The different types of force characteristics such as heavy versus light forces and continuous versus intermittent force and the type of tooth movement influence the rate and force of orthodontic tooth movement. Heavy forces generally results in pain development, necrosis of cellular components within the PDL and undermining resorption of alveolar bone adjacent to the affected tooth (Proffit, Fields, & Saver, 2013). A study carried out by Alikhani et al. , observed a linear relationship between the magnitude of force and the expression of inflammatory markers when orthodontic force was first applied.
However, the inflammatory response plateaued with high magnitude of orthodontic force. This plateau occurred between the force level of 10 and 25CN. Therefore, the results established that 25cN is an excessive force for tooth movement. Furthermore, the study has explained that in response to a high force magnitude, a saturation point in the biological response is reached and thereby no further increase in inflammatory markers or tooth movement will be observed.
Thus, the idea of increasing magnitude forces to escalate the biological response and hence, increasing rate of tooth movements is limited. For any further increase of rate of tooth movements, the saturation of biological response needs to be overcome by other approaches (Alikhani et al. , 2015). In contrast to heavy forces, lighter forces are harmonious with survival of cells within the PDL and allow for a relatively painless frontal resorption remodelling of the tooth socket.
Clinically, in orthodontic practice, the aim is to generate tooth movements via frontal resorption while acknowledging that certain areas of PDL necrosis and undermining resorption will most likely ensue. In addition to the magnitude of force applied, he duration of force is also essential for tooth movement. Experiments involving animals have indicated that increased levels of cyclic adenosine monophosphate (CAMP), a vital second messenger for cellular functions and differentiation, emerge after four hours of continuous pressure result in tooth movements.
The amount of time essential to elicit a biological response in animals is comparable with a human’s biological response to removable appliances. Therefore, if a removable appliance is worn for less than four to six hours a day, it will fabricate no orthodontic effects. Consequently, when the ppliance is worn above this duration threshold, tooth movements will occur. Both force magnitude and duration of the applied force is important to allow for optimal tooth movements. Therefore, a synergistic balance between the two factors needs to be considered.
The elements of light force and prolonged force results in modification in the PDL chemical environment and steady progression of tooth movement will result from frontal resorption. However, if force is heavy and continuous, tooth movement will be hindered until undermining resorption can remove the bone adjacent to the tooth to allow or tooth movement into a new position. When this occurs, the tooth will promptly alter its position and the constant force will continue to compress the tissues, inhibiting repair of the PDL.
This heavy continuous force may cause destruction to periodontal structures and the tooth itself if a period of regeneration is not provided. Therefore, heavy continuous force should be avoided, but heavy intermittent force is tolerable, though it less effective than light continuous force (Proffit, Fields, & Saver, 2013). Another key factor to consider for optimal tooth movement is the type of orthodontic tooth movement. There are a few types of tooth movements to acknowledge such as tipping, rotation, and bodily movements also known as translation.
For different types of orthodontic tooth movements, there are contrasting optimum forces. The ideal forces for the various tooth movements are: tipping at 35-60gm, bodily movement at 70-120gm, and rotation at 35-60gm. From experiments with animals and clinically obtained experience with humans, it is suggested that lighter forces are better for smaller teeth. Furthermore, if two forces are applied simultaneously to the crown of a tooth, the movement of the root apex and crown can e translated in the same direction and amount.
It is evident that to produce the same pressure in the PDL and hence, the same biologic response, twice as much force is needed for translation than for tipping (Proffit, Fields, & Saver, 2013). When categorizing data according to type of tooth movement, duration and magnitude of force, care should be taken as optimal forces are often influenced by individual variability (Ren, Maltha, Kuijpers-Jagtman, 2003). There have been many studies and experiments on exploring the optimal rate and force for orthodontic tooth movements.
However, the ideal rate and force is still yet to be fully nderstood as there are many issues to combat before a definitive result is obtained. There are many factors, independent of force magnitude, which affects the extent of tooth movement. Factors may be intrinsic involving differences in the shape of the root and alveolar bone, or bone density. Conversely, it could be caused by extrinsic factors such as occlusal forces and chewing habits.
Furthermore, individual difference between subjects makes it hard to obtain a sample of individuals with similar anatomical features, age, gender, and malocclusion. The intensity of an individual’s biological response ay also differ between subjects and differ at different stages of tooth movement (Alikhani et al. , 2015). Individual differences in the characteristics of tooth movements are likely to be linked to the variation in cellular activity within the PDL and alveolar bone.
Additionally, localised differences in the expression of cytokines and growth factors may also influence tooth movements. Therefore, undesired or inefficient tooth movements during orthodontic treatments may occur as a result of individual variation in biologic response. Another factor that contributes to the inconsistent results for ideal force and ate for tooth movements is the structural changes in the alveolar bone and periodontal tissues during the different phases of tooth movements.
Data gathered from experimental studies of tooth movements are commonly obtained over a relatively short period. Thus, data acquired are usually pertaining to the early phase and not the linear phase whereby true orthodontic tooth movement is considered to take place (Ren, Maltha, & Kuijpers-Jagtman, 2003). This could cause slight inaccuracies when information is used for optimal treatment. Even in this decade, the question of optimal force for rthodontic tooth movement remains unresolved.
Research continues for the optimal force for orthodontic tooth movement not only for ideal tooth movements but to prevent unwanted side effects such as root resorption. Biological reactions to externally applied mechanical stimuli has manifested its way into the spotlight as one of the main methods to determine ideal force and rate of tooth movements. There is no one ideal force for tooth movements. The duration and magnitude of force depend on the type of tooth and tooth movement. There are currently many uncontrolled factors that deviate ideal rate nd force results from optimal orthodontic tooth movement.
Therefore, in the future, well-controlled and standardised experiments or clinical research should be carried out beyond the early phases of treatment and include data from the linear phase. Furthermore, with improved technology in the future, a computer simulation model may be able to reproduce local tissues’ stress and strain responses and to calculate tooth movement when orthodontic forces are applied (Ren, Maltha, & Kuijpers-Jagtman, 2003). Collectively, these alternatives may provide a better understanding of techniques to improve orthodontic treatment.
