Common sense mechanics
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The document outlines a 16-part series on "Common Sense Mechanics" in orthodontics. It discusses various orthodontic mechanics principles like forces, moments, torque, and their clinical applications. Key topics include the diving board concept to control forces, differential torque mechanics, archwire-bracket relationships, extraction mechanics, and various malocclusion treatments. The goal is to help orthodontists understand and apply basic mechanics principles in a common sense manner.
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Common sense mechanics
- 1. COMMON SENSE MECHANICS 101 Dr. Rabab Khursheed
- 2. CONTENTS ‡ Common Sense Mechanics Part 1 -Introduction -Visual inspection -Simple rule ‡ Common Sense Mechanics Part 2 -Forces and Moments -Cue Ball concept -Translation, Rotation -forces and moments on teeth -Lingual root torque ‡ Common Sense Mechanics Part 3 -Static Equilibrium -Requirements ‡ Common Sense Mechanics Part 4 -Cross Bites -Expansion/contraction overlays
- 3. ‡ Common Sense Mechanics Part 5 -Diving board concept -Cantilever principle -Constant load versus constant deflection ‡ Common Sense Mechanics Part 6 -Clinical application of the diving board concept ‡ Common Sense Mechanics Part 7 -Distalization with differential torque -Class II correction without head gear or elastics ‡ Common Sense Mechanics Part 8 -Wire bracket relationships ‡ Common Sense Mechanics Part 9 -Extraction mechanics ‡ Common Sense Mechanics Part 10
- 4. ‡ Common Sense Mechanics Part 11 -Simultaneous cuspid and bicuspid retraction ‡ Common Sense Mechanics Part 12 -Applying principal to total treatment -Class I non extraction ‡ Common Sense Mechanics Part 13 -Class II division 2 ‡ Common Sense Mechanics Part 14 -Class III malocclusion ‡ Common Sense Mechanics Part 15 - Types of cross bites
- 5. "Common Sense Mechanics" Thomas F. Mulligan- series of articles in JCO (Sep. 1979 – Dec. 1980) Based on fact that no appliance exists which allow an orthodontist to treat Maloclussion without adding the necessary ingredient of "Common Sense" to the mechanics instituted for correcting the malocclusion
- 6. • Refinement in appliances may ↓ the physical effort, but will not eliminate the need for the orthodontist to • Think • understand • Apply basic principles of mechanics in a common sense manner
- 7. Visual Inspection Method • Often confuses the orthodontist in attempting to determine with reliability what forces are present • The visual method seems to be so obvious, but faulty conclusions. • Not to be misled by determining forces present through the visual inspection method Common Sense Mechanics, Part 1 Volume 13 : Number 09 : Pages (588-594) 1979
- 8. A. Vertical Plane of Space. B. Horizontal Plane of Space.
- 9. • What is the force acting on the Molar ??
- 13. A SIMPLE RULE • If the bend is located off center, 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 • Another way to think of it is this: The short segment points in the opposite direction of the force. This is certainly different than visual inspection might lead us to believe.
- 14. Determine only the forces p Same wire as below Same wire as above Same wi as abov
- 16. t this time. ame wire as below Same as below Same as above 13 Figure 1-14
- 19. FORCES AND MOMENTS • Force is nothing but push or pull
- 20. When line of action of force passes through any point other than the COR This turning effect of force is also known as “Moment of Force”
- 21. MOMENT= FORCE X DISTANCE
- 24. • You can "sense" a force when you bend a wire, but you cannot "sense" torque. Because the latter is simply a product of force times distance, as previously discussed, the distance (length) is just as effective as the force. If the force passes through the center of resistance, no perpendicular distance is involved. Therefore, regardless of the magnitude of the force, there is no moment. What does all of this mean?
- 31. • Extrusive force present on the molar teeth acts at the molar tubes which lie, buccally to COR. This force times distance results in molar Lingual Crown Torque.
- 32. When lingual root torque is placed into the incisor section, a long segment & a short segment is produced as with tip back bend.
- 35. When one observes an effect, he should be able to interpret the cause and vice versa. We should also begin to recognize that such force systems should not be routinely considered as undesirable side effects except for the orthodontist who is unaware of their presence and therefore is not prepared to prevent undesirable effects as well as to utilize the systems effectively when indicated. If lingual crown torque is desired, it should be permitted to act. If undesirable, it can be prevented with a lingual arch, a rectangular wire, or whatever means the operator chooses.
- 36. STATIC EQUILIBRIUM • When forces acting on an object which is at rest are balanced, then we say that the object is in a state of static equilibrium. The resultant of these forces equals zero • If we, desired to convert this "dynamic" state to a state of statics, either shift the unequal weights or the fulcrum point on the board
- 37. REQUIRMENTS FOR STATIC EQUILIBRIUM • The sum of all the Vertical Forces present must equal zero. we must deal with extrusive components of force during overbite correction. • the sum of all Horizontal Forces present must equal zero . This is why we cannot correct a unilateral crossbite with a single horizontal force • The sum of the Moments acting around ANY point must also equal zero We may produce heavy torques in a given area and little or no torque elsewhere, but when added around any given point, they should equal zero. REQUIREMENTS FOR EQUILIBRIUM 1. Sum of the Vertical Forces equals zero. 2. Sum of the Horizontal Forces equals zero. 3. Sum of the Moments around a Common Point equals zero. um
- 38. Golden Rule The archwire when fully engaged always relults and/or tries to achieve a sate of static equllibrium.
- 39. • With two equal moments at either end of the archwire, the system is in balance.
- 40. • With two unequal moments at either end of the archwire, the system reaches a balance, but seems to be unbalanced and with the entire unit rotating counterclockwise.
- 41. • the unequal moments create (in this case) an extrusive force on the incisor and an intrusive force on the molar. The sum of these forces is zero, but the configuration causes the entire unit to rotate clockwise.
- 42. A full strapup with a reverse curve of Spee
- 44. ARCH LEVELLING • Do you sometimes observe the posterior teeth moving lingually for no "apparent" reason during arch leveling procedures? • During arch leveling procedures, we frequently observe responses that may be undesirable. They often occur unexpectedly and in various forms. • As we can see, there is a reason for all responses. • Whenever we witness responses for "no apparent reason", we have failed to recognize the cause, and as a result made our treatment somewhat more difficult. The recognition of causes permits us to utilize as well as avoid certain types of tooth movement. • When leveling an arch, it has already been shown that in a full strapup, intrusive forces act through the molar tubes, producing buccal crown torque on the molars. Do you sometimes observe the posterior teeth moving buccally for no "apparent" reason during arch leveling?
- 47. CROSS BITES • If an individual molar, or an entire buccal segment in crossbite • when we observe a buccal segment in crossbite, it may be a unilateral crossbite, or in most of cases a bilateral crossbite with a lateral mandibular shift
- 48. OVERLAYS • 0.36 mm over lays in 0.45 mm head gear tubes • The force provided will be equal and opposite— not unilateral in direction as might appear to be the case when the overlay is inserted into one tube and observed • the over corrected or worsened side will eventually return to normal by relapse , wheras the corrected side will remain corrected.
- 51. COSMETIC OVERLAY
- 53. THE DIVING BOARD CONCEPT • It is not that we use the diving board in force control, but the mental image should permit us to recall more vividly the advantages involved in utilizing the factor of "length" in our archwires. • There is a formula that says that stiffness— or load/deflection rate— is inversely proportional to the cube of the length. Formulas of this kind often seem confusing and of little use to the orthodontist, as well as difficult to remember. • To make all of this useful and a little easier, let us analyze the situation more closely. First of all, stiffness is the amount of deflection we get from a given load (force). Stiffness or load/deflection rate α 1/ L3
- 54. • The formula tells us that if we are dealing with a cantilever (such as a diving board), by doubling the length stiffness is reduced to one-eighth. By doubling the length, only one-eighth the force will be required to produce the same deflection or the same force acting at double the length will produce eight times as much deflection. A.When the length of the diving board is doubled, only one-eighth the force is required to produce the same amount of deflection. B. The same force acting at twice the length will produce eight times as much deflection
- 55. • Load on diving board produces bending moments along the board, with the maximum moment being located closest to the point of attachment In orthodontics, we often refer to this moment as the "Critical Moment", as it is the largest moment involved and is often responsible for breakage in an archwire at that particular point
- 58. • when we place a given bend, we must determine what angle is necessary to produce the desired load (force). It also requires that we must know the length of wire between brackets and tubes. We can resort to reference tables or we can go through "trial and error" until we arrive at the bend which gives us the force we want. If, instead, we choose to place a "constant" bend (angle), we find that we create variable loads (forces)
- 59. CLINICAL APPLICATION • As we know, small interbracket distances can produce very high magnitudes of force with the so called "light wires • Bypassing teeth is one method of increasing interbracket distance. Individuals often use single wing brackets for this purpose, but when all teeth are banded all of the time and an archwire engaged in every bracket automatically, there is little alternative for reducing force levels
- 60. • we recognize that length load (force). If we double the length of wire, we reduce the force per unit of deflection to one- eighth. • Therefore, if we bypass bicuspids and cuspids during overbite correction, and use a wire with tipback bends at the molars, we have in effect created a "diving board”. • If the tipback activation is constant, such as a 45° angle, then as the distance doubles, so does the deflection
- 61. • Therefore, although the load per unit of deflection is reduced to one- eighth, the unit of deflection is doubled, resulting in a net force of one- fourth (2 × 1/8 = ¼). However, it is quite evident that the length of wire is increasing much more than "twice", and therefore the net intrusive force on the anterior segment is dramatically reduced. • With wire sizes of .016, the magnitudes at times become so low, you wonder if "anything" will happen with the overbite. It is common to have forces in the range of 20-30 grams and lower. If we apply a total force on an incisor segment of 30 grams (intrusion), for example, we produce equal and opposite forces on the molars. But, one-half goes to each molar, meaning that each molar in this example would incur only 7½ grams of force— enough to allow the molars to erupt during vertical growth, but not enough to overcome the forces of occlusion.
- 62. DIFFERENTIAL TORQUE center bend. These similarities will be extr predicting force systems. Cuspid Root Torque Molar Tip-Back Lingual Root Torque Molar Tip-Back Figure 1-23 Figure 1-24
- 63. ROW BOAT EFFECT "rowboat effect“- tendency for the maxillary teeth to move forward during anterior lingual root torque
- 64. DISTALISATION • The opposite of the row boat effect
- 65. • This "distalization" tendency -easy to check simply by observing the unbanded cuspids and their change in axial inclination. • The cuspid crowns tip distally as they are forced back as a result of the thrust being received at the crown level.
- 68. CLASS II CORRECTION WITHOUT HEADGEAR OR ELASTICS Class II correction is coincidental during overbite correction using tipback bends esp. in U/arch.. The amount of headgear treatment originally planned is either reduced, sometimes even eliminated.
- 70. WIRE / BRACKET RELATIONSHIPS • The relationship of the archwire to the brackets and tubes, prior to engagement, offers valuable and interesting information • If a straight wire is placed over angulated brackets, a certain angular relationship develops between the wire and the plane of the bracket slot
- 71. • The same wire/bracket relationship can be created by a bend in the wire or a straight wire in relation to a malocclusion.
- 72. CENTERED BEND
- 73. Applying the requirements for static equilibrium. • If all four forces (activational) are equal- first requirement for static equilibrium is fulfilled.
- 74. Moment : A + D = 0 Moment : B + C = 0
- 76. STEP BEND
- 79. OFF-CENTER BEND
- 82. EXTRACTION MECHANICS • Diffrential torque is produced by a tip back off- center bend • Depending on the angle at which the wire with an off-center bend crosses the bracket, and the length of the long segment • moment produced by the longer segment can be • clockwise • counterclockwise • nonexistent
- 84. • Bends given intraorally using a tweed loop forming plier , to create differential torque .
- 86. BICUSPID RETRACTION 4 4 5 E Extraction of
- 91. TOE IN BEND OR CURVE
- 97. • Extractions- 4 4 4 5
- 99. • This type of case requires certain compromises to be established beforehand, such as the willingness to leave "some“ lower rotations, which will encourage more overbite and overjet following treatment
- 100. TYPES OF ANTERIOR CROSSBITES
- 101. SINGLE TOOTH CROSS BITE
- 102. CONCLUSION • Common sense mechanics is a very easy to use system if we as orthodontists understand how it works. It is much more convenient to predict precise movements of teeth and prepare in advance for any unwanted tooth movement. • This does not require us to use different appliances, or discard a different technique, but certain modifications brought about to the mechanics can be a great deal of help.
- 103. REFERENCES • Common Sense Mechanics Part 1 • Common Sense Mechanics Part 2 • Common Sense Mechanics Part 3 • Common Sense Mechanics Part 4 • Common Sense Mechanics Part 5 • Common Sense Mechanics Part 6
- 104. • Common Sense Mechanics Part 7 • Common Sense Mechanics Part 8 • Common Sense Mechanics Part 9 • Common Sense Mechanics Part 10 • Common Sense Mechanics Part 11 • Common Sense Mechanics Part 12 • Common Sense Mechanics Part 13 • Common Sense Mechanics Part 14
- 105. • Common Sense Mechanics Part 15 • Common Sense Mechanics Part 16
Editor's Notes
- born in Grand Forks, North Dakota, served in the army in the far east , and received his D.D.S. degree from Marquette University in 1960. his orthodontic practice in Phoenix, Arizona, where he has since resided and practiced. His many interests outside of orthodontics have included boating, sailing, flying and jogging.
- Appliances are being refined and will continue to improve with the passage of time. Which is good, but the danger lies with the individual who fails to recognize that the refinement of appliances may reduce the physical effort 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..
- Part 1 Over bite returns So, let us proceed to determine what forces, if any, are present on the particular tooth in question. Disregard the moments altogether and ask only whether there will be an intrusive or extrusive force present— or no force at all
- Before we begin, we should know that forces can act in the vertical plane of space, and the horizontal pla
- A number of two teeth illustrations to help us understand and quickly determine directions of forces . On the molar? On the cuspid?
- On the molar on the cuspid?
- let us move from the vertical planes of space and proceed to ask questions similar in the buccal and lingual planes of space
- When ever this passes thorough the center of resistance – translation Center of mass – of a free body is a point in the body where the whole of its mass is concentrated, it is also called as valance point. Center of resistance is the center of mass of a restrained body.determined by root lenth, alveolar bone level, and number of roots.
- The tendency for a force to produce a rotation is called moment .it is determined by the magnitude of force multiplied with the perpendicular distance from the line of action.
- Units of moments is newton (gm x mm)
- ADD couple force photograph
- Tug of war because of bigger moment , which will be explained later in the row boat effect
- the net forces will be intrusive on the molar and extrusive on the incisor. Therefore, if we are hoping for overbite correction, but increased our lingual root torque to this point, we can expect our overbite to increase instead of decreasing. So, we might decide, if we know this beforehand, to either increase the molar tipback bend, decrease the amount of lingual root torque on the incisor segment, or a combination of each, in order to assure ourselves of a net intrusive force on the incisor segment for overbite correction.
- If two individuals are of equal weight and equal distance from the fulcrum , theyw ill be in complete balance
- attempting anterior intrusion, we produce anterior and posterior forces with equal and opposite extrusive forces occurring through the bicuspid areas
- Intrusive forces acting on molar tipps it buccaly
- Extrusive forces acting through molar when a 2x4 appliance is used,
- These are heavy wires overlaying the main arch wire
- But, the overalay movement is very rapid-usually about three to six weeks
- It dosnt show when one smiles
- Part 5
- For a given bend determine angle necessary to produce the desired force & to know length of wire between brackets and tubes
- anterior-posterior arch length varies from patient to patient, when bicuspids and cuspids are bypassed the length becomes a variable and, thus, so do the magnitudes of the intrusive and extrusive forces at each end of the archwire However, the entire range of force is so low that low magnitudes of force may pose a greater problem than attaining higher levels of force. In fact, it may even require going to archwire of greater diameter to produce a required force and desirable response.
- PART 7
- Crown movement precedes root movement If overbite interferes, at the time, with the distal crown movement (tendency), mesial root movement of the molars will occur.
- Notice the teeth tipped back along the mandibular plane … in the pic below see how the tooth have come to normal upright position after appliance removal
- particularly where molars require some uprighting, the combination of "E" space with that gained mechanically is significant. The tipped-back teeth, while uprighting, are continuing to erupt along a new longitudinal axis, and thus give a "net gain" when they finally attain their upright position. So there is gain of an extra 1-1½ millimeters notice in the second picture the second molar is banded instead of the first , only to increase the lenth of the arch wire and bring about good use of the diving board principle
- Girl presented to the author with a class II molar relationship . Because of the extreme deep bite and significant mesial crown tipping of the molar the difrrential torque mechanism with a tip back bend was used . And head gear treatment was given just prior to finishing of treatment
- Centered bend
- there are no horizontal forces necessary to engage the wire into the brackets,so the second principle is fulfilled Thrd requirement states that moments should add up to zero .
- A produces a clockwise moment equaling the cunter clockwise moment produced by d B produces a smaller moment which is counter clock wise and opposite to c , smaller moment because the inter bracket distance is reduced . When we add these moments it is equal to 0
- Force A and Force D each produce clockwise moments equal in magnitude and opposite in direction to the counterclockwise moments produced by Forces B and C. In spite of the fact that Forces B and C act at smaller distances, balance is maintained due to their greater magnitudes of force. The important thing to realize is that the net activational forces at each bracket are unequal, unlike the center bend
- Therefore to achieve equilibrium to compensate for these unbalanced moments there will be forces acting on either end( white arrows )
- Part 9-14 Just as a review
- .022 × .028 slots and .045 headgear tubes, the initial wires used were 0.16 class 1 molar relationship/ canine relationship Off center bends placed mesial to molar bracket
- The non anchor side is farther away from the offcenter bend. As it moves back , it tips, upon reaching the bend the system gets converted to a center bend system, which helps uprighting the tooth roots now. A molar toe in is also given to counteract molar rotation because of the elastics.
- Class I molar relation with deep overbite , missing lower left 2nd bicuspid, but first bicuspid is almost in contact with lateral incisor. Notice the tissue blanching because of unerupted cuspid Initiallu 0.16 wires were used, and a off-centerbend just distal to lower left molar was given and the 1st bicuspid was retracted . The off center was converted to a center and the roots uprighted
- On the lower left side the wire has a step down bend to act as a space maintainer, meanwhile aslso having a center bend to upright the roots of the 1st premolar. On the right side you can see the molar and the bicuspid are ready for protraction . Here no tip back bend is given mesial to molar, but it is given distal to the canine and an 0.18 wire is used, a 0.020 wire can also be used for this purpose.because the tendency for molar to rptate is more on a lighter wire
- Class 2 div 1 case with removal of upper first bicuspids and lower 2nd bicuspids .maximum anchorage in lower arch . Bends were placed ntraorally and in the upper arch u can see thebend mesial to the 2nd bicuspid, where as in lower arch mesial to molar . After achieveing a class 1 canine relation, his intial plan was -decided on protracting the lower molars ,
- But he decided to retract the lower cuspids fut=rther more, which created anterior spacing. He then again close the lower space and protracted the molars.
- In the following articles he demonstrated how the unconventional method of treatment could lead to good results as well , only because of application of comman sense. This is a class 1 non extraction case with crowding in lower anteriors and liguoversion of lower right bicuspids, the maxilarry and mandibular lingual cusps are high as we can see, and muligan explained that we can understand which direction to apply the force on the teeth according to which cusp we see higher. The lower molars required a lingual tipping, which would bring about the illusion of the upper molars being in buccoversion ,we can also see that the lower lingually tipped bicuspids require buccal force to upright.
- 0.16 wire was placed in a 2x4 appliance . Due to the tip back bend, the molars had an eruptive force, which cause the lower molars to have a lingual tip, which as predicted brought the upper molars into a buccoversion. The lower anterior crowding was corrected by the space gained due to the lower molars tipping back, . Then a constricted overlay of 0.36 diameter was given to constrict the maxillary arch
- Class 2 div 2 malocclusion To be noted in this case is the overbite, and the buccoversion of the 2nd moars in the maxillary arch what is interesting in this case that the line of force is assumed to pass lingually to the incisor, but the cephalometric tracing show this is not the case. The case was treated with an upper first bicuspid extraction and 0.16 wire . An intrusion arch was given in the upper anteriors, only then the lower anteriors were strapped up.
- An 0.18 arch wire with distal extensions for force application on the second molar is given and activated intraorally with a tweeds loop forming pliar . The distal extensions were then cut off once the desired lingual crown movement was achieved Finally after this, a rectangular arch wire of 19x 25 dimension was fabricated with anterior lingual root torque in the upper anteriors.
- Class 2 subdivision open bite case . Left side in class 2 and right side in class 1 in the second figure we can see the amount of anterior spacing created by retraction of cuspids, except here on the lower left side , were retracted only part way followed by molar protraction. To close the bite, up and down anterior elastics were used. No wires were used except round wires, except for the class 2 div 2 case
- Three types of class 3 cases, pseudo, purely dental, one is a combination of skeletal and dental, and the last purely skeletal. This case is an atypical class 3, anterior mandibular shift while closure.first 4 bicuspids were extracted, maxillary incisor alighntment was achived by saving anchorage, but then after alighntment the maxillary molars were protracted.here it was intended to retract the mandibular incisors sufficiently to eliminate mandibular shift .
- A skeletal and dental class 3 malocclusion , notice the dish face appearance due to midface deficiency . A non extraction treatment was instituted, with a clear intention to expant the maxillary tooth , and to correct the molar relation as much as possible with class 3 elastics. Coil springs were given to advance the maxillary incisors, and after 6 and a hlf months, a mild overbite has been achieved , the maxillary molars have also moved slightly distally due to the prolonged coil spring.an expansion over lay was given for correction of the cross bite bilaterally. After one year of appliance removal, the expected relapse has occurred, though the pverbite has remained as corrected in this patient.
- The patient was asked to keep the lower tooth out of the way.