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Friction – Etiology & Management in SWA

INDIAN DENTAL ACADEMY Leader in continuing dental education

One of the most common methods of translating a tooth orthodontically - sliding mechanics. Mesiodistal tooth movement - by guiding a tooth along a continuous arch wire with the use of an orthodontic bracket. Disadvantage - friction - resist the movement.

Friction is defined as a force that retards or resists the relative motion of two objects in contact, and its direction is tangential to the common boundary of the two surfaces in contact. Frictional force - 2 sliding surfaces Îą to the force - surfaces are pressed together. Ffr = u Ă— F. The value of u (the coefficient of friction)

ď ą Static Frictional forces - smallest force

needed to start a motion of solid surfaces with respect to each other. ď ą Kinetic frictional force - force needed to

resist the sliding motion of one solid object over another at a constant speed.

Several variables - directly or indirectly contribute - friction – b/w - bracket & wire; They are:  Arch wire.  Material.  Cross-sectional shape/size.  Surface texture.  Stiffness.

 Ligation of arch wire to bracket.  Ligature wires.  Elastomerics.  Bracket.     

Material. Slot width and depth. First order bend (in-out). Second order bend (angulation). Third order bend (torque).

 Orthodontic appliance.  Interbracket distance.  Level of bracket slots between adjacent teeth.  Forces applied for retraction.  Intraoral variable.    

Saliva. Plaque. Acquired pellicle. Corrosion.

 Static frictional force = coefficient of static

friction x resultant normal force;  Kinetic frictional force = coefficient of kinetic friction x resultant normal force.  The coefficients of static and kinetic friction, generally having magnitudes between zero and one,  depend upon -relative roughness of the

contacting surfaces.

Prososki etal (AJO-1991) states that surface roughness influences friction most directly when  dry, unlubricated sliding occurs or when only

meager lubrication is present.  geometry of roughness,  orientation of roughness features, and  relative hardness of the two contacting surfaces. Friction tends to be highest for very rough or very smooth surfaces.

 Sliding mechanics- biologic tissue response

and tooth movement - applied forces overcome the friction at the bracket-wire interface.  High levels of bracket-wire friction may result in  binding of the bracket  little or no tooth

movement.  binding of an anterior tooth under retraction  loss of anchorage. The most desirable and ideal situation, - little or no friction - b/w bracket and wire.

 Proffit etal considers frictional resistance in

orthodontic appliance to be multifactorial,  It is α force with which the contacting

surfaces are pressed together  Affected by the nature of the surface at the

interface  Independent of the apparent area of contact

ďƒ˜ role of asperities (limited number of small spots at

the peak of surface irregularities) - contributing factor. ďƒ˜ These elevated areas carry the entire load between

two surfaces and may undergo plastic deformation with appropriate force. ďƒ˜ Applied load determines the true contact area.

 coefficient of friction is 

α shear strength of the junction &

1/ α yield strength of the material.

 The interlocking of large and pointed asperities or

‘plowing’ of asperities into opposing surfaces -  friction.

Arch wire: ď ą Material: Garner et al (AJO-1986) – found significantly larger frictional force with beta-titanium and nitinol when compared with stainless steel. Differences in surface smoothness - account for the differences in friction.



Beta - Titanium

Drescher et al - SEM study - between diverse wire materials.  SS and Elgiloy - smooth surface texture,  NiTi, TMA, - extensive surface roughness.  Surface texture - friction magnitude in edgewise mechanics.  Effective force has to increase by twofold (stainless steel) to sixfold (TMA) to overcome bracket-to-wire friction.

 Tidy DC - fixed appliance in vitro to simulate

tooth movement in a previously aligned arch.  Nitinol and TMA (beta-titanium) >frictional forces -2x & 5x – of SS.  SS arch wires may be used in preference to nitinol or TMA arch wires to reduce the friction in sliding mechanics.

Kapila et al (AJO- 1990) greater magnitude and more frequent variation in frictional forces per unit distance of bracket travel with NiTi and ß-Ti wires than with SS or Co-Cr wires.  Higher mean frictional forces - NiTi and ß-Ti wires.  surface roughness of these alloys > SS or Co-Cr in SS brackets

 Archwire Dimension:  Tidy found that wire dimension and slot size had little effect on friction.  Vaughan etal - The frictional forces  with  

rectangular wire than with round wire, and  wire size   frictional force.

 Pizzoni - friction occurring in sliding mechanics

as being influenced by the bracket design, wire material and wire cross section.  He concluded that round wires have lesser friction than rectangular wires,

 Kapila et al(AJO 1990) -

Stainless steel, Co-Cr, and ß-Ti wires ↑ bracketwire friction with increase in wire size. Increase in size of NiTi wires - no significant effect on - friction between bracket and wire – in 0.018 inch narrow single br..

 Surface properties:  Ryan et al- (AJO 1997) - effects of ion

implantation on the rate of tooth movement.  ion-implanted wires - > movement than their

untreated counterparts.  The ion-implantation process -  stress fatigue

and hardness of the material  the friction.

ď ą Brackets: Drescher et al -study found narrow brackets to intensify friction by enhancing tipping movements. This implies a preference for the use of medium or wide brackets in arch-guided tooth movement, particularly in cases in which excessive mesiodistal tooth translation is required

 Andreasen and Quevodo, - study to evaluate the

frictional forces in the 0.022 X 0.028” edgewise bracket system.   

Multiple round and rectangular SS wires, brackets of three different widths, four bracket wire angulations.

both wet and dry conditions

 Tipping the bracket &larger wires -  friction,  Bracket width & wet and dry conditions were

found to be insignificant

ďƒ˜ Tidy studied the effect of load, bracket width, slot

size, arch wire size, and material. ďƒ˜ The forces acting on the surface of the tooth root were simulated by a single equivalent force acting at the center of resistance of the root. The couple produced by the two-point contact with the arch wire counters the moment of this force about the arch wire.

 The movable bracket was fitted with a 10 mm

power arm - weights - hung –force acting at the center of resistance of the tooth root. The length of the power arm - distance from the slot to the center of resistance of a typical canine tooth.  The movable bracket was suspended from the load cell of the testing machine, while the baseplate moved downward with the crosshead on which it was mounted.

Friction α applied load and

1/ α bracket width. The friction was greatest for narrow brackets. Wide brackets and stainless steel arch wires may be

used in preference to nitinol or TMA arch wires to reduce the friction in sliding mechanics.

 Kapila et al.- investigated –  Frictional properties of Stainless steel (SS), cobalt-

chromium (Co-Cr), nickel-titanium (NiTi), and βtitanium (β -Ti) wires of several sizes were tested in narrow single (0.050-inch), medium twin (0.130inch) and wide twin (0.180-inch) stainless steel brackets in both 0.018 and 0.022-inch slots.  frictional force  - wider brackets  Due to the higher force of ligation - the greater stretching of elastic ligatures on wider brackets.

Vaughan et al –  Overall friction of sintered stainless steel brackets 40% to 45% < conventional cast stainless steel brackets.  Pratten et al- frictional resistance of ceramic and SS brackets + SS and NiTi wire. Ceramic brackets  frictional resistance than SS brackets when used in combination with either SS or NiTi arch wires.

 Dickson etal- experimental polycrystalline

ceramic bracket with a SS insert and compared conventional & SS bracket.  The exptl. bracket -  frictional resistance and the ceramic bracket - 0˚ angulation. No sig. diff. between the two ceramic brackets at 10˚,  frictional resistance than SS bracket. Stainless Steel insert slot - experimental bracket behave more like a stainless steel bracket rather than a conventional ceramic bracket.

 Madhav.M and Jyothindra Kumar compared the

frictional properties and debonding characteristics of gold inserted slot Luxi™ bracket system and stainless steel inserted Clarity™ bracket system and compared them with stainless steel Gemini™ bracket.  Metal inserted ceramic brackets - frictional properties as good as stainless steel brackets.  Luxi™ - least kinetic friction  Clarity™ bracket - highest value, of the three bracket systems evaluated for both 50 gms and 100 gms load.

 Ligation.  Edwards et al- ligation techniques - on the static

frictional resistance of stainless steel brackets and archwires - dry and wet conditions.  No significant differences in frictional resistance

were found between conventionally tied elastomeric modules and stainless steel ligatures. Teflon-coated ligatures - lowest frictional forces.

 David etal- ( AO – 95) - static frictional resistances

between  Teflon- coated stainless steel and clear elastomeric ligatures –with 

SS, polycrystalline ceramic and single crystal ceramic 0.022-inch slot brackets, SS and NiTi archwires, 0.018 inch and 0.016 × 0.022 inch.

 Friction was measured in the dry state at bracket-

archwire angulations of 0, 5, 10, and 15 degrees.  Teflon-coated SS ligatures - friction than elastomeric ligatures regardless of bracket type, archwire type, or bracket-archwire angulation.

ď ą Self ligation. SPEED ApplianceSpring-loaded, Precision, Edgewise, Energy, and Delivery, all of which describe features of the design.

Berger (AJO â&#x20AC;&#x201C; 1990) â&#x2020;&#x201C; force - required to move rectangular steel or round braided arch wires - a standard distance self-ligation SPEED bracket < the elastomeric and the steel-tie ligated "A"-Company and American Orthodontics bracket systems.

 Activa brackets -fully

programmed preadjusted brackets that were introduced in 1986- Irwin Pletcher.  The arch wire -retained - resilient clip retaining groove gingival to the arch wire.  The friction is < elastomeric rings and conventional brackets.

 Shivapuja etal (AJO1994) –

Compared three self-ligating bracket systems to conventional SS brackets and ceramic brackets + polyurethane elastomeric and SS tie wire ligation.  Self-ligating bracket systems -  frictional

resistance,  chairtime for arch wire removal and insertion.

 Saliva.  Stannard et al (AJO 1986)- compared the friction of wires under dry and wet conditions.  artificial saliva -  the coefficients of friction for stainless steel, beta-titanium, and nickel-titanium compared to dry conditions.  Thought to occur from  atomic attraction among ionic species.  Water and other polar liquids -  adhesion or attraction among polar materials and  friction.

 Baker et al – (AJO 1987)-  of force necessary

to move the teeth in a saliva medium as compared to a dry medium.  Kusy et al –(AO 1991) - coefficients of friction in the dry and wet (saliva) environment for stainless steel, cobalt-chromium, nickel titanium, and beta-titanium wires against either stainless steel or polycrystalline alumina brackets.

 In the dry state - coefficients of friction -  

stainless steel combinations     beta-titanium wire combinations.  In the wet state, stainless steel combinations -  0.05 over the dry state. beta titanium -  50% of the values in the dry state. Attributed to the adhesive and lubricious behavior of the saliva.

 Tselepsis et al-(AJO 1994)- investigated frictional

resistance between brackets and arch wires for –  arch wire, brackets, angulation, and lubrication.  Lubrication significantly reduced the frictional

resistance (up to 60.5%) for both 0° and 10° bracket-to-arch wire angulation

Conclusion.  Friction has been a problem for orthodontists

ever. Many efforts have been made to increase the efficiency of tooth moving mechanics by reducing or eliminating the friction, but to marginal success.  Orthodontist’s dream would be to move the teeth in a frictionless system, effortlessly & efficiently.

Friction – etiology & management in swa/ dental implant courses by Indian dental academy  
Friction – etiology & management in swa/ dental implant courses by Indian dental academy  

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