The Mechanics of tooth movement in orthodontic practice

# force# centerof resistance # center of rotation # type of tooth movement # type of forces # tipping # torquing# controlled and uncontrolled tooth mob
Cement # continuous, interrupted, intermittent forces

1. The Mechanics of tooth movement Dr. Shweta A. Kolhe BDs. Mds . (Ortho) phd scholar Associate professor Vyws dental college and hospital, Amravati

2. Introduction Every body continues in it’s state of rest or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it ( Sir Isaac Newton) – and teeth are no exception. Mechanics – defined as that branch of engineering science that describes the effect of force on a body.

3. Force Force can be defined as an act upon a body that changes or tends to change the state of rest or of uniform motion of that body. Force is vector, having definite magnitude, a specific direction and a point of application. f =ma. ( unit is newton or gram millimeter /second 2 ) In clinical practice it is either a push or a pull.

4. Resultant force Multiple forces can be combined to form a resultant force by vector addition. Determined by the law of parallelogram of vectors by connecting the vectors ‘ head to tail’ while maintaining the length and direction of the line of action.

5. Stress and strain Stress is the force applied per unit area / external force acting upon a body. Strain is the internal distortion per unit area. Strain can be expressed in the form of a change in either the external dimension or internal energy of the body. Stress and strain are inter-related terms.

6. Couple Couple is a pair of concentrated forces having equal magnitude and opposite direction with parallel but non-collinear line of action. Or Couple is two parallel forces of equal magnitude but opposite in direction. Couple = force x distance. ( unit is grams milimeter )

7. A couple when acting upon a body brings about pure rotation around the center of resistance. A couple exerts no net force on the center of resistance because the two forces that comprise it are opposite in direction and cancel each other.
Couple acts to rotate, tip, or torque the Tooth around the center of resistance.
Moment of couple is the rotational tendency of the couple.

8. Couple – two forces of same magnitude, parallel to each other and acting in opposite direction. Clinical application of a couple in treatment of rotations.

9. Engaging a wire in an angulated bracket slot results in a couple that helps in uprighting a tooth. Engaging A rectangular wire into a slot produces a couple that is used in producing the required torque. Couples are applied by engaging a wire in an edge wise bracket Slot.

10. Center of resistance Every body or free object behaves as if it’s mass is concentrated at a single point on which it can be perfectly balanced. For physical calculation, this point can be taken as the point where the whole body weight is concentrated and can be termed center of gravity or mass.

11. Center of resistance of tooth can be defined as that point on the tooth when a single force is passed through it, would bring about it’s translation along the lie of action of the force.

12. Center of resistance of tooth is constant. In a single rooted tooth it lies between one third and of one half of the root, apical to the alveolar crease While in a multi-rooted tooth the center of resistance lies between the roots, 1-2 mm apical to the furcation. The center of resistance exists for - single tooth, units of tooth, the entire dental Arch and the jaws.

13. Factor which can change the position of the center of resistance - Root length- longer the root, the center of resistance will be placed more apically. Alveolar bone height – if the alveolar crest is higher, the center of resistance will be placed more coronally .

14. Moment Moment can be defined as the measure of rotational potential of a force with respect to a specific axis. Moment not only produce a linear motion but also produce rotation. Moment = Magnitude of force x distance Unit is gram millimeters .

15. There are two variables that determine the moment of force – the magnitude of force and the distance from the center of resistance. Either one of these two variables can be manipulated to produce the desired force systems.

16. Center of rotation Center of rotation is a point, about which a body appears to have rotated, as determined from it’s initial and final positions. Center of rotation is a variable point and changes according to the type of tooth movement. In case of controlled crown tipping, the center of rotation – at the root apex. In case of perfect translation – at infinity.

17. Center of rotation At root apex – during controlled tipping. At incisal edge – during torquing. Outside root – during Intrusion & extrusion. At away from root apex – during uncontrolled tipping.

18. Type of tooth movement The prime motive of orthodontic treatment is to move the teeth into more favourable and corrected positions. Variety of tooth movement in all the 3 planes – sagittal, coronal, and transverse. Tooth movement within oral cavity can be listed – 1. Tipping. 2. Bodily movement 3. Intrusion 4. Extrusion 5. Torquing 6. Uprighting

19. Tipping Tipping is a simple type of tooth movement where a single force is applied to the crown Tipping force results in Movement of the crown in the direction of the force and the root in the opposite direction. Tipping can be of two type – Controlled Tipping Uncontrolled tipping

20. Controlled tipping It occurs when a tooth tips about a center of rotation at its apex. There is a lingual movement of the crown with minimal movement of the root in labial direction.

21. Uncontrolled tipping It describes the movement of a tooth that occurs about a center of rotation apical to and very close to the center of resistance. It is characterized by the crown moving in one direction while the root moves in the opposite direction.

22. Bodily movement If the line of action of an applied force passes through the center of resistance of a tooth. All the points on the tooth will move an equal distance in the same direction signifying a bodily displacement or translation.

23. Intrusion and extrusion Intrusion is the bodily displacement of a tooth along it’s long axis in an apical direction. Extrusion is the bodily displacement of a tooth along it’s long axis in an occlusal direction.

24. Rotation Rotations are labial or lingual movement of a tooth around it’s long axis.

25. Torqueing Torqueing can be considered as a reverse tipping characterized by lingual movement of the root.

26. Uprighting During orthodontic treatment, the crowns of certain teeth will be tipped in a mesio -distal direction with the roots tipped in the opposite way. Tipping these roots back to get a parallel orientation is termed uprightng

27. Tooth movements can be classified basically into three: 1. Pure translation
2. Pure rotation
3. Generalized rotation.

28. Pure translation Occurs when all points on the tooth move an equal distance in the same direction. This is brought about when the line of action of an applied force passes through the center of resistance of a tooth. Pure translation of three types : Intrusion Extrusion Bodily movement ( mesio - distal, labio - lingual)

29. Pure rotation A displacement of the body, produced by a couple, characterized by the center of rotation coinciding with the center of resistance i.e. the movement of points of the tooth along the area of a circle, with the center of resistance being the centre of the circle. Pure rotations can be divided into 2 types: Transverse rotation Long axis rotation

30. Transverse rotation – Those tooth displacements during which the long – axis orientation changes e.g. Tipping and torquing. Long – axis rotation – Here the angulation of the long – axis is not altered. E.g. Rotation of tooth around it’s Long axis. Generalized rotation - . Any movement that is not pure translation or rotation can be described as a combination of both translation and rotation. E.g. Seen during routine clinical practice..

31. Types of forces As is well stated by the pioneers in the subject, if malocclusion is the disease in orthodontics, force is definitely it’s medicine. It is therefore very important for the clinician to use his knowledge and experience in determining the type, amount and direction of force required to bring about efficient treatment results. Based on the duration of application – Continuous force Intermittent force Interrupted force

32. Graph showing the different types of force

33. Continuous force It is an active orthodontic force that decrease little in Magnitude between appointment periods. E.g. Light wire appliance. For an appliance to deliver continuous force, the appliance components should be highly flexible and the activation must be done to a relatively low force level. This is because continuous forces are expected To bring about direct resorption of the root socket.

34. Intermittent force It is an active orthodontic force that decays to zero magnitude or nearly so prior to the next appointment. E.g. Removable active plates.

35. The appliance components should have high stiffness and the initial activation should be twice the Expected corresponding soft – tissue deformation. Due to a relatively high activation, a greater force is exerted on the teeth. lead to undermining resorption and corresponding tooth movement.
Once the tooth has moved, the force will decay considerably so that repair of the necrosed soft tissue and resumption of blood supply occurs within the periodontium.

36. Interrupted force It is an orthodontic or orthopedic force that is inactive for intervals of time between appointments.

37. For an interrupted force to be delivered, the pre-requisites include:
1.It should deliver heavy forces.
2.There should not be any force decay.
3.There should be a specific magnitude-time pattern for pattern for example, 200-300 gms of force 10-14 hours a day.
4.The inactive period of each day must be sufficient to keep the periodontal ligament healthy over the total period of time of use of the appliance.

38. Thinking in the normal way, it might occur to us that a continuous force may bring about a continuous movement or an increased force may lead to an Increased tooth movement. It often exhibits, cyclic, long-term magnitude-time pattern. E.g. Force exerted by an extra- oral appliance worn only at night. But these assumption are far from reality both In the clinical and cellular context. The rate of tooth movement is highly dependent upon the complex biologic responses to the various types of forces that are yet to be clearly understood.