B iom - echanics incanine retractionandanterior retraction INDIAN DENTAL ACADEMY Leader in continuing dental education
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INTRODUCTION
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Introduction
Extraction of teeth for the purpose of retraction of anteriors or for the correction of crowding is very common in orthodontics. The extraction space need to be closed. However, different cases require different treatment plans:
Total anterior retraction Only mesiallization of posteriors. Equal retraction and mesialization Or any combination www.indiandentalacademy.com
Introduction
Most approaches to space closure describe the technical features of the appliance,
Loops, Wire-bracket interactions Placement and size of elastics
… yet little attention has been devoted to the biomechanical principles of space closure.
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Introduction
Orthodontic space closure should be individually tailored based on the diagnosis and treatment plan. In this seminar I will cover,
the various theoretical aspects of space closure and methods to close the extraction site based on biomecanical concepts Sliding method Loop method.
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Two Schools of Thoughts for retraction of anteriors following Premolar extraction Separate canine and anterior retraction En-mass retraction
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First School of Thought ď Ž
ď Ž
ď Ž
The first school of thought states that canines and incisors are retracted separately to conserve anchorage The principle is that by retracting fewer teeth at a time, less stress is placed on the posterior anchorage. Moreover, by adding teeth to the posterior segment one can enhance the anchorage
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First School of Thought
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First School of Thought
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First School of Thought
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First School of Thought ď Ž
However, the anchorage is taxed twice with a separate canine and incisor retraction, as opposed to once with en masse retraction.
ď Ž
The posterior segment does not know how many teeth are being retracted, whether it is a single canine or an entire anterior segment; it merely responds according to the force system involved.
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Second School of Thought ď Ž
ď Ž
The second concept is that of en-masse retraction, where the canines and incisors are retracted together. Here, the method of anchorage is based on the types of tooth movement in the posterior and anterior segments and does not entirely depend on the number of teeth in each segment (e.g., translation or root torquing in the posterior segment Vs controlled tipping in the anterior segment). www.indiandentalacademy.com
Second School of Thought
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Second School of Thought
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Basics of Biomechanics Force Moment Couple Moment-to-Force Ratio
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DEFINITION ď Ž
Mechanics is the discipline that describes the effect offeres on bodies
ď Ž
Bio-mechanics refers to the science of mechanics in relation to biologic system.
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FORCE Force is a vector quantity, which acts on a body to change the state of rest or motion.
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POINT OF APPLICATION
Center of Mass Center of Gravity Center of Resistance
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CENTER OF MASS
ď Ž
ď Ž
It is a point in a body in a gravity free environment, which behaves as if whole mass is concentrated at that point. The center of mass is a point at which body can be balanced. Center of mass of a object in gravity is called center of gravity.
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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.
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The center of resistance of a tooth is dependent on
tooth length morphology number of roots level of alveolar support
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Moment Is defined as a tendency to rotate
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MOMENT OF A FORCE ď Ž
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.
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A moment may be referred differently in orthodontics.
Rotation Tipping Torquing
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Factors controlling the Moment: ď Ž
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). M =F x D M = Moment of force.
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CENTER OF ROTATION ď Ž
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
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The center of rotation can be at the center of resistance
apical to the center of resistance at the root apex at infinity At tip of the crown
Their position will determine the type of tooth movement.
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COUPLE Two equal and opposite, non-collinear forces
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MOMENT OF COUPLE ď Ž
Instead of a single force, we applied two forces equal in magnitude and opposite in direction at a distance, the moment thus created is called the moment of a couple
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ď Ž
The two forces cancel out any tendency for the body to move, but the moment created by the two forces does not cancel each other.
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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.
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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
Couple-1.swf
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Couple-3.swf
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Couple – Clinical point
Canine Bracket
Anterior Bracket www.indiandentalacademy.com
Factors controlling the Moment of a Couple
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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 d1 OR M= F2 x d2
F1
F2
d1 d2 F2
F1
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Couple – Clinical point
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MOMENT-TO-FORCE RATIO
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MOMENT-TO-FORCE RATIO ď Ž
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
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MOMENT OF FORCE (CLOCKWISE) ď Ž ď Ž
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).
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COUNTER-CLOCKWISE MOMENT
ď Ž
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 = Force (X) d = distance
F(X) x d = M(X) www.indiandentalacademy.com
M(X)
ď Ž
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.
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ď Ž
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.
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ď Ž
In addition to the force applied, a couple may also be engaged intentionally to partially correct, completely correct, or over-correct this tendency.
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ď Ž
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.
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Uncontrolled Tipping
8----Uncont-Tip.swf
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Controlled Tipping
9---Controlled-Tip.swf
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Translation
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Root Movement
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Pure Rotation
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ANCHORAGE
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ď Ž
Storey, Smith and Stuwed have show that :
5-55% of the total extraction space can be taken up by the anchor molar. ď Ž According to Salzman during space closure following extraction some amount of anchor loss will definitely take place.
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Begg stated that a differential force application (reciprocal light force) moves the anterior teeth while posterior teeth remain stationery.
ď Ž
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Classification of anchorage requirements during space closure Group A anchorage: ď Ž this category the critical maintenance of the posterior tooth position. 75% or more of the extraction space is needed for anterior retraction.
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Group B anchorage: ď Ž this category describes relatively symmetric space closure with equal movement of the posterior and anterior teeth to close the space. This is often the least difficult space closure problem.
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Group C anchorage: ď Ž this category the non critical anchorage. 75% or more of the extraction space is needed for posterior protraction.
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Total extraction space
C Posterior
Anterior
(mesial surface of 2nd PM)
(distal surface of canine) C www.indiandentalacademy.com
Anchorage balance during space closure
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RECIPROCAL SPACE CLOSURE (Group-B) ď Ž
If anchorage has been properly planned and controlled during earlier stages of treatment, reciprocal space closure can be the method of choice in many cases.
ď Ž
Theoretically, this leads to a 50:50 movement of incisors and molars, which is clinically acceptable in many cases, especially if the spaces are small.
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RECIPROCAL SPACE CLOSURE
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Space closure in maximum anchorage cases – crowding (Group-A)
Most of the premolar extraction space is used to relieve crowding in these cases.
careful anchorage control early in treatment, the space closure stage is normally minimal, because the available space has mainly been used to relieve crowding.
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RECIPROCAL SPACE CLOSURE
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Space closure in maximum anchorage cases – protrusion (Group-A)
Good anchorage control is needed. First premolars are normally chosen for extraction. Second molars are included in the set-up if possible. Palatal bars and lingual arches may be used during the alignment stage, to restrict mesial movement of molars. If patient cooperation is available, a headgear may also be considered, sometimes with Class III elastics.
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GROUP - A
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GROUP - A
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SPACE CLOSURE IN MINIMUM ANCHORAGE (Group-C)
These are cases where there is only mild crowding or protrusion. It is necessary to close remaining spaces mainly by mesial movement of molars. Second premolars may be chosen for extraction in this type of case,
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METHODS OF SPACE CLOSURE
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Methods Of Space Closure:
Driftodontics Orthodontics
Removable Fixed appliance
Sliding mechanics (Frictional method) Loop mechanics (Non-frictional method) Contineous looped arch Segmented loop
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It is important to restrict the Removable appliance treatment because of
Co-operation of patient Maintenance of good oral hygiene Poor control of tooth movement.
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RETRACTION WITH FIXED APPLIANCE Sliding mechanics Loop mechanics
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Retraction With Fixed Appliances:
Canine retraction:
Sliding mechanics Loop mechanics
Anterior retraction
Sliding mechanics Loop mechanics
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FRICTION MECHANICS
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FRICTION MECHANICS ď Ž
In friction mechanics, an elastic chain or thread is attached to the tooth and a continuous archwire is placed.
ď Ž
Elastic chain is the force component of the retraction assembly, with wire bracket interaction producing the moment components.
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Advantages of Friction Mechanics
The complicated wire bending is not required, making initial wire placement less timeconsuming. This can enhance patient comfort. Chances of going wrong is very less. Vertical force consideration can be avoided, so there will not be any first order or third order side effects.
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Disadvantages Of Friction Mechanics
Any thing that adds the friction slows the movement of teeth along the archwire. Chances of loosing anchorage is higher Chances of loosing anterior torque is higher Anterior can extrude while separate canine retraction
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Factors Affecting Sliding Mechanics 1.
WIRE MATERIAL: cobalt chromium, beta titanium and nickel titanium wires produces more friction than stainless steel wires.
2.
WIRE CROSS SECTION: rectangular wire produces more friction than round wires.
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3.
WIRE DIMENSIONS: larger diameter wires produces more friction than smaller
4.
BRACKET COMPOSITION: The composition of bracket eg., ceramic brackets, causes more friction than the stainless steel bracket.
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CANINE RETRACTION WITH SLIDING MECHANICS
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CANINE RETRACTION
Minor Cuspid Retraction
Major cuspid retraction
Uncontrolled tipping of 1-2 mm
Controlled tipping or translation of more than 3 mm.
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Minor Cuspid Retraction Minor Cuspid Retraction :- uncontrolled tipping of the canine, when 1-2mm of arch length per side is required. It can be carried out with the use of lace backs.
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Bennett and McLaughlin introduced lacebacks to reduce or minimize these side effects of “Roller-coster” effect. These are constructed using 0.009” or 0.010” ligature wire, tied in figure-of-eight fashion. They extend
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Walking canines: ď Ž
ď Ž
This involves slight initial tipping of the canine against the alveolar crest followed by a period of rebound caused by the binding of the arch wire with the bracket which causes the root to move distally. With elastic force this rebound time does not occur due to the presence of a continuous tipping force.
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Major cuspid retraction
Controlled tipping or translation of more than 3 mm of arch length per side is required. Where the canine’s axial inclination is ideal, it is preferable to translate the tooth. As the canine is retracted, the anterior crowding unravels. The lateral incisors tends to move distally due to pull of the trans-septal fibers.
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MOMENT
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COUNTER MOMENT
MOMENT www.indiandentalacademy.com
MOMENT
COUNTER MOMENT
The Desired Criteria As Follows :
Control of its first order position (rotation and labiolingual position) The center of rotation should be at the root apex or at infinity Optimal rate of tooth movement Minimal discomfort to the patient
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An elastic chain may be attached to the power arm on the cuspid bracket.
To pass the force as close to the center of resistance of the canine as possible, to achive translation movement.
When the force passes away the center of resistance.
The tooth experiences a moment in two planes of space.
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1.
Moment causes a mesial-out rotation of the tooth. 1.
2.
The mesial-out moment is an undesirable side effect.
Distal tipping of the crown. 1.
The distal tipping contributes to retraction by causing binding of the archwire, which in turn produces a moment that results in distal root movement. www.indiandentalacademy.com
Due to force of the elastic chain, the canine tooth initially tips distally,
followed by a period of “rebound” due to the leveling effect of the archwire- bracket interaction, which causes distal uprighting of the root.
REFRRED TO AS “WALKING OF THE CANINE”
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INCISOR RETRACTION MECHANICS
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INCISOR RETRACTION (SLIDING MECHANICS)
In friction or sliding mechanics an elastic chain or thread is attached to the tooth and a continuous archwire is placed. The elastic chain is the force component of the retraction. The wire-bracket interaction produces the moment
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The magnitude of this moment which causes the distal root movement depends on the
size shape material of the wire width of the bracket
Therefore, wires with a high load-deflection rate produce greater force when they are deflected and hence produce a greater moments.
Force is directly proportional to moment www.indiandentalacademy.com
Canine retraction
The wider the bracket, the greater is the is the moment
The distance at which the wire binds with the bracket increases
As the tooth uprights, the moment decreases until the wire no longer binds. The crown then slides along the archwire until distal crown tipping again causes binding.
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Process is repeated until tooth is retracted or the elastic force is dissipated Roth-retraction.swf
ROTH – RETRACTION
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MBTretraction.swf
MBT - RETRACTION
comparision.swf ali-sliding.swf
COMPARISION
ALI
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Different designs of Sliding techniques
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Passive tiebacks ď Ž
Before starting space closure, it is recommended that the rectangular steel .019 / . 025 wires be left in place for at least 1 month with passive tiebacks. This allows time for torque changes to occur on individual teeth and for final leveling of the arches, so that sliding mechanics can proceed smoothly when active tiebacks are placed.
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Active tiebacks using elastomeric modules ď Ž ď Ž ď Ž
In daily clinical practice, these are simple, economical, and reliable. Placement is not difficult and can be delegated routinely, with few complications. Active tiebacks using elastomeric modules are preferred for space closure in most cases.
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There are three methods of placing active tiebacks with elastomeric modules.
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Type one tieback.
Active tie-backs consist of an elastic module and ligature tie combination. www.indiandentalacademy.com
Type Two tieback.
2
Active tie-backs consist of an elastic module and ligature tie combination. www.indiandentalacademy.com
Type Three tieback.
2
Active tie-backs consist of an elastic module and ligature tie combination. www.indiandentalacademy.com
‘V’ BEND PLINCIPLE
When we place a bend in the middle of the wire and engage it into the brackets two equal and opposite moments are produced.
when an offset bend is placed in the wire like the anchor bend in Begg, a differential moment is created
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ď Ž
ď Ž
a greater clockwise moment in the posterior and a smaller anticlockwise moment in the anterior segment The side with the greater moment shows extrusion and there is intrusion on the side with the lesser moment.
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Frictionless Mechanics
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Frictionless Mechanics: ď Ž
ď Ž
In frictionless mechanics, the teeth are moved without the brackets sliding along the archwire. Retraction is accomplished with loop or springs. The force of a retraction spring is applied by pulling the distal end through the molar tube cinching is back.
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ď Ž
ď Ž
The moment is determined by the wire configuration and by the presence of pre-activation or of gable bends, which produce an activation moment. In general, the more wire gingival to the bracket, the more favorable the activation moment and therefore better the overall M/F ratio.
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ADVANTAGES OF FRICTIONLESS MECHANICS
Precise control over the anterior and posterior anchorage. The tooth will move only to the limit to which the loop is activated. Differential tooth movement is possible. Retraction loops or springs offer more controlled tooth movement than friction.
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DISADVANTAGES OF FRICTIONLESS MECHANICS : ď Ž
A good understanding of the mechanics is required when using retraction loops or springs, because minor error in mechanics can result in major errors in tooth movement.
ď Ž
More wire-bending skill and chair time are required than with sliding mechanics.
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ď Ž
Retraction loop may be uncomfortable to some patient, specially with less vestibular depth.
ď Ž
Retraction loops produce an undesirable mesial out moment when individual teeth are retracted.
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DIFFERENT FORCE SYSTEMSof Loop Mechanics
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: ď Ž
ď Ž
The force system of an orthodontic appliance determine the type of tooth movement expressed. The force act in all three planes of space (first , second and third order) creating moments.
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ď Ž
Alpha moment:- this is the moment acting on the anterior teeth(often termed anterior torque).
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ď Ž
Beta moment:- this is the moment acting on the posterior teeth. Tip-back bends placed mesial to the molars produce an increased moment.
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ď Ž
Horizontal forces:- these are the mesio-distal forces acting on the teeth.
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ď Ž
Vertical forces:- These are intrusive – extrusive forces acting on the anterior or posterior teeth. These forces generally result from unequal alpha and beta moments.
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CANINE RETRACTION BY FRICTIONLESS MECHANICS
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FIRST LET US SEE IN THE ANTERIOR SEGMENT
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Alpha and beta bends.
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Alpha and beta bends.
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Alpha and beta bends.
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NOW LET US SEE IN THE POSTERIOR SEGMENT
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T loop
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ď Ž
The T loop spring described by Burstone and subsequently refined or modified is a simple yet effective device for controlled space closure.
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The use of the T-loop for major cuspid retraction is recommended. It fulfills all the requirements of a retraction assembly.
It is easily fabricated at the chair. It is inexpensive. It is resistant to deformation. It is easily contoured for comfort. It has a adequate M/F ratio for controlled tipping or translation. Its M/F ration can be easily adjusted.
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CRITERIA FOR OPTIMAL SPRING
It should possess low load deflection rate. It should deliver optimal force. It should deliver proper M/F ratio which determines proper C.rot. Less bulky Hygienic
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ď Ž ď Ž
It is made form 0.017 x 0.025 inch TMA. The spring is first made passive from the auxiliary tube of the first molar to the cuspid slot, it must be remembered that the canine slot is slightly occlusal to the auxiliary tube of the first molar bracket that is why distal vertical leg of Tloop is kept 1 mm short.
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ď Ž
ď Ž
When more canine retraction is desired the Tloop is placed as close to the canine as possible. When more buccal segment protraction is desired, the T-loop is positioned closer to the molar auxiliary tube (Ace. to Marcotte).
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CONFIGURATION FOR THE BASIC T-LOOP SPRING 10mm
2mm 4mm
5mm 1mm
?mm
2mm
?mm
Pre-activation bends www.indiandentalacademy.com
PRE-ACTIVATION BENDS
1st bend
2nd bend
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3rd bend
4th bend
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6th bend
5 bend th
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EN MASS RETRACTION WITH FRICTIONLESS MECHANICS
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According to Charles Burstone, the moment-toforce ratio needed for translation is about 10:1. A regular 10 mm high vertical loop offers a moment-to-force of only 3:1 when it is activated by 1mm. In order to get a moment-to-force of 10:1, which is needed for translation, the activation should be reduced to as low as 0.2 mm but the force levels will not be sufficient to bring about retraction.
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In order to increase the moment ,the height of the vertical loop can be increased. The loop design can be varied, for example a T-loop has a higher moment-to-force ratio than a regular vertical loop. The most effective way to increase the momentto-force ratio is by placing pre-activation or gable bends.
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CLINICAL CONSIDERATION
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Alpha and beta bends.
200
150
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Alpha and beta bends.
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ALPHA AND BETA MOMENTS
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When Beta Bend Is Greater Than Alpha Bend What Happens ?
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GREATER BETA MOMENT
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CONCLUSION
Indeed it is a daunting and challenging job to treat open bite cases by now you will agree on that point I think.
“A thing started nicely is half work done” so taking this guideline and putting meticulous attention to biomechanics, I think we can very successfully finish retraction.
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THANK YOU
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CONCLUSION
Indeed it is a daunting and challenging job to treat open bite cases by now you will agree on that point I think.
“A thing started nicely is half work done” so taking this guideline and putting meticulous attention to biomechanics, I think we can very successfully treat the very difficult open bite cases . www.indiandentalacademy.com
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INTRODUCTION
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Common: _ _ _ _ Differences: _ _ _ _ www.indiandentalacademy.com
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Need for modification MOMENT
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Need for modification MOMENT
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Translation Bracket Features COUNTER MOMENT
MOMENT www.indiandentalacademy.com
MOMENT
COUNTER MOMENT
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Common: _ _ _ _ Differences: _ _ _ _ www.indiandentalacademy.com
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