The edgewise arch (square arch) technique is described here as the typical method of fixed orthodontic treatment. The edgewise technique involves working a square, spring hard archwire that is adapted to the desired width and form of the dental arch with special arch formers and is connected to the teeth with fixing components.
The fixing components are square brackets and tubes for fixing the ideal archwire (orthodontic ideal arch) (Figs 10-89 and 10-90). The standard edgewise bracket has a square, horizontal slot, but twin brackets with two adjacent slots can also be used.The brackets are matched to the different tooth shapes in size, width, and base curvature. The fixation components can be fixed onto a cemented band or directly to the tooth by a direct bonding technique.
When banding teeth, the band must not damage the marginal border and tooth, which is achieved by good gingival and occlusal fit. The cement is used not only to fix the band but also for hygiene reasons because it fills the "gap" between tooth and band.
Acrylic, ceramic, or steel brackets are used in the bonding technique. Retention in the enamel is prepared by acid etching (eg, with phosphoric acid) of the teeth. The bracket is bonded to the etched tooth surface with a uniformly thin layer of adhesive.
In the edgewise technique, the archwires for each individual tooth are bent under tension: The edgewise (square) arch is first adapted to the ideal dental arch, then the wire is bent roughly into the position of the malpositioned tooth but only approximately. Once the wire is firmly attached to the tooth in the bracket, the tooth will be pulled in the direction of tension of the wire (Fig 10-91).
The edgewise wire does not sit fully in the bracket but is lifted out slightly. It is held in the bracket guide with small, taut elastomeric modules (like rubber bands). Forces now act on the tooth in two ways: the spring force of the tightened edgewise arch and the pulling force of the elastomeric modules. The forces act three-dimensionally on the tooth via the wire to correct the position of individual teeth, sections of the dental arch, or the entire dental arch.
The concept of the ideal arch supposes that in an optimal masticatory system every tooth is correctly positioned in terms of anatomical contact points. Different prefabricated archwires are available for different individual sizes of dental arch. As well as narrow ones, there are wide versions for the maxilla and the mandible: ellipses, parabola, U-shaped, and V-shaped. Each archwire is divided into symmetric segments: anterior, canine, premolar, and molar segments. The placement of the fixing components is determined by this arrangement.
Fixing components can be placed in a variety of ways. Placement on the vestibular and the lingual surfaces of the teeth is possible.
The bracket is positioned in the middle on the dental crown while the horizontal bracket slot lies parallel to the incisal edge or the cusp tip so that, if the archwire were straight, the teeth would be in the regular position in relation to each other. The purpose of an edgewise approach is to guide the bracket position in the direction of the individual ideal arch with the orthodontic arches employed.
At the start of treatment, a highly elastic archwire is used to guide the teeth continuously in the desired direction; each new pair of archwire comes closer and closer to the ideal arch form. Each new archwire is stronger than the previous one and is prebent more precisely. The gradual alignment of the rows of teeth is characteristic of the edgewise technique.The archwire can be passively guided past individual teeth that are not to be incorporated into the ideal arch until a later stage.
There are various passive and active loops available as special components for individual correction of the position of single teeth in relation to the occlusal plane:
- Vertical loop for correction in the horizontal plane
- Horizontal loop for correction of extruded or intruded teeth
- Delta loop for correction of horizontal and vertical anomalies of posterior teeth
- Combination helix for correction in the vertical and horizontal planes for alignment and rotation
- Closing loops and bull loops for space closure in the mandible and maxilla
Figures 10-92 and 10-93 illustrate banded arches in the edgewise technique.