Biomechanics of Tooth Movement by DrNayAung

1. Biomechanics of Tooth Movement
Nay Aung, BDS PhD
17.12.2022

2.  Biomechanics is the science concerned with the effects of forces acting on the human body.
 A basic understanding of the biomechanics of tooth movement will help the student to
understand the effects an applied force will have on the direction of tooth movement.

3.  Single teeth, groups of teeth and the facial bones have a center of resistance.
 This is the point in a body at which resistance to movement is concentrated.
 If a force is applied directly to the center of resistance of an object, bodily movement will
occur.
 For a restrained object, such as a tooth, the center of resistance does not coincide with the
center of mass and is determined by the shape of the tooth and properties of the alveolar
bone and periodontal ligament.
Center of resistance

4.  The center of resistance should be visualized in all planes of space.
 For a single rooted tooth, with normal periodontal attachment, the center of resistance is
approximately halfway down the root surface.
 For multi-rooted teeth, the equivalent point is in the area of the root furcation.
 Alveolar bone loss results in apical movement of the center of resistance.
Center of resistance
(A) The center of resistance (red circle) of a single-
rooted tooth shown in
(i) buccolingual
(ii) Mesiodistal and
(iii) Occlusal plane.
(iv) The center of resistance of a molar is located in the
furcation region.

5.  The center of resistance of the maxilla is thought to lie in the area of the premolar roots.
 It is an important point to appreciate when applying headgear forces for maxillary restraint.
Center of resistance

6.  Orthodontic tooth movement is dependent on the application of forces.
 A force is a vector that can be described by its magnitude and direction and can be broken
down into its individual components.
 This can help determine the effects a force will have on the direction of tooth movement.
Forces, moments and couples
(B) The resultant force can be broken down into its
various components. Here the resultant force has both
a horizontal and vertical component that will influence
the direction of tooth movement.

7.  The moment of a force is the component of the force that tends to cause rotation.
 Mathematically, the size of a moment of a force is equal to the magnitude of the applied force
multiplied by the perpendicular distance between the point of application and the center of
resistance.
Forces, moments and couples
(C) The size of a moment is equal to the magnitude of
the force (F) multiplied by the distance from the tooth’s
center of resistance (d) from which it acts.

8.  This is of relevance to orthodontics because a force cannot be applied directly to a tooth’s
center of resistance, which lies along the root, and must be applied to the crown.
 This will produce a moment which will cause the tooth to rotate.
Forces, moments and couples
(D) Forces applied at a distance from the center of
resistance result in rotation of the tooth.
This should be considered in all planes of space:
(i) Buccolingual
(ii) Mesiodistal and
(iii) Occlusal.

9.  A couple is produced when two equal and opposite forces act to cause rotation.
 The size of a couple is equal to the magnitude of the forces applied multiplied by the distance
between them.
 Couples are commonly generated during fixed appliance treatment between an archwire and
bracket slot.
 They can be used to alter the inclination of teeth or to act in the opposite direction to the
moment of an applied force to produce bodily tooth movement.
 The term torque is often used in orthodontics to describe a moment or couple.
Forces, moments and couples
(E) An archwire in a bracket slot can be used to apply a
couple in the:
(i) Mesiodistal (ii)buccopalatal (iii) occlusal planes.
(iv) The couple created between the bracket slot and
archwire can be used to control the tipping caused by
the moment of the force (F).
In this way, bodily tooth movement can be achieved.

10.  The basic types of tooth movement are tipping, bodily movement and rotation.
 Tipping occurs when the crown of a tooth moves more than its root in a given direction.
 This form of movement is common during orthodontic treatment as the moment created by
the applied force will tend to cause tipping.
 It is important to remember that when a tooth tips, the crown and root move in opposite
directions.
Types of tooth movement

11.  Bodily movement occurs when the crown and root of a tooth move an equal distance in the
same direction.
 This type of tooth movement can only be produced with fixed appliances where a couple can
be created between an archwire and the wall of a bracket slot to control tipping that
accompanies the application on an orthodontic force to the crown of a tooth.
Types of tooth movement

12.  When a mesial or distal force is applied to the labial surface of a tooth, the moment of the
force will cause rotation.
 This is commonly encountered during fixed appliance treatment.
 It is good practice to tie teeth firmly to the archwire to prevent unwanted rotation, when such
forces are applied.
Types of tooth movement

13. Thank you so much for your kind attention.