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In Clinical Orthodontics BIOMECHANICS INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com www.indiandentalacademy.com

INTRODUCTION :

INTRODUCTION www.indiandentalacademy.com

INTRODUCTION:

INTRODUCTION Orthodontic problems are the result of mechanical forces, and their correction depends on mechanical forces. The force systems in the face can form or deform, and their conscious control is a continuing challenge in orthodontics. www.indiandentalacademy.com

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Altering the balance of forces can arrest or reverse progressive deformities during growth, and it can correct many of their effects even in the adult. www.indiandentalacademy.com

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Thus understanding of the fundamentals of mechanics must be the starting point for understanding orthodontics. www.indiandentalacademy.com

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The principles of force analysis are the basic tools of the mechanical engineer, & their application is universal. www.indiandentalacademy.com

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In applying them to oral environment, one combines engineering with dentistry, which requires a mixed terminology that is partly foreign to each discipline: BIO LOGY + MECHANICS _______________ BIO-MECHANICS www.indiandentalacademy.com

BIOMECHANICS:

BIOMECHANICS Laws & Terminologies www.indiandentalacademy.com

Laws & Terminologies This chapter is divided into following parts::

Laws & Terminologies This chapter is divided into following parts: Force Direction Magnitude Point of application (Center of Resistance) Moment Moment of force Factors controlling the moment of force Center of rotation Center of rotation and type of tooth movement Couple Moment of couple Factors controlling the moment of couple Moment to Force ratio www.indiandentalacademy.com

Mechanical Forces:

Mechanical Forces The two broad classes of mechanical force are: Static Dynamic At any moment the oral structures can be considered to be in a state of static balance. www.indiandentalacademy.com

Force :

Force Definition: an act upon a body that changes or tends to change the state of rest or the motion of that body. www.indiandentalacademy.com

Magnitude of force:

Magnitude of force Increasing the amount of force will increase the amount of a free body displacement. However, it is unclear how force magnitude is related to the rate of tooth movement , which is biologically controlled phenomenon. www.indiandentalacademy.com

Point of Application:

Point of Application Center of Mass Center of Gravity Center of Resistance www.indiandentalacademy.com

Center of Mass::

Center of Mass: Each body has a point in its mass, which behaves as if the whole mass is concentrated at that single point, which we call it the center of Mass in a gravity-free environment. www.indiandentalacademy.com

Back to Earth:

Back to Earth www.indiandentalacademy.com

Center of Gravity::

Center of Gravity: The same is called center of Gravity in an environment where gravity is present. www.indiandentalacademy.com

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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

Center of Resistance:

Center of Resistance It is a point at which resistance to tooth movement is concentrated. It is at the approximate midpoint of the embedded portion of the root. www.indiandentalacademy.com

Center of Resistance:

Center of Resistance Center of Gravity Center of Resistance www.indiandentalacademy.com

Where is the Center of resistance of… :

Where is the Center of resistance of… Single Tooth ? Anterior Segment ? Full Upper Dentition ? Center of Resistance www.indiandentalacademy.com

Moment :

Moment Is defined as a tendency to rotate www.indiandentalacademy.com

Moment of force:

Moment of force We can apply a force only 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, called as moment of force. www.indiandentalacademy.com

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In orthodontic terminology we refer to moment as Rotation Tipping Torquing www.indiandentalacademy.com

Factors controlling the Moment::

Factors controlling the Moment: Moment (M) Force (F) Perpendicular Distance (d) M = F x d www.indiandentalacademy.com

Center of Rotation:

Center of Rotation www.indiandentalacademy.com

Center of Rotation & Center of Resistance:

Center of Rotation & Center of Resistance Changing the point of force application and its relation to the center of resistance of the tooth will create: Uncontrolled tipping Controlled tipping Root Movement Bodily Movement Center of Rotation at infinity www.indiandentalacademy.com

Couple :

Couple Two equal and opposite, non-collinear forces www.indiandentalacademy.com

Couple :

Couple 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

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If the two forces of the couple act on opposite sides of the center of resistance , their effect is additive. However, if they are on the same side of the center of resistance , their effect is subtractive. www.indiandentalacademy.com

A clinical example:

A clinical example Additive Subtractive www.indiandentalacademy.com

Explanation:

Explanation Resultant moment = Fd 1 + Fd 2 F 1 x d 1 = Fd 1 F 1 F 2 d 1 F 1 F 2 d 1 F 2 x d 2 = Fd 2 d 2 d 2 www.indiandentalacademy.com

Factors controlling the Moment of a Couple:

Factors controlling the Moment of a Couple Moment (M) One of the forces (F1 or F2) Moment arm of the couple (d) M= F1 x d OR M= F2 x d F1 F2 d F1 F2 d www.indiandentalacademy.com

Couple – Clinical point:

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. www.indiandentalacademy.com

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Irrespective of point of application of a couple on a body or a tooth, the body will experience a moment & it will rotate around it’s center of resistance. www.indiandentalacademy.com

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Depending on the plane in which the couple is acting , this rotation has been called “rotation” (first order), “tipping” (second order), or “torque” (third order) in orthodontics. www.indiandentalacademy.com

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I am sure all of you already familiar with the above mentioned points. I will assume that from now onwards you are familiar with terminologies such as, Force Moment of a force Couple Moment of a couple www.indiandentalacademy.com

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Next we will cover- Moment to Force Ratio And I am sure many have this question: Which moment? Whether moment of force or moment of couple? www.indiandentalacademy.com

Moment-to-Force ratio:

Moment-to-Force ratio The ratio of counter-balancing moment produced to net force that is applied to a tooth. www.indiandentalacademy.com

An example::

An example: In order to retract an incisor tooth we apply a force on the crown of the tooth. This force tends to move the center of resistance of the tooth, however it also creates a moment of force (clockwise). F = Force d = distance (X) F x d(X) = M(X) F M(X) www.indiandentalacademy.com

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An counter-clockwise moment can be generated easily by applying a couple. Note couple generates a moment irrespective of center of resistance of the tooth. F(X) x d = M(X) M(X) F = Force (X) d = distance www.indiandentalacademy.com

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The two moments (i.e. the moment of force and the moment of couple ) cancel out any tendency for the rotation of the tooth, thereby allowing the force to move the center of resistance of the tooth. www.indiandentalacademy.com

Bodily movement of a tooth requires a moment-to-force ratio of 8:1 or 10:1, depending on the length of the root, BUT WHY?:

Bodily movement of a tooth requires a moment-to-force ratio of 8:1 or 10:1 , depending on the length of the root, BUT WHY? 10 mm 20 mm M : F 10 : 1 M : F 20 : 1 www.indiandentalacademy.com

Tooth Movement:

Tooth Movement Orthodontic Application of Moment to Force Ratio www.indiandentalacademy.com

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As we have seen whenever a force is applied at the crown of a tooth, a tendency for the tooth to rotate , tip or torque (a moment) is also created. www.indiandentalacademy.com

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The force at the bracket is equivalent to a force at the center of resistance plus a moment that will cause the tooth to tip. www.indiandentalacademy.com

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In addition to the force applied, a couple may also be engaged intentionally to partially correct, completely correct, or over-correct this tendency. www.indiandentalacademy.com

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By varying the ratio of moment to force applied to teeth, the quality of tooth movement can be changed among uncontrolled tipping, controlled tipping, translation and root movement. www.indiandentalacademy.com

Uncontrolled Tipping:

Uncontrolled Tipping At the CENTER OF RESISTANCE At the BRACKET NET MOMENT www.indiandentalacademy.com

Controlled Tipping:

Controlled Tipping At the CENTER OF RESISTANCE At the BRACKET NET MOMENT www.indiandentalacademy.com

Translation:

Translation At the CENTER OF RESISTANCE At the BRACKET NET MOMENT ZER www.indiandentalacademy.com

Root Movement:

Root Movement At the CENTER OF RESISTANCE At the BRACKET NET MOMENT www.indiandentalacademy.com

Pure Rotation:

Pure Rotation At the CENTER OF RESISTANCE At the BRACKET NET MOMENT www.indiandentalacademy.com

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Thank you For more details please visit www.indiandentalacademy.com www.indiandentalacademy.com