Fixed Appliances and Orthodontic Instruments
Orthodontic appliances that are bonded to teeth and cannot be removed by the patient are termed ‘fixed appliances’. Depending on the type and design, fixed appliances can be used for anchorage, correction of some skeletal or jaw discrepancies and for the treatment of dental irregularities. Examples of some of the fixed appliances for jaw corrections are the Herbst®, Forsus™ or palatal expanders that are discussed further in Chapters 6, 7 and 8. Braces are fixed appliances used for treating dental misalignment and disharmony. The focus of this chapter is on braces, and provides a brief summary of the history of braces and a review of some of the most commonly used instruments and ligatures. Precise tooth movement is carried out with the use of fixed appliances. Successful treatment outcomes are achieved with braces in reduction of overbite, multiple tooth movement, relief of crowding, space closures, correction of rotations and more.
The interaction between the metal attachments on the tooth called bracket and the arch wire determines the direction of the movement. There is an extensive variety of materials and designs for brackets and arch wires. Wires are classified according to their cross‐section as either round or rectangular (Figure 5.1). Interactions between the round wire and the bracket slot allows tipping in buccolingual directions with a degree of mesiodistal tipping and rotational positioning. Rectangular wires are used after a period of using arch wires with a round cross‐section to gain better control of the tooth movement. The rectangular wires completely engage in the bracket slot thus permit bodily tooth movements.
Unlike removable appliances, fixed appliances allow several tooth movements such as rotations, extrusion, intrusion, uprighting and torque. The difference between up righting the roots and torque is the direction of root movement. Uprighting is made possible by mesiodistal movements along the length of the arch wire. Torque is made possible by labiolingual movements with right‐angle bends on the arch wire. Specific movement and torque is made with various types of bends categorised as first‐order, second‐order and third‐order bend. The orthodontist will use specific pliers to bend stainless steel or beta‐titanium wires to complete final root and crown adjustments. Most bracket systems have prescribed palatal root torque within the bracket slot. Once engaged with a rectangular arch wire, the root moves palatally and the crown moves labially. In some instances, orthodontists may bond a bracket upside down to achieve the opposite effect and move the root labially as the crown moves palatally.
Evolution of Bracket Systems
In the 17th century, a French dentist, Pierre Fauchard (known as the father of orthodontics) introduced orthodontics with an arch‐shaped metal band. This introduction to orthodontics was suitable for tipping teeth with minimal rotational control. In the 20th century, Edward Hartley Angle (known as the father of modern orthodontics) introduced the Edgewise appliance after devising the classification of malocclusion (see Chapter 3). Over the years, Angle developed four appliances to treat patients without extractions. The objective behind these systems was to expand the arch form rather than using extractions to relieve crowding. The evolution of the Angle appliances is:
- E‐arch: the molar teeth were banded and every tooth was ligated to a heavy labial arch wire to deliver heavy interrupted forces. This appliance only allowed tipping of the teeth.
- Pin and tube: more teeth in the arch were banded as well as the molars. A tube was soldered on to the bands to allow the heavy arch wire to pass through it. The pins had to be repositioned frequently to achieve the desired tooth movement.
- Ribbon arch: the tubes on the band were modified with incorporation of a vertical slot and was termed bracket. A gold wire was engaged through the bracket slot and held in place with a pin to deliver light continuous forces. This appliance had great success yet presented with limited root control.
- Edgewise: an evolution from the ribbon arch appliance is the edgewise system that provides much better crown and root control. After several experiments, the dimensions and orientation of the bracket slot altered. The edgewise system has a horizontal (90‐degree) bracket slot. The width of the slot in the edgewise brackets are either 0.018 inch or 0.022 inch. A better control of the tooth movement is achieved as the wire closely fits in the bracket slot. The most popular brace systems known today are based on the edgewise appliance.
An Australian orthodontist Percy Raymond Begg launched the Begg technique on the fundamentals of the ribbon arch appliance. This technique was also designed for extraction cases. The edgewise technique focused on expanding the arch form without extractions and bodily movement of the teeth. However, the Begg technique involved a two‐step tipping of the crown and uprighting the roots to achieve the desired tooth position with extractions to create the required space. The modifications made to the ribbon arch appliance to launch the Begg technique were:
- replacement of precious metal to stainless steel in the appliance.
- rotated the design of the bracket upside down.
- incorporated the use of auxiliary springs to adapt better root control.
Over the years, there has been several modifications made to the edgewise system yet all follow the same principle introduced by Angle. The non‐extraction edgewise system was challenged by one of Angle’s students, Charles H. Tweed. He introduced the use of edgewise appliances in combination with extractions and he formulated anchorage as a crucial aspect of a successful treatment. To enhance the efficiency of the edgewise appliance, the straight wire appliance was invented by Dr Lawrence Andrew and evolved by Dr Roth. This innovative technique incorporated torque and prescription angulation in the bracket slot to compensate for specific tooth anatomy. Integrating various angulations in different planes of space within the bracket design greatly reduced the need for several arch wire changes and bends in the final stages of treatment. However, bends in the wire for final detail is still critical to achieve the desired and ideal tooth position that varies in individuals. Other systems used a single bracket for every tooth and compensated for specific tooth movements using auxiliaries and repetitive bends in the arch wire. Owing to the prescriptions and variations in the design of the brackets, only one system can be used in an individual for optimum results. The bracket systems are differentiated on the basis of the slot width. The two main systems used are 0.018 inch and 0.022 inch with a depth variation between 0.025 inch and 0.032 inch.
A revolution from the conventional brackets occurred with launching of self‐ligating brackets in the early 1930s (Figure 5.2). Over several years, the design and mechanism of self‐ligating brackets evolved and became more popular with the improved designs as treatment efficacy were enhanced. The self‐ligating brackets have built‐in gates or clips incorporated into the bracket to engage the arch wire without the need of modules. These brackets portray a significant improvement in oral hygiene due to less plaque accumulation without the presence of elastic modules holding the wire in place (Pellegrini, 2009).
Self‐ligating brackets are differentiated in two categories based on their ligation mechanism (Brauchli et al., 2012) as:
- Passive self‐ligation: a gate built in the bracket to transform the open slot into a tube. This ligation mechanism offers a low friction system. Examples of passive self‐ligating brackets include the Damon® System and SmartClip™ SL3 self‐ligating appliance system.
- Active self‐ligation: a clip is incorporated in the bracket design that exerts pressure on the arch wire, thus enhances the effect by offering high control on tooth movement. The active clips provides enhanced rotational control and torque expression. Examples of self‐ligating brackets are the Speed System™, In‐Ovation® R and BioQuick®.
In the 1970s, lingual orthodontic appliances were introduced for better aesthetics. These appliances are used by many practitioners and require modified dexterity and specific lingual instruments. Initially, the desired outcomes were difficult to achieve because indirect vision made adjustments difficult. However, with development of software programs, customised brackets and wires for patients improved treatment outcomes dramatically (Figure 5.3). This innovation is used by many specialists, based on their training and experience.
Components of Fixed Appliances
Precious metals and their alloys were first used in orthodontics before the Angle era. One of the most vital properties of these alloys were good corrosion resistance. However, poor tensile strength and flexibility were disadvantages. In 1919, Dr. F Hauptmeyer combined steel and chromium to introduce stainless steel with improved physical properties to replace gold in orthodontics. This combination has been widely used since the 1930s and are much more cost effective than gold. Arch wire can be manufactured from:
- copper nickel titanium
- nickel titanium
- beta titanium
- chrome cobalt
- stainless steel
- gold alloy
- alpha titanium.
The physical properties vary depending on the shape, size and type of material. Fixed appliances contain active components and passive components. The active components of orthodontic appliances bring about the necessary movement. Examples of active components are arch wires, separator rings, elastics and coil springs. The passive components of fixed appliances deliver and transfer the force of active components; examples are brackets, bands, molar tubes and accessories such as modules.
The necessary physical properties of arch wire include stiffness, strength, flexibility, resilience, ductility, formability and biocompatibility. Stress is the ability to disperse the force load internally. There are three types of stress, which depends on the direction of force:
- Tension (tensile strength).
- Compression (compressive strength).
- Shear stress.
Tensile stress results in elongation of the object, compressive stress condenses the object and shearing stress is the sliding of two objects against one another. Stress is associated with straining of the object. Strain is the distortion that occurs from stress and is categorised as elastic and plastic. Elastic strain reverses upon removal of the force and plastic strain is a permanent distortion to the object. The ratio of tensile stress to tensile stain is known as modulus of elasticity. This indicates the flexibility and stiffness of an object. The higher the modulus of elasticity, the higher the stiffness. Strength and stiffness are closely associated with range. Range is the elastic strain limitation prior to reaching a permanent internal deformation. The strength, stiffness and range of the arch wires are highly dependent on composition, shape, cross‐section and size.
Resilience is another crucial physical property. The release and springback of energy that is absorbed by an object during stress is known as resilience. This generally happens before the object reaches its distortion limit. Ductility is the ability to undergo tensile forces and to withstanding plastic strain without breakage. Ductility decreases as the temperature increases. Malleability is undergoing plastic strain with compressive stress. Malleability increases with high temperatures. Formability is withstanding permanent distortions without material failure.
The ideal characteristics for an arch wire are low stiffness, high range, high strength and high formability. It may be difficult to gain all of these characteristics in one material, so various wires with different characteristics and materials are employed for specific purposes and to achieve the definite desired tooth movement.
The arch wire communicates the biomechanical forces through the brackets and tubes. The aim is to begin treatment with light continuous forces and gain effective tipping with round wires. The initial wire must have:
- sufficient flexibility
- low stiffness
- high range
- freely move within the bracket slot.
An ideal arch wire for the initial stages of treatment must be biocompatible and should offer high resilience to spring back to its original shape upon force application. This allows sufficient alignment and precise tooth movement to be achieved with minimal distortions to the wire. The arch wires composed of nickel titanium alloy offer excellent elasticity and shape memory. Shape memory and thermoelasticity is the ability to regain initial form after deformation. The composition of the most commonly used arch wire is nickel, titanium, copper and chromium, and is available in round and rectangular cross sections. This wire is used as the initial wire in orthodontics due to its suitable physical properties.
The use of tightly fitted rectangular wires can create undesirable movements due to its effect on the root apex. However, better torque control is effectively achieved with arch wires that fully engage against the bracket slot. Stainless steel arch wires can be used from mid‐treatment and for final finishing and detailing. Beta‐titanium wires offer the most suitable physical properties for final detaining. The thickness and formability of these wires make them extremely effective. The detailing wires can be bent in the desired shape in the absence of material failure. As the diameter of the wire increases, the rigidity and stiffness also increases.
There have been several inventions in arch wire material and design to improve treatment efficacy, patient comfort, with the aim of minimising the need for frequent wire adjustments. Examples of some other arch wires include:
- Optiflex was a new invention to provide highly aesthetic clear arch wires. It can be used in cases of moderate crowding in the early stages of treatment. These tooth‐coloured wires are composed of three optical fibres: silicon dioxide, which brings about tooth movement, silicon resin, which enhances moisture resistance and strength, and nylon, which adds strain resistance.
- Bioforce arch wire is another common shape memory alloy wire. It can undergo plastic deformation and return to its original shape. This thermodynamic arch wire contains gradient force across the arch wire, applying lower forces to the anterior section and increases towards the posterior and plateauing at the molars. It is an aesthetic wire and, with its force delivery, eliminates the need for frequent wire changes.
- Australian (Wilcock) arch wires are heat‐treated stainless steel arches developed by Begg. Based on the resiliency of the wire, the Australian wire is categorised as round, regular, regular plus, special, special plus, premium, premium plus and supreme grades (Pelsue et al., 2009). The resiliency increases from regular to supreme. These wires are highly resistant to permanent deformation.
Synthetic elastomers and latex elastics are widely used in orthodontics. Latex elastics are composed of natural rubber and are used for intermaxillary traction. These elastics are also manufactured without latex for patients with latex allergy. The synthetic elastomers are composed of polyurethane rubber and are widely used for intramaxillary movement. Elastics vary in force and magnitude depending on several factors such as alveolar bone condition, patient cooperation and the movement required the choice of elastics vary. Different movements are achieved with several types of elastic patterns (discussed in Chapter 4).
Nickel titanium coil springs provide continuous forces to open or maintain the space with open coils and closed coils, respectively.
Most brackets are composed of stainless steel containing chrome and nickel. There are plastic and ceramic brackets deigned for an aesthetic pleasing appearance. Plastic brackets are composed of polycarbonate and plexiglass. The problems associated with these types of brackets are poor physical properties and discolouration. The forces applied distort the bracket slot and the desired movements are not achieved, as the forces are not transmitted to the teeth appropriately. To resolve this issue, the plastic brackets were designed with stainless steel slots. Ceramic or fiberglass reinforced the material to improve the physical properties and colour stability. Ceramic brackets are made up of monocrystalline and polycrystalline. These brackets are highly fragile and have a high rate of breakage. Owing to the occlusal bearing in the posterior region and the risk of breakage, clear brackets are not manufactured for molars. The lowest friction is shown to be in ceramic reinforced composite brackets. Ceramic brackets can greatly damage enamel during the debonding process, due to the chemical retention mechanism of the resin in the orthodontic adhesives and the bracket base. To minimise enamel damage, the bonding mechanism is modified to a mechanical mode.
The brackets are designed with a mesh base of different types and patterns for better retention and bonding. There are various adhesives that can be used for orthodontic bonding with different modes of polymerisation (chemical, light or dual cured). Composite resin or resin reinforced glass ionomer cement are commonly used and the setting of the adhesives must be followed as recommended by the manufacturer. Brackets can be bonded to teeth with two different techniques, known as direct bonding or indirect bonding. Direct bonding is placement and positioning of each bracket directly on the enamel surface. Indirect bonding technique involves prepositioning the brackets on the cast models manually or on digital models using software programs. Once the ideal bracket positions are confirmed on the models by the orthodontist, plastic trays are fabricated over the brackets. Every specialist may have their own unique method of bonding. However, methods for bonding brackets using direct bonding (Figure 5.4) and indirect bonding (Figure 5.5) are summarised in Box 5.1.