Types of tooth movement in orthodontics (4th BDS)

Types of tooth movement in orthodontics
Copyright by Ortho Department, Yangon

1. Types of orthodontic tooth movement Biomechanics of tooth movement Types of orthodontic tooth movement

2. Mechanics Mechanics is the science which deals with the action of forces on the form and motion of bodies. Biomechanics Biomechanics is the study of the action of forces delivered by the orthodontic appliances on the biologic tissues such as the teeth, periodontal ligament and the bones

3. An orthodontic appliance is a system, storing and delivering forces against the teeth, muscles or bone and creating a reaction within the periodontal ligament and alveolar bone that causes movements of the teeth or alters bone morphology or growth .

4. Orthodontic appliances

5. Force Force is an energy or strength brought to bear on an object causing a change in shape or motion. Force is a vector matter. It may be represented by its vectors as : - magnitude - Direction - line of action. - sense. - Point of application.

6. Center of gravity (CG) or center of mass (CM). Every object or free body has one point where all of the weight is concentrated at this point. The object would be perfectly balanced, if supported, on this point as if the rest of the object does not exist. This point is known as the center of gravity (CG) or center of mass (CM).

7. Center of resistance. (C-ris) Unlike free body, movement of a tooth is constrained by the periodontal attachments to the roots, and the alveolar bone. Therefore in orthodontic biomechanics, a point analogous to the center of gravity or center of mass is used. This point is termed center of resistance. (C-ris)

8. Center of mass F Center of gravity Center of resistance

9. Tooth Movement Basically, there are three types of movement. - Translation. (Bodily movement) - Pure rotation. - Combined rotation and translation.

10. Translation. Translation or bodily movement occurs when all points on the tooth move an equal distance in the same direction.

11. Pure rotation . Rotation indicates movement of points of the tooth along the arc of a circle, with the center of resistance being the center of the circle. Center of resistance

12. Combined rotation and translation. Any movement that is not pure translation or rotation can be described as combination of rotation and translation.

13. Effects of forces on tooth movements The movement of a tooth depends upon the relationship of the line of action of a force to the center of resistance of a tooth. Cris

14. If the line of action of a force passing through the center of resistance the tooth will respond with pure translation or bodily movement in the direction of the line of action of the applied force. (Translation or bodily movement) Cris Line of action of force

15. If the line of action of the applied force does not pass through the center of resistance , the center of resistance will translate as if the force did pass through it, but the tooth will also rotate , since the applied force produces a moment about the center of resistance. The resulting movement is combined translation and rotation. ( Tipping movement ) Force Moment Translation

16. The magnitude moment is equal to the force multiplied by the perpendicular distance from the line of action to the center of resistance. 100 gm 8 mm Moment = Force x perpendicular distance = 100 x 8 = 800 gm-mm 100 gm Force = 100 gm Tooth movement depends on the relative degree of force and moment.

17. A single force (a force with a single point of contact) cannot produce pure rotation. A system of forces that can produce rotation of a tooth is called force couple . A couple consists of two forces of equal magnitude , acting in parallel but opposite direction and having different points of application.

18. A force couple produce moment (Mc) inducing rotation about center of resistance (Cris) whose magnitude is the product of the distance between the lines of force composing the couple (dc) and their magnitude. F F dc Moment = F x dc

19. ORTHODONTIC TOOTH MOVEMENT

20. Orthodontic Tooth Movement. Tipping movement (Combined rotation and translation) Translation or bodily movement Rotational movement Intrusion Extrusion. Uprighting Torqueing

21. Optimal force level The level of orthodontic force, just enough to stimulate cellular activity without completely occluding the blood vessels in the periodontal ligament. The various force levels are required for different types of orthodontic tooth movements.

22. The force applied at the crown of a tooth does not determine the biologic reactions in the PDL. The force is distributed to the area of the PDL that are under compression. (force per unit area of PDL compression) Therefore, the smaller the area of PDL under compression, the greater the force will be distributed and the larger the area, the lesser the force. The area of compression of the PDL depends upon the types of tooth movement. Therefore different optimal force levels are required for various types of tooth movement. Why does optimal force level varies for different types of tooth movement?

23. Optimal force for orthodontic tooth movements. Type of tooth movement Optimal force (gm) Tipping 35 to 60 Translation (bodily movement) 70 to 120 Root uprighting 50 to 100 Extrusion 35 to 60 Intrusion 10 to 20

24. Tipping movement (combined rotation and translation) If the force does not pass through the CR a tooth will translate as well as rotate around the CR. Orthodontically, tipping movement is produced when a single force (from a spring or labial bow from a removable orthodontic appliance) is applied against the crown of a tooth. A tooth rotates around the center of resistance as well as translates along the line parallel to the line of force. The maximum pressure is created at the alveolar crest on the opposite side of the force application and at a side near the root apex on the same side of the force application. The pressure is gradually reduced to minimum towards the center of resistance.

25. Loading diagram for tipping movement

26. Loading diagram consists of two triangles covering in two areas where its concentration is high in relation to the force applied to the crown. According to loading diagram, only one half of the PDL area that could actually be loaded is under compression. Therefore optimal force required to tip the tooth is quite low, approximately 35 to 60 gm. The force for translation of tooth at CR would be much below the optimal force level so that translation movement is insignificant.

27. Bodily movement. Most orthodontic force system apply force at the bracket cemented to the crown of a tooth. Therefore when a force is applied at the bracket an equivalent force system at the CR would be force (causing translation) and a moment of force (causing rotation). For bodily movement the force system applied at the crown must be equivalent to the force and moment of force at the CR. Therefore force system having a force and a couple at the bracket is needed so that a tooth can be moved bodily.

28. Moment = Force x Perpendicular distance from the point of application

29. Loading diagram for bodily movement

30. In bodily movement total PDL area is uniformly loaded from alveolar crest to apex on the opposite side of force application, creating rectangular loading diagram. Therefore to produce the same pressure in the PDL and the same biologic response as for tipping, twice the amount of force would be required for bodily movement. The optimal force level for bodily movement would be 70 to 120 gm.

31. Rotation movement. Rotation of tooth around its long axis requires the application of a force couple to the crown.

32. There is no compression of the PDL area instead all the ligaments are under tension. However, since the tooth roots are triangular or oblong shape some areas of the PDL may be compressed. Therefore, optimal force levels for the rotation movement would be about the same as tipping.

33. Extrusion movement. Ideally, the extrusion would produce no area of compression within the PDL. All the PDL would be under tension and stimulate bone deposition necessary to maintain tooth support. Some areas of compression in PDL may occur depending upon the morphology of root. Therefore, optimal force level would be about similar to tipping movement. 35 to 60 gm. Use of heavy force should be avoided.

34. Intrusion movement When a tooth is intruded the whole of the supporting structure is under pressure with no area of tension. The force is concentrated over a small area at the apex resulting in resorption of bone around the root apex. Therefore, extremely light force (10 to 20 gm) is needed to produce appropriate pressure within the PDL during intrusion. Resorption of root is likely to occur if heavy force is used.

35. Loading diagram for intrusion.

36. Root uprighting (Controlled tipping) Mesiodistal movement of particularly of root. (more root movement than crown.) Require the application of force couple to the crown in such a way that the fulcrum lies within the crown. Optimal force required would be 50 to 100 gm.

37. Root torqueing Labial or palatal movement of apices is referred to as torqueing. Torqueing requires the application of a force-couple to the crown in such a way that the fulcrum lies within the crown.

38. 4 E r F 3 L F = The force delivered for a given deflection E = Elastic modulus r= radius L= Wire length