FRICTIONLESS MECHANICS
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Friction, in general, is the resisting force on the relative lateral motion of solid surfaces, fluid layers or material elements in contact. In orthodontics, friction is the state of drift between archwire and the bracket slot. It is a factor which causes the loss of forces provided during retraction.
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Important definitions ď Ž
FORCE is defined as an act upon a body that changes or tends to change the state of rest or motion of the body. Force is a vector it has both magnitude and direction. Direction consists of two properties – a line of action and a sense. In case of understanding of tooth movement along with magnitude and direction, point of application of force is important. The forces are indicated by straight arrows. www.indiandentalacademy.com
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A MOMENT is defined as the product of the force times the perpendicular distance from the point of force application to the center of resistance, and thus is measured in units of gmmm (or equivalent). If the line of action of an applied force does not pass through the center of resistance, a moment is necessarily created.
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MOMENT OF FORCE: When a force is applied at any point other than through the center of resistance in addition of moving the center of resistance in direction of the force, a moment is created. In case of tooth, since it is embedded in the alveolar bone, we cannot apply force directly on CRES, but can apply force on the exposed part of the tooth, which is at a distance from CRES. Therefore with a single force we invariably create a moment called as moment of force. A moment may be referred as, Rotation, Tipping or Torquing. www.indiandentalacademy.com
Contents
Introduction to frictionless mechanics Anchorage classification Biomechanics of looped archwire retraction 1. Design of loop 2. Biomechanical considerations Advantages & disadvantages of loop mechanics
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Advantages of a loop Various types of loop designs Methods of frictionless mechanics other than the use of loops Conclusion
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WHAT IS FRICTIONLESS MECHANICS яБо
Retraction is accomplished with the help of loops or springs. Loops
{TEARDROP LOOP}
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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. 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.
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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. 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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VARIOUS LOOP DESIGNS THAT HAVE BEEN TRIED FOR FRICTIONLESS SPACE CLOSURE
Ray D. Robinson (1915) was the first to document the use of loops in orthodontics Dr. Harry Bull advocated a squashed vertical loop with an .0125”/.025”edgewise wire opened the distance of a ‘thin dime’.
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Since that time various loop designs have been advocated – 1. Rickett’s canine retractor 2. PG (Paul Gjessing) retraction spring 3. Delta loop 4. Closed vertical loop 5. Bull loop 6. Open vertical loop 7. The R (Rectangular) loop 8. Vertical loop with helix 9. Omega loop 10. T-Loop 11. Opus loop www.indiandentalacademy.com
12. T-loop retraction archwire (continuous and segmental) 13. Double keyhole 14. Box loop 15. Double delta
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RICKETT’S CANINE RETRACTOR
This is a combination of a double closed helix and an Extended crossed T made with blue Elgiloy wire. It delivered 30-50gms per mm of activation.
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ACTIVATION Activated by pulling 3-4mm each adjustment by pulling the wire through the tube and locking it with a simple bend. There should be 90 degree of Gable bend in the canine region
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Advantages of Rickett’s Retractor Rapid space closure Only a few weeks of wearing Disadvantages of Rickett’s Retractor Bulky and irritating to soft tissues Difficult to use in the lower arch because it extends into chewing area
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PG { PAULGJESSING } RETRACTION SPRING
Paul Gjessing (Denmark) introduced this spring in 1985. Design: -
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The canine retraction spring was constructed in 0.016�/0.022� SS wire The principle element is a double helix 10mm in height It was introduced to reduce the load deflection rate of the spring The mesial and distal extensions of the looped archwire are angulated in both horizontal and vertical plane The posterior curvature is adjusted to deliver the force magnitude of 15-25gms per side. www.indiandentalacademy.com
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Advantages Reduced load deflection rate Reduced vertical height The rounded form avoids load concentration
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BEFORE RETRACTION
AFTER RETRACTION
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DELTA LOOP
The design similar to that of opening loop William R. Proffit (1993) 0.016”/0.022” SS wire used in .018” slot and 0.018”/0.025” SS wire in .022” slot
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VERTICAL LOOP
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Vertical loop with HELIX
The advantage of Helix in a vertical loop is that it increases the working range Closed Helix
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Open Helix
Bull loop
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OPEN VERTICAL LOOP
Morris & Bruce It is used to open the spaces It is activated by closing the legs
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OMEGA LOOP
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THE ASYMMETRIC T LOOP
James J. Hilgers (1992) This loop allows simultaneous bite opening and space closure. The anterior portion is smaller and engages the lateral incisor bracket.
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THE OPUS LOOP/STANDARD OPUS
Dr. Raymond E. Siatkoski This specialized spring can deliver sufficiently high “inherent M/F ratio” within the range of 89 to produce en masse translation without giving the pre-activation bends.
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Groups of teeth can be moved more accurately to achieve predetermined anteroposterior treatment goals for esthetics and stability
The distinct advantage of Opus loop is that it is free of residual moments and produces the periods of “true rest” when deactivated.
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OPUS LOOP
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THE RECTANGULAR LOOP CHARACTERISTICS: 1. Can be used for first, second and third order corrections 2. Since the loop is inserted in at least two brackets, it represents a static force system. 3. The clinician can determine the moment-to-force ratio delivered to the active unit. 4. 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.
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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 2nd 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
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A = B = C www.indiandentalacademy.com
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 0.025� and a 0.018� 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.
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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
α
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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
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Continuous T loop archwire
Given by Stoner, thus also called as Stoner Tloop archwire This loop is used when retraction and intrusion of the upper incisors is desired. Archwire used is of .016x.022 inch stainless steel and comprises of a T-loop between upper lateral and canines.
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Construction The mesial leg of the T- loop should lie 1 mm posterior to the loop bracket of the lateral incisor. The loops are activated by to 1mm by sliding the archwire to the posterior brackets and buccal tubes and bending them up. If we need anterior bite opening tooo then a reverse curve of spee can be formed in the archwire. With every visit only 1 mm of activation can be done and the visits can be repeated in the intervals of 3 weeks www.indiandentalacademy.com
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Double keyhole loop
Introduced by John Parker of Almeda, California. When spaces are present anterior as ell as posterior to canines and need to be closed by front backward or back forward then this loop is incorporated in continuous archwire. The archwires generally used are .019”x.025” The double keyhole loop archwire also doesn’t allow the canine to rotate during the space closure i.r.t. extraction site. www.indiandentalacademy.com
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Box loop
Generally used in cases of molar protraction as well as anterior retraction. The loop bypasses the tooth anterior to molar. The mesial leg of the loop is adjacent to canine while the distal leg is mesial to the molar tube.
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METHODS OF FRICTIONLESS RETRACTION OTHER THAN CONTINUOUS LOOP MECHANICS 1. 2.
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Drum spring retractor for canine retraction Separate canine retraction with cuspid to cuspid bypass. Retraction with utility arches The three piece intrusion and retraction arch.
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DRUM SPRING RETRACTOR FOR CANINE RETRACTION ď Ž
Constant force without the need for reactivation through an intraoral appliance
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Force spring with a hook fabricated to deliver force of 50gms Other parts are a drum, a spring box and a central pin soldered o molar band Assembled and soldered to molar band and activated by pulling the end of the spring The force level is always at 50gms and there is no need for reactivation. www.indiandentalacademy.com
SEPARATE CANINE TO CANINE RETRACTION WITH CUSPID TO CUSPID BYPASS
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Cuspid to cuspid bypass is used to prevent 1. Prevent rotation 2. Actively derotate teeth when there is space 3. alter arch-width 4. eliminate side effect of vertical forces
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The indications for cuspid to cuspid bypass are Cases requiring bilateral symmetrical canine retraction where distal-in rotation must be prevented Cases with canines with different vertical levels Cases with bilateral or unilateral canine rotation
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A rigid wire at least 0.017”/0.025” is stepped down 3 to 4mm mesial to canines and around the incisors This allows for the simultaneous bracketing of the incisors
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The arch form is made longer and wider than the initial 3-3 distance in order to counteract any constrictive forces caused by the force of retraction. The T-loop is then engaged in Burstone Cuspid bracket and the retraction is started
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RETRACTION WITH UTILITY ARCHES
In 1950s Robert Rickett’s developed the lower step down arch, also popularly known as Rickett’s Utility arch, to hold the buccal segment upright during retraction and also for lower incisor intrusion with light continuous forces.
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It is known as 2/4 appliance because it engages only molars and incisors It has multiple uses in various stages of orthodontic treatment.
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Types of utility arches Passive utility arch Intrusion utility arch Protrusion utility arch Retrusion utility arch The retrusion utility arch is used in either the mixed or permanent dentition to achieve retraction and intrusion of incisors by incorporating loops in the archwire. www.indiandentalacademy.com
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THREE PIECE INTRUSION ARCH
It was given by Burstone for simultaneous Intrusion and retraction. The force of application for intrusion is lingual to centre of resistance so there is no flaring of anteriors as seen in continuous arch intrusion
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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 www.indiandentalacademy.com helix at the molar tube, 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
THREE-PIECE INTRUSION ARCH
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CONCLUSION
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This is only a simplistic presentation of extremely complex mechanism It is impossible to calculate the required force magnitude for every patient because there are many variables 1. Different tooth sizes and inclinations 2. Different arch sizes which affect the length of wire spans 3. Change in the arch size as the tooth movement takes place 4. Individual biomechanical responses 5. Limitations of the material properties.
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Although bracket design and proprietary treatment protocols are broadly used in clinical circumstances, achieving predictable and stable orthodontic results requires more than simply selecting a particular bracket system The fundamental basis of orthodontic treatment remains the application of mechanical forces to produce desirable tooth movement. Today’s orthodontist needs the knowledge of both friction and frictionless mechanics No single technique suits every situation. There are specific indications for both. www.indiandentalacademy.com
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The foremost thing in orthodontics is Discipline Discipline in diagnosis Discipline in treatment planning Discipline in use of appliance system Discipline in mechanics Discipline in management of patient’s orthodontic needs
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REFERENCES
CONTEMPORARY ORTHODONTICS: WILLIAM R. PROFFIT
BIOMECHANICS IN CLIICAL ORTHODONTICS RAVINDRA NANDA BURSTONE CJ, HANLEY KJ: MODERN EDGEWISE MECHANICS SEGMENTED ARCH TECHNIQUE. SMITH RJ, BURSTONE CJ: MECHANICS OF TOOTH MOVEMENT. AM J ORTHOD 85:294-307, 1984. KOENIG HA: OPTIMIZING ANTERIOR AND CANINE RETRACTION. AM J ORTHOD 70:1-19 1976.
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