Friction in orthodontics.

Conventional wisdom suggests that resistance to sliding (RS) generated at the wire-bracket interface has a bearing on the force transmitted to the teeth. The relative importance of static and kinetic friction and also the effect of friction on anchorage has been a topic of debate. Lot of research work has been done to evaluate the various factors that affect friction and thus purportedly retards the rate of tooth movement. However, relevancy of these studies is questionable as the methodology used hardly simulates the oral conditions. Lately studies have concluded that more emphasis should be laid on binding and notching of archwires as these are considered to be the primary factors involved in retarding the tooth movement. This article reviews the various components involved in RS and the factors affecting friction. Further, research work should be carried out to provide cost effective alternatives aimed at reducing friction.

When straight-wire mechanics are used in orthodontics, the resistance to sliding (RS) generated at the wire-bracket interface greatly influences the character of the force transmitted to teeth.[1] This resistance is believed to reduce the efficiency of orthodontic appliances and hence result in slower tooth movement.

Considerable research has been done to determine the sources of friction and to find means of reducing it. It has been determined experimentally that friction is affected by the design of the appliance, masticatory forces, and the presence of saliva.[2]

Friction in orthodontics occurs at multiple contact points along the archwire. Variables affecting friction between components of fixed appliance are reviewed which include the bracket, the archwire, the ligation mechanism, and biological factors.

Friction is defined as the force (FR) that opposes a movement when an object moves tangentially against another.[3] As two surfaces in contact slide against one another, several forces arise. The frictional component (F) is directed in a tangential direction to the surfaces in contact. Normal force component (N) is directed perpendicular to the contacting surfaces. Friction is directly proportional to the normal force and described by the equation F = μN, where μ = the coefficient of friction.

Static and Kinetic Friction

All materials have two coefficients of friction; static coefficient; and kinetic coefficient.[3] Sliding between bracket and wire in the oral cavity occurs at a low velocity as a sequence of short steps rather than as a continuous motion.[1] In such conditions, the distinction between static and kinetic frictional resistance is arbitrary because these two forms of friction are dynamically related.[2] In orthodontics, a tooth undergoing a sliding movement along an archwire goes through many tipping and uprighting cycles, moving in small increments. Therefore, orthodontic space closure depends more on static friction than on kinetic friction.[4]

All surfaces are more or less irregular, and the physical explanation of friction is in terms of the true area of contact, which is determined by asperities[5] [Figure 1], and the force with which the surfaces are forced together.

Figure 1

Asperities

Resistance to Sliding

Friction is only a small part of the resistance to movement as a bracket slides along an archwire. Kusy and Whitley[6] divided RS into three components: (1) Friction, static or kinetic (FR), due to contact of the wire with bracket surfaces; (2) binding (BI), created when the tooth tips or the wire flexes, so that there is contact between the wire and the corners of the bracket [Figure 2]. When a force is applied to a bracket to move a tooth, the tooth tips in the direction of the force until the wire contacts the corners of the bracket, and binding occurs; and (3) notching (NO), when permanent deformation of the wire occurs at the wire-bracket corner interface. This often occurs under clinical conditions [Figure 3]. Tooth movement stops when a notched wire catches on the bracket corner and resumes only when the notch is released.

Figure 2

Binding of archwire with bracket wings

Figure 3

Notching of archwire

The contributions of friction, binding, and notching to RS can be understood best by considering the three stages in the active phase of moving teeth:[7]

The first is the early stage of sliding as the tooth tips and contact of the wire with the corner of the bracket begins to occur; both friction and binding contribute to resistance to sliding: RS = FR + BI

In stage 2, the contact angle increases between the bracket and the wire, when binding is the major source of resistance and friction becomes inconsequential: RS = BI

In stage 3, if the contact angle becomes steep enough, notching of the wire occurs, and both friction and binding become negligible: RS = NO.

Friction and Anchorage

Conventional wisdom states that an orthodontist must apply added force to overcome friction, the result of which can be increased anchorage loading and subsequent anchorage loss.[89] This concept has motivated our specialty to seek techniques to reduce friction and, consequently, reduce the potential for increased anchorage loss.[9] However, Southard et al.[10] claim that emphasis on using reduced-friction (e.g., self-ligating) brackets during sliding mechanics to help preserve posterior anchorage is unwarranted and based more on bracket salesmanship than on orthodontic biomechanics. According to him, if the teeth are free to slide along the archwire, friction between brackets and archwires does not increase anchorage loading.

Factors Affecting Friction

Vaughan et al.[11] has reviewed several variables that can directly or indirectly contribute to the frictional force levels between the bracket and the wire and are listed as follows:

Archwire

Bracket

Ligation

Biological factors.

Archwire

Kusy and Whitley[6] described the effects of wire size on friction by describing the critical contact angle between the wire and bracket slot. As the diameter of the wire increases, the free space in the slot decreases and the amount of tip required to achieve the critical contact angle decreases. They claim friction is greater in wires of greater diameter because the critical contact angle is met with less tip in the bracket. In addition to the critical contact angle increasing friction, the larger wires are stiffer and there is a greater likelihood that the slot will cause notching of the wire.[12]

Several studies show that rectangular wires produce greater friction than round wires but only in certain circumstances. Drescher et al.[13] and Frank and Nikolai.[1] found that the occluso-gingival dimension of the wire was the most critical dimension affecting friction. They found that a 0.020” wire produced more friction than a 0.019 × 0.025” wire at low and high angulations. Conversely, the 0.018” wire produced less friction than the 0.017 × 0.025” wire. They stated that friction is a complex interplay between many factors, three of which are occluso-gingival size, cross-sectional shape, and the bending stiffness.

When a stainless steel (SS) wire is drawn across a SS bracket, the SS-SS combination has been shown to produce the least amount of frictional resistance. Elgiloy and NiTi wires produce more friction than SS, but in similar amounts, while titanium molybdenum alloys (TMA) produces the greatest amount of friction.[1613] Frank and Nikolai[1] demonstrated that at bracket angulations of 6 and 10°, a NiTi wire has less friction than a SS or Elgiloy wire. They attribute this to the difference in stiffness between NiTi and SS and Elgiloy. They state that Young's modulus of elasticity for NiTi is one-sixth the value of the other two alloys therefore reducing normal forces at the bracket-wire interface under greater deflections.

Kusy et al.[14] studied six titanium-based or TMA-type archwires (Beta III, Resolve, CNA, TMA, low-friction ion-implanted TMA or TMAL, and TiMolium). They concluded that whether binding occurs or not, frictional resistances appear independent of surface finishes or ion implantation. Despite the vendors’ advertisements, other issues, e.g. dimensional tolerances, quality assurance, and cost might be more important than surface finishes or frictional resistances.

Brackets

Stainless steel has been the most popular material in orthodontics. Kapila et al.[15] evaluated friction between edgewise SS brackets and orthodontic wires of four alloys (SS, Co-Cr, NiTi, and B-Ti). Mean frictional forces with conventional cast SS brackets ranges between 40 and 336 g. Vaughan et al.[11] compared the frictional characteristics of sintered SS bracket with an ordinary one. They concluded that kinetic friction produced by a sintered SS bracket is 45% of the frictional force produced by conventional SS bracket.

With the increase in demand for esthetics in dentistry, orthodontic suppliers have been developing brackets made of different materials that are more esthetic than SS. Ceramic, polycrystalline alumina, single crystal alumina, and polycarbonate brackets have been produced to meet this demand. In addition, titanium brackets are available that claim to be more biocompatible than SS at withstanding the oral environment. Kusy et al.[16] compared the frictional characteristics of SS and titanium brackets. They concluded that the optical roughness of Ti bracket was more than that of SS brackets. With regard to the coefficient of friction, Ti brackets favored comparably with their SS counterparts. Ceramic brackets produce nearly twice as much friction compared to the SS brackets.[1718]

To overcome the increased friction of ceramic brackets, some manufacturers have incorporated a SS slot into the ceramic bracket. No significant difference was found between the SS brackets and the ceramic bracket with a SS slot.[19] Currently, available zirconia brackets offer no significant improvement over alumina brackets with regard to their frictional characteristics.[20] Frictional force generated by silica-insert ceramic brackets was found to be significantly lesser than conventional ceramic brackets and was just equivalent to the force generated with SS brackets.[21]

With the SS mode of ligation, composite brackets had similar force levels when tested with the SS wires. Considering both ligation techniques, the composite bracket was observed to produce the lowest friction. In contrast, the SS and polycrystalline ceramic brackets were associated with higher levels of friction.[22] In Tip edge mechanics, diagonal wedges of the brackets are removed [Figure 4]. This bracket design reduces the RS. In Begg mechanotherapy, extremely loose fit between a round archwire and narrow bracket reduces the RS.

Figure 4

Tip-edge bracket

Studies done to evaluate the effect of bracket width and inter-bracket distance have produced contradictory results. Tidy compared the two slot sizes and found that the size of the slot made no significant difference in the amount of friction produced.[23] Kusy and Whitley[6] suggest that clinicians must be more precise in initial leveling and aligning with a 0.018 slot due to the greater potential for binding.

Ligation

Iwasaki et al.,[24] using an intra-oral device, calculated that 31–54% of the total frictional force generated by a premolar bracket traveling along a 0.019 × 0.025 inch SS archwire was due to the friction of ligation. SS ligatures were used universally for the greater part of the 20th century until the introduction of elastomeric ligatures. Frank and Nikolai[1] compared the two ligation mechanisms and found that frictional resistance increased as the ligature applies greater force to the wire. They found that there was an insignificant difference between elastomeric ligation and a SS ligature tied with a force of 225 g. Chimenti et al.[25] compared the frictional resistance seen with different sized elastomeric ligatures. They concluded that there were no significant differences between small and medium sized ligatures. Frictional force produced was found to be 13–17% greater with large sized elastomers.

Polymeric coated super slick ligatures were introduced in 2000. Manufacturers claimed that these ligatures generated lesser RS than conventional elastomers. It was claimed that coated modules produced 50% less friction than all other ligation methods except self-ligating brackets. However, when the different bracket and elastomeric module combinations were compared by Griffiths et al.,[26] significant differences were observed. In all but two combinations, round modules provided the least RS, rectangular modules the greatest and super slick modules in between the two.

Recently Teflon coated ligatures are being used with ceramic brackets to negate the esthetic problems associated with SS ligature wires and frictional disadvantages of transparent elastomers.[27]

A new low force ligation system made of special medical polyurethane was introduced recently that markedly reduces the friction produced.[28] The wire is free to slide as in a passive self-ligating bracket and it is claimed to cause lesser discomfort to the patient.

Self-ligation

Many practitioners have touted the many benefits of self-ligating brackets since Stolzenberg introduced the Russell attachment in 1935. It is claimed they are more hygienic, more efficient during adjustments, and even reduce treatment time because of reduced friction.[29] Within the last 10 years, there has been an explosion of new self-ligating bracket designs and interest by the orthodontic community [Figure 5].

Figure 5

Low force ligation system

The debate over whether a self-ligating bracket should have an active or passive ligation mechanism has been around since their development. Proponents of an active clip claim that it provides a “homing action” on the wire when deflected, providing more control with the appliance.[30] Those who advocate a passive clip state that there is less friction in the appliance during sliding mechanics because the slot provides more room for the archwire, and they provide no active seating force.[29]

When a small round wire lies passively in the slot, the self-ligating brackets produce significantly less friction than conventionally ligated brackets.[31] Design of the self-ligating mechanism can affect friction when teeth have first-order misalignments. Furthermore, as the size of the wire increases, the friction produced by an active bracket will become more than that produced by its passive counterpart.[32]

Biological factors

Baker et al.[33] had studied the effect of saliva on friction and concluded that human saliva reduced the frictional force by 15–19%. However, Kusy et al.[34] suggested that saliva can act as a lubricant or an adhesive depending on the archwire-bracket combination. They also said that artificial saliva was the least effective fluid in reducing friction when compared to human saliva and water.

A variable that likely plays a role in orthodontic friction are the forces of occlusion. With teeth contacting thousands of times a day during chewing, speaking, and swallowing, it is likely that the teeth and the orthodontic appliance are repeatedly moving in relation to one another. Braun et al.[8] added random perturbations to the bracket or wire to assess their effects on frictional resistance. They found that each time the bracket or wire was tapped, the frictional resistance was essentially reduced to zero. They concluded that, while masticatory forces did reduce frictional resistance, they did so unpredictably and inconsistently.

Conclusion

Whether friction is really a bane to orthodontics is a subject open to debate. However, a clinician should look beyond friction and realize that it is just a small part of RS. The current methodologies employed to study the effects of friction on orthodontic biomechanics are inadequate and simulate oral conditions poorly. Only improved methodologies can shed more light on this subject.