# Fudamentals of Biomechanics and Biomechanics of Levelling and Aligning(Includes Biomechanics of...

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Fundamentals of Biomechanics including Mechanics of Leveling and AligningDr. Meenakshi Vishwanath

Contents IntroductionTerms and definitionsPrinciples of biomechanics One couple systemTwo couple systemLeveling and aligning -Begg and PAE systems

Introduction Mechanics-is the discipline that describes the effect of forces on bodies.

Biomechanics-study of mechanics as it affects the biologic systems.

Application of mechanics to the biology of tooth movement.

Introduction History

Studies concerning the biology of tooth movement -1930sThe study of mechanics and effect on periodontium-1950sVarious methods to study mechanics Laser holography, Photo-elasticity, Complementary Strain Energy, F E M . .

IntroductionOrthodontic tooth movement Force on the teeth.

Knowledge of mechanical principles and governing forces- necessary for the control of orthodontic treatment.Basis of orthodontic treatment-clinical application of biomechanic concepts

Proper mechanical force system = medications Treatment success.

Terms and definitionsCentre of Mass- All objects (finite) behave as if the entire mass is concentrated onto a single point.

Applicable in force - free stateBehaviour- Predictable if forces acting in relation to this point is known.

Terms and definitionsCentre of Gravity- objects subject to gravitational force Cmass / Cg - Balance point

Terms and definitionsCentre of Resistance- analogous to the Cmass for restrained bodies.Function of a body in a system of constraints-supporting tissues.

Terms and definitions

Terms and definitionsCres depends on-Root length & MorphologyNumber of rootsLevel of alveolar bone support

Terms and definitionsVarious authors differ in the estimation of Cres.Methodology.For single rooted teeth-At 50% of root length-Proffit,NikolaiB/w 50%-33% of root length-Smith and BurstoneAt 33% of root length-BurstoneB/w 25%-33% of root length-NandaThe Cres of facial bones, entire arches of teeth, or segments can also be estimated

Terms and definitionsMultirooted teeth-

Maxillary anterior dentiton Cres - maxillary anterior teeth-distal to lateral incisor-NandaIncorporation of lateral incisors-small distal shift,canines-significant distal movement-Burstone & SachdevaMutirooted-close to bifurcation of the roots NandaTrifurcation-Upper I molar- Worms, Isaacson and Speidel

Terms and definitions

Cres of -maxilla-slightly inferior to orbitale-Nanda

Postero-superior ridge of the pterygomaxillary fissure registered on the median sagittal plane-Tanne et al

Terms and definitionsDetermination of the centers of resistance of all the individual teeth and groups of teeth using FEA.Conclusions-Longer the root, the more apically placed was the Cres.

The Cres of all teeth were slightly apical to the centroid of the teeth.

The Cres of Mand. Premolars lie at the same level, Cres of I Max. Premolar is more apical to that of the II premolar.

Terms and definitionsThe Cres of the maxillary and Mandibular molar lies at the tri/bifurcation respectively.

Intrusive forces on groups of teeth-Cres shifts posteriorly as more number of teeth were included in the segment.

For retractive forces on groups of teeth-Cres shifts coronally as more number of teeth were included in the segment.

Terms and definitionsPrecise location-not known, conceptual awareness needed.

Relationship of force systems to Cres of tooth-type of tooth movement

Terms and definitionsCentre of Rotation- a point around which an object rotates. -The geometric point about which no movement occurs

Point around which an object seems to have rotated as determined from its initial and final positions.

Terms and definitionsMethod for determining centre of rotation

Terms and definitionsCan be at any point ON or OFF the tooth

If there has been no rotation-infinity

If tooth has followed an irregular path-several centers of rotation

Principles of biomechanicsScalars and vectors

Force- vector -a load applied to a object that will tend to move it to a different position in space

F=mass x acceleration Newtons or gm.mm/sec2

Orthodontics- (mm/s2 )-irrelevant

Principles of biomechanics

1 Newton=101.937 grams (or) 1 gram=.oo981 N

1 Pound=16 Oz =.4536 Kg

1Oz =28.35 grams

Principles of biomechanicsDirection and magnitude of force -Origin/point of applicationMagnitudeSense/DirectionLine of action

Principles of biomechanicsMultiple vectors can be combined through vector additionSum of 2 or more vectors- Resultant

ResultantForce 1Force 2

Principles of biomechanicsDifferent points of application- Forces can be combined using the law of transmissibility of force.

When considering the external effects of a force on a rigid body the force may be considered to have a point of application anywhere along its line of action

Principles of biomechanicsResolving a force into vectors-

Principles of biomechanicsMoment Rotational tendency of a force that is not passing through the centre of resistance.

The magnitude of the moment=Force x lar distance of line of action (force) to the Cres

Principles of biomechanicsUnit - Gram millimeters (Newton millimeters)

Principles of biomechanics M=F x D

Principles of biomechanics

Principles of biomechanicsImportant to distinguish between force and moment.Cue ball concept

Principles of biomechanics

Couple consists of two forces of equal magnitude, with parallel but non-collinear lines of action and opposite senses. - Two equal and opposite parallel forces separated by a perpendicular distance. -Applied moments Pure rotation about the centre of resistance

Principles of biomechanicsMagnitude of a couple=Mag.of 1 force x perpendicular dist b/w them Unit-Gram.mm

Translational effects cancel out each other

Principles of biomechanicsMoment arm of a couple-

Principles of biomechanicsCouples result in pure rotational movement regardless of where the couple is applied on the object.

50 x 10=-500gm-mm50x30=1500gm-mm

500gm-mm is negative Thus 1000gm-mm just as in the previous case

Principles of biomechanicsClinically Centre of rotation coincides with the centre of resistance.

Principles of biomechanicsForces indicated by-

Moments indicated by-Moment of force-rotational tendency of a single force that does not pass though the Cres-(linear movement occurs)

Moment of couple-two forces that produce pure rotation (no linear movement)

Principles of biomechanics

Application of force/couple usually is at the brackets

Predicting the type of tooth movement-determine the Equivalent force system at the of resistance.

Equivalent force system- Analysis that replaces the applied force at the bracket with its equivalent at the at the Cres

Principles of biomechanicsThe 2 extreme situations-Force exactly at the centre of resistance-only linear movement (translation).Forces that produce pure rotation.

All situations in between the two produce some translatory and some rotatory movements.

So what are the forces at the Cres and what type of tooth movement results?

Principles of biomechanicsDetermining the force system at the CresPlace force vector at the Cres (maintain magnitude and direction)Calculate -moment of forcePlace MF - centre of resistanceApplied moment placed at the CresThe MF and applied moment added net momentResulting force - expected tooth movement 100gm10mm1000gmm1000gmm

Principles of biomechanics

Principles of biomechanics

Principles of biomechanics

Biomechanics Bio-mechanicsCmass /gravity/resCentre of rotationForces and MomentsCouple Moment of force/Moment of coupleEquivalent force system

Principles of biomechanicsEquivalent force system- It is the forces at the centre of resistance that determine how teeth move.

The force system placed at the bracket is equated at the Cres to determine the tooth movement.

Helps in-Prediction of the tooth movement

Equivalent force system- Analysis that replaces the applied force at the bracket with its equivalent at the the Cres

Principles of biomechanics

Principles of biomechanicsTypes of tooth movement-infinite variety but can be categorized into basic types uncontrolled Tipping controlledTranslation Root movementRotation

Principles of biomechanicsEach type of tooth movement is due to the variation in the amount of applied moment and force.

The ratio of the applied counter moment and the applied force experienced at the Cres is called the Moment- to- force ratio .

The M/F ratio of the applied force and moment determines the type of movement at the centre of ro

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