I read the comprehensive response of the AJO-DO expert reviewers to the debatable self-ligating bracket claims in the “Readers’ forum” section in the August 2010 issue (Marshall SD, Currier GF, Hatch NE, Huang GJ, Nah HD, Owens SE, Shroff B, Southard TE, Suri L, Turpin DL. Ask us. Self-ligating bracket claims. Am J Orthod Dentofacial Orthop 2010;138:128-31). I agree that self-ligating brackets are a controversial topic in the orthodontic literature these days and that much of the interest is in response to reports comparing the benefits of self-ligating systems with conventional edgewise brackets—whether this information comes from marketing materials, nonrefereed sources, or refereed journals. Two main questions that were addressed in the “Ask Us” column regarded the comparison of the clinical forces and the effect of friction between self-ligating and conventional brackets. I found it interesting that there was generalization when it came to conventional brackets, although different types of ligation can be used with conventional brackets. Many studies show that the type of ligation used with conventional brackets is a critical factor that affects the seating force of the archwire and therefore the force produced by the archwire and also the amount of force lost in friction. All types of ligatures, both elastomeric and stainless steel, apply a force that pushes the archwire against the base of the slot. However, higher archwire seating forces were found with elastic ligation than with steel ligation and lower seating forces with loose stainless steel ligation than with tight stainless steel ligation. Different seating forces were also found with various elastomeric modules. Therefore, ligation could influence the force exerted by the archwire through the seating force it exerts on the archwire. Another factor that could influence the force level is the resistance to sliding encountered with elastic ligation; a study evaluating the 3-dimensional orthodontic force system suggested that adding elastic ligation increased the forces in the 3 planes.
In the same way, ligation could affect friction. Studies showed that self-ligating brackets produce lower friction compared with conventional brackets only when coupled with smaller archwires, and that the difference between the 2 types of brackets diminishes as the wire size increases. A recently published in-vitro study found no statistically significant differences between the stainless steel ligation method and the Damon MX self-ligating bracket. That study found also that an increase in wire size led to an increase in friction in all bracket-archwire combinations.
Orthodontists constantly seek treatments that are simpler, easier, faster, and more comfortable for their patients, and it seems, in that realm, that steel ligatures have been abandoned. The important question here is whether we need to reintroduce the steel ligature to orthodontics.
Although laboratory studies do not emulate the clinical situation, sometimes we have no choice other than to rely on them. Since no in-vivo studies have evaluated friction between bracket and archwire, and since it is difficult to measure stresses and strains in the periodontal ligament of loaded teeth directly, laboratory studies are useful to investigate these areas, especially when focusing on the comparative aspects of the study.