BIOMECHANICS IN ORTHODONTICS
INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com www.indiandentalacademy.com
BIOMECHANICS IN ORTHODONTICS Orthodontic therapy depends on the reaction of the teeth and more generally the facial structures to gentle but persistent force. In orthodontics, biomechanics is commonly used in discussion of the reaction of the dental and facial structures to orthodontic force, whereas, mechanics is reserved for the properties of the strictly mechanical components of the appliance system. COMMON TERMINOLOGIES: FORCE MASS CENTER OF MASS (C.M.) CENTER OF RESISTANCE (Cres) CENTER OF ROTATION (Crot) MOMENT COUPLE (MF) MOMENT OF FORCE www.indiandentalacademy.com MOMENT OF COUPLE (MC)
FORCE: A load applied to an object that will tend to move the object to different position in space. Units: Newton Force equal to mass times acceleration. F = ma In metric system, it is usually measured in grams or ounces. In orthodontics forces are obtained in a variety of ways. Deflection of wires, activation of springs, elastics and magnets are the common means of producing orthodontic force. MASS AND WEIGHT: The mass of any body is the quantity of matter it contains. The quantity of mass of a particle can be measured by weighing the particle, since mass and weight are proportional but not the same. www.indiandentalacademy.com
CENTER OF MASS (C.M.): The point at which the mass of a body may be considered to be concentrated is known as center of mass. In other words the center of mass is an object’s balance point. E.g. in a space, if one could push a box in line with its center of mass, the box will translate away as its entire mass is concentrated at that single point. Practically speaking one can predict the behavior of any body in space if one knows forces in relation to their center of mass.  CENTER OF RESISTANCE (Cres): For an object in free space, the center of resistance is the same as the center of mass. If the object is partially restrained as in case of tooth embedded in bone, the center of resistance is equivalent to the balance point for restrained bodies.
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The center of resistance depends upon, · Root length and morphology · Number of roots · Level of alveolar bone support. The exact location of Cres for a tooth is not easily identified. Although Cres for single rooted teeth with normal alveolar bone levels is about one fourth to one third of the distance from the cemento-enamel junction, to the root apex. Cres for multirooted teeth lies just below the furcation area, i.e. 1-2 mm apical to the furcation. Cres of maxilla lies in the area of postero-superior aspect of zygomatico-maxillary suture. Although its precise location is typically unknown, it is important to have a conceptual awareness of Cres in selecting and activating an orthodontic appliance. The relationship of the force system acting on the tooth to the Cres determines the type of tooth movement expressed. It is the point through which pure force will produce only translation i.e. all the points on the tooth moving in parallel straight line.
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CENTER OF ROTATION (Crot): It is the point around which rotation actually occurs when an object is being moved. Depending upon the force system applied, the center of rotation may vary. E.g. In case of controlled tipping center of rotation will be at root apex while in case of perfect translation it will be at infinity. Â MOMENT: Is product of force times the perpendicular distance from the point of the force application and to the center of the resistance. Moment = Force x Perpendicular distance from Cres to point of force application Thus it is measured in the unit of gm-mm. www.indiandentalacademy.com
COUPLE: Two forces equal in magnitude and opposite in direction produce couple. The result of applying two forces in this way is a pure moment, since translatory effect of the forces cancels out. A couple will produce pure rotation, spinning the object around its Cres. While combination of a force and a couple can change the way an object rotates while it is being moved.
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MOMENT OF FORCE (MF): If the line of action of an applied force does not pass through the center of resistance, the force will produce some rotation. The potential for rotation is measured as moment and the magnitude of the moment is equal to the magnitude of the line of force multiplied by the perpendicular distance of the line of the action of force to the Cres. The direction of moment can be determined by continuing the line of action of the force around the Cres. Unit of MF is gm-mm. Two factors determine MF. 1) Magnitude of force 2) Distance. www.indiandentalacademy.com
MOMENT OF COUPLE (MC): Another method of achieving rotational movement is through the MC. A couple is two parallel forces of equal magnitude acting in opposite direction separated by a distance. The magnitude of a couple is calculated by multiplying the magnitude of force by distance between them. Unit: gm-mm. The direction of the rotation is determined by following the direction of either force around the Cres to the origin of the opposite force. The moment of a couple is the product of one of the forces times the distance between the two forces. This distance is called “the moment arm of the couple�. When the tooth is embedded in alveolar bone, we cannot apply a couple with one force on the crown and the other force on the root. We all know and have experienced that the force required to turn a bigger wheel is considerably lesser than that required to turn a smaller wheel. The reason for that being, in the bigger wheel the moment arm is larger and hence the force to generate a moment of www.indiandentalacademy.com particular magnitude is less.
In order to retract incisor we apply a force on the crown of the tooth. This force creates a moment, as it is away from the center of resistance and will cause tipping. To keep tipping of the tooth to a minimum we have to create a moment on this tooth in a direction opposite to that created by the force. This can be done easily by applying a couple having an anticlockwise moment.
A force of 100 gm acting at a distance of 10 mm from the Cres of a tooth, produces a clockwise or negative moment of 1000 gm-mm which will cause the tooth to tip. Since tipping is undesirable, we must generate a counter balancing moment of 1000 gm-mm so that a bodily movement is obtained.This can be achieved by twisting the anterior segment of the rectangular wire and fitting it into a rectangular slot. Once the wire is engaged in the bracket slot it generates an “Inherent moment of couple�, which is nothing but the couple produced within the wire itself. In a rectangular wire, the moment arm is the depth of the bracket,which is very small. www.indiandentalacademy.com
Since moment is force times the distance the force is equal to moment divided by the distance. Thus it requires a large force to generate the counter balancing moment.
Inherent moment of a couple
‘d’
Inherent couple acting at a distance from the Cres producing secondary moment of a couple
From our clinical experience we know that such heavy forces are not required to achieve bodily movement. The reason for that being, the moment of couple generated by torquing the rectangular wire acts at a certain distance from the Cres of the tooth. This again produces a moment of couple called “secondary moment of couple”. This secondary moment of a couple adds to the inherent moment of a couple generated by the rectangular wire. www.indiandentalacademy.com
TYPES OF TOOTH MOVEMENT: Basic tooth movements are categories into, 1. Tipping 2. Translation 3. Root movement 4. Rotation Each movement is the result of variation of the applied moment and force (either by magnitude or point of application). Tipping: Is greater movement of the crown of the tooth than that of the root. Crot is apical to the Cres. Tipping can be further classified on the basis of the location of the center of rotation as Uncontrolled tipping and Controlled tipping. www.indiandentalacademy.com
Uncontrolled tipping A horizontal force at the level of bracket will cause movements of the root apex and crown in opposite directions. This is simplest type of tooth movement. It requires single force and no applied moment.
Crot lieswww.indiandentalacademy.com just below the Cres.
Controlled tipping It is achieved by an application of force to move the crown, as done in uncontrolled tipping and application of a moment to control or maintain the position of the root apex. Crot lies at the root apex
M/F ratio = 7:1 www.indiandentalacademy.com
Translation: This type of tooth movement is also known as ‘bodily movement’. Translation of a tooth takes place when the root apex and crown move the same distance and in the same direction. A horizontal force applied at the Cres of a tooth will result in this type of tooth movement. However, the bracket where the force application takes place is at a distance from the Cres. This force alone applied at the bracket will not result in translation. To achieve translation at the level of the bracket, a couple of forces are required that are equivalent to the force system through the Cres of tooth. Point of force application – Cres Center of Rotation – Infinity. M/F = 10:1 www.indiandentalacademy.com
Root movement (TORQUE): Root movement is achieved by keeping the crown of a tooth stationary and applying a moment and force to move only the root. Root movement is termed as ‘torque’. Point of force application – a point apical to the Cres Center of Rotation – at the incisal edge or bracket.
M/F = 12:1
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The simplest way to determine how a tooth will move is to consider the ratio between moments created when a force is applied to the crown of a tooth (moment of force MF) and the counter balancing moment generated by a couple within the bracket (moment of couple Mc). MC/MF = 0 Pure tipping (tooth rotates around the Cres). 0 < MC/MF < 1 Controlled tipping (Inclination of tooth changes but the Crot is displaced away from the Cres and the root and crown move in the same direction.) MC/MF= 1 Bodily movement (equal movement of crown and root) MC/MF > 1 Torque (Root apex moves further than www.indiandentalacademy.com crown)
Pure rotation: This type of tooth movement occurs when tooth rotates about its center of resistance. A couple is required to produce pure rotation.
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Intrusion and extrusion: It is tooth movement in axial direction. Intrusion is the bodily displacement of a tooth along its long axis in an apical direction. Extrusion is bodily displacement of a tooth along its long axis in an occlusal direction.
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FORCE SYSTEMS: In order to achieve the described tooth movements, the proper force system is a critical requirement. The following factors related to the force system are potentially under the control of the clinician. 1. Moment-to-force ratio 2. Constancy of forces and moments. 3. Magnitude of forces and moments. Moment-to-force ratio: In order to produce tooth movement other than uncontrolled tipping by applying a force system only at the bracket, a single force alone is insufficient; a rotational tendency (a moment) must be applied at the bracket. The proportion of rotational tendency (moment) to the force applied at the bracket will determine the type of tooth movement. This is represented by M/F at the bracket. Moment-to-force ratio plays an important role in anchorage control. By varying the moment-to-force ratio applied to the anterior and posterior segments during space closure after bicuspid extractions, the amount of forward displacement of the posterior segments can be controlled. www.indiandentalacademy.com
Force constancy: If we accept the assumption that a relatively constant force within an optimal range produces the most desirable type of tooth movement, then we will have to design the active components of an appliance such that they have desirable spring properties as follows. A) Load deflection rate of the spring appliances. B) Frictionless force application system. Load deflection rate: Refers to the amount of force produced for every unit of activation of an orthodontic wire or spring. The lower this rate, the more constant is the force as the tooth moves and the appliance is deactivated. Four major design parameters available to the clinician to vary the load deflection rate are: 1. Wire cross-section. 2. Wire length. 3. Wire material. 4. Wire configuration. Â Load deflection rate varies directly as the fourth power of the diameter of a round wire and as the third power of the depth of a rectangular wire. Â www.indiandentalacademy.com
L.D.R. Îą wire cross section
Therefore, reducing the cross section of the wire can significantly reduce the load deflection characteristics of an orthodontic appliance. Also the size must be such as to prevent permanent deformation during mastication, thereby restricting the wire cross-section within the maximal elastic strength of the wire. On the other hand those parts of the appliance that are concerned with preservation of anchorage require a relatively rigid wire with a large cross-section. The effect a rigid wire attachment between anchorage teeth is to enhance the anchorage potential of these units by producing a more advantageous stress distribution in the periodontal structure and to prevent the movement of the anchorage unit. www.indiandentalacademy.com
Wire Length: The wire length changes the load deflection rate inversely as the third power. L.D.R. Îą
1 Wire length
Therefore, small increase in length of wire can also dramatically reduce the load deflection rate. In continuous arch multibanded appliance, the inter-bracket distance between adjacent teeth dictates the wire length to a great extent, although some length can be added to the wire by using loops. Long wire with a longer inter-attachment distance delivers a more constant force magnitude as well as a more constant force direction as the teeth move to the new desired positions. www.indiandentalacademy.com
Wire material: For designing appliances, stainless steel alloys are in common use today. In order to improve the characteristics of the stainless steel wire, multi-stranded wires with greater flexibility (reduced load-deflection rate) have been introduced. Alloys such as NiTi and Beta titanium with low modulus of elasticity and high spring back have radically changed appliance design. Â Â Wire configuration: In order to best utilize the effect that wire length has on load deflection rate, the design of the wire configuration should be carefully considered. By placing more wire at the regions where bending deflections are the greatest and at the regions where the bending moment is large, the load deflection rate can be optimally reduced www.indiandentalacademy.com
Frictionless force appliance system: In order to achieve constant force and moment levels, sliding frictional forces must be eliminated or reduced. Â Force and moment magnitude: The magnitude of force and moment is the third parameter with respect to force system. The accuracy in determining and maintaining force and moment levels become more critical in achieving the desired treatment goals. Thus the appliance with low load deflection rate is the system of choice in accurately calibrating these levels. A small error in activation of spring with a high load deflection rate will result in a larger error in the activation force. In addition to the consideration of tissue damage, force and moment magnitude are important in anchorage control. Distributing the force over more teeth can reduce the stress levels on the anchor units www.indiandentalacademy.com
To achieve a more advantageous pattern of stress distribution in the periodontium of the anchor units, use of heavy rigid arch wires is recommended so as to allow all the teeth in the anchor unit to react uniformly. Applying a moment there by, preventing the tipping and permitting the anchor units to translate can further enhance this. Therefore, carefully monitoring the magnitude and M/F ratio (of the force system), stress levels in the anchorage unit can be controlled and thereby reduce the use of an extra oral appliance which requires patient cooperation. Â Biomechanical considerations serve not only to explain the effect of an orthodontic appliance but also to detect side effects of therapy and to assist in planning strategies for the avoidance or therapeutic exploitation of these side effects. Efficient orthodontic treatment requires that sound www.indiandentalacademy.com treatment plans be carried with sound mechanical plans.
â&#x20AC;&#x2DC;Vâ&#x20AC;&#x2122; BEND MECHANICS Appliances are being refined and will continue to improve with the passage of time. This is good, but the danger lies with the individual who fails to recognize that the refinement of appliances may reduce the physical effort put forth in treatment, but will not eliminate the need for the Orthodontist to think, understand and apply basic principles of mechanics to his treatment. This means that regardless of how well we understand mechanics and regardless of how much the appliance is refined, we are dealing with a biologic environment whose variation in response will continue to challenge the Orthodontist in many ways. Â www.indiandentalacademy.com
CENTERED AND OFF-CENTERED â&#x20AC;&#x2DC;Vâ&#x20AC;&#x2122; BENDS If the bend is located OFF-CENTRE, there will be a long segment and a short segment. When the short segment is engaged into the bracket or tube, the long segment will point in the direction of the force produced on the tooth that will receive the long segment. The short segment points in the opposite direction of the force that will be produced on the tooth that receives the short segment.
Fig.1
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Fig.2
If the bend is in the CENTRE, there no longer exists a long or short segment. Therefore, no force is produced because when the wire is engaged in the brackets, forces cancel each other leaving pure moments - not a bad situation when we wish to parallel roots following space closure, or rotate teeth equally and oppositely.
Fig.4
Fig.3 www.indiandentalacademy.com
FORCES AND MOMENTS Whenever a force passes through the Cres of a body, there is no moment produced and therefore no rotational tendency. However, when a force acts away from the Cres, a moment is produced and a rotational tendency occurs. The moment produced is equal to the perpendicular distance from the line of force to the Cres of the tooth.
Cres of a tooth
Moment of a force
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CUE-BALL CONCEPT Â No left or right rotation is produced when the force is applied through the centre of the cue ball. A force offcentre causes the cue ball to rotate as well as move forward in a straight line.
Force throâ&#x20AC;&#x2122; Cres no rotation
Force off-centre rotation + forward movement
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Equal and opposite forces (couple) -pure rotation
DIFFERENTIAL TORQUE When the arch wire with the tip back bend is tied into the brackets and tied back at the molar tubes different magnitudes of moments are produced which are referred as differential torque.
But, again that is not all that is taking place. There are other forces and moments taking place at the same time, which will produce molar extrusive forces, incisor intrusive forces, molar mesial root torque significantly larger than the incisor lingual root torque, and molar lingual crown torque. www.indiandentalacademy.com
Molar – Extrusion + mesial root tip + lingual crown toque Incisor – Intrusion + labial crown torque
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STATIC EQUILIBRIUM Understanding the concept of equilibrium is crucial to understand the mechanics of tooth movement. Static equilibrium is a valuable application of Newton’s laws of motion to the analysis of force system delivered by orthodontic appliances. Equilibrium is defined as “a state of balance between or among the opposing forces, resulting in the absence of acceleration.” The concept has its basis in Newton’s first and second law of motion.
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1. LAW OF INERTIA: “A body at rest remains at rest and a body in motion remains in uniform motion in a straight line unless acted upon by the external force”. 2. Law of acceleration: “The acceleration of body is directly proportional to the applied force and is in the direction of the straight line in which the force acts.” 3. Law of action and reaction: “For every action there is always opposing and equal reaction.” www.indiandentalacademy.com
Three requirements are automatically fulfilled whenever static equilibrium is established. 1.The sum of all vertical forces must equal zero. This is why we must deal with extrusive components of force during overbite correction. Since we cannot eliminate these forces, we must learn to control them.
2.The sum of all horizontal forces must equal zero. This is why we cannot correct a unilateral cross bite with a single horizontal force.
  3.The sum of moments acting around any point must also equal zero. www.indiandentalacademy.com
A Symmetric V- bend creates equal and opposite couples at the brackets with no forces, because the associated equilibrium forces at each bracket are equal and opposite, and therefore cancel each other out. A symmetrical V-bend is not necessarily half way between two teeth or two groups of teeth. If two teeth are involved but one is bigger than the other (e.g. a canine and lateral incisor), equal and opposite moments would require placing the bend closure to the large tooth, to compensate for the longer distance from the bracket to its centre of resistance
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An Asymmetric V-bend creates a greater moment on one tooth or unit than the other. As the bend moves toward one tooth, the moment on it increases and the moment on the distant tooth decreases. When the bend is one-third of the way along the inter bracket span, the distant tooth receives only a force, with no moment. If the v-bend moves closer than the one-third point to one of the teeth, a moment in the same direction is created on both teeth, instead of opposite moments
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Step bend is a combination of two off-centre bends with short sections bent in opposite directions.
It creates two couples in the same direction regardless of its location between the brackets. The location of a V-bend is a critical variable in determining its effect, but the location of a step bend has little or no effect on either the magnitude of the moments or the equilibrium forces. www.indiandentalacademy.com
DETERMINATE Vs INDETERMINATE FORCE SYSTEMS Â Force systems can be defined as statically determinate, meaning that the moments and forces can readily be discerned, measured and evaluated, or as indeterminate. Statically indeterminate systems are too complex for precisely measuring all forces and moments involved in the equilibrium Typically, only the direction of net moments and approximate net force levels can be determined. Determinate force systems, therefore, are advantageous in orthodontics because they provide much better control of the magnitude of forces and couples. For all practical purposes, determinate systems in orthodontics are those in which a couple is created at one end of an attachment, with only a force (no couple) at the other. www.indiandentalacademy.com
This means that a wire that will serve as a spring can be inserted into a tube or bracket at one end, but must be tied so that there is only one point of contact on the other. When the wire is tied into a bracket on both ends, a statically indeterminate, two couple system is created. In orthodontic applications, one couple systems are created in two conditions 1) Cantilever spring applications 2) Auxillary intrusion / extrusion arches Molar – Intrusive force 50 g Buccal crown torque Mesial crown tip (20 mm x 50 g = 1000 g-mm.) Canine – Extrusion 50 g L Lingual crown torque
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Two couple systems are created when intrusion utility arch is tied into the incisor brackets as well as in transpalatal and lingual arches. Molar â&#x20AC;&#x201C; Extrusion Distal crown tip Incisors -Intrusion Labial crown torque
However the precise magnitude of forces and couples cannot be known.
The basis of orthodontic treatment lies in the clinical application of biomechanical concepts, which explain the mechanism of action of orthodontic appliances. If these biomechanical principles are applied to mechanotherapy, not only may treatment time be reduced, but one could also develop more individualised treatment plans for achieving more predictable results. Cognitive application of biomechanical concepts in the delivery of orthodontic care can be beneficial in achieving efficient and effective treatment. www.indiandentalacademy.com
INTRODUCTION The three dimensional control of single teeth exhibiting severe positional anomalies is a common challenge for the Orthodontist. A through clinical diagnosis, careful treatment planning and appropriate appliance design is necessary if successful outcome is to be achieved. Super elastic wires, elastic threads and chain elastics have made the straight-wire appliance technique very efficient for the correction of minor discrepancies. In contrast, the segmented arch approach enables the orthodontist to generate well-defined force systems that lead to highly controlled tooth movement. By segmenting the appliance, an optimal force system can be applied to the active unit and to the tooth to be moved, while the reactive forces are transferred to an anchorage unit consolidated to withstand the undesirable forces. Correction of positional discrepancies of single teeth is highly predictable, and any undesirable side effects can be minimized. A low load-deflection rate enables the clinician to deliver relatively constant forces and moments throughout the orthodontic treatment, and allows tooth movement to proceed without frequent monitoring and appliance adjustments. When maximum control of tooth movement is desired, rectangular loops are the first choice for intra-segmental alignment, due to their simplicity and large range of activation. www.indiandentalacademy.com
When a straight wire is inserted into the brackets of malaligned teeth an odd combination of forces and moments, such as an inconsistent force system where moments are desirable yet forces detrimental, or vice-versa may develop. Minor bends, such as step and â&#x20AC;&#x2DC;Vâ&#x20AC;&#x2122; bends, have been recommended with the purpose of delivering the desired moment-to-force ratio on one side of the bend. As the wire is always in a state of equilibrium, a balancing force system acts on the other side of the bend. The creative bend involves only a minimum amount of wire. Consequently, the spring-back and the activation range of the bends are low, and the load-deflection rate high. In addition, the slightest change in tooth position will lead to change in the desired force system. Only minor discrepancies, therefore, should be corrected by using such bends.
FIRST ORDER BEND
SECOND ORDER BEND www.indiandentalacademy.com
The simultaneous correction of major discrepancies in three planes of space requires an appliance that delivers a specific force system, which is relatively constant and within a large range of activation. This can be obtained by loops individually designed for the correction of specific irregularities.
ADVANTAGES OF SEGMENTED ARCH TECHNIQUE: 1.
2. 3. 4. 5. 6. 7.
Segmentation offers the possibility of using multiple wire cross-sections and materials within the same arch. This permits a great deal of versatility in the selection of proper wire for a given tooth movement for an optimal force. Segmentation increases the distance between points of force application. This lowers the load-deflection rate of the wire. The active and reactive forces occur between segments so that the forces can be distributed over many teeth. Alignment can be achieved without initial flaring and hence round tripping of teeth can be minimized. A segmented arch can be prefabricated so as to not only increase office efficiency but also give greater accuracy to the Orthodontist in force control. The force levels can be easily calibrated. All segments need not be routinely replaced as treatment progresses thereby wire duplication can be kept to a minimum. Early visits may be longer because of the larger number of arch wires placed at the start of active treatment in the segmented arch technique hence improvingwww.indiandentalacademy.com clinical efficiency.
ADVANTAGES OF A LOOP: 1. The inconsistency of the force system developed by a SWA can be avoided by using loops. 2. The addition of wire length into the appliance while maintaining the wire size reduces the load-deflection rate. 3. Greater constancy of force. 4. Since the distribution of the wire with respect to the bracket determines the moment-to-force ratio, and tooth movement is produced by the deactivation of the loop itself, friction is not an issue. 5. It is possible to design a loop in such ways that forces and moments are dissociated to generate many combinations of moment and force. www.indiandentalacademy.com
A
A
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6.The desired combination of moments and forces can be reached by choosing different points of force application, controlling the horizontal dimension of the loop or by angulating the horizontal arm of the loop. 7.The rectangular loop has a low load-deflection rate and a large range of activation. 8.Combining wires of different dimension can produce composite loops. For correcting major rotations or tipping, the combination loops are advantageous as their working range is large.
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TYPES OF LOOPS USED FOR INTRA-SEGMENTAL ALIGNMENT:
1. Vertical loop 2. L-loop 3. T-loop 4. Rectangular loop www.indiandentalacademy.com
THE RECTANGULAR LOOP For a given tooth movement, only one combination of force and moment can be considered correct. The active components of the orthodontic appliance must be designed in such a manner that they posses a low loaddeflection rate and a frictionless force application system.
CHARACTERISTICS:
1. 2. 3. 4.
5.
Can be used for first, second and third order corrections Since the loop is inserted in at least two brackets, it represents a statically indeterminate force system. The clinician can determine the moment-to-force ratio delivered to the active unit. All combination of moments and forces can be produced. The direction of moment generated at the loop depends on the point of force application in relation to the horizontal dimension of the box. The point at which the moment changes sense is called â&#x20AC;&#x2DC;point of dissociationâ&#x20AC;&#x2122;. At this point, no relationship exists between moment and force. The localization of this point depends on the length as well as the dimension of the wire. www.indiandentalacademy.com
FABRICATION OF ‘R’ LOOP:
Fabrication of R loop for the 2nd premolar correction: Step 1: Measure the distance between mesial of molar tube and the distal of 2 nd premolar bracket (D) Step 2: The ‘R’ loop is fabricated using the formula A = B = C each being equal to half of D. Note: Distance D for any tooth is measured from the distal of the bracket (of the tooth to be corrected) to the mesial of the bracket (of the tooth distal to it). A
B
C
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A=B=C
VARIOUS ACTIVATIONS:
‘R’ loop can be effectively used for the correction of : Rotation First order discrepancies Second order discrepancies
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COMPOSITE LOOPS:
Differences between the stiffness of the active and reactive units can be varied producing Composite loops. E.g., Combining 0.017 x 0.025â&#x20AC;? and a 0.018â&#x20AC;? TMA wires Depending on the point of welding, this will displace the point of dissociation from the geometrical center of the loop. When correcting major rotations or tipping, the composite loops are advantageous as their working range is large. They can also be designed for a correct moment-to-force combination. www.indiandentalacademy.com
‘T’ LOOP Characteristics: 1. 2.
3.
Made of 0.017”x 0.025” TMA wire No side determination be made, however, the alpha leg (anterior leg) of the T loop is longer than beta leg (posterior leg) by 1mm to compensate for the difference of height between the bracket of the canine and the auxillary tube of the molar. The central position of the loop can be calculated by the formula
D=L-A 2 Where, D = distance from either the molar auxillary tube or the canine to the center of the loop L = distance from the molar auxillary tube to the canine vertical tube (or center of the bracket) A= activation of the spring www.indiandentalacademy.com
10 mm 2 mm 4 mm BETA (POSTERIOR) SEGMENT
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5 mm ALPHA (ANTERIOR) SEGMENT
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PREACTIVATION CHECK LIST: 1. Check the neutral position of the loop (0 mm). 2. Determine the amount of activation. 3. From the center of the T, mark distance D on both arms of the spring. Place a vertical bend gingivally 5mm anterior to the mark on the anterior leg. 4. Check for comfort and passivity and necessary adjustments are made to achieve the same. 5. Placement of Alpha and Beta preactivation bends: Preactivation bends are placed at six points in the spring •Bend 1: is at the mesial loop of the T loop. A light wire plier is placed between the looped gingival arms of the Tspring with the round beak along the inside of the loop, which is then opened approximately 300. www.indiandentalacademy.com
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•Bend 2: is placed at the distal loop of the T-spring and the loop is opened by 300 as was done for bend 1. •Bend 3: is placed 2mm mesial to the anterior leg of the Tloop, which is bent approximately 250 to the base arch wire. •Bend 4: is placed 2mm distal to the posterior leg of the Tloop, which is bent approximately 250 to the base arch wire. •Bend 5: is made 2mm mesial to the anterior leg of the T-loop, which is bent approximately 250 to the base arch wire. •Bend 6: is made 2mm distal to the posterior leg of the T-loop, which is bent approximately 250 to the base arch wire. Note : all bends are made around the round beak of the plier. www.indiandentalacademy.com
Perform trial activation: grasping the spring with two pliers, one just anterior to bend 5 and other just posterior to bend 6, the activation force is applied to the spring. The spring is then checked on the template for superimposition. Anti-rotation bends: are placed after the trial activation. Anti-rotation bends are placed at 4 different points, 2 at alpha position and 2 at beta position. •Bend 1: is made by placing the plier along the gingival portion of the Tloop approximately 1 mm from the mesial loop. The lower portion of the T-loop is then pulled to the buccal so that it makes an angle of 250 to its original position. •Bend 2: is placed 2 mm mesial to the anterior leg of the T-loop and the loop bent approximately by 250 in the same manner as the first bend. •Bend 3: similar to bend 1, approximately 1 mm mesial to the distal loop. •Bend 4: similar to bend 2, approximately 2 mm distal to posterior leg. The number of anti-rotation bends may be based on clinical judgement. Additionally lingual elastics can be used to prevent rotation of the canines. www.indiandentalacademy.com
ANTIROTATION BENDS 1
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OCCLUSAL VIEW
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ATTRACTION SPRING TMA 0.017X0.025” R CENTERED ∆
mm
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
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gm
gm
0.0 28.4 50.1 72.1 94.7 116.7 140.7 162.7 185.0 208.8 232.9 257.5 281.8 307.3 333.4
4.9 5.4 5.5 6.1 6.6 7.2 8.0 8.6 9.4 10.1 10.9 11.6 12.3 12.9 13.6
gm- mm
Mα
gm- mm
Mβ
gm/mm
Mβ
Mα/F
Mβ/F
1361.9 1464.0 1556.1 1641.1 1724.1 1801.2 1875.8 1943.5 2009.5 2074.9 2131.2 2187.5 2243.8 2293.8 2348.8
-1410.4 -1501.8 -1583.7 -1663.4 -1740.3 -1815.7 -1887.1 -1955.1 -2019.9 -2085.9 -2145.6 -2206.3 -2261.6 -2316.6 -2367.6
00.0 55.1 43.6 44.1 44.8 43.7 47.8 45.0 44.6 46.9 49.4 48.9 48.6 50.7 52.0
00.0 52.0 31.2 22.8 18.2 15.4 13.3 12.0 10.9 9.9 9.2 8.5 8.0 7.5 7.0
00.0 53.4 31.7 23.1 18.4 15.6 13.4 12.0 10.9 10.0 9.2 8.6 8.0 7.5 7.0
∆ - ACTIVATION FH - HORIZONTAL FORCE FV – VERTICAL FORCE Mα - ALPHA MOMENT Mβ - BETA MOMENT www.indiandentalacademy.com
mm
mm
OVERBITE INTRODUCTION The deep bite can be defined by the amount and percentage of overlap of lower incisors by the upper incisors. The overbite may be calculated as a percentage of the clinical crown height of one of the mandibular central incisors. Fleming showed that between 9 and 12 years of age the overbite is usually increasing, whereas in the period between age 12 and adulthood it is decreasing. No sex differences were noted. More over, the amount of vertical overbite is closely associated with some craniofacial dimensions. The study also determines that ramus length is one of most important dimensions associated with the amount of overbite.
Correction of dental overbite can be achieved by: 1. 2. 3. 4.
Genuine intrusion of the anterior teeth Extrusion of posterior teeth Combination of anterior intrusion and posterior extrusion. The skeletal vertical dimension, the AB relationship, the occlusal plane cant desired after treatment, growth and muscular factors, dictate which method is used to correct the deep bite. www.indiandentalacademy.com
According to Burstone, there are six principles governing deep overbite correction by intrusion: 1. Use of optimal magnitude of force and the delivery of this force constantly with low load-deflection rate springs. 2. Use of point contacts in the anterior region. 3. Position of force-careful selection of the point of force application with respect to the Cres of all the teeth to be intruded. 4. Selective intrusion based on anterior tooth geometry. 5. Control over the reactive units by formation of a posterior anchorage unit. 6. Inhibition of the eruption of the posterior teeth and avoidance of undesirable eruptive mechanics. www.indiandentalacademy.com
BIOMECHANICS OF INTRUSION: An intrusive force through the center of resistance of any tooth will intrude the tooth without producing any labial or lingual rotation of the tooth being intruded. The center of resistance of anterior teeth can be estimated to be located near the geometric center of their roots. Ideally while intruding, we want the intrusive force through the Cres of the tooth or group of teeth to be intruded. If the intrusive force is labial to the center of resistance, a moment is produced which flares the crowns labially while the roots move lingually. www.indiandentalacademy.com
To prevent this flaring, the intrusion arch is tied back posteriorly.
Patients presenting with procumbent incisors must be handled somewhat differently during intrusion, because the intrusive force is farther from the Cres, a much greater moment occurs and much more lingual root movement occurs. This is contraindicated because the roots are too far lingually placed initially. www.indiandentalacademy.com
This situation can be handled in one of the two ways: 1. One approach is to retract the anterior teeth first and produce more upright axial inclinations and then proceed with intrusion. 2. The second approach is to apply the vertical force lingual to the center of resistance either with a continuous intrusion arch or a three-piece intrusion arch. Indiscriminate leveling with a continuous arch or with a sectional wire can produce undesirable side effects. Many times the overbite is corrected not because intrusion has been accomplished, but because extrusion and altering the cant of the occlusal plane has occurred. If this happens, it may be impossible to achieve intrusion through intra-oral mechanics. accomplished, but because extrusion and altering the cant of the occlusal plane has occurred. If this happens, it may be impossible to achieve intrusion through intra-oral mechanics. www.indiandentalacademy.com
According to Vanden Bulcke: 1. For an anterior segment comprising two central incisors, the Cres was located on a projection line parallel to the mid-sagittal plane on a point situated on the distal half of the canines. 2. For an anterior segment comprising four incisors, the Cres was located on a projection line perpendicular to the occlusal plane between the canines and first premolars. 3. For a rigid anterior segment that included the six anterior teeth, the Cres was situated on a projection line perpendicular to the occlusal plane distal to the first premolar. www.indiandentalacademy.com
Control of reaction forces exerted by intrusion arches: The best control of the anchorage unit is accomplished by minimizing the forces used for intrusion. The largest effect upon the anchorage unit will be a result of the moment produced by the intrusive force, which is large because of the large moment arm present. Two significant side effects are produced: First side effect alters the plane of occlusion of the buccal segments. It is caused by the moment produced by the intrusion arch on the buccal segments. In the maxilla, the plane of occlusion steepens and in the mandible it flattens. www.indiandentalacademy.com
To minimize this side effect several steps can be taken. 1. Keep the forces of intrusion as low as possible. 2. Have as many teeth as possible incorporated in the anchorage unit. 3. Do as much retraction initially as possible to decrease the length of the moment arm. 4. An occipital headgear with the force directed anterior to the Cres of the posterior anchorage unit can negate the moment.
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Second side effect is produced by equal and opposite extrusive force upon the buccal segments. This force is acting at the auxillary tube of the molar, which is buccal to Cres of molar and / or buccal segment.
This force will produce a moment on the molar to tip buccal segment lingually with the roots moving buccally. This side effect is prevented by the use of lingual arch, which maintains the axial inclinations and the arch widths. RELAPSE AFTER DEEP BITE CORRECTION: Changing the lower anterior face height in adult patients due to extrusion of molars is not an advisable clinical procedure. According to some authors, intrusion of lower incisors may not be the ideal treatment with respect to stability. Some claim that the establishment of an appropriate inter incisal angle is advisable in an attempt to prevent deep bite relapse. www.indiandentalacademy.com
INTRUSION ARCHES
PARTS OF AN INTRUSION ARCH ASSEMBLY: The posterior anchorage unit The anterior segment The intrusion arch itself
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CONTINUOUS INTRUSION ARCH: Is fabricated from 0.017” x 0.025” / 0.016” x 0.022” TMA wire. The 0.018” round TMA stops can be welded or helices may be bent in the wire mesial to the molar tubes, which serve as tiebacks to prevent flaring of the teeth undergoing intrusion. A gingival step is placed mesial to the canine bracket to avoid this bracket upon activation of the intrusion arch. The step is made no greater than required to attach the intrusion arch anteriorly without canine interference. Anteriorly, the intrusion arch is tied to the wings of the brackets of the incisors, not into the slot of the bracket itself.
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THREE-PIECE INTRUSION ARCH
Parts: 1. The posterior anchorage unit 2. The anterior segment with a posterior extension 3. The intrusion cantilevers 4. An elastic chain. The anterior segment is bent gingivally distal to the laterals, then bent horizontally, creating a step of approximately 3 mm. The distal part extends posteriorly to the distal end of the canine bracket, where it forms a hook. This anterior segment should be made of 0.019” x 0.022” / 0.017” x 0.025” SS wire. The intrusion cantilevers are fabricated from 0.017” x 0.025” TMA wire. The wire is first bent gingivally mesial to the molar tube (and then helix is formed if SS wire is used). On the mesial end of the cantilever, a hook is bent through which the intrusive force can be applied to the anterior segment. The cantilever is then activated by making a bend mesial to the helix at the molar tube,www.indiandentalacademy.com and then cinched back.
An elastic chain can be attached to the hook of the anterior segment to the molar tube to redirect the forces in a posterior direction. www.indiandentalacademy.com
CANINE INTRUSION A cantilever from the auxillary tube of the molar tied to the canine bracket as a point contact can be used. Because the intrusive force is applied labial to the Cres of the canine, the canine can flare labially during intrusion. To prevent this, the cantilever is bent to the lingual to give a lingual force.
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FORCE VALUES FOR INTRUSION TOOTH MOVEMENT
FORCE (gm)
INTRUSION
PER SIDE
2 UPPER CENTRAL INCISORS
15 – 20
30 –40
4 UPPER INCISORS
30 – 40
60 – 80
6 UPPER ANTERIORS
60
120
2 LOWER CENTRAL INCISORS
12.5
25
4 LOWER INCISORS
25
50
6 LOWER ANTERIORS
50
100
2 UPPER CANINES
25
-
2 LOWER CANINES
25
-
MOLAR EXTRUSION
60 – 100
120 – 200
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TOTAL IN MIDLINE
ROLE OF FRICTION IN SLIDING MECHANICS Friction is the relative roughness of two surfaces in contact. It is the force that resists the movement of one surface past another and acts in a direction opposite the direction of movement. When two surfaces in contact slide or tend to slide against each other, two components of total force arise. One of these is the frictional component, which is parallel in direction to the intended or actual sliding motion and opposes the motion. The other component, known as the normal force, is perpendicular to one or both contacting surfaces. During canine retraction, the relationship of the bracket to the wire changes at different stages of treatment. Therefore, the magnitude and direction of the associated frictional and normal components of contact forces will also vary with time. Once movement has been initiated, friction does not depend on the surface areas in contact or on the velocity of their relative motion. Â The coefficient of friction can be described mathematically as the frictional force that resists motion, divided by the normal force that acts perpendicular to the two contacting surfaces. There are two coefficients of friction for a material. One is the coefficient of static friction, which reflects the force necessary to initiate movement, and the other is the coefficient of kinetic friction, which reflects the force necessary to perpetuate this motion. It takes more force to initiate motion than to perpetuate it. Therefore when sliding mechanics is used some of the applied force is dissipated as friction, and the remainder is transferred to supporting structures of the tooth to mediate tooth movement. Maximum biological tissue response occurs only when the applied force is of sufficient magnitude to adequately overcome friction and lie within the optimum range of forces necessary for movement of the tooth www.indiandentalacademy.com
Variables affecting frictional resistance during tooth movement A. Physical 1. Arch wire a. Material b. Cross-sectional shape/size c. Surface texture d. Stiffness 3. Bracket a. Material b. Manufacturing process c. Slot width and depth d. Design of bracket e. First-order bend f. Second-order bend g. Third-order bend B. Biological 1. Saliva 2. Plaque 3. Acquired pellicle 4. Corrosion
2. Ligation of arch wire to bracket a. Ligature wire b. Elastomerics c. Method of ligation 4. Orthodontic appliances a. Interbracket distance b. Level of bracket slots between adjacent teeth c. Forces applied for retraction
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WALKING OF THE CANINE Sequence of canine movement during retraction with sliding mechanics is as follows, 1. The normal component of force (N) and the frictional resistance to movement (f). 2. Â The bracket tips until the diagonally opposing corners of the bracket contact the wire. 3. The wire deflects producing a couple to upright the tooth. This emphasis why elastomeric chains used for canine retraction should not be changed frequently. Thus the canine is retracted bodily by alternate movements of tipping and root uprighting.
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BYPASS ARCHES MULLIGAN’S BYPASS ARCH Is fabricated from 0.016” stainless steel wire. Bracketing the anterior teeth may be delayed, or the anterior section of the wire can be stepped up to avoid interference. An anchor bend is placed in front of the molar, second bicuspids are sometimes temporarily not bonded to increase the distance and therefore the differential torque. Toe-in bends or lingual elastics are placed to offset the tendency for mesiolingual rotation of the molars. The tip-back bend is an off-center bend hence produces two unequal moments. The larger moment (crown distal and root mesial) lies at the bracket / tube containing the short segment. The moment at the bracket / tube containing the longer segment may be clockwise, counter clockwise or non-existent. Whatever be the moment at the long segment, the net effect is dominated by the short segment and is always crown distal and root mesial. A power chain is tied directly from the molar to the cuspid, while the second bicuspid is tied individually with an ‘O’ ring. This allows a greater range of force. The anchor unit should remain relatively upright, while the non-anchor unit should undergo tipping until arch wire binding occurs. Once binding occurs, the roots will respond to the moments produced by the arch wire, until binding stops and crown movement is resumed. As the cuspid continues to move distally, the bend automatically approaches the center of the wire, until finally when the extraction site is closed, the bend is centered. As the off-centered bend moves towards the center during space closure, the differential torque begins to gradually disappear, and becomes equal and opposite torque when the bend is finallywww.indiandentalacademy.com centered.
BURSTONE’S CANINE-TO-CANINE BYPASS ARCH Uses, 1. Prevent rotation 2. Actively derotate teeth when there is sufficient space 3. Alter arch width 4. Eliminate side effects from vertical forces. A rigid wire, preferably 0.021” x 0.025” SS or at least 0.017” x 0.025” SS, is stepped gingivally 3 to 4 mm mesial to the canine and around the incisors. This allows for simultaneous bracketing and alignment of the incisors as the canines are retracted. The vertical step down also incorporates additional wire length, which allows the step to be bent lingually or labially, depending upon patient comfort and the need to keep the anterior wire away from the incisors as the canines retract. In some patients, no gingival step is made if the incisors are not to be bracketed until canine retraction is completed. Gradual first order reverse curvature is placed to produce moments rotating the canines distal-out. The amount of wire length distal to the canines is determined by the amount that the canines must be retracted to relieve the anterior crowding. The increased length provides a means of extending the segment anteriorly to avoid contacting the incisors during retraction. To avoid lip irritation and patient discomfort, the wire should only be 1.5 mm labial to the incisors. This distance can be adjusted each visit by advancing the wire as the canines retract. www.indiandentalacademy.com
REFERENCES
Burstone C.J. ‘Rationale of the segmented arch’. Am.J.Orthod 48:805-822,1962 Burstone C.J. ‘The mechanics of the segmented arch technique’.Am.J.Orthod 36:99120,1966 Burstone C.J. ‘Segmented arch approach to space closure’. Am.J.Orthod 82:361-378,1982 Smith R.J.,Burstone C.J.‘Mechanics of tooth movement’. Am.J.Orthod 85:294-307,1984 Nelson K.R.,Burstone C.J.,and Goldberg A.J. ‘Optimal welding of orthodontic wires’. Am.J.Orthod 92:213-223,1987 www.indiandentalacademy.com
R. Issacson ‘Activating a 2 X 4 Appliance’ Am.J.Orthod 63(1): 17-24,1993 Burstone & Koeing: ‘Creative wire bending’-Force system from step and V bends. Am.J.Orthod 93: 59-67,1988 T. F. Mulligan ‘Common sense mechanics’ JCO’ Sept.79-Dec 80 Lindaeur S.J., Issacson R.J. ‘One couple system’ Sem.Orthod 1: 12-24,1995 Kula K.,Philips C.,Gibilaro A. and Proffit W. R. ‘The effect of ion implantation of TMA archwires on the rate of orthodontic sliding space closure’. Am.J.Orthod 114: 577-585,1998 www.indiandentalacademy.com
Issacson R.J. ‘Two couple orthodontic appliance system’Torquing arches. Sem.Orthod 1:31-36,1995 Rebellato J. ‘Two couple orthodontic appliance system’transpalatal arches. Sem.Orthod 1:44-54,1995 Siatowski R.E. “Continuous arch wire closing loop design,optimization and verification’- Parts I and II Am.J.Orthod 112:484-495,1997
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Burstone. C.J. ‘ Mechanics of segmented arch technique’. Graber and Vanersdall. ‘Current principles and techniques’. Nanda R. ‘Biomechanics in clinical orthodontics’. Marcotte M.R. ‘Biomechanics in orthodontics’. Proffit W.R. ‘Contemporary Orthodontics’.
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