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Biomechanics in Clinical Orthodontics:

Biomechanics in Clinical Orthodontics Mechanics of tooth-movement Mechanics of Orthodontic Appliance 2 nd part

Predicting the Force Systems of Orthodontic Appliances:

Predicting the Force Systems of Orthodontic Appliances INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com

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In orthodontics we use various components (wires, elastics, etc.) to bring about desired tooth movement. Each component is a system by itself, and produces its own force(s) and couple(s).

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The force systems produced by orthodontic appliance activation must be resolved separately from the actual forces and moments that individual teeth will experience at their respective center of resistance.

Visual inspection method: :

Visual inspection method: It is a simple method used by a majority of orthodontists. For determining what forces are present, the arch wire is fully engaged into a bracket or tube and possible force system is evaluated by eyeballing.

Simple example:

Simple example

Slightly complex example:

Slightly complex example

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This method is based on laws of static equilibrium and if not understood correctly can confuse the orthodontists further. It is this method that so often leads us down the road to faulty conclusions. Many a times, it is because we don’t apply the simple concept of static equilibrium.

Static Equilibrium & Orthodontic Appliance:

Static Equilibrium & Orthodontic Appliance

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We know that according to Newton’s third law , every action has an equal and opposite reaction. In spite of knowing this, many orthodontists tend to forget to apply this law in their daily orthodontic mechanics in a simple and practical manner.

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Three requirements are accomplished automatically whenever static equilibrium is established. They are: The sum of all forces present must equal zero. The sum of all moments present must equal zero. The sum of all forces and moments (together) present must equal zero.

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Zero

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When we engage an arch wire fully, we are generating unequal moments. Forces are also generated to keep the system balanced. Therefore, all the moments and forces will sum up to zero (i.e. always results in static equilibrium ).

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ZERO ZERO ZERO

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Looking at the two unequal moments in 2 nd figure, it appears that the entire unit would rotate counter-clockwise. ZERO

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But by considering the forces generated we see that forces by themselves would cause the unit to rotate clockwise. Actually these are equal and opposite force and their sum equals zero. ZERO

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It is impossible to design an appliance that defies (disobey / confront) the laws of physics.

Next …:

Next … Now using the simple method of visual inspection we can classify the orthodontic appliance into various groups.

Mechanical Classification of Orthodontic Appliances:

Mechanical Classification of Orthodontic Appliances

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Orthodontic appliances can be divided into following three categories depending on their biomechanical mode of action (forces and moments): No couple appliance system One couple appliance system Two couple appliance system

No Couple Appliance System:

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 no moments are generated.

No Couple Appliance System:

No Couple Appliance System A B C

One Couple Appliance System:

One Couple Appliance System

One Couple Appliance System:

One Couple Appliance System 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 into the bracket/tube.

Statically Determinate:

Statically Determinate 1000 gmmm 50 gm 20mm 50 gm

Biomechanics of Begg Appliance :

Biomechanics of Begg Appliance All over the world, the Begg appliance is giving way to the new genre of appliances. In India, we are still churning out a huge number of Begg cases and all of us are well aware of the quality of results.

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We have managed to make Begg appear so simple that even non-orthodontists have tried putting their hand in the “Pie” (without knowledge). Who is to blame for this?

Biomechanics of Begg Appliance :

Biomechanics of Begg Appliance As we understand today the Begg appliance is a good example of single couple system. Stage I arch wire

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It is not just the base archwire which can be explained in terms of single couple system, but other components of Begg appliance works in a similar fashion, such as Torquing auxillaries Up-righting Rotation springs Let us see these ….

Up-righting Springs :

Up-righting Springs

COUPLE in Horizontal plane: :

COUPLE in Horizontal plane:

Uprighting spring (Horizontal Problem):

Uprighting spring (Horizontal Problem)

COUPLE in Vertical plane: :

COUPLE in Vertical plane:

COUPLE in Vertical plane: :

COUPLE in Vertical plane:

COUPLE in Vertical plane: :

COUPLE in Vertical plane:

One point contact :

One point contact

Uprighting spring (vertical problem):

Uprighting spring (vertical problem)

Uprighting spring (vertical problem):

Uprighting spring (vertical problem)

Uprighting spring:

Uprighting spring With … 1 st Premolar extraction  2 nd Premolar extraction 

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To overcome the problem of uprighting springs it was suggested to use Conventional  0.018” or 0.020” special plus Refined Begg 0.018” or 0.020” premium wire It was thought that such a stiff wire will prevent bending of archwire, but still it can rotate the base archwire within the slot of bracket.

Torquing auxillary:

Torquing auxillary

Torquing auxillary:

Torquing auxillary

Stage III (Antero-posterior problem):

Stage III (Antero-posterior problem)

Stage III (Antero-posterior problem):

Stage III (Antero-posterior problem)

Torquing auxillary (Antero-posterior problem):

Torquing auxillary (Antero-posterior problem)

Cinch-back in Stage III:

Cinch-back in Stage III

Torquing Auxillary (antero-posterior problem):

Torquing Auxillary (antero-posterior problem)

Torquing Auxillary (antero-posterior problem):

Torquing Auxillary (antero-posterior problem)

Toe-in bend :

Toe-in bend

Torquing springs (vertical problem):

Torquing springs (vertical problem)

Torquing springs (vertical problem):

Torquing springs (vertical problem)

Torquing springs (vertical problem):

Torquing springs (vertical problem)

Torquing springs (vertical problem):

Torquing springs (vertical problem)

Torquing springs (vertical problem):

Torquing springs (vertical problem) Intrusion forces on the posterior teeth especially the molars Intrusion force buccal to center of resistance of these teeth lead to intrusion of buccal cusps And transverse expansion of posterior teeth

Torquing springs (Transverse Problem):

Torquing springs (Transverse Problem)

Torquing springs (Transverse Problem):

Torquing springs (Transverse Problem)

Transverse flexibility of Stage III base archwire :

Transverse flexibility of Stage III base archwire 

Stage III arch-wire:

Stage III arch-wire Incorporation of numerous bends in base archwire was done to prevent various side effects of auxillaries. Upper stage III base archwire Lower stage III base archwire

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In vertical plane Gable bend distal to canine Mild anchor bend (in few selective cases) In transverse plane Constriction of upper arch Toe-in bend Molar offset bend In sagittal plane Cinch-back distal to molar tube

Gable bend (only)!:

Gable bend (only)! WHY?

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After we have Understood the advantages and side-effects of each individual components of Begg mechanotherapy, let us evaluate the whole system. Let us start with Stage I & Stage II of Begg mechanotherapy

Stage I & Stage II:

Stage I & Stage II “Bio-mechanically, Begg mechanotherapy is most efficient in the first two stages”. And I am sure everybody agrees with me!!! Why So ?

Stage I arch-wire:

Stage I arch-wire Exceptionally good anchorage control Couple at molar tube  high moment in posterior segment Bite opening mechanics Extrusion of posterior anchorage tooth due to high moment created Intrusion forces on anterior segment

Stage I arch-wire:

Stage I arch-wire

Stage III:

Stage III Very poor anchorage control Couple at incisors  high moment in anterior segment Bite deepening mechanics (Advantageous in selected patients e.g. open-bite cases) Side effect  extrusion of anterior teeth due to high moment created by auxillaries Side effect  intrusion forces on posterior segment Why So ?

Uprighting springs + Torquing springs + vertical bends in base archwire =:

Uprighting springs + Torquing springs + vertical bends in base archwire =

Addition of all uprighting springs:

Addition of all uprighting springs

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Scrutinization of the literature, reveals that there was little or no consideration provided to counteract the side effects of the stage III. For example the commonest problem of flaring of the molars and their intrusion was given no consideration.

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Hardly anybody has suggested, the use of anchor bend in the posterior segment in order to generate higher moments (in opposite direction) to the moments generated in anterior region. This simple bend (the trade mark of Begg appliance) was forgotten when starting stage III.

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And even to control the proclination of the upper incisors due to horizontal effects of the auxillaries, a (mild) Class II elastics was suggested. However class II elastic would add up to the extrusion component of the auxillaries on the incisors, worsening the bite-deepening effect . Let us see…

vertical problem:

vertical problem

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Anchor bend was suggested in the lower arch only in cases where a Class II elastic was to be used. What about using anchor bend in upper arch in stage III? But first, let us see the effects of anchor bend..

Solution for Problems in Stage III:

Solution for Problems in Stage III We saw that the moment generated in stage I and II was in the posterior segment and therefore excellent anchorage control.

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Let’s apply this basic knowledge to the stage III which has all of its moments concentrated in the anterior region , causing Proclination of dentition Extrusion of incisors Let us see this …

Stage III arch wire with anchor bend:

Stage III arch wire with anchor bend

The Tragedy of Begg technique:

The Tragedy of Begg technique Inefficient understanding of Biomechanics Cook book approach Dogmatic view Clinician’s inefficiency …….. ….. ... .

Conclusion :

Conclusion

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Forces and couples are applied to teeth to move them in the desired directions. Tooth movement is monitored regularly to assure that treatment proceeds smoothly and positively.

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Unwanted effects are corrected by adjustments along the way. The final result is achieved by a series of well-planned mechanical interventions that initiate and sustain a controlled biologic reaction.

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The components of comprehensive orthodontic treatment such as, preliminary alignment, overbite control, space closure, root paralleling, and finishing, rely on a series of biomechanical principles and process which need to be understood.

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The choice of appliances and techniques used by practitioners varies radically among individuals, but the fundamental forces and moments they produce are universal. Appliance will always act according to the laws of physics.

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Understanding the basic biomechanical principles involved in effective controlled tooth movement makes the final outcome more predictable and consistent.

Biomechanically Yours:

Biomechanically Yours Prof. K. Sadashiva Shetty Principal & Head Dept. of Ortho. Bapuji Dental College Davangere THANK YOU

Visual inspection method: :

Visual inspection method: It is a simple method used by a majority of orthodontists. For determining what forces are present, the arch wire is fully engaged into a bracket or tube and possible force system is evaluated by eyeballing.

PowerPoint Presentation:

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