2
nd part
Mechanics of tooth-movement Mechanics of Orthodontic Appliance
1. 2.
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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.
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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.
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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.
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We know that according to Newton’s third law, 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: 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 (together) present must equal zero. www.indiandentalacademy.com
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). equilibrium
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ZERO
ZERO
ZERO
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Looking at the two unequal moments in 2nd 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.
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Now using the simple method of visual inspection we can classify the orthodontic appliance into various groups.
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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
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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.
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A
B
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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.
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1000 gmmm
50 gm
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50 gm
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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?
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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 ….
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Flash Player Movie
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With … 1st Premolar extraction Flash Player Movie
2nd Premolar extraction Flash Player Movie
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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 Begg0.018” 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.
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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
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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 www.indiandentalacademy.com
In vertical plane
In transverse plane
Gable bend distal to canine Mild anchor bend (in few selective cases) Constriction of upper arch Toe-in bend Molar offset bend
In sagittal plane
Cinch-back distal to molar tube
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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
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“Bio-mechanically, Begg mechanotherapy is most efficient in the first two stages�. And I am sure everybody agrees with me!!!
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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
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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
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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. effect Let us see…
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Anchor bend was suggested in the lower arch only in cases where a Class II elastic was to be used.
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What about using anchor bend in upper arch in stage III? But first, let us see the effects of anchor bend..
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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 …
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Flash Player Movie
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Inefficient understanding of Biomechanics Cook book approach Dogmatic view Clinician’s inefficiency …….. ….. ... . www.indiandentalacademy.com
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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 wellplanned 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.
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Prof. K. Sadashiva Shetty Principal & Head Dept. of Ortho. Bapuji Dental College Davangere
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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.
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