Role of friction in orthodontics /certified fixed orthodontic courses by Indian dental academy


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Role of friction in orthodontics /certified fixed orthodontic courses by Indian dental academy

  1. 1. Role of Friction in Orthodontics INDIAN DENTAL ACADEMY Leader in continuing dental education
  2. 2. Contents Introduction Static & dynamic friction Laws of friction Sliding mechanics effect of bracket archwire ligation biological factors
  3. 3. What is FRICTION ? A force resisting the relative displacement of two contacting bodies in a direction tangent to the plane of contact . Because of friction ,part of the mechanical energy intended for movement of the two bodies relative to each other is dissipated as thermal energy
  4. 4. Static friction is the component of frictional force that has to be overcome to initiate motion Dynamic (kinetic) friction is the component of frictional force that has to be overcome to maintain motion . static frictional force is usually higher than dynamic frictional force
  5. 5. It is the force that resists the movement of one surface past another and acts in a direction opposite the direction of movement. Friction may exist between : - Two solid surfaces - Solid fluid interface -Liquid/fluid layers
  6. 6. Types of friction 1. Rolling or sliding 2. Static or dynamic When two surfaces in contact slide or tend to slide against each other, two components of total force arise Frictional component Normal force
  7. 7. Normal force :- • Perpendicular to one or both contacting surfaces and also to the frictional component. • Fixed surface on which the block rests responds only to the weight of the block with an upward force, perpendicular to the plane contact area. • This force is symbolized by N. Also signifies the force pushing two surfaces together
  8. 8. Frictional force :- • Parallel in the direction to the intended or actual sliding motion and opposes the motion.
  9. 9. 2. Static and Dynamic Friction :- - Resistance that precludes actual motion is termed static friction. - That which exists during motion is called Dynamic friction. Both static and dynamic forms of sliding friction are of orthodontic interest.
  10. 10. Frictional coefficient(µ ) –the law of friction theorized by coulomb states that the magnitude of the frictional force F is equal to the product of normal force N acting perpendicular to the contact surface multiplied by frictional coefficient The frictional coefficient depends on the surface roughness of the combination of the materials involved .it does not depend on the area of contacting surfaces and varies only slightly with velocity of movement
  11. 11. Coefficient of static friction : Reflects the force necessary to initiate movement. Coefficient of Kinetic friction :- Reflects the force necessary to perpetuate the motion. It takes more force to initiate motion than to perpetuate it.
  12. 12. . LAWS OF FRICTION : As early 17th and 18th centuries, Amontons and coulomb were formally investigating frictional forces. From their efforts, fundamental laws of friction evolved :- a) Frictional force (f) is proportional to the applied normal force (N) multiplied by the coefficient of friction (µ) i.e f = µ N. b) Frictional force (f) is independent of the apparent area of contact b/n two sliding surfaces.
  13. 13. This is because all surfaces, no matter how smooth, have irregularities that are large on a molecular scale and real contact occurs only at a limited number of small spots at the peaks of surface irregularities
  14. 14. ASPERITIES : spots called Asperities, Carry all the load b/n two surfaces. Even under light loads,, local pressure at the asperities may cause appreciable plastic deformation of small areas bcoz of this the true contact area is to a considerable extent deter­ mined by the applied load and is directly proportional to it
  15. 15. Surfaces with relatively large asperity wavelengths may be considered smooth but not flat, whereas, surfaces with relatively short asperity wavelengths may be considered flat but not smooth. With preparation methods such as machining, milling, grinding, or lapping, peak height and wavelength may vary
  16. 16. Frictional force is independent of the sliding velocity (v) i.e the so called coulomb’s 3rd law.
  17. 17. Orthodontic tooth movement during space closure is achieved through two types of mechanics :-   Friction FrictionFree
  18. 18. Friction Mechanics (Sliding Mechanic) Involves either - Moving the brackets along an archwire - Sliding the archwire through brackets and tubes.
  19. 19. ROLE OF FRICTION IN SLIDING MECHANICS : Most fixed appliance techniques involve some degree of sliding between bracket and archwire. When sliding mechanics are used, friction occurs at the wire bracket interface. Some of the applied force is therefore dissipated as friction and the remainder is transferred to supporting structures of the tooth to mediate tooth movement.
  20. 20. 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
  21. 21. in vitro resolution of static and kinetic frictional resistance into separate and distinct phases is arbitrary and potentially misleading because at low velocity, such as exists in orthodontics, static and kinetic frictional resistances are dynamically related.
  22. 22. The first two laws are usually obeyed in orthodontics whereas the third usually is not. In wire/bracket couples of stainless steel/stainless steel and NiTi/ stainless steel, the third law is obeyed; however for Co-Cr/SS and B-Ti/SS couples, the values of u slightly increase and markedly decrease respectively with velocity.
  23. 23. if a stationary mass (M) is at equilibrium (at rest with zero velocity V0) on a solid flat surface (S), the contact between the mass and the surface is the result of a normal force (F) acting on the mass The normal force F may be the net system force or the weight of mass M. The interfacing area between mass M and surface S may be approximated as the nominal area by the macroscopic surface area of M in contact with S
  24. 24.
  25. 25. To overcome the static frictional resistance from the rest position, a minimum pulling or shear force (f), which is parallel to the contact surface of the nominal area, is required to move mass M at velocity V. This static frictional resistance (fs) is equal to the normal force or load (F) multiplied by a coefficient of static friction .Once a steady sliding motion of a constant velocity (Vc) is achieved, then a minimum force to overcome the kinetic frictional resistance (fk) is required to maintain velocity Vc of mass M. The kinetic frictional resistance is equal to the normal force or load (F) multiplied by a coefficient of kinetic friction (fk).
  26. 26. Static friction (occurring instantaneously up to the onset of sliding) and kinetic friction (occurring continuously after the onset of sliding) are two distinct phases that by definition cannot coexist. The classic Amontons-Coulomb laws relate static and kinetic friction as follows: 1. s and k are independent of F and area; 2. s and k are materials dependent; and 3. usually k< s.
  27. 27. The two distinct frictional phases (ie, fs and fk are defined by zero velocity Vo and constant sliding velocity Vc, respectively. the transition from zero velocity Vo to constant sliding velocity Vc must involve an acceleration of mass M. The acceleration phase is of particular interest when Vc approximates Vo (ie, low velocity friction).
  28. 28. Refinement of the Amontons-Coulomb principles is required especially when it becomes evident that Vc approximates Vo. Apparent deviation from the Amontons- Coulomb principles includes time-dependency of the static coefficient S(t) (ie, static coefficient as a function of time) and velocity-dependency of the kinetic coefficient K(V) (ie, kinetic coefficient as a function of velocity).
  29. 29. The Stick-Slip Phenomenon At low speeds a “Stick - slip phenomenon” may occur as enough force builds up to shear the junctions and a jump occurs, then the surfaces stick again until enough force again builds to break them. A single stick-slip cycle involves a stick state associated with elastic loading of the system, followed by a sudden slip corresponding to stress relaxation.
  30. 30. Static Coefficient as a Function of Time {s (t)} the static coefficient of friction varies as a function of increasing time t before the onset of sliding. Increases in the coefficient of static friction, as a function of stick time vary over a wide time interval range the longer a mass M is at rest on a flat surface S (or an archwire at rest on a bracket) the greater the resistance to pulling force f parallel to the contact surface of nominal area.
  31. 31. Age strengthening of localized point contacts among the asperities of mass M and surface S is correlated with experimental observation of slow plastic deformation occurring at the stressed asperities, leading to an increasing effective interface area as a function of time When plastic deformation occurs at the level of the softer asperities, the frictional force becomes a function of shear stress localized to point contacts among surface asperities of mass M and surface S As a result, increases in both area and shear produce a proportionate increase in
  32. 32. . Assuming that the normal force F (ie, load) remains constant, the coefficient of static friction increases as a function of time
  33. 33. Kinetic Coefficient as a Function of Velocity The second refinement of the Amontons- Coulomb principles is that constancy of the kinetic frictional coefficient is dependent on maintenance of a steady sliding velocity Vc. Different materials exhibit unique kinetic frictional characteristics as a function of velocity within very low velocity ranges, most materials exhibit decreasing coefficients of kinetic friction as the low-velocity range increases (ie, velocity weakening).
  34. 34. subsequent velocity strengthening as velocity progressively increases beyond the low-velocity range.
  35. 35. At low velocity, such as occurs with in vivo tooth movement, steady sliding instability may lead to oscillations of motion characterized by cycles of sticking and slipping. Stick-slip motion, as observed over a broad velocity range in frictional sliding, can potentiate consequences resulting in noise (chatter), energy loss (friction), surface damage (wear), and component failure (breakage).
  36. 36. Stick-slip processes are caused when the frictional force does not remain constant as a function of some other variable such as distance, time, or velocity Orthodontic evidence of repetitive stick-slip oscillations at the archwire-bracket interface may be inferred from scanning electron micrographs that reveal permanent deformation of archwires subjected to intermittent binding and sliding at bracket surfaces.
  37. 37. Loss of Applied Force Orthodontic tooth movement is dependent on the ability of the clinician to use controlled mechanical forces to stimulate biologic responses within the periodontium. it has been concluded that the rate of tooth movement increases proportionally with increases in applied force up to a point, after which additional force produces no appreciable increase in tooth movement.
  38. 38. With orthodontic mechanotherapy, a biologic tissue response with resultant tooth movement will occur only when the applied forces adequately overcome the friction at the bracket wire interface. mechanotherapy to move a tooth via a bracket relative to a wire results in friction localized at the bracketwire interface that may prevent the attainment of an optimal force in the supporting tissues.
  39. 39. The portion of the applied force lost because of the resistance to sliding can range from 12% to 60%. If frictional forces are high, the efficiency of the system is affected, and the treatment time may be extended or the outcome compromised because of little or no tooth movement and/or loss of anchorage. the amount of frictional resistance will impact on the moment to- force ratios of the teeth and, consequently, their centers of rotation.
  40. 40. When the archwire and the bracket have clearance, classical friction exists as the only component to the resistance to sliding. When clearance disappears and an interference fit occurs between the bracket and the arch wires, binding arises as a second component to the resistance to sliding superimposed on the classical friction.
  41. 41. Movement of the crown mostly precedes displacement of the root because a tipping moment is placed on the crown of the tooth This tipping leads to increased friction from binding between the archwire and bracket restricting movement of the entire tooth.
  42. 42. Friction is influenced by - the nature of contacting surface ,but is independent of area of contact,this is due to the interlocking of surface irregularities -the extent to which asperites on the harder material plough into the surface of the softer material Total frictional resistance is the sum of -Force necessary to shear all junctions -Resistance caused by interlocking roughness -Ploughing component of total frictional force
  43. 43. Orthodontic Model of friction - BETWEEN ARCHWIRE AND BRACKET Sliding friction is generated between arch wire and bracket when -       The wire “guides” the bracket during M-D movement of an individual tooth. -       the wire is slipped through posterior crown attachment in, e.g:- Retrac­tion of anterior dental segment.
  44. 44. Possible Components of this force are :- -       Engagement of arch wire in brackets that are out alignment. Ligatures pressing the wire against base of slot -      Active torque in rectangular wire -      Bodily tooth movement in which tipping tendency is resisted by two point contact between the bracket and archwire.
  45. 45. The relative magnitudes of these components of frictional force vary according to the clinical situation
  46. 46. VARIABLES AFFECTING FRICTIONAL RESISTANCE DURING TOOTH MOVEMENT A) PHYSICAL :- 1) Archwire a. Material b. Cross sectional shape/size. c. Surface texture. d. Stiffness. 2) Ligation of archwire to bracket Ligature wires. Elastomerics Method of ligation : Method of tying, bracket designs to limit force of ligation, self ligating brackets
  47. 47. ) Bracket: a.    Material b.     Manufacturing process : Cast or sintered stainless steel. c.    Slot width and depth d.    Design of bracket : Single or twin e.    First order bend ( in - out) f.     Second order bend (angulation) gThird order bend (torque
  48. 48. 4. Orthodontic appliance a. Interbracket distance. b.Level of bracket slots between adjacent teeth. c. Forces applied for retraction. B. BIOLOGICAL 1. Saliva 2.Plaque. 3.Acquired pellicle. 4. Corrosion
  49. 49. With so many variables affecting frictional force, it is difficult to accurately determine them in a clinical situation. the problem is further complicated by wide array of brackets, wires and ligatures available that provide a multitude of combinations for use during various stages of orthodontic treatment.
  50. 50. EXPERIMENTAL METHOD USED TO STUDY FRICTION : 1. SIMULATED TOOTH MOVEMENT : Most of studies within orthodontic literature have carefully simulated different clinical conditions b/n bracket and archwire to measure sliding frictional resistance. 2. SURFACE ROGHNESS : Some studies have quantified surface roughness of various bracket and archwire materials. Most common method of estimating surface roughness – SPECULAR REFLECTANCE Involves determination of amount of light that is reflected back from a surface.
  51. 51. Smooth surface :- Reflects much of light shone on it in a narrow pattern. Rough surface :- Scatters light and reflects it back in amore dispersed pat­tern. 3. CONTACT FLATS :- Coefficients of friction have also been evaluated using orthodontic wire held between two parallel plates (Contact flats) made of material similar to that used in orthodontic brackets such as SS, polycrystalline alumina or Teflon various levels of normal force were applied to plates and wire is pulled through them to measure friction generated.
  52. 52. 4. DESCRIPTIVE STUDIES :- These have involved discussion of frictional resistance of brackets and wires based on clinical experience and anecdotal information.
  53. 53. EFFECT OF BRACKET MATERIAL, DESIGN AND MANUFACTURING PROCESS ON FRICTION :- Various Bracket materials today available are :- 1. Stainless steel Cast Sintered 2. Ceramic brackets Polycrystalline alumina Single Crystalalumina (SCA) (i.e Sapphirc) 3. Zircoma brackets 4. Plastic brackets
  54. 54. 1. Stainless Steel Brackets :- Most popular bracket material Stainless steel brackets are associated with lowest frictional force values amongst the available bracket materials
  55. 55. Kapila et al (1990) Evaluated friction b/n Edgewise SS brackets and orthodontic wires of 4 alloys (SS, Co- Cr, NiTi and B-Ti) Mean frictional forces with conventional cast stainless steel brackets ranges between 40-336 g.
  56. 56. Level of frictional forces observed in :- 0.018inch SS brackets – Ranged from 49g with 0.016 inch SS wires in narrow single brackets to 336g with 0.017 x 0.025 inch B-Ti wires wide twin brackets. 0.22 inch SS brackets – Friction ranged from 40g 0.018 inch SS wires in narrow brackets to 222g with 0.019 x 0.025 inch NiTi wires in wide brackets. Several SS bracket wire combinations generated low levels of frictional forces less than 100g.
  57. 57. SINTERED STAINLESS STEEL BRACKETS Sintering : Process of fusing individual particles together after compacting them under heat and pressure. Sintering allows individual bracket to be premolded in a smooth stream­lined manner. The SS particles are compressed in a contoured smooth rounded shape as apposed to older casting procedure in which milling or cutting pro­cess left sharp angular brackets that were bulky and rough.
  58. 58. Sintered edgewise brackets RMO Mini Taurus. RMO Mini Taurus Synergy Unitek Mini Twin. Sintered SS brackets produce significantly lower friction than cast stainless steel brackets overall friction of sintered SS brackets is approx 40% - 45% less than friction of conventional cast stainless steel brackets
  59. 59. 2) CERAMIC BRACKETS : With ceramic brackets, most of wire size and alloy combinations with both 0.018 and 0.022 inch slot sizes demonstrate significantly higher fric­ tional forces than with SS brackets.
  60. 60. Characteristics of Ceramic Bracket Material or Slot Surface Texture: Highly magnified views have revealed numerous generalized small indentations in the ceramic bracket slot while the SS bracket appeared relatively smooth. Hardness of the material All currently available ceramic brackets are composed of Aluminium oxide. Aluminium oxide is extremely hard.
  61. 61. The rough but hard ceramic material is likely to penetrate the surface of even a steel wire during sliding, creating a considerable resistance and this is worse with titanium wires The interaction of metal wire - ceramic slot interface leads to leveling of ceramic slot. This results in drop in friction as ceramic peaks are removed and valleys become clogged with metal
  62. 62. Types of Ceramic brackets : -      Single Crystal alumina (SCA) -      Polycrystalline alumina (PCA)
  63. 63. monocrystalline alumina :- Single crystal ceramic brackets are derived from large single crystals of Alumina which are milled into desired shape and dimensions by ultrasonic cutting, diamond cutting or combination of two techniques. Because Alumina is third hardest known material, this procedure is difficult and may explain granular and putted surface of ceramic brackets seen in SEM
  64. 64. Polycrystalline brackets - have also been observed under SEM to possess very rough surfaces which actually scribed grooves into the archwire Monocrystalline brackets were observed to be smoother than PCA brackets but their frictional properties were comparable. The most apparent difference b/n polycrystalline and single. crystal brackets is their optical clarity. Single crystal brackets are noticeably clearer than PCA brackets which tend to be translucent
  65. 65. Clinical significance :- Combination of metal archwires and Ceramic brackets produce high magnitudes of frictional force; therefore greater force is needed to move teeth with ceramic brackets compared to SS brackets in sliding mechanics.
  66. 66. Since ceramic brackets on anterior teeth are often used in combination with SS brackets and tubes on premolar and molar teeth, retracting canines along archwire may result in greater loss of anchorage because of higher frictional force associated with Ceramic than SS brackets. To reduce frictional resistance Ceramic brackets with smoother slot surfaces and consisting of metallic slot surface are avaliable
  67. 67. ) ZIRCONIA BRACKETS : Besides high friction, Ceramic brackets have very low fracture resistance Due to their brittle nature even smallest crack or flaw can propagate rapidly through the material. Zirconia brackets have been offered as an alternative to ceramic brackets since surface hardening treatments to increase fracture toughness are available for Zirconium oxide.
  68. 68. Frictional coefficients of Ziconia brackets were found to be greater than or equal to those of poly crystalline alumina brackets in both dry and wet states (Keith et al , 1994). Surface changes consisting of wire debris and surface damage in Zirconia brackets after sliding of archwires were also observed.
  69. 69. ) PLASTIC BRACKETS : In an attempt to create an esthetic bracket with lower frictional resistance and easier debonding features than ceramics a varity of new, ceramic reinforced plastic brackets with or without metal slot inserts have been introduced.
  70. 70. Trade Name Company Type E.g: Silkon American Orthodontics Plastic reinforced with ceramic Spirit Ormco Plastic with metal insert Image GAC Plastic reinforced with glass Clarity Unitek Ceramic with metal insert Cermaflex TP Metal with plastic base
  71. 71. Plastic brackets can deform because of compression from ligation and thus binding of the wire, and higher frictional resistances were recorded than stainless steel brackets. Recently introduced composite brackets with and without metal slots faired better in friction studies showing lower frictional resistance than both ceramic and stainless steel brackets in one of the studies.
  72. 72. EFFECT OF BRACKET WIDTH ON FRICTION Effect of bracket width on friction has been controversial - Some studies have found that altering bracket width made no difference in friction (Peterson et al 1982, Andereasen et al, 1970). - Frictional resistance has been reported to increase with increase in bracket width (Tidy 1989, Drescher et al, 1989). - Whereas others found that frictional resistance decrease as bracket width increased. • Franks and Nikolai (1980) :- Related greater friction with wider brackets to the fact that binding occurs frequently with wider brackets.
  73. 73. Omana et al suggested that with a narrow bracket the tooth could tip considerably before binding could occour,and once binding occurs it was of severe nature Kapila et al (1990 and Ogata et al (1996) : Suggested that with a wider bracket the elastomeric ligature used was stretched more than with a narrower bracket which exerted a greater normal force on the wire
  74. 74. Bracket Width and Interbracket Distance (IBD) Bracket width is closely related to IBD Narrower the bracket ↓ Greater the length of interbracket wire ↓ Greater the flexibility of wire ↓ Decrease in stiffness of wire ↓ Greater chance of binding with more flexible wire.
  75. 75. Bracket slot size may not influence the frictional resistance, some studies suggested that frictional resistance decreased as slot size increased from 0.018 inch to 0.022 inch because of reduced binding probably from increased wire stiffness. And also because of the increased play in the slot with final
  76. 76. ADDITIONAL DESIGN FEATURES IN BRACKETS TO REDUCE FRICTION Bumps on the bracket slot walls and floor which decreased surface contact with the wires, help decreased friction in bracket wire interface. (Ogata etal)
  77. 77. Begg brackets—haveachieved low friction by virtue of an extremely loose fit betweena round archwire and a very narrow bracket, but this is at thecost of making full control of tooth position correspondinglymore difficult. Some brackets with an edgewise slot have incorporatedshoulders to distance the elastomeric from the archwire and,thus, reduce friction, but this type of design also producesreduced friction at the expense of reduced control.
  78. 78. EFFECT OF SECOND ORDER DEFLECTION OF FRICTION Second order defection of wire b/n brackets held in series can have significant effects on brackets wire friction. - Several studies have found that increasing the angulation between bracket and wire produced greater friction.
  79. 79. Frank and Nikolai (1980) :- Found that frictional resistance increased in a nonlinear manner with bracket angulation. With brackets out of alignment archwire stiffness, strongly influences forces normal to the points of contact and hence friction. In a well aligned arch forces that result from archwire deflection are not important and friction is largely independent of archwire stiffness
  80. 80. Ogata et al (1986)Evaluated the effects of different bracket wire combinations and 2nd order deflections on kinetic friction. The brackets were offset deflecting wire in increments of 0.25 mm As 2nd order deflection increased frictional resistance increased for every bracket wire combination - With lower deflections a smooth sliding phase appeared in which friction increased in approximate a linear manner.
  81. 81. As deflection increased further a binding phase occured in which friction increased at a much greater rate and was not necessarily linear. Binding generally occured between 0.75 and 1.00 mm of 2nd order deflection. The relationship between frictional resistance and second order angulation may not be linear and may become more important as the angulation increases. The active configuration for binding occurred between 3 to 7°.
  82. 82. When tipping occurs the frictional resistance of nickel-titanium has been reported to be less than stainless steel, Because of the lower modulus of elasticity of nickel-titanium compared with stainless steel, lower normal force that was induced by binding occurred resulting in less resistance to sliding.
  83. 83. Active third order torque with rectangular wires would increase the friction even more. Similarly, greater friction with larger rectangular wires results from the possible introduction of torque because an 0.021x 0.025” wire has 3.9° of play compared with an 0.018 x0.025” wire that has 14.8° of play when engaged into an 0.022” bracket slot
  84. 84. Effect of ARCHWIRE ) WIRE ALLOY:- The role of wire alloy in frictional characteristics of sliding mechanics has been extensively studied. most studies have found SS wires to be associated with the least amount of friction and Beta titanium with the most. from lowest to highest friction SS, Co - Cr, NiTi and B-Ti.
  85. 85. Frank and Nikolai (1980) :- Found that SS wires had less friction than nickel titanium at non binding angulations, but as the angulation increased and binding was present reverse was true.
  86. 86. SURFACE TEXTURE :- Specular reflectance studies have shown that SS wires have the smoothest surface followed by Co-Cr, B-Ti and NiTi wires in order of increasing surface roughness Since B-Titanium had the most friction but was not the roughest Kusy and Whitly concluded that one cannot use surface roughness as an indicator of frictional characteristics in sliding mechanics.
  87. 87. NiTi has greater surface roughness Beta Ti has greater frictional resistance. as the titanium content of an alloy increased its surface reactivity increases and surface chemistry is a major influence on frictional behaviour. β-Ti at 80% Titanium has higher coefficient of friction than NiTi at 50% titanium there is enough titanium reactivity for wire to “COLD WELD” itself to steel bracket and therefore β-Ti wire exhibits more of stick slip phenomenon.
  88. 88. ION IMPLANTATION :- - Alteration of the surface of titanium wires by implantation of ions into the surface. - Gas ions (Nitrogen and Oxygen) are implanted in to the wire surface resulting in a surface that is extremely hard. - Ion implantation produces no interface b/n the coating and the wire neither does it alter the dimensions of the wire. Burstone and Farzin Demonstrated that ion implanted β-Ti wires produced about the same level friction as SS wires.
  89. 89. Braided arch wires Berger (1990) Studied friction produced by 0.0175 inch braided archwire in a 0.022 slot and found very high friction levels. -      1.5 times compared to 0.018 inch round SS wire with elastomeric ligation. 5 times with stainless steel ligation This can be attributed to interwoven pattern and irregular surface of Braided arch wire. Mechanical interlocking of the archwires with the edges of the bracket slot increase friction as the wire moves relative to the bracket. ,efforts to reduce friction with teflon coating are being made
  90. 90. WIRE SIZE - Several studies have found that an increase in wire size is to be associated with increased bracket wire friction . The main reason for the increase in friction as the wire size increased can be attributed to an increase in the stiffness of the wire.Wires of greater stiffness will create a greater normal force with binding of the archwire with the edges of the bracket. . .
  91. 91. Rectangular wires produce more friction than round wires. At nonbinding angulations the contact area between bracket and archwire is important factor in friction and would therefore expect more friction with rectangular wire.(nanda) placement of a rectangular wire can dramatically increase the friction because of the concomitant increase in wire stiffness.(tidy) At greater angulation of the bracket, the determining factor is the point at which the wire contacts the edge of the bracket
  92. 92. With round wires bracket slot can “bite” into the wire at one point ,causing an indentation in the wire With rectangular wire the force is distributed over a large area i.e the entire faciolingual dimension of the wire, resulting in less pressure and therefore less resistance to movement Frank and Nikolai (1980) Found that an 0.020 inch wire was associated with more friction than 0.017 x 0.025 inch wire
  93. 93. ) ROLE OF WIRE STIFFNESS AND CLEARENCE Mechanically speaking orthodontic archwires are elastic beams supported at either one or both ends. WIRE STIFFNESS DEPENDS UPON :- - Diameter or cross section of the wire - Length of beam. e.g Doubling length of cantilever beam decreases stiffness by 8 times. - By altering interbracket distance stiffness of wire can be altered.During canine retraction in a premolar extraction case the increased inter­ bracket span of the unsupported wire over the extraction site decreases the stiffness of wire.
  94. 94. Retraction force therefore has a greater chance of deflecting the wire resulting in buckling. -      To prevent such deflections of the wire that may increases friction and chances of bracket binding, diameter of wire should be increased to compensate for decrease in stiffness when interbracket span is greater than normal. -       Another reason for not using flexible small size archwires during sliding canine retraction is that flexible small size archwires can deflect as canine crown tips distally which can lead to incisor extrusion.
  95. 95. CROSS SECTIONAL DIMENSION IN DIRECTION OF BENDING 0.017 x 0.022 inch wire placed edgewise is more springy in vertical dimension than when placed in ribbon mode Drescher et al (1989)Stated that vertical dimension of the wire was an important factor in frictional resistance . Nature of end supports of a beam Rigidly supported beam at both ends has stiffness 4 times as compared to cantilever beam.Therefore During sliding space closure the wire therefore should be tied into the supporting brackets tightly to increase stiffness. eg During canine retraction premolar and lateral incisor brackets should be tied tightly to the archwire
  96. 96. Clearance of arch Wire:- Adequate clearence should be provided between bracket and wire to prevent binding. Clearance or play in 2nd order i.e tipping depends upon Slot size, Bracket width , Archwire size 3rd order play in rectangular arch wires. In 0.018 slot→ 16.7° for 0.016 x 0.016 wire 4.5° for 0.017 x 0.025 wire. 0.022 27.4° for 0.016 x 0.022 inchwire. 2° for 0.0215 x 0.028 wire.
  97. 97. EFFECT OF LIGATION TECHNIQUE ON FRICTION:- The normal force exerted by ligature has a significant influence in determining the frictional resistance developed within an orthodontic appliance. Ligation technique signifies the force that pulls the wire into the bracket. Elastomeric modules: -      Affected by the oral environment. -      Demonstrate stress relaxation with time Stainless steel ligatures: Can be tied either too tight or too lose.
  98. 98. Properties of an ideal ligation system  be secure and robust; ensure fullbracket engagement of the archwire; exhibit low friction betweenbracket and archwire; be quick and easy to use; permit highfriction when desired; permit easy attachment of elastic chain; assist good oral hygiene; be comfortable for the patient
  99. 99. Edwards et al (1995) : Compared the effect of 4 ligation techniques. -      E modules tied conventionally and in figure 8 pattern. -      Stainless steel ligatures. -      Teflon coated ligatures. static frictional resistance greatest in figure of 8 emodules-      No significant differences between frictional resistance offered by conventionally tied E-modules and steel ligature.teflon coated ligatures produce lowest friction
  100. 100. Shivapuja et al (1994) : E-modules produced greater frictional resistance as compared to steel ligature ties. This combined with rapid rate of decay for these E-modules and their predliction for harboring large quality of plaque suggests little merit in their use especially in sliding mechanics
  101. 101. NEW SLICK ELASTOMERIC MODULE SYSTEM: A new slick elastomeric module system incorporating metafasix technology (TP orthodontics) has recently been introduced claims to combine ease of use with low friction. The new slick E - modules reduced friction by upto 60% compared with their regular counterparts when tied normally
  102. 102. BRACKET DESIGNS LIMITING FORCE OF LIGATION: Three brackets were introduced to restrict the amount of force placed on wire by the ligature. -      American friction free bracket (Am. Orthod) -      GAC shoulder bracket (GAC central I slip NI) -      RMO synergy bracket (RMO) These brackets generated lower mean frictional forces at 2nd order deflections of 0.00 & 0.25 mm than conventionally ligated brackets
  103. 103. eg.Synergy bracket: Includes 6 wings 3 on each side of bracket slot. The lateral wings may be included in ligation for correction of rotation of teeth but only center wings may be ligated during sliding mechanics to reduce force of ligation.
  104. 104. SELF LIGATING BRACKETS: Orthodontic brackets are now available that possess the feature of self ligation.First Edgewise self ligating bracket Russelhock (1946).
  105. 105. SELF LIGATING BRACKET SYSTEMS: LIGATIONS SYSTEM * Edgelok bracket (Ormco) – 1972 Sliding cap * Speed bracket (Strite industries) - 1980 Spring clip * Activa bracket (A company) - 1986 Lever arm. Damon – 1994 vertical slide
  106. 106.
  107. 107. BERGER (1990) Compared between speed bracket and stainless steel bracket revealed that friction with self ligating bracket was between 12% and 23% that of stainless steel bracket irrespective of wire shape and ligation technique. The unique anatomic characteristic associated with speed bracket - highly resilient and flexible spring clip was determined to be causative factor in critically lowering level of applied force.
  108. 108. Shivapuja et al (1994) Self ligating brackets displayed significantly lower level of friction both static and dynamic as compared to conventional ligating system. Significantly less chair side time was required for archwire removal and insertion with self ligating system as compared to conventional ligating systems. Kapur found dramatically lower frictionwith both stainless steel and nickel-titanium wires for Damonbrackets compared to conventional brackets
  109. 109. Advantages of self ligating system more certain fullarchwire engagement; low friction between bracket and archwire; less chairside assistance; faster archwire removal and ligation
  110. 110. Gac in ovation bracket similar in design to speed bracket with sliding spring clip, Damon sl 2 with vertical slide
  111. 111. the clip places a diagonally directed lingual force on the wire, which does not contribute to any third order interaction between the wire corners and the walls of the bracket slot, which is the origin of torquing force This increases the ‘slop’ between the rectangular wire and the slot, and also reduces the moment arm of the torquing mechanism
  112. 112. Activa bracket with clip Clip retaining groove is visible on the gingival surface.
  113. 113. With low friction, the net tooth-moving forces are morepredictablylow and the reciprocal forces correspondingly smaller Lower net forces deflect archwires less and, therefore, facilitaterelease of binding forces between wire and bracket, enhancingsliding of brackets along a wire.
  114. 114. BIOLOGICAL FACTORS: I. Effect of saliva on kinetic friction: It has been suggested that saliva substitute serves as an excellent lubricant in sliding of the bracket along the wire. BAKER ET AL (1987): Using an artificial saliva substitute found a 15% - 19% reduction in friction. KUSY ET AL (1991): Found that saliva could have lubricious as well as Adhesive behaviour depending on which archwire bracket combination was under consideration
  115. 115. SSWIRES: Showed an adhesive behaviour with saliva and a resultant increased in coefficient of friction in wet state. β-TI wires: In wet state kinetic coefficients of friction were 50% of the values in dry state.when sliding through SS brackets ,the titanium rich oxide layer in β-Ti archwire breaks down,reacts,adheres and breaks away ,resulting in a stick-slip phenomenon Hypothesis: Saliva probably acts by preventing solid to solid contact.
  116. 116. CLINICAL SIGNIFICANCE: In an adult patient: H/O of Xerostomia or decreased salivary, Oral radiation therapy, Anticholinergic medication. Should be noted as possible factors in varying force levels necessary to move teeth.
  117. 117. Surface Characteristics Affecting Friction Most metals are subject to oxidation and an associated oxide layer growth. Friction between specific sliding metallic surfaces significantly decreases with proportionate increases in oxide layer thickness, although sliding characteristics in the presence of an oxide layer vary from material to material. .bioflims may reduce the coefficient of friction by producing a boundary lubrication effect through salivary protein adsorption and plaque accumulation
  118. 118. CLINICAL SIGNIFICANCE OF FRICTION Based on information gathered from studies of friction several points of clinical significance can be identified. An appreciation of magnitude of friction is crucial for the orthodontist who employs sliding mechanics during treatment -      With best of wire bracket combinations atleast 40g of friction must be included in force applied to initiate tooth movement.
  119. 119. New bracket designs and manufacturing techniques have been introduced to decreased the amount of friction generated between wire and bracket slot. -      Sintered SS brackets. -      Bracket designs limiting force of ligation Self ligating brackets Clinicians using esthetic tooth colored brackets - important to know the level of friction generated by these brackets before initiating tooth movement
  120. 120. Selection of wire shapes and sizes: e.g.: 0.018 inch SS steel best choice for canine retraction in 0.022 slot. - If overall torque control is required: 0.016x 0.022 inch - 0.018 slot. 0.019x0.025 inch - 0.022 slot. Archwire can also be thinned down in region distal to canine so as to further facilitate movement. Care must be taken not to over reduce the wire dimensions which could decreased strength of wire
  121. 121. Complete leveling of arch - important factor in reducing friction during tooth movement. e.g.: Space closure using 0.019 x 0.025 wire in 0.022 slot. Before starting space closure rectangular wires need to be place for atleast 1 month. -       To ensure proper leveling and freedom from posterior torque pressure. Sliding mechanics can proceed smoothly
  122. 122. To optimize use of sliding mechanic sufficient time must be allowed for distal root movement to occur. -      A common mistake is to change the E-chain too often thus maintaining high force levels and a M/F ratio that produces distal tipping only.
  123. 123.                       Thank you For more details please visit