Systematic Review

By Dr. Benedetta Toni , Dr. Debora Loli , Dr. Emanuela Coppotelli
Corresponding Author Dr. Benedetta Toni
Sapienza University of Rome - Department of Oral and MaxilloFacial Sciences, - Italy
Submitting Author Dr. Debora Loli
Other Authors Dr. Debora Loli
Sapienza University of Rome - Department of Oral and MaxilloFacial Sciences, - Italy

Dr. Emanuela Coppotelli
Sapienza University of Rome - Department of Oral and MaxilloFacial Sciences, - Italy


self ligating brackets, frictional resistance

Toni B, Loli D, Coppotelli E. A solution to the problem of frictional resistance with self-ligating system: a systematic review. WebmedCentral ORTHODONTICS 2017;8(11):WMC005399

This is an open-access article distributed under the terms of the Creative Commons Attribution License(CC-BY), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Submitted on: 15 Nov 2017 12:59:00 AM GMT
Published on: 15 Nov 2017 05:53:02 AM GMT


During the early stages of orthodontic treatment, the self-ligating system is more efficient because a reduction of friction in the interface archwire-bracket occurs. The friction is determined by three components: friction, binding and notching; it depends on the critical contact angle defined, for each pair of wire and brackets, where they contact in two diametrically opposite slot’s points.


In the last few years international orthodontic literature has analyzed the friction in straight-wire mechanics.

A series of methods have been proposed with the aim of limiting the frictional restraints that contrast tooth movement at the bracket archwire-ligature, such as self-ligating brackets .1- 4

 Advantages of self-ligating brackets include reduced friction, full and secure wire ligation, improved oral hygiene, anchorage conservation, chairside time savings and improved ergonomics, quicker treatment times, and longer appointment intervals.5

From the patient’s perspective, the self-ligating appliance are generally smoother, more comfortable, and easier to clean because of the absence of wire ligature; reduced chair time is another significant advantage.1-5

Several authors have investigated the efficiency of self-ligating appliances during the early stages of treatment. They underline as the duration of treatment, which is decreased, is correlated to low friction levels between archwire and bracket.1,5,6,7.

In the sliding mechanics, the tissues respond to the application of orthodontic forces with little dental movements which are a succession of states near to equilibrium and occur only if the forces overcome the frictional resistance between the bracket and the wire.8 It’s important that the frictional resistance is very low.9 In orthodontics the frictional resistance is the force that occurs when an object moves in contact with another, in their interface. In our case the frictional resistance is given between bracket and archwire.

It is produced by the sum of three components: friction, binding and notching,10 and depends on the critical contact angle defined for each pair of wire and bracket where they contact in two diametrically opposite slot’s points. 11,12  


The reduction of friction represents a major challenge for the clinician that should be aware of the variables that influence it. The aim of this study is to analyze them. We have used as source Pubmed and Scopus in order to perform a systematic review. Keywords used are: self-ligating, friction, binding, notching, sliding mechanics. We selected 24 articles from the literature.


Friction is determined mainly by the nature of ligation.13 In an in-vivo study, Iwasaki et al 14confirmed that, during sliding mechanics, 30% to 50% of the total friction force generated by a premolar bracket traveling along a 0.19 X 0.25 stainless steel archwire is due to the friction of the ligation.

For the important concept of friction, the orthodontist must consider these following variables: entity of crowding and leveling; design, slot, mesio-distal extension and materials of bracket; section, dimensions and materials of archwire.

Regarding the first point, when crowding and leveling increase, the inter-bracket distance decreases and there are major possibilities of creating sharp angles between archwire and slot.15,16,17 Therefore the degree of crowding defines the levels of orthodontic forces to be applied at the initial aligning phase.18 The crowding is evaluated by the Little's irregularity index proposed in the seventies years. We can distinguish mild, mean and severe crowding. This is an index based on the measurement of five contact points at the level of the upper front teeth.19

The resistance to sliding is created by the strenght of slot-ligature system, which is different according to the configuration (active, passive, interactive), and bracket’s material.20

Passive self-ligating brackets don’t create the pressure on the wire, which is free to flow and express less friction than other brackets. Active brackets have elements like clips or springs, so there is an interaction between the brackets and the archwire since the earliest moments of orthodontic treatment. Frictional forces developed from them are greater than passive brackets.21

The design of brackets with smooth entrances reduces sliding resistance22.

The mesio-distal increment of bracket results in an increase in sliding resistance due to inter-bracket distance reduction22.

Ceramic brackets show the highest sliding resistance levels22.

The size of archwire influences the friction. It increases as the wire section increase.15,22. The efficiency of the self-ligating systems is maintained with smaller archwires that are able to slide freely in the slot. This aspect is lost during the progression of the treatment23,24 as a result of the larger filling of the slot because the size of the archwire increases. The archwire has a major contact’s surface with the bracket and minor "play" within it.

Super-elastic Ni-Ti archwire have less sliding resistance than Beta-Titanium and Steel ones22.

The sliding resistance is higher in the rectangular section archwire22.


Advantages of self-ligating brackets include reduced friction, full and secure wire ligation, improved oral hygiene, chairside time savings and improved ergonomics, quicker treatment times, and longer appointment intervals. In conclusion, we can say that the reduction of the friction is strongly influenced by three variables: the amount of crowding and leveling, the features related to the bracket and the features related to archwire. Friction increases when the inter-bracket distance decreases and the crowding increases. Active brackets demonstrate greater frictional resistance than interactives and passives ones, though they have more control over dental movements. Finally, it’s evident that archwire size gradually increases the frictional resistance


  1. Franchi L, Baccetti T, Camporesi M, Barbato E. Forces released during sliding mechanics with passive self-ligating brackets or nonconventional elastomeric ligatures. American Journal of Orthodontics and Dentofacial Orthopedics, 2008, vol.133 (pg. 87-90).
  2. Baccetti T,  Franchi L,  Camporesi  M, Defraia E. Orthodontic forces released by low-friction versus conventional systems during alignment of apically or buccally malposed teeth .European Journal of Orthodontics, Volume 33, Issue 1, 1 February 2011, Pages 50–54.
  3. Pizzoni L, Ravnholt G, Melsen B. Frictional forces related to self-ligating brackets, European Journal of Orthodontics, 1998, vol. 20 (pg. 283-291).
  4. Henao SP, Kusy RP. Frictional evaluations of dental typodont models using four self-ligating designs and a conventional design, Angle Orthodontist, 2005, vol. 75 (pg.75-85).
  5. Turnbull NR, Birnie DJ. Treatment efficiency of conventional vs self-ligating brackets: effects of archwire size and material. Am J Orthod Dentofacial Orthop 2007;131:395-9.
  6. Fleming J. Self-ligating brackets in Orthodontics: A systematic Review. Angle Orthod. 2010.
  7. Chen et al. Systematic review of self-ligating Brackets. Semin Orthod Dentofacial Orthop. 2010.
  8. Burrow. Friction and resistance to sliding in orthodontics: A critical review. Am J Orthod Dentofacial Orthop. 2009.
  9. Damon. The Damon Low- Friction Bracket: A Biologically Compatible Straight-Wire System. JCO 1998.
  10. Kusy RP,Whitley JQ.  Resistance to sliding of orthodontic appliances in the dry and wet states. Influence of archwire alloy, interbracket distance and bracket engagement. J. Biomed Mater Res. 2000.
  11. Kusy RP, Whitley JQ. Influence of archwire and bracket dimensions on sliding mechanics: derivations and determinations on the critical contact angles for binding. Eur Orthod 1999.
  12. Kusy RP. Ongoing innovations in biomechanics and materials for the new millannium. Angle Orthod. 2000.
  13. Schumacher HA, Bourauel C, Drescher D. The effect of the ligature on the friction between bracket and arch. Fortschr Kieferorthop 1990;51:106-16. 2.
  14. Iwasaki LR, Beatty MW, Randall CJ, Nickel JC. Clinical ligation forces and intraoral friction during sliding on a stainless steel archwire. Am J Orthod Dentofacial Orthop 2003;123:408-15.
  15. Crincoli et al. Frictional forces during sliding of various brackets for malaligned teeth: an in vitro study. The Scientific World Journal. 2013.
  16. Ong et al. Efficiency of self-ligating vs conventionally ligated brackets during initial alignment. Am J Orthod Dentofacial Orthop 2010.
  17. Liv et al. Effects of bracket design on critical contact angle. Angle Orthod. 2013.
  18. Macauley et al. Using Little’s Irregularity Index in orthodontics: Outdated and inaccurate? Journal of Dentistry 2012.
  19. Pillai et al. Comparison of frictional resistance between archwire and different bracket system: an in vitro study. J Pharm Bioall Sci 2014.
  20. Reddy et al. A comparative in vivo evaluation of the alignment efficiency of ligation methods: a prospective randomized clinical trial. Eur J Dent 2014.
  21. Brauchili et al. Active and passiveself-ligation- a myth? Angle Orthod. 2011.
  22. Proffit W.R “Contemporary Orthodontics” Edra Masson (fourth Italian edition) 2013.
  23. Rinchuse et al. Self-ligating brackets:present and future. Am J Dentofacial Orthop. 2007.
  24. Pandis et al. Active or passive self-ligating brackets? A randomized trial controller of comparative efficency in risolving maxillary anterior crowding in adolescents. Am J Orthod Dentofacial Orthoped 2010.

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