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Mechanics of tooth movement INDIAN DENTAL ACADEMY Leader in continuing dental education

Scalers &vectors • Force (F=MA)_force is that which changes or tends to change the state of rest of a body or of uniform motion along a st line Force acts in three dimensions of space

Resultants of orthodontic force Forces with common point of application

Forces with different points of application Law of transmissibility of forces =external effect of force on a rigid body may be considered to have a point of application anywhere along its line of action

Resolving a force in to components

Force & movement

• Center of gravity ~A point on which a free body can be perfectly balanced (C of mass) • Center of resistance~A point through which the line of action of force passes, producing translation

Determinants of center of resistance • • • • • • •

Size &shape of the crown & root\roots periodontal attachment level &density of alveolar bone multirooted 1~2mm apical tofurcation 50% of root Proffit & Nikolai 50~33% Smith & Burstone 25~33% Nanda

Types of tooth movement • Translation-pure bodily movement • Pure rotation-It indicates movement of points of the tooth along the arc of a circle with cent of resist as cent of circle • Combination

Translation • If the line of action of an applied force passes through the center of resistance of a tooth it responds by translation in the direction of line of action

Moment of a force • Moment (potential for rotation) = magnitude of force * perpendicular distance from line of action to point of resistance

Direction of moment (positive &negative )

Combined translation & rotation

Rotations / Couples Couple consists of two forces of equal magnitude , with parallel but noncolinear lines of action & opposite senses

Moment of couple = magnitude of forces *

dist between them

AntiCl wise~10*50=500 gm mm Cl wise ~30*50 =1500gm mm Net ~1500-500 =1000 gm mm Force *dist ~50*20 =1000gm mm Couple is a “free vector”

Recap • Force ~

physical entity applied to an . . object • Moment ~turning tendency of force (pt of . reference ) • Couple ~inherent moment not relative to . any point

Center of rotation • Center of rotation # a point about which a body appears to have rotated

Center of rotation &tooth movement

• Hurd & Nikoli ~Point which moves the least on the line of long axis of tooth • Hocevar ~geometric point about which no moment has occurred

Force system &center of rotation • Only net force translation (infinity ) • net moment only rotation (cent of resist ) • moment &force ( ? )

Equivalent force systems • Sums of forces in x direction are identical • Sum of forces in y direction are identical • sum of moment about any point are identical

Moment about any point

Clinical situation of canine distalization

Center of rotation

Clockwise apical Counter clock wise incisal

All put together

M/F ratio &tooth movement


Units millimeter

summary • • • • • • • •

Orthodontic forces are vectors They can be combined or divided They produce translation ,rotation ,combination couple center of resistance center of rotation M/F ratio key to controlled tooth movement Understanding of mechanics controls one of the

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