Mechanism of tooth movement during braces

Physiology of tooth movement – ​​compression zone – extension zone

Schwartz proposed the pressure hypothesis in 1932. This is the simplest and most widely accepted hypothesis. According to this hypothesis, whenever a tooth is subjected to an orthodontic force it creates a zone of extension and a zone of compression. Alveolar bone is resorbed when the tooth root exerts compressive force on the periodontal ligament area for a certain period of time, that is, the side that is subjected to the compressive force. New bone is deposited when a stretching force acts on the periodontal ligament fibers, such as the stretched side.

Force - Force vector

Definition

Orthodontic force is any impact that creates a change in tooth movement. Force is measured in ounces, grams or Newtons (about 100 grams per Newton on Earth because the acceleration due to gravity is considered constant and equal to 9.807/s2). 

In orthodontics, the unit of force is usually grams 

Direction and magnitude of a force 

Because a force is a vector, the direction of the force is represented by an arrow pointing in the same direction as the tooth movement. Magnitude is expressed by the length of the arrow by convention. 

Force application point

The ​​force application point is the location where the force acts on the object, by convention the starting point of the arrow. The position of the force application point is related to the center of the object's mass (center of mass) because this exact position will determine the object's movement tendency, whether it is translation or rotation when affected by this force. 

Center of resistance (CR)

Center of resistance is an important concept in orthodontics because the tooth is not a free object, the tooth root is held in the alveolar bone by the ligaments around the tooth. 

The center of mass (center of mass) and the center of resistance are not in the same location. CR is located apically compared to the center of mass. It is a mathematical point at which all resistance to movement is said to be concentrated. Force system calculations involving translational and rotational mobility are performed in relation to the center of resistance. 

The center of resistance varies with each tooth and depends on the condition of the surrounding periodontal tissue. In teeth with healthy periodontal tissue, the center of resistance is usually located near the mid-root level. 

The center of resistance refers to all forces acting on the teeth. For a tooth, it includes forces from the periodontal ligament (PDL), blood vessels, bone, and connective tissue. 

The center of resistance can be studied on an individual tooth, or on a group of teeth if they are bonded together (where they appear as a larger mass). 

Importance of the center of resistance: When applying forces to teeth, it is imperative to evaluate their three-dimensional impact and the resulting movements that occur when this force system is applied to the teeth. 

When a patient has alveolar bone loss, the alveolar crest moves toward the apex. The center of resistance of the tooth will move significantly towards the tip, and the distance from the bracket to the center of resistance increases almost twice. 

In orthodontics, any doctor wants to achieve a force that passes through the center of resistance. To achieve this, controlling tooth root torque is a priority factor. This is only achieved when using the appropriate slot and torque on the braces for each case.

Center of rotation

Center of rotation is the point around which the object rotates. The center of rotation changes according to the position of the center of resistance and the force acting on the object. Pure rotation occurs when the center of rotation coincides with the center of resistance. Pure translational motion occurs when the center of rotation is an infinite distance from the center of resistance.

Types of tooth movement

Teeth move in three spatial dimensions. It is important to be aware of the different types of movement possible when planning treatment to take into account desired and unwanted tooth movements. 

1. Translational motion: In translational motion, all points on the object move in the same direction and with the same intensity. The center of rotation is located at an infinite distance from the tooth because there is no rotation. 
2. Pure rotation occurs when an object rotates about its center of resistance 
3. Uncontrolled tilt: When a force is applied to the crown of a tooth, the crown moves in one direction, while the root moves in another direction. In this case, the center of rotation is near, or located apically compared to the center of resistance, so the teeth rotate around the center of resistance

1. Controlled tooth tilt: The center of rotation is at the tip of the tooth root. It involves a torque and a force, and the tooth rotates around the center of rotation
2. Tooth root movement occurs when the center of rotation is at or near the occlusal edge, and rotation occurs around this point. Then the tooth crown moves less than the tooth root. 
3. Subsidence and extrusion involve tooth movement along the long axis. In this type of motion, the center of rotation is infinite (because there is no rotation). In other words, teeth rotate around a point at an infinite distance so we do not see any rotation, only translation or subsidence/extension motion. 

 To achieve the desired movements, the doctor needs to choose the appropriate wire and bracket system for each case, individualizing it for each orthodontic patient. In addition, you can refer to implant tooth structure to better understand the most popular tooth beautification solution today.

Physiological force - pathological force

Physiological force

Physiological force in orthodontics is usually a continuous light force, achieving the desired tooth movement and few side effects. This force is only achieved when the doctor has high professional qualifications, masters orthodontic mechanics and controls the case well. Besides, it is necessary to have the patient's absolute cooperation, to come to the examination on time, to limit bracket peeling and to have good oral hygiene. 

To get a perfect treatment result, orthodontists need effective support from technology:

Smilestream software

Support diagnosis, avoid missing problems, measure the resistance of cortical bone to ensure the force is applied to move teeth in the bone, minimizing tooth root resorption and bone loss. An optimal point of this software is to support doctors in choosing personalized braces parameters, suitable for each specific case through simulating tooth movement with different braces systems. 

Autolign software

An orthodontic treatment result is only good when it is maintained stably over a long period of time. This depends a lot on the wire shape chosen during the orthodontic process. With Autolign, the doctor can easily define the initial shape of the dental arch, thereby assigning the most suitable personalized arch wire for the patient.

The teeth on the jaw all move in 3 dimensions in space. To ensure teeth move in the right direction, we need to have a realistic view of each type of movement. With Autolign, doctors no longer need to vaguely imagine but can accurately determine each movement (direction of movement and distance) using 3D simulation video.

With the support of these software, the case will be well controlled from the beginning, achieving good and stable results after treatment. In addition, for those of you who are wondering can braces be placed on implants you can ask your doctor for further answers.

Non-physiological force

Occurs when orthodontic forces create many unwanted movements, causing complications such as bone loss and tooth root loss. This happens when too much force is used, the brackets and wires are not properly selected to suit each case, the patient is late for appointments many times/lates for a long time, has poor oral hygiene and has periodontal problems (damage to the tissues around the teeth).

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