Orthodontics tooth movement is achieved by delivering a constant controlled force to the teeth. The duration of the force applied is the key factor for successful tooth movement rather than the force magnitude. The aim of the orthodontics treatment is to achieve good occlusion with minimal side effects. Several factors should be considered during the orthodontic treatment such as force type, force magnitude and the duration of treatment to avoid the undesirable effect.
Proffit have defined the orthodontic tooth movement as a biological response due to the changes in the physiological equilibrium of the dentofacial complex when an external force is applied.
Theories of orthodontic tooth movement
Teeth are positioned in harmony with the oral environment to maintain their position. The applied forces will cause histological changes during teeth movement. These forces will cause bone resorption on the pressure side and deposition on the tension side, most importantly that the capillaries remain patent on the compressed side to allow cell proliferation and avoid the formation of hyalinized zone
On the other hand bone formation will occur at the tension side due to the increased periodontal ligament width and the proliferating fibroblast and osteoprogenitor cells. Osteoblast will be formed from the proliferated osteoprogenitor and will deposite osteoid and result in bone formation.( systematic review)
Several theories have been proposed to explain orthodontics tooth movement. The main theories are :
The biomechanical theory is mainly based on experiments and cellular response observation explaining the biological events during orthodontic tooth movement.
The earliest evidence supporting the role of Prostaglandin in orthodontic tooth movement was provided by Hang (Hang et al 1976). He noticed that mechanical distortion of the cell membrane in a culture dish will increase the synthesis of Prostaglandin. Later Harrel supported these finding in his in vitro. Harrel showed that mechanical distortion will generate Prostaglandin and cyclin adenosine monophosphate (c AMP).HARREL 1977
Other researchers ( rodan et al 1975, davidaritch and shanfield 1975) have found that mechanical distortion will cause changes in the intracellular nucleotides changes.
The biomechanical theory was based on these finding and proposed that mechanical strain of the cell membrane will activate the cell signaling cascade. Initially phospholipase A2 will be activated which will initiate the metabolism of arachidonic acid. The leukotrienes and the Prostaglandins will be synthesized in response to the metabolism of Arachidonic acid. The synthesis of Prostaglandin will increase three time after five minutes. The Prostaglandin then activates the G proteins receptors on the cell membrane which will initiate a second messenger signaling cascade causing a cellular response leading to bone remodeling.
Yamasaki provided further evidence to support this theory by designing a three phase split mouth study to investigate the effect of administrating Prostaglandin on orthodontic tooth movement. One side was injected with Prostaglandin and the contra lateral served as the control.
Phase one involved the movement of the upper first premolar buccaly. The rate of tooth movement was doubled on the injected side when compared to the control side.
Phase two involved retraction of the canine into the upper first premolar space using sectional contraction loops. The findings were similar to phase one.
The third phase involved the retraction of the canine with the routine mechanics. The rate of movement was 1.6 faster on the injected side than the control side.
No adverse effects were recorded in the gingiva or the alveolar bone. Yamasaki et al 1984
This theory proposes that the pressure applied to the tooth will be transferred to the adjacent alveolar bone which will respond by bending and causing small electrical current produced by electrons being transferred from deformed crystal structure to another. The electrical current will activate the osteoclast and osteoblast and result in bone remodeling required for tooth movement (mcdonald 1993)
This theory was supported by Baumrind’s split mouth study on rats. Baumrind showed that the tooth crown will displace ten times more than the reduction of the periodontal ligaments on the pressure side. The difference in the amount of displacement between the crown and periodontal ligament has lead to the assumption that the alveolar bone deflects more readily than the periodontal ligament. Considering the amount of crown deflection and the periodontal ligament changes, it can be concluded that lower forces can be used to produce bone deflection which will create changes in the periodontal ligaments.(baumrind 1969)
Several studies on animals and human investigating an endogenous electric signals, bioelectric potential, showed that the application of low voltage direct current will modify the bioelectric potential and cellular activity causing faster tooth movement when compared to a control group.(giovanelli s â€¦.ref 9 p324)
Davidovich showed that by applying electrical current ( 15 Âµ amps) combined with force of 80 g will enhance bone resoprtion near the anode and bone deposition near the cathode when compared to the control.( Davidovich et al 1980)
Heller and Nanda demonstrated that periodontal ligaments are less likely to undergo tensile strain or transfer the force directly to the alveolar bone.( Heller and Nanda 1979)
Piezoelectric signals characterize by a fast decay rate even if the force is maintained as the crystals will remain stable. If the force was removed the crystal will return to the original shape and an equivalent signal and opposite in direction will be created.
The role of stress generated signals during normal mastication is well documented in the literature in maintaining the alveolar bone. On the other hand, the constant orthodontic forces will create a brief signal which will not create a prominent stress generated signals. These signals have little if anything to do with tooth movment.Profit text book
Pressure tension hypothesis
Classic hypothesis proposed by Oppenheim, Sandstedt and Schwarz based on histological research.
This theory proposes that tooth movement will occur in the periodontal ligaments and the collagen fibers will create a pressure and tension sides transferring the applied forces to the adjacent alveolar bone. The forces should be less than the capillary blood pressure to maintain the blood flow and avoid bone necrosis.
On the pressure side, the periodontal ligament will display disorganization and the cell replication will decrease in response to the vascular constriction. On the tension side the periodontal ligament will be stretched and increase the cell replication.
Baurmrind 1969 report showed statistically significant increase in cell replication during tooth movement and there was a reduction in collagen formation rate on the tension and pressure side.
Heller and Nanda (1979) interfered with the collagen function and metabolism by administrating lathyritic agent beta aminoproprionitrile and showed that normal tooth movement will occur in periodontal ligaments with disrupted collagen fibers. Their findings demonstrated that periodontal ligaments are less likely to undergo tensile strain or transfer the force directly to the alveolar bone.
Orthodontic tooth movement phases
Once the orthodontic force is applies to the tooth the bone remodeling process will begin. During the first six to eight days there will be an initial period of rapid movement due to the periodontal ligaments compression and tooth displacement within the periodontal ligament. The blood supply will be reduced or cut off producing hyalinized zone; a vascular cell free zone. In the second phase; the lag phase; tooth movement will be minimal or will stop completely due to the hyalinized zone. On the histological level Retain ( 1957, 1960) have reported that the a vascular cell free zone will be formed even with minimal force and the a vascular cell free zone will occur more with short roots. The lag in tooth movement varies between four to twenty days according to the applied force; with light forces the lag phase will be relatively short and it will increase with heavier forces.
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The periodontal ligaments will reorganize to remove the hyalinized zone by phagocytosis; foreign body gaint cells, macrophages, fibroblast and pre-osteoclasts will be recruited from the neighboring undamaged alveolar bone marrow cavities and the periodontal ligaments. Once the avascular cell free zone is removed tooth movement will commence again; the last phase. Tooth movement usually begins forty days after the initial force application. Recent study by Von Bohl demonstrated that the hyalinized zone will be formed during the last phase and it is more frequent with high forces and have no effect on orthodontic tooth movement at this stage as the bone remodeling process will continue at a certain rate independently from the force magnitude. Von Bohl concluded that the formation of a vascular cell free zone is apart of the orthodontic tooth movement process. His report supported the pervious finding of Owman moll et al 1996 and Vas leeuwen 1999
Orthodontic force magnitude
Orthodontic forces can delivered through the use of fixed appliances, removable appliances , TAD, extra orally such as head gear â€¦etc.to achieve the desired tooth movement different force magnitude will be required. The recommended forces are:
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