Ligature Systems and Friction

Ligation is the method by which the archwire is held in the bracket slot, and it strongly shapes the friction that arises when a tooth slides along the wire. Traditional elastomeric or stainless steel ligatures press the wire into the slot, whereas self-ligating brackets use a built-in clip or gate. Because much sliding mechanics works against this resistance, ligation and friction are central to how fixed appliances behave.

Definition

Ligation is the means of securing the archwire in the bracket; friction, or resistance to sliding, is the force opposing movement of a tooth along the wire, arising from the contact between wire, bracket, and ligature.

Scope

This topic covers how archwires are ligated, the concept of resistance to sliding and its components (classical friction, binding, and notching), the variables that influence it (wire alloy and size, slot dimension, ligation force, lubrication by saliva), and the comparison between conventional and self-ligating systems. It is a reference description of these mechanics, not treatment guidance.

Core questions

• What methods are used to ligate an archwire to a bracket?
• What is resistance to sliding, and how do friction, binding, and notching contribute to it?
• Which variables increase or decrease frictional resistance?
• How do self-ligating and conventionally ligated brackets compare in friction and in clinical outcomes?

Key concepts

• Elastomeric and stainless steel ligatures
• Self-ligation (active and passive)
• Resistance to sliding
• Classical (Coulomb) friction
• Binding and notching
• Effect of saliva and lubrication
• Wire size, slot size, and angulation effects

Mechanisms

When a tooth slides along an archwire, the resistance it meets has several components. Classical friction arises from the normal force pressing the wire against the slot and ligature; binding occurs when the wire contacts the slot corners at higher angulations; and notching can occur when permanent deformation of the wire-slot contact locks movement. The size of these effects depends on wire alloy and cross-section, slot dimensions, the force with which the ligature holds the wire, and lubrication by saliva (Kusy, 1997; Kusy & Whitley, 2000; Rossouw, 2003). Self-ligating brackets aim to reduce the ligation force, and passive designs in particular present low resistance when the wire is small, although at higher angulations binding tends to dominate regardless of ligation type (Chen et al., 2010; Proffit, 2018).

Clinical relevance

Ligation and friction are central to how sliding mechanics and appliance systems are described and compared. This entry explains the mechanics for reference and education and does not direct the management of any individual case.

Evidence & guidelines

Laboratory studies show that self-ligating, especially passive, brackets reduce friction with small wires, but a systematic review found that this does not translate into consistent, clinically important advantages in overall treatment outcomes; at higher wire engagement binding dominates and minimises differences between systems (Kusy & Whitley, 2000; Chen et al., 2010).

History

Concern with friction grew as sliding mechanics became common in preadjusted appliances. Kusy and colleagues characterised resistance to sliding in the dry and wet states and separated its frictional, binding, and notching components, providing the conceptual framework still used today. Self-ligating brackets, marketed partly on reduced friction, renewed interest in the topic and prompted systematic appraisal of their clinical benefit.

Debates

Does reduced friction with self-ligation improve treatment?

Self-ligating brackets reduce friction in vitro with light wires, but evidence does not show consistent clinically important gains in treatment outcomes, partly because binding dominates resistance once larger wires are engaged.

Key figures

• Robert P. Kusy
• P. Emile Rossouw

Seminal works

• kusy-1997
• kusy-2000
• chen-2010

Frequently asked questions

What is resistance to sliding in orthodontics?

It is the total force opposing the movement of a tooth along an archwire, made up of classical friction plus binding and notching that occur as the wire contacts the bracket slot.

Do self-ligating brackets really reduce friction?

In laboratory tests they reduce friction with small wires, especially passive designs, but once larger wires are engaged binding dominates, and systematic reviews have not shown consistent clinically important outcome advantages.