In the name of GOD

Biomechanic and types of tooth movement Presented by: Dr Somayeh Heidari Orthodontist

Reference: Contemporary Orthodontics Chapter 8 William R. Proffit, Henry W. Fields, David M.Sarver Mosby

Definition of terms

Force a load applied to an object that will tend to move it to a different position F = m.a its units are Newtons or gram. Millimeters/ Seconds 2 is usually measured in weight units of Grams or ounces a force is a vector Line of Action Origin / Point of application Sense / Direction

Resultant

Center of resistance a point at which resistance to movement can be concentrated for an object in free space is the same as the center of mass for a partially restrained object, determined by the nature of the external constraints

The center of resistance for a tooth is at the approximate midpoint of the embedded portion of the root: about halfway between the root apex and alveolar bone crest.

Location of the center of resistance depends on the alveolar bone height, root length, morphology and number of roots.

Moment a force (F) acting at a distance from the center of resistance (d) m = F × d (gm-mm) moment of a force results in some rotational movement it tends to rotate the object around the center of resistance it is precisely the situation when a force is applied to the crown of a tooth

F Distance (d) M = F × d Moment of Force

Clinical examples of moments of the forces

Couple two parallel forces equal in magnitude and opposite in direction that separated by a distance the result is a pure moment will produce pure rotation around the center of resistance couple = magnitude of force(s) × distance between forces

F F d M M 50 g 8 mm 400 g.mm Couple force = F × d

Center of rotation the point around which rotation actually occurs when an object is moved If a force and a couple are applied to an object, the center of rotation can be controlled

Types of tooth movement

 optimum orthodontic force will stimulate cellular activity without completely occluding blood vessels in the PDL.  the PDL response is determined not by force magnitude alone, but by force per unit area, or pressure.  the distribution of force within the PDL differs with different types of tooth movement

Pressure = Force / Area

Tipping  the crown and the root move in opposite directions  greater movement of the crown than the root  the center of rotation is apical to the center of resistance  based on the location of the center of rotation is classified into uncontrolled and controlled tipping

Uncontrolled Tipping  the simplest form of orthodontic movement  produced when a single force is applied against the crown  the tooth will rotate during movement  the PDL is compressed near the apex (same side as the force) and at the alveolar crest (opposite side from the force)

 maximum pressure is at the alveolar crest and the root apex  progressively less pressure is created as the center of resistance is approached  only one-half the PDL area is loaded  high pressure in the two areas is concentrated  force must be kept quite low: not exceed approximately 50 gm  M/F ratio = 0:1 to 5:1 (average root length and 100% alveolar height)

Controlled Tipping  the center of rotation is at the root apex  by application a force to move the crown and a moment to control the apex  concentration of stresses at the alveolar crest  minimal stress at the root apex  M/F ratio = 7:1

Translation  also known as “bodily movement”  the root apex and crown move in the same direction and the same distance  PDL area is loaded uniformly  twice as much force is required for translation as for tipping

 a horizontal force applied at the center of resistance will result in translation  two forces are applied simultaneously (a couple and a force are required)  the center of rotation is at infinity  M/F ratio = 10:1

Rotation  pure rotation requires a couple  no net force acts at the center of resistance  in theory, rotational force can be more larger than other movements  in fact, rotational movement without tipping is essentially impossible  appropriate forces for rotation are similar to those for tipping

Extrusion  ideally would produce no area of compression in the PDL, only tension  practically the tooth tipped at all while being extruded  even if compressed areas avoided, heavy forces in pure tension would be undesirable unless for extraction  about the same magnitude of force as those for tipping

Intrusion  successful intrusion can be accomplished, only if very light forces are applied  the force concentration is in a small area at the root apex  the tooth probably will tip somewhat as it is intruded

Root movement  keeping the crown stationary and applying a moment and force to move the root  the center of rotation is at the incisal edge  root movement requires a large moment  concentration of stress is in the apex area  M/F = at or above 12:1

Optimum forces for orthodontic tooth movement Force* (gm)Type of movement 35-60Tipping Translation Root uprighting 35-60Rotation 35-60Extrusion 10-20Intrusion *values depend n part on the size of the tooth, smaller values appropriate for incisors, higher values for multirooted posterior teeth.

Effects of force duration and force decay

 animal experiments after about 4 hours, cyclic nucleotide levels increased  clinical experiments threshold for tooth movement is in the 4-8 hours range  longer force maintenance increasingly effective tooth movement  fixed appliances are more effective unless the removable appliance used almost all the time

Duration of force has another aspect, related to how force magnitude changes as the tooth responds by moving. From this perspective, orthodontic force duration is classified by the rate of decay as: Continuous – force Interrupted – force Intermittent - force

Continuous force

Interrupted force

Intermittent force

theoretically light continuous forces produce the must efficient tooth movement. the heavier forces are physiologically acceptable only if: force levels decline so that there is a period of repair and regeneration before the next activation or the force decreases at least to the point that no second and third rounds of undermining resorption occur

 Experiments has shown that orthodontic appliances should not be reactivated more frequently than at 3 week intervals.  A 4 to 6 week appointment cycle is more typical in clinical practice.  Undermining resorption requires 7 to 14 days : equal or longer period is need for PDL regeneration and repair before force is applied again.

Thanks for your attention