Forces in orthodontics /certified fixed orthodontic courses by Indian dental academy

1. FORCES IN ORTHODONTICS INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com www.indiandentalacademy.com

2. • Force : A load applied to an object that will tend to move it to a different position in space.(Proffit) • The correct unit used to express forces is Newtons(N) • In orthodontics, forces commonly expressed in grams. www.indiandentalacademy.com

3. OUTLINE • • • • • • • • • • • Forces Moments Couples Moment-to-force ratio Tooth movement Center of resistances Center of rotation Moment of the couple Moment of the force One-Couple force system Two-Couple force system www.indiandentalacademy.com

4. FORCE The two broad classes of mechanical force are: Static: which deals with bodies at rest, and Dynamic: which deals with moving bodies. At any moment the oral structures can be considered to be in a state of static balance. Forces are vectors, having both direction & magnitude. Point of application of force is also important in the understanding of tooth movement. www.indiandentalacademy.com

5. • Point of Application · Center of Mass · Center of Gravity · Center of Resistance www.indiandentalacademy.com

6. Tipping - When a force is applied on this pencil below its center of mass, it tips and moves forward. www.indiandentalacademy.com

17. • The center of gravity of the tooth is located more towards the crown of the tooth as the mass of the tooth is concentrated more coronally www.indiandentalacademy.com

18. Translation - If the force is passing through the center of mass, the whole body moves in a straight line, in a parallel fashion. Here all the points of the body get displaced equally from the initial position. This is called bodily movement or translation www.indiandentalacademy.com

36. • As the root of tooth is partially embedded in the bone, its Center of Gravity shifts apically and is then referred to as the Center of Resistance. The center of resistance is at the approximate midpoint of the embedded portion of the root. www.indiandentalacademy.com

37. www.indiandentalacademy.com

38. Moment • A moment is defined as a tendency to rotate. • A moment (M) is the product of the force (F) times the perpendicular distance from the point of force application to the center of resistance (d). Therefore it is measured in hybrid units such as grams-millimeters (gmmm). www.indiandentalacademy.com

39. • Moment of the force: When a force is applied at any point other than through the center of resistance, in addition to moving the center of resistance in the direction of the force, a moment is created. Since the tooth is embedded in the alveolar bone, we can never apply a force directly on the center of resistance. We can only apply a force on the exposed part of the tooth, which is at a distance from the center of resistance. Therefore with a single force in a typical clinical situation, we invariably create a moment, which is called as moment of force. A moment may be referred differently in orthodontics, as “Rotation”, “Tipping” or “Torquing” www.indiandentalacademy.com

49. If the distal force is Applied coronal to the Center of resistance, the Center of resistance of the Tooth will move distally And the crown of the tooth Will tip distally( tipping) www.indiandentalacademy.com

51. If the distal force is Applied coronal to the Center of resistance, the Center of resistance of the Tooth will move distally And the crown of the tooth Will tip distally (tipping) www.indiandentalacademy.com

55. If an intrusion force is applied Buccal to the center of resistance The crown will torque facially (Torquing) www.indiandentalacademy.com

61. • Since the value of a moment is the product of a force value (such as grams) multiplied by the linear value (such as millimeters) therefore either increasing the magnitude of the force or applying the same force even further from the center of resistance will increase the tendency for rotation. A)either increasing the magnitude of the force or B)applying the same force even further from the center of resistance will increase the tendency for rotation. www.indiandentalacademy.com

62. • • Center of resistance (circle) and center of rotation (star). In order to understand the magnitude of rotational tendency we have to understand another terminology, the center of Rotation and for the measurement of the moment we need to understand relation of center of rotation to the center of resistance of the tooth. Center of Rotation: Now, if we draw the long axis of the tooth in its initial and final positions, we will find that both these lines intersect at a point. This is the point around which the tooth rotates and is called the center of rotation. www.indiandentalacademy.com

63. • Center of Rotation and Center of Resistance: The center of rotation can be at the center of resistance, apical to the center of resistance, at the root apex or at infinity. Their position will determine the type of tooth movement www.indiandentalacademy.com

64. • Uncontrolled tipping: In this situation, when the force is applied, the crown moves in one direction and the root moves in the opposite direction. Here the center of rotation lies somewhere near the center of resistance of the tooth. This is referred to as uncontrolled tipping. www.indiandentalacademy.com

65. • Controlled tipping: In this case, the center of rotation lies close to the apex. Here the crown moves in one direction but the root position remains the same or gets minimally displaced. This is referred to as controlled tipping. www.indiandentalacademy.com

66. • Translation or bodily movement: In this situation, where the long axis of the tooth in its initial and final positions does not meet, the center of rotation lies at infinity. This is referred to as translation or bodily movement. www.indiandentalacademy.com

67. • Couple Two equal and opposite, non-colinear forces are called a couple. www.indiandentalacademy.com

89. • The two forces cancel out any tendency for the center of resistance of the pencil to move, but the moment created by the two forces does not cancel each other. The pencil therefore, rotates about its center of resistance regardless of the point of application of the couple. www.indiandentalacademy.com

90. • The moment of a couple is the product of one of the forces times the distance between the two forces. This distance is called the moment arm of the couple. A couple applies a pure moment to a tooth. www.indiandentalacademy.com

91. • The concept of couple actually encompasses all situations involving rotation. In other words any rotational tendency is related to couple. But it may be said that a moment (rotational tendency) can be achieved with a single force. The moment thus created is called the moment of the force. www.indiandentalacademy.com

92. • Consider a cantilever beam, apply a force of one kilograms to its free end. In such a case in which a specific fulcrum is known, a counterforce equal to the applied force must be present at the fulcrum to maintain equilibrium, and that force completes a couple. • Special note should be made of the important difference between a fulcrum and an axis because they are often confused. www.indiandentalacademy.com

93. • Axis: An axis is any point about which rotation takes place; it is dependent only on the influence of outside forces and may be located anywhere within or outside the rotating object. • Fulcrum: A fulcrum is a physical support such as an axle or the pivot under a lever. A fulcrum is an actual point of application of force, either applied or reactive. www.indiandentalacademy.com

94. • Couple- Clinical Point When the tooth is embedded within the alveolar bone we cannot apply a couple with one force on the crown and the other force on the root. We can apply a couple only on the exposed part of the tooth. Using this couple mechanism we can achieve various tooth alignment procedures, irrespective of the center of the resistance of the tooth. • Depending on the plane in which the couple is acting, this rotational tendency (moment) has been called “rotation” (first order), “tipping” (second order), or “torque” (third order) in orthodontics www.indiandentalacademy.com

95. In orthodontics,couples are applied www.indiandentalacademy.com

96. • Moment-to-Force Ratio The ratio of the counter-balancing moment produced to the net force that is applied to a tooth will determine the type of tooth movement that will occur. This is called the moment-to-force ratio. www.indiandentalacademy.com

97. • If a bracket is 10 mm from the center of resistance, a force applied at the bracket causes the tooth to tip because of a moment that is 10 mm times the magnitude of the force. To counter- act this tendency to tip, a couple in the opposite direction with a moment 10 times the magnitude of the force would need to be applied in addition to the force. This would be an applied momentto-force ratio of 10:1, resulting in displacement of the tooth as if the force alone had been placed through the center of resistance Example: A lingual force of 80g applied 10mm from center of resistance with a counter clockwise Mc of 800gmm and M:F ratio of 10:1 leads to Translation. www.indiandentalacademy.com

98. • Uncontrolled tipping: M/F ratio< 5:1 causes uncontrolled tipping of the tooth, in which crown and the root apex move in opposite directions. www.indiandentalacademy.com

104. • Controlled tipping: When the M/F ratio is between 5:mm and 8:1 mm, it causes more of controlled tipping of tooth,in which the root apex remains stationary and the crown moves. www.indiandentalacademy.com

109. • Root movement: The M/F ratio of 12:1 mm causes root movement of the tooth.The crown remains stationary, while only the root moves. www.indiandentalacademy.com

120. • Pure rotation: If only a couple, and no force is applied to a tooth, the tooth will rotate around its center of resistance and the tooth will not translate. Because the action of a couple doesn’t depend on its point of application, a pure moment always acts at the center of resistance. Using only a couple we www.indiandentalacademy.com

129. • Static Equilibrium: According to Newton’s third law, every action has an equal and opposite reaction. Requirements for static equilibrium: Three requirements are accomplished automatically whenever static equilibrium is established. They are: 1. The sum of all forces present must equal zero. 2. The sum of all moments present must equal zero. 3. The sum of all forces and moments present must equal zero. www.indiandentalacademy.com

130. • Orthodontic appliances can be divided into following three categories depending on their mode of action (forces and moments): 1. Equal and opposite force system 2. One couple appliance system 3. Two couple appliance system www.indiandentalacademy.com

131. • No Couple Appliance System: • Simplest orthodontic appliance, an elastic band stretched between two points of attachment is the best example. This produces forces of equal magnitude on either end but opposite in direction. Elastic band (orthodontic appliance) is in equilibrium and neither couples nor moments are generated. Equal and opposite force system www.indiandentalacademy.com As far as

132. One Couple Appliance Systems [Statically Determined Systems]: www.indiandentalacademy.com

133. • One end of the appliance experiences couple (bracket or tube) and other end is tied as a point contact (which can not produce a couple). It is statically determinate because the magnitudes of the forces and moments produced can be determined clinically after the appliance is inserted in to the bracket/tube. This can be done by inserting the appliance into the bracket and measuring the force required to activate the wire to the site it will be tied as a point contact. • Because there is only one site of engagement, the direction of the couple at that site depends on the angle formed between that bracket and the wire. www.indiandentalacademy.com

134. Two couple appliance system [Statically undetermined Systems]: Here both the ends of the appliance are engaged into attachments (brackets or tubes). A couple, therefore, may be generated by the wire at either or both attachment sites. In this case, inserting the wire into one attachment and using a gauge to measure the force required to deflect the wire to the other bracket will not necessarily give an accurate assessment of the force produced by the appliance. Because of the inability to measure force systems produced by two couple appliances clinically, they are referred to as being statically indeterminate. www.indiandentalacademy.com

138. The force systems produced by two-couple orthodontic appliance depends on both the wire geometry and bracket angulation relationships. • Force systems from a two couple appliance: Straight segment of wire in misaligned brackets results in equal and opposite couples. www.indiandentalacademy.com

139. • As angulation of one of the brackets reduces apart from the couple generated, forces results. www.indiandentalacademy.com

140. • When angulation of one bracket reduces to the half of that other, no couple is experienced at the bracket with smaller angulation and forces are consequently increased. www.indiandentalacademy.com

141. • If angulation of one bracket decreases to zero(parallel to the archwire), couple in the same direction as other bracket will result and forces are even greater www.indiandentalacademy.com

142. • When both the brackets are equally angled in the same direction, forces at the two brackets are equal and in the same direction with forces at a maximum. www.indiandentalacademy.com

143. • Similar situation is seen when the wire is angulated in relation to the bracket. • When the apex of V-bend is half way between the brackets the wire exerts equal and opposite couples at the both attachments and no forces. www.indiandentalacademy.com

144. • As the V-bend is moved off center, the couple nearer the bend increases and the couple further from the bend decreases.Forces result to maintain appliance equilibrium. www.indiandentalacademy.com

145. • When the V-bend is at the 1/3rd the distance between the brackets, a couple is only present at the bracket nearer the bend.No couple is experienced at the brackets further from the bend and forces result to maintain equilibrium. www.indiandentalacademy.com

146. • When the V-bend is very close to one bracket, couple in the same direction is experienced at both brackets, with larger couple at the bracket closer to the bend.Forces increase as the bend is moved further off center. www.indiandentalacademy.com

147. • A step bend , regardless of where it is placed results in equal couples in the same direction at both attachments.Forces are at maximum. www.indiandentalacademy.com

148. Conclusion The choice of appliances and techniques varies radically among individuals, but the fundamental forces and moments they produce are universal .Appliance will always act according to the laws of physics. Understanding the basic principles involved in effective controlled tooth movement makes the final outcome more predictable and consistent. www.indiandentalacademy.com

149. Thank you www.indiandentalacademy.com Leader in continuing dental education www.indiandentalacademy.com

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