tipping molar, causing rapid tooth movement due to the attraction of numerous osteoclasts to this site (Hashimoto et al, 2001).

The reports cited above suggest that the extent of tissue remodelling and the rate of tooth movement can be significantly influenced by numerous factors capable of interacting with paradental cells. However, if our goal is to complete orthodontic treatment successfully and at the shortest possible amount of time, then we should avoid moving roots into areas from which they would have to be retrieved later.

When mechanical loads are applied to intact tissues in vivo or in vitro, the tissues usually become distorted (strained). In the case of the skeleton, loads like gravity prompt cells to arrange the architecture of the bony structural features in a way that would resist redundant loads. This phenomenon is known as “Wolff’s Law,” defined by Julius Wolff in 1892. However, when bone cells are subjected to non-redundant loads, such as orthodontic forces, the cells are activated, and remodelling of the alveolar process ensues, which facilitates tooth movement. In vivo applications of compressive loads to ulnae in turkeys and roosters by Lanyon and Rubin (1984) revealed that extensive osteogenesis can be evoked by short term dynamic (intermittent) forces. In those experiments, the optimal load magnitude was 2,000-4,000 microstrain, and its daily duration was 10-20 minutes. These findings suggest that orthodontic forces would be most effective when applied for brief periods, rather than continuously. This assumption was found to be correct in an experiment in rats by Gibson, King, and Keeling (1992). In that experiment, maxillary molars were subjected to mesial-moving forces for 1 hour, 1 day, or 14 days. Teeth exposed to only 1 hour of force application continued to move mesially for 14 days, and achieved 75 per cent of the movement reached by the teeth that had been subjected to orthodontic forces continuously for 14 days.

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