Like ordinary fixed appliances, a self-ligating bracket consists of a bracket base and a body containing slots and tie-wings ( Fig. 2.1 ). The difference between conventional and self-ligating brackets lies in the way in which the archwire is engaged in the slot. In self-ligation, the bracket itself contains a clip or other mechanism, which is used instead of either elastic or metal ligatures.
Just like conventional brackets, self-ligating brackets really only serve one function: they are the junction between the element generating the force (wire or auxiliary) and the tooth—so that they are simply a means to an end. The use of self-ligating brackets has given rise to a number of treatment philosophies, which are believed to offer significant advantages over ordinary ligation. However, it is important to remember that the tooth is not aware of how the force is being applied to it—whether it is by self-ligation or ordinary ligation.
A number of challenges that apply to traditional brackets also apply to self-ligating brackets: the fit of the bracket base to the tooth, the precision of the archwire slot, etc. There are few differences between self-ligation and ordinary ligation, as the method of production for the two systems is identical. Depending on how self-ligating brackets are manufactured, there may be a number of technical issues with the locking mechanism, which are described in greater detail in the section on “Rotation and Friction” below.
An ideal self-ligating bracket should have the following characteristics:
Anatomically appropriate curvature of the bracket base including retention and undercut
Marking of the vertical and horizontal axis
An appropriately designed layout for good bracket positioning
The bracket should be identifiable for each individual tooth (color coding or laser engraving)
Hooks should be available for the application of elastics
Precise slot dimensions (either 0.018 or 0.022)
Robust self-ligating mechanism
Twin tie-wings for engaging elastic chains or elastic modules
Additional auxiliary slots
The bracket base connects the bracket to the tooth and therefore must have retentive elements such as mesh, undercuts, or other retentive features which allow for good band strength. The adhesive enters the undercuts and allows mechanical retention, which should be resistant to everyday masticatory forces on the one hand, but should still be capable of being debonded without damaging the enamel surface on the other.
Shape of the Base
An ideal base should follow the curvature of the respective tooth surface for a good fit. This should enable the operator to place the bracket securely in the appropriate position on the tooth without rocking. A poorly fitting base can result in unprecise torque, angulation, and rotation once the full-sized wire is completely engaged. In order to produce an appropriately fitting bracket base, the manufacturer needs to pay attention to a number of factors.
The buccal surfaces of individual teeth show only very minor anatomical variations. An anatomically preformed bracket base is ideal and will fit well in the majority of cases. A precisely fitting base needs to take into account both the occlusal–gingival and also the mesiodistal curvature of the tooth surface. This is a challenge from the manufacturing point of view as a tooth surface is not built with a uniform curvature and a single radius like a circle, where a bracket can be positioned anywhere on the surface with equally good results. A tooth surface has many diverse radii and curvatures, depending on the location on the surface-and this applies to both the occlusal–gingival and mesiodistal directions ( Fig. 2.2 ).
The importance of the congruence of the bracket base and the surface of the tooth has been known for a long time. Most manufacturers now offer brackets that have different surface characteristics with increased or decreased convergence. These convergences were originally determined by cross-sectional analysis of teeth that were cut in order to measure the curvature. It was therefore only possible to obtain a small number of convergences per analyzed tooth; due to the intense labor involved, the sample size per tooth type was usually small. Despite this, the results from the original studies are still often used in the manufacturing of bracket bases even today. Modern three-dimensional reconstructions of tooth surfaces are nowadays used in computer models and this method allows better correlation of the bracket base with the actual surface of the teeth, due to the increased number of teeth that can be analyzed and averaged ( Fig. 2.3 ). Some manufacturers use this technique to design and construct their bracket bases and therefore claim to produce better-fitting bracket bases than others, but it is important for the bracket base to be manufactured in such a way that the data obtained can be used in a meaningful way. This is most likely to be possible with metal injection molding (MIM) or ceramic injection molding (CIM). Both of these techniques allow the individualized and fitted shape to be transferred when the bracket is produced. A number of bracket manufacturers produce a bracket base from premanufactured plates, which are then bent into the desired shape. In a separate step, this bracket base is then connected to the bracket itself (see the section on “Bracket Body” below). It is not possible to produce the ideal surface characteristics that a bracket should have using these techniques. This is due to the very small size of the bracket base, resistance to deformation by the metal itself, and manufacturing issues with the application of forces to the small surfaces.
Mismatches between the bracket surface and the tooth have to be compensated for by the adhesive, which may lead to poor bracket positioning and in turn result in incorrect tooth positioning.
Positioning errors can also result from canting the bracket or from migration of the bracket between positioning and polymerization. This may lead to poor slot orientation and in turn to undesired tooth movement ( Fig. 2.4 ).
The ideal orthodontic bracket adhesive should have two main properties: on the one hand, it should ensure a sufficient bond strength to be able to withstand the everyday stresses of mastication and manipulation. On the other hand, it should also allow easy removal of the bracket without damage to the enamel. As these two properties are diametrically opposed, orthodontic adhe-sives compromise by trying to deliver an adequate bond strength for most clinical situations-neither too strong nor too weak.
Most studies would agree that the minimum bond strength necessary for orthodontic treatment is in the range of 8–10 MPa.8 , 15 More frequent bracket failures can be expected if the bond strength values are below this. At retention values above 20 MPa, there is a greater risk of enamel fracture on debonding.12 , 15
The Opal bracket (Ultradent) had a shear bond strength of only 4 MPa when the adhesive suggested by the manufacturer was used ( Fig. 2.5 ). This was insufficient to withstand everyday stresses and strains.4 Other brackets have a retention strength of more than 20 MPa, and these values are similar to those used in restorative dentistry. Brackets that produce retention values of that order may therefore pose a greater risk to the integrity of the enamel on debonding.4 As a general rule, the weak point during debonding shifts from the bracket-adhesive (at lower values) to the adhesive–tooth interface.13 , 15
There is an extensive literature on the shear bond strength of brackets, but it is difficult to compare the different studies. Even in studies that use similar brackets and adhesives, the results are often not comparable, as there are too many variables that are not standardized—such as different teeth (bovine or human), the thickness of the adhesive, the direction and type offorce application (torsion or shear force), and so forth. The German Institute for Standardization (DIN) recommends a standard methodology for testing the shear bond strength of adhesives for orthodontic attachments (DIN 13990), which could lead to the standardization of research findings if it was internationally accepted.
The most important factors that affect the bond strength are the adhesive used, the chairside workflow, and the quality of the retention surface.
The bond between the bracket base and the adhesive can be obtained in a number of ways. For metal brackets, it is usually mesh, laser-etching, or other retentive elements that provide the undercuts necessary for mechanical retention. In addition, for ceramic brackets chemical retention is usually achieved through silane coupling. Generally, the bracket base should come with a retentive pattern that allows the adhesive to sit in the undercuts, creating a tight mechanical connection between the bracket base and the tooth. With metal injection molding, a number of bracket bases can be created in this way, some better than others ( Fig. 2.6 ).
The acid-etching technique and the bonding of brackets with composites has been the standard for many years now. A more recent trend using self-etching primers improves workflow and time efficiency by up to 60%, as claimed by some manufacturers. Having fewer steps in the process means fewer potential mistakes during enamel conditioning, which should improve treatment outcome ( Fig. 2.7 ). Self-conditioning primers contain acidic hydrophilic monomers, which etch the surface of the teeth and at the same time apply a very thin coating of unfilled composite (primer). A number of investigations have shown that although none of the currently available self-etching primers achieve the same depth of action as 35% phosphoric acid, comparable shear strengths result.2 , 7 , 8 , 22 However, due to the reduced penetration depth, the fracture point on debonding is typically located at the enamel-adhesive interface, which means that less composite residue is left on the tooth surface.3 , 15 The advantages of self-etching primers in orthodontics are the reduced surface erosion they cause during conditioning and the enamel proximity of the fracture point on debonding, and factors that make this primer attractive in the clinical setting.1 , 2
The body of the bracket consists of the tie-wings and the bracket slot, but in the case of self-ligating brackets, it also houses the ligation mechanism. It is therefore challenging to keep the overall size of a self-ligating bracket small but strong enough to withstand everyday stresses without distorting. Depending on the manufacturer’s preferences, bracket frames can either be produced in one piece or can be assembled from various parts using laser welding or soldering ( Fig. 2.8 ). The latter technique-assembling bracket bodies from a number of components—can lead to spontaneous disassembly during treatment, and there is an increased tendency for corrosion to develop between the parts. There may also be a tendency for plaque to accumulate on the adjoining structures.
Self-ligating bracket bodies can be classified into either a tie-wing design or a block design. The former is the classic twin design, and with self-ligation it is accompanied by a locking mechanism. The four tie-wings allow additional modules such as elastic chains to be attached over the archwire if needed. Brackets in the block design group do not allow additional elements to be attached over the archwire, and the body is simply used as the retention mechanism for the self-ligating complex ( Fig. 2.9 ). However, it is still possible with this design to use elastic thread or chain by placing it under the archwire, which in turn will hold the auxiliaries in place. The disadvantage of this approach is that attaching and removing the auxiliaries is relatively cumbersome, as it requires removal of the wire at each appointment.
Either colored markers or laser etching are commonly used to identify the bracket and proper positioning ( Fig. 2.10 ). Color coding is easier and quicker to read than laser engraving, but the latter lasts longer and is more resistant to wear. These markings are also used with conventional brackets, but with most self-ligating brackets both the vertical axis and the horizontal bracket slot are covered by the self-ligating mechanism, making it sometimes difficult to identify the ideal bracket position. Some manufacturers counter this by adding additional horizontal and vertical markings on their brackets.
Some self-ligating brackets (e.g., Discovery SL) do not have the bracket slot in the middle of the bracket, and it is very important to remember this during bracket placement.