Friction in orthodontics /certified fixed orthodontic courses by Indian dental academy


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Friction in orthodontics /certified fixed orthodontic courses by Indian dental academy

  1. 1. Friction in orthodontics
  2. 2. INDIAN DENTAL ACADEMY Leader in continuing dental education
  3. 3. Contents Introduction Components of friction Active and passive configuration Critical contact angle Effect of bracket material on friction Effect of wire material on friction Effect of bracket design on friction Effect of bracket ligation on friction Effect of arch wire size on friction Resistance to sliding in leveling and aligning Resistance to sliding in canine retaction Effect of saliva on friction Conclusion
  4. 4. Introduction The straight wire appliance was the first orthodontic mechanism that was based on sliding mechanics . When one moving object contacts another tangentially friction at the interface resists the movement . Consequently , orthodontists have to apply more force to overcome the frictional force to achieve the desirable result, due to which there is more patient discomfort and pain and also increases anchorage demands.
  5. 5. • A. Factors effecting frictional resistance during tooth movement. PHYSICAL 1. Archwire . Material cross section size and shape surface texture stiffness 2. ligation of arch wire and bracket ligture wire elastomerics method of ligation 3. Bracket material Manufacturing process Slot width and depth Design of bracket Second order angulation Third order bend
  6. 6. 4.Orthodontic appliance interbracket span level of bracket slots b/w adjacent teeth forces applied for retraction B BIOLOGICAL 1 saliva 2. Plaque 3. corrosion
  7. 7. Two main phenomena that resists sliding movement are fricton and binding • FRICTION – is classically described as a force acting tangentially at the surface of two moving bodies in contact. • Friction acts parallel to and opposes the tooth movement. • Friction can be static or kinetic. • Static friction is force required to start the movement . • Kinetic friction is the force required to maintain movement started.
  8. 8. Components of resistance to sliding (RS) 1 st component – classical friction (FR) –is a product of co –efficient of friction (µ) and normal force. Co –eff of friction- it is the objects frictonal proportionality constant. i.e surface roughness of the material. N- is the amount of force acting perpendicular to the surface of the object such as ligation force.on the bracket.
  9. 9. • 2 nd component – Binding • SECOND ORDER ANGULATON – it is the angle b/w base Of The wire (vertical dimension of wire) and the bracket.{θ}. • Critical contact angle – the level where the wire contacts both the ends of the bracket slot. .{θc}. • When the second order angulation increases to critical angle binding occurs .
  10. 10. • Passive configuration- FR exsists has an only component when the arch wire and bracket have clearance, in this angulation b/w arch wire and bracket is less than critical angulation. • Active configurationwhen clearance is absent and interferance occurs (θ = θ c) binding occurs. Under these conditions two forces exsists i.e. N and binding
  11. 11. • Factors determining critical contact angle are- • Slot size • Bracket width • Arch wire size • Engagement index – size/ slot • Bracket index – WIDTH / SLOT
  12. 12. Bracket width Wire size Slot size
  13. 13. 3rd component - notching
  14. 14. Influence of arch wire and bracket dimensions on sliding mechanics : derivations and determinations of the critical contact angles for binding ( robert .p. kusy and whitley -1999) Derived an equation size/ slot = width/ slot(sin θc)+cos θc. In this equation size/slot defines the engagement index .This index defines the fraction of the bracket slot filled by the arch wire . width/ slot defines the bracket index – the number of times the bracket width is more than slot dimension. Toghether , these two dimensionless indices define all that is necessary to determine θc as the point at which the binding starts. This study derived the maximum and minimum engagement and bracket indices possible,
  15. 15. • . For maximum bracket index - large bkt width/ small bkt slot = size= 250 mil/18 = 13.9 For minimum bracket index - small bkt width/ large bkt slot size=125mil/22=5.7 The range bracket index is 5.7 to 13.9 For maxi engagement index – large wire size/ small bkt slot= 16/18=.86 For mini engagement index – small wire size/ large bkt slot= 14x22=.64 The range for engagement index is .5 to1 When the nominal parameters of arch wire and bracket used in sliding mechanics were estimated for critical contact angle .three important conclusions were drawn
  16. 16. 1) Narrow bracket showed θc double the value when compared to the wider brackets. 2) Smaller bracket slot showed decreased θc value .Hence, more precise aligning and leveling is required before retraction. 3)Smallest wires used for retraction i.e. 16 size wire in 22 slot, 125mil width . θc =2.8 degrees. Same wire in 18 slot showed θc =0.9 degrees. Even in the best scenario the practitionar must align and level so that the angulation b/w wire and Bracket is within the range of 1-4 degrees or else binding increases and sliding ceases. To accomplish the best scenario most easily within the strength and stiffness requirements. The bracket width and wire size should be small and bracket slot should be larger.
  17. 17. INFUENCE OF BINDING OF THIRD ORDER TORQUE TO SECOND ORDER ANGULATION (ROBERT P. KUSY 2004) As base dimensions of arch wire increases, bracket width decreases, bracket slot size decreases the critical contact angle for binding decreases. But when torque is incorporated into the wire ,the height of the wire is also considered. ( depth of the wire). As the torque angle is increased, clearance b/w arch wire and bracket decreases reducing the critical contact angle. Thus, the chances of binding.
  18. 18.
  19. 19. By incorporating the torque in the posterior region would help to preserve the anchorage by increasing the resistance to sliding in posterior segment.
  20. 20. stainless steel brackets • These are the most popular brackets till today. • Friction is minimum in these brackets due to smoother surface. • Sintered stainless steel brackets had low friction then cast stainless steel brackets due to more smoother surface texture.
  21. 21. CERAMIC BRACKETS demonstrated significantly higher frictional forces than ss brackets.highly magnified views revealed numerous small indentations in ceramic brackets. monocrystalline alumina brackets had smoother surface than polycrytalline bracket,but their frictional characteristics are comparable. since ,greater forces are required to slide the teeth. Caution in preserving anchorage must be exerted in such situations. ceramic brackets with metal slots showed decreased friction as wire is contacting the smoother metal slot.
  22. 22. Zincornia brackets • Zincornia brackets has been offered as an alternative to the ceramic brackets since surface hardening treatments to increase fracture toughness are available for zincornium oxide . However, the frictional co-efficients for these brackets were found to be greater than or equal to polycrystalline brackets in both wet and dry states. Surface changes consisting of wire debris and surface damage in zincornia brackets after sliding of wires were observed.
  23. 23. Titanium brackets Introduced in response to reports of the corrosion of stainless steel brackets and increased sensitivity to nickel content of the alloy. It is proven to be biocompatible. It is very rough as the titanium content of the alloy is increased. More chance of cold weld formation with the titanium brackets at bracket wire interface which increases resistance to sliding.
  24. 24. Plastic brackets In an attempt to make a esthetic bracket with low frictional resistance and easier debonding features than ceramic ,a wide variety of new ceramic reinforced plastic brackets with or without metal slots were introduced. several studies showed that when these brackets were tightly ligated with steel ligatures deformed slightly to squeeze the bracket slot thereby increasing friction.
  25. 25. Tip edge brackets Tip edge bracket design- since, diagonally wedges of bracket is removed from each side which allows the slot size change from 0.022 slot to 0.028 slot as the tooth tips. This bracket design decreases the resistance to sliding during tooth movement.
  26. 26. WIRES Specular reflectance studies have shown that stainless steel wire have smoothest surface followed by co –cr, beta–ti, niti in the order of inceasing surface roughness. Beta titanium wires may form micro-welds in dry states and further increase the frictional forces. frank and nicoli found that stainless steel wires had least friction at non binding sites ,but as angulation increased and binding was present , the reverse was true.
  28. 28. Co-efficient of friction and surface roughness Kusy et al used laser spectroscopy to study surface roughness of orthodontic wires . Among the four wire alloy types that are commonly used in orthodontics , stainless steel appeared to be lowest followed by cobalt chromium, beta titanium and nickel titanium. kusy and whiteley were the first to look at the effect surface roughness on friction coefficient . The results showed that low surface roughness was not sufficient condition for low friction co efficient. For ex: beta titanium has decreased roughness than niti wires but frictional force resistance is more for beta titanium.
  29. 29. Ion implantation Gas ions like nitrogen and oxygen are implanted into the wire surface, resulting in a surface that is hard and creates a considerable compressive force at atomic level. This improves the surface characteristics and reduce co=-efficient of friction. Burstone and farzin –nia demonstrated that ion implanted beta titanium wires produced the same level friction as stainless steel,
  30. 30. • BioForce®, Nickel Titanium (NiTi) wires have transformed the orthodontic profession. The current trends in orthodontic treatment are low force, reduced friction, and shorter treatment time. BioForce uniquely addresses all of these issues, all within one wire. • BioForce is the only single strand, superelastic orthodontic arch wire that provides forces that range gradually from 80 grams in the central to 320 grams at the molars: specific biologically determined forces to move specific teeth. • All in One Arch Wire. At any point on the wire the force is near constant.
  31. 31. • The Quest for a Low Friction System!! . The truth is that Friction is not simply a component of the bracket alone but is instead tied to the interaction of the bracket slot and the wire within that slot. The wire is as responsible for Friction as the bracket. BioForce With Ion Guard will reduce friction in your current system!! Cu NiTi Has 78% more friction Than IonGuard. IonGuard = Greater BioCompatability Quicker retraction Similar to Steel smoothness No Breakage over Wide Spans. Bio Compatibility / No measurable nickel release
  32. 32. Round vs rectangular wires. Several studies have found that an increase in wire size increase bracket wire friction. Rectangular wires produce more friction than round wires. At non binding sites contact area between arch wire and bracket is an important factor in friction hence more friction with rectangular wires. At binding sites with rectangular wire the force is distributed over a large surface area resulting in less pressure and less resistance to movement. At binding sites with round wires the bracket slot can bite the wire causing indentations resulting in more friction.
  33. 33. Ligation techniques. Normal ligation force ranges from 50 to 300 grams. Edward et al in 1995 compared the frictional forces produced elastomeric modules applied conventionally or figure of eight , stainless steel ligatures and teflon coated stainless steel when used for arch ligation. Figure of 8 configuration appeared to create highest friction. No significant difference between normal conventional module and stainless steel ligature. Teflon coated stainsteel had lowest frictional force. Even the composition of the ligature is another variable in determining the co_efficient of friction.
  34. 34. As the elastomeric ligatures are polyurethane based polymers , studies have shown that when exposed to oral environment they undergo stress relaxation and hydrolytic decompensation over time which will effect the properties of the module. Frictional forces by ligation can be reduced by pre streching the module , or by using stainless steel ligatures or using self ligating brackets. Backing of one quarter turn after tying steel ligatures.
  35. 35. Conventional ties such as O-rings and stainless steel ligatures make using optimal forces impossible due to friction and binding. Elastomeric O-rings will lose half their elasticity within days of initial tie in, thus compromising tooth control. O-rings are extremely plaque retentive and greatly increase the number of microorganisms attached to appliances during treatment, increasing the incidence of decalcification during treatment.
  36. 36. Self ligating brackets • The first self ligating bracket was the Russel lock. • Self ligating brackets are ligatureless bracket system that have mechanical device built into the bracket to close off the edgewise slot . • These brackets show low frictional resistance. • They are 2 types – 1) Active-spring clip which presses against archwire. 2) passive- slides which does not press against wire and produces less friction. • { This difference in friction is seen only when the bigger sized arch wire is used.}
  37. 37. Smart clip
  38. 38. • It Begins with Low Force • .* Damon System brackets are designed to allow the clinician to use these optimum low forces throughout all phases of treatment. This is only possible with a completely passive system. • Damon brackets produce significantly less friction than conventional or active self-ligating appliances. By greatly reducing the amount of friction in the Damon bracket system, low-force archwires can work to peak expression, thereby stimulating a more biologically compatible tooth movement. Low forces throughout treatment also mean greater patient comfort.
  39. 39. Effect of arch wire and material on the resistance to sliding of self ligating brackets with second order angulations in the dry state. ( robert p kusy 2002) The results of this study showed that when the wire is passive ,the ligation force applied by slides of bracket was absent. But when the clips were used the amount of ligation appeared to be proportional to dimensions of the wire relative to lumen of the slot. When clearance no longer existed , i.e. when the second order angulations increased more than critical contact angle . The rate of binding was independent of these bracket designs.
  40. 40. A new low force ligation system (jco 2005) • • • This article describes an alternative to self ligating systems a ligature that markedly reduces the friction b/w the arch wire and bracket. The slide ligature is made of special polyurethane ,is applied in the sameway as a conventional elastomeric ligature .Like a passive self ligating it forms a fourth wall and allows the archwires to slide freely in the slot while transmitting its full force to the teeth.
  41. 41. •This ligature also forms the buffer b/w the bracket and soft tissues considerably improving patient comfort.
  42. 42. Sliding mechanics often occurs in one of the 2ways ; 1) During the space closure at the extraction site ( mesio distally). Dr . kusy 2) Aligning and leveling of the misaligned teeth. ( labio lingually, occlusogingivally, mesio distally)
  43. 43. Importance of wire stiffness and clearance during canine retraction. Stiffer wires are less springy and deflect less for a given force .stiffness is proportional to modulus of elasticity and wire dimensions The interbracket span is increased due to extractions which further reduces the stiffness The retraction force therefore has a greater chance of deflection and result in buckling and increases friction. To compensate , the wire size should be increased. Stiffness of the wire is dependent on vertical dimensions of the wire ex. 17x25 ( 17 is the vertical dimension.
  44. 44. • Another reason for not using flexible wires during retraction is that flexible wires can deflect as the canine crown tips distally which would result in extrusion of the centrals. During canine retraction it ia advisable to use 19x25 wire which provides adequate stiffness and clearance with acceptable tip control during space closure.. Movement of the brackets along the 0.021x0.025inch wires ss produced three times more friction than 0.019x0.025 ss wire due to decreased clearance. The overall means being 3.0N and 1.2N respectively. An increase in bracket tip from 1 to 3 degrees almost quadrupled the friction. With 0.021x0.025 wire at 3 degrees tip friction lock was seen and ceased sliding completely. (Moore and Rock 2004,EJO)
  45. 45. Biomechanical consideration Effect of point of application of force Mesial and distal tipping of the teeth is dependent upon the the location of the force application relative to the centre of resistance and biological consideration of the teeth. Tipping of the bracket produces pressure at the contact areas b/w arch wires and bracket with a resultant increase in the frictional resistance. Farther away the retraction force is applied from the centre of resistance greater is the moment arm and greater the tipping and binding.
  46. 46. Aligning and leveling During the initial stages of treatment when the teeth might be severly misaligned relative to each other . Small flexible wires provide low resistance to sliding ,by increasing the intra bracket space and critical contact angle, which allows the greater portion of the applied force available for unraveling the teeth.
  47. 47. Intrabracket space and inter bracket span: critical factors in clinical orthodontics .(SChudy and SChudy 1989) • Intrabracket space- ( slop, play) is the space around the wire. • Inter bracket span – is the amount of wire between two adjacent brackets. • The results also demonstrate several general facts that the inherent behavior of the wire and the effect of the appliance modification on that behavior as follows. • • • • less force and greater range result from use of 1) smaller wires. 2) narrow brackets. 3) increased intra bracket space.
  48. 48. They are many positive manifestations of more intrabracket space. 1) less patient discomfort 2) less complex wires 3) fewer arch removals 4) faster and more efficient leveling 5) reduction of permanent sets. The only disadvantage of increased intrabracket span is lose of torque control. Round wires used over a longer period of time allow buccal and lingual rolling of posterior teeth and labial dumping of anterior teeth. Advancing to rectangular wire early in treatment makes it possible to control axial inclinations. In addition to increased flexibility and range , the intra bracket span reduces the amount of friction b/w archwire and bracket
  49. 49. Resistance to sliding of stainless steel multistrand archwires and comparision with single stranded leveling wires.(rucker and kusy 2002). Niti wires have low stiffness because the modulus of elasticity is 20% of stainless steel. Alternatively for the same wire dimensions and material, a multistrand wire has low stiffness to single stranded wire of ss. This study compared the frictional behavior of the single stranded niti and stainless steel and multistranded wires of ss in wet and dry conditions. Multistranded wires used were 1.3 stranded round wire 2. co axial wire. 3. 3 stranded rectangular wire. 4. 8 stranded rectangular wire.
  50. 50. Results of this study showed thatIn passive configuration co- eff of friction in presence of saliva was same as dry state in stainless steel wires, more for multistrand wires ( adhesive effect), and was less with niti wires ( lubricating effect). In passive configuration the co eff of friction was more for niti wires than single and multistanded stainless steel due to rougher surface of niti. In active configuration has the binding component of the friction dominates the co eff of friction , Which is dependent on the stiffness of the wire. Niti single stranded wires show low resistance to sliding than single stranded wires of ss for the same dimension of wire. Multistrand wires have low stiffness than niti wires and hence low resistance to binding .Co axial wtre produced low RS.
  51. 51. INFLUENCE OF CERAMIC BRACKETS AND STAINLESS STEEL BRACKETS ON NOTCHING OF ARCHWIRES DURING CLINICAL TREATMENT.(KUSY 2000) • Notching – is defined as observed mechanical damage to an archwire that occurs during later stages of binding which manifests itself as recognizable defects of varying number, pattern and severity. • the direction and movement determines the type of damage and subsequent appearance of NO p, NO s,No n. • Fourteen different defect patterns are only required to describe notching. • Specific wire aspects ( lingual aspect) and anatomical regions ( canines and incisors ) are more prone to notching.
  52. 52.
  53. 53. • Two mechanisms of notch formation are suggested • 1) fretting- in which the tooth movement is in vertical plane often causes pairs of shallow parabolic effects . • 2) sliding- in which the translational movement in the horizontal plane often leaves a elliptical defect. • • The notch activity and severity were nearly three times more greater in ceramic brackets than in stainless steel brackets. • Over 1/3 rd of the notches in ceramic brackets had severity numbers greater than 3 and penetrated at least ¼ of the wire.
  54. 54. Three main other reasons that interferes with sliding mechanics • 1) involuntary irregularities introduced into the arch wires while construction. • 2) corrosion of the arch wires and brackets which effect the polished surface. • 3) appearance of adherent concretions due to hard tartar and calculus.
  55. 55. Frictional resistance in orthodontic brackets with repeated use.( ram.s. nanda 1999) • This study measured and compared the level of frictional resistance generated by repeated and non- repeated brackets to evaluate the bracket slot will influence the frictional resistance. • The results of the study show that there was a distinct trend for the mean frictional force to be higher with repeated use of the brackets . • The grooving and wearing of the brackets and mechanical interlocking at the bracket wire interface frictional resulted in higher resistance.
  56. 56. vibration Vibration is a motion that creates forces. chewing food bolus requires forces and creates vibrations which are of low frequency and high magnitude. Clenching of teeth produces vibrations of high frequency and low magnitude. These vibrations that stimulate teeth can impede motion or facilitate it. Eg. These vibrations can help an arch wire to jump out of notched wire region occurred due to excessive angulations. These are undeterminable.
  57. 57. Effect of saliva on kinetic friction • Saliva serves as an excellent lubricant in the sliding of the bracket along the wire. • Kusy et al 1991 found that saliva could be lubricous as well as adhesive behaviour depending upon which archwire bracket combination is used . • The kinectic co eff of friction of the beta titanium wires in wet state were 50% of the vaues in the dry state. When sliding through stainless steel wire , titanium rich oxide layer in beta titanium wires break down , adheres and reacts breaks away , resulting in stick-slip phenomenon.
  58. 58. Conclusion Sliding mechanics is a necessary accompaniment in orthodontic practice .The clinician requires complete understanding of its complexities. The clinician must consider bracket material, design and wire alloy size ,material ,as well as resistance factors .The multitude of possible appliance combinations in sliding mechanics posses a serious challenge in producing a force system that is optimal for tooth movement. Mechanical and biological factors must be considered in producing the appliance best suited for the patient. The level of force systems must be take into account the frictional force levels in order to successfully achieve the treatment objectives.
  59. 59. Thank you