Biological effects of orthodontic forces.
To obtain a tooth movement it is necessary to practice a force of a suitable intensity for a sufficiently long period of time.
The intensity of the force applied on the tooth has a relative value as it is transmitted through a chain of mediators that can significantly alter the effect of it.
The key parameter is the intensity of the force per surface's unit of the alveolar bone (pressure).
If the intensity of the force exceeds the elastic tissue like it happens when there's a trauma, it causes a rift where there's less resistance. If the elastic limit is not exceeded the bone bends proportionally to the force's intensity.
The minimum pressure which can induce tooth movement is equal to 30gr/cm which roughly corresponds to blood pressure capillary.
After 20-30 hours in the side of alveolar bone, the direct bone resorption begins and it will not come to an end until the force won't be exhausted while the speed of this last one increases with the increasing of the force.
If the applied force exceeds 100gr/cm, occurs ischemia (Hyaline necrosis), that appears after 36 hours' application of force.
The duration of this necrosis can vary from two to four weeks depending on the age of the individual. During this period of time, the movement of the tooth is prevented and if the force continues to act it will result the indirect bone resorption.
The root resorption depends on the direction of motion and is proportional to the applied force and time of application but not at the speed of movement.
A move that quickly produces is a sign of low tissue resistance and therefore low risk of root resorption.
The response to a persistent force applied to the teeth depends on its entity. The goal is induce tooth movement predominantly through a mechanism of front resorption, even knowing that inevitably, despite our best efforts, some areas will still have an necrosis of the periodontal ligament and indirect resorption.
Fig.1: In experimental animals can be observed changes in blood flow in the PDL infusing a dye into the vascular bed before sacrificing the animal. The vessels are filled by the dye, so it is easy distinguish the size. (A) PDL normal Spraying. Note that the darker areas indicate the blood flow. (B) A force of 50 g on PDL. Note the reduction of spraying and the presence of a certain blood flow in the compression zone. © A heavy force that produces an almost complete closure of the blood flow in compressed area. This sample was taken in horizontal section; Note that the vessels were compressed in the area PDL, which is in the direction of movement of the tooth. The cells are missing in the compression area and area is sometimes called hyalinization for its resemblance to hyaline cartilage. (Courtesy of Dr FE Khouw.)
Figure 2: On the opposite side to the direction of movement of the tooth, the area of the PDL is wider and the blood vessels are dilated. In the area where the PDL is stretched as a result of the shift we observe the blood vessels dilated and partially filled .
Fig.3: Schematic representation of the increase of blood vessels's compression due to increase of pressure in the PDL. On specific value of continuous pressure, vessels are completely occluded and there is a sterile necrosis of the periodontal tissue.
Pressure tension Theory
It is based on the classical hypothesis that cell differentiation and eventually tooth movement are controlled by chemicals signals rather than electrical. There are no doubts that chemical messengers intervene in the succession of events leading to bone remodeling and tooth movement.
According to this theory, a continuous force causes a movement of blood flow in the PDL through an action of the tooth movement within the alveolus, which means a compression of the ligament in some areas and a traction in others. Blood flow decreases in the areas of compression (Fig. 1) of the PDL, while the same or increased in areas of strength (Fig 2). In areas where the PDL is particularly stretched, blood flow may be reduced temporarily. Modifications of spraying are associated with rapid chemicals changes. For example, the oxygen tension decreases in the areas of compression and may tend to increase in areas of tension; similarly, in a few minutes variations in other metabolites can be registered . These chemical changes can stimulate, directly or through other biologically active substances, a special cell differentiation. According to this hypothesis, the tooth movement can be explained in three stages: (1) alteration of blood flow resulting from the pressure exerted on the PDL, (2) synthesis or release of chemical messengers, (3) activation of specialized cells. (Table 1)
Effects of forces of different magnitudes
The greater is the force applied on the PDL, the higher is the reduction of the flow inside until come to a complete collapse of blood vessels and subsequent ischemia (Fig. 3). This result was obtained in experimental animals in which the increase of the force applied to a tooth is accompanied by a reduced perfusion of the areas of the PDL subjected to compression (Fig 1 and 2).
Comparaison of the sequence of events that occur by applying a light and heavy orthodontic forces .
When at the tooth is applied a light and continuous force, a reduction of blood flow through the PDL partially compromise is observed; this occurs as soon as the fluids are pushed outside the periodontal space following the movement of the tooth in the alveolus (in a few seconds). In few hours, chemical changes occurring induce a different model of cellular activity. In animal tests it was found increased levels of cAMP, the "second messenger" of many functions involved in cell differentiation; this occurs approximately 4 hours after application of a continuous pressure .The same interval of time was necessary in humans to obtain a first response to the application of removable devices. In fact, if these are worn for less than 4-6 hours a day, they do not induce any orthodontic effect; beyond this threshold, the teeth begin to move.
These events occur during the first hour of applying a continuous force to a tooth, ie in the period between the beginning of the pressure and tension exerted on the PDL and the appearance of second messengers few hours later.
Prostaglandins and interleukin-1beta
Recent experiments have shown that short period of time from the application of pressure, the values of prostaglandins and Interleukin-1beta in the PDL increase; in particular it seems that the prostaglandin is an important mediator of cellular response. Changes in cell shape may play an important role. It is believed that the release of prostaglandins occurs when cells are mechanically deformed (the release of prostaglandins may be a primary response not secondary to the pressure stimulus). It is likely that the mobilization of membrane phospholipids, leading to the formation of inositol phosphate, is another way to get to the cellular response. Other chemical messengers are involved, particularly those of the family of cytokines, but also nitric oxide (NO) and other regulators of cell activity. From the moment there are drugs that can affect the levels of prostaglandins and other potential chemical messengers, it is reasonable to think that the response to orthodontic force can be pharmacologically modifed.
Bone apposition and resorption
To obtain a shift of the tooth, it requires the involvement of osteoclasts, which remove bone in the area of compression of the PDL . It is also necessary that the osteoblasts affix new bone on the side of power and intervene by the remodeling in areas affected of bone resorption on the compression side. The prostaglandin E has the interesting property to stimulate either the activity of osteoblastic or osteoclastic,and therefore it is probable that it is the main chemical mediator of tooth movement. Parathyroid hormone, when injected, is able to induce osteoclast differentiation in a few hours; this response is much slower when the stimulus consists of the compression of the PDL. In this case, in fact, pass even 48 hours before the first osteoclasts are observed within or close to the PDL. Cell kinetics studies show that these cells arrive in two waves, and this suggests that the first wave comes from a local population, while the latter comes through the blood supply, from more distant areas. These cells attack the adjacent lamina through a process of frontal bone resorption, which is usually followed by tooth movement. Meanwhile, although with some delay, osteoblasts (locally differentiated from progenitor cells present in the PDL) form new bone at the side of tension and begin the task of remodeling on the side of compression.
Effects on the PDL
The situation changes when the applied force is high enough to totally occlude the blood vessels in an area of the PDL. In this case we obtain , rather than cell differentiation, sterile necrosis of the compression zone. In orthodontic practice it is difficult to avoid that the pressure does not result in avascular areas of the PDL and has been shown that periodical interruption of pressure on the teeth, while the same is maintained for a sufficient number of hours to produce a biological response, could help to ensure a degree of tissue viability. When performing this type of necrosis, bone remodeling happens through the intervention of cells from adjacent non-affected areas. However, these cells begin to invade the area of necrosis with a delay of several days. It is important to note that osteoclasts appear in the bone marrow around and begin to attack the external side of the bone tissue adjacent to necrosis. This process is described as indirect resorption. (It appears a latency period before the removal of bone. At that point, the tooth moves abruptly and, if we keep applying a heavy force, there will be a re- latency period before a new cycle of indirect resorption .)