From Volume 7, Issue 4, October 2014 | Pages 114-117
Advances in orthodontic technology, especially those that address the aesthetic problem of metal or visible braces, have boosted the demand for adult orthodontics. Dental clinicians should be familiar with the advantages and limitations of the new aesthetic appliances coming onto the market.
Adult orthodontic treatment has increased dramatically in the last two decades, mainly due to the technological developments that have made orthodontic appliances more available and aesthetically acceptable. These advances include:
Currently, there are two types of aesthetic bracket, the ceramic/porcelain and the plastic bracket systems. Although the plastic bracket appliance is not popular nowadays, it still has some uses in certain cases.
These were first used in orthodontics in the early 1970s, and are manufactured from acrylic or polycarbonate. The main drawbacks are weak bonding to enamel and deformation under pressure (creep), especially if the torquing load is over 2000 gm/mm, making the brackets impractical for torque expression.1 Other disadvantages include:
Some manufacturers have introduced a metal-reinforced acrylic bracket to overcome some of these problems. It is considered an improvement over plain acrylic brackets with regard to creep resistance but slot imperfection is still considered to be problematical, as with the pure acrylic brackets.5
This type of aesthetic bracket is subdivided into polycrystalline, monocrystalline, metal-reinforced and zirconia brackets. Polycrystalline brackets were introduced in the 1980s and are made from aluminium silicate particles mixed with a binder, heated to 1800°C and then injected into a mould. The slot is then refined with diamond, ultrasound, or laser cutters.6 They can be produced relatively simply in large quantities and so are low cost. However, the moulding production causes some imperfections in their slot accuracy, causing high friction during orthodontic sliding mechanics.7,8,9 Metal-reinforced polycrystalline brackets, incorporating a metal slot, are a successful modification of the polycrystalline bracket and go some way towards addressing the problem, by reducing friction and also introducing a weakness in the base, which allows for easier debonding.10
Monocrystalline brackets are made from synthetic sapphire which is milled using diamond tools, heated to over 2100°C to relieve internal structural stress, then cooled slowly.11,12 These brackets have fewer imperfections, better optical clarity and produce lower friction than the polycrystalline type. However, their production is costly and they still have a low fracture resistance.4,13
Finally, Zirconia (ZrO2) brackets are similar in their properties to polycrystalline brackets, being opaque and cheaper than monocrystalline brackets.14 It is claimed that they produce less friction with sliding mechanics when used with stainless steel or Nickel Titanium wires (NiTi) in comparison to polycrystalline brackets.15
In spite of the aesthetic advantages of the ceramic brackets, their drawbacks have been discussed extensively in the literature.4,13,16
One of the main problems is an inherent error in bracket positioning owing to difficulty in the visual differentiation between bracket and tooth material. Manufacturers have tried to overcome this issue by using temporary bracket markers or bracket jigs.15
If metal and tooth-coloured elastic ligatures are used with ceramic brackets, they tend to cause discoloration which nullifies the aesthetic advantage of the ceramic brackets. To overcome this problem, Teflon coated ligatures, ‘white’ elastomeric modules, thin metal ligatures or even self-ligating ceramic brackets (Damon ‘Clear’) can be used.
A further disadvantage of ceramic brackets is the lack of ductility and high toughness which may lead to fracture.17 The tie wings are the weakest area of resistance to the torsional forces exhibited by rectangular wires.4 For this reason, ceramic brackets are not recommended in cases where heavier forces may be used, as in orthognathic or deep overbite cases.18
As mentioned, milling or machining the brackets results in a rougher slot surface texture8 when compared to injection-moulded stainless steel brackets. Although the manufacturers tried to overcome this problem by incorporating a metal slots,14 studies have suggested that there is no advantage in using this modification,19 whilst other studies show even more friction.11 This may be due to the difficulty in adapting the metal slot to the ceramic slot and to their different expansion coefficients. Other modifications include glazing the slot with silica or introducing a bump in the slot to reduce the friction. Research has shown this latter modification does not improve efficiency.20 Thorstenson and Kusy recommend the use of closing loops rather than sliding mechanics, avoiding bonding premolar teeth or changing the archwires at each visit to provide a newer and smoother sliding surface.19
Ceramic is nine times harder than enamel and is second only to diamond4 so that wear of incisor enamel had been reported in cases treated with ceramic brackets.21 Clinical recommendations to avoid this problem include avoidance of teeth with obvious occlusal interference and, if possible, restriction of ceramic bracket usage to the upper arch. In deep bite cases, or where there is occlusal interference, lifting the bite using glass ionomer cement to dis-occlude the occlusion is recommended.22 When ceramic brackets are used in Class II division 2 cases, proclination of the upper labial segment before bonding the lower incisors is highly beneficial. Other operators prefer using softer polycarbonate brackets on the lower incisors, in deep overbite cases, or rubber ligatures as cushions to avoid heavy contact with palatal enamel.4
Another shortcoming of ceramic brackets is the high bonding strength to enamel, hence it is not recommended to use ceramic brackets with heavily restored, non-vital or cracked enamel teeth to avoid iatrogenic teeth fracture during debonding. Grooves or undercuts in the base of the bracket, weaker bonding resin or metal meshes in the base, are modifying features in an attempt to overcome this problem. However, metal bases may cause shadowing and poor aesthetics; opaque bonding resin with macro-filler can be used to overcome this weakness 18,23,24,25
In addition to the risk of enamel fracture, there is a possibility of brackets fracturing during debonding with consequent inhalation or swallowing.24 Residual ceramic fragments on the enamel surface need removal with a diamond bur, with associated heat generation which can damage the pulp.26,27 Russell recommended the use of protective spectacles to protect the eyes and applying the debonding pliers to the mesial and distal of the bracket, keeping a finger over the bracket to prevent it becoming a projectile.4 Larmour et al advised undermining the edges of the bracket to reduce the bonding strength of the bracket, provide a plier's holding point and to reduce the bonding strength.28 Specialized debonding techniques such as manufacturer specific pliers, electro-thermal debonding pliers, ultrasonic, laser-aided debonding (CO2 laser), and chemical debonding agents could also be used.4,29,30 Finally, the extra cost of ceramic brackets in comparison to metal is an important factor which should be taken into consideration.
These were first developed by Kurz in the USA and Fujita in Japan.31,32 Generally speaking, lingual appliances offer benefits, including improved aesthetics, less visible enamel decalcification and improved overbite reduction.33 The key elements in an acceptable lingual orthodontic appliance system include the ability to align teeth efficiently, easy precise positioning of the brackets with accurate indirect bonding technique, as well as the ability to accept a straight archwire. 34
There are many types of lingual orthodontic systems mentioned in the literature, among these are the two dimensional (semi pre-adjusted system), three dimensional (fully pre-adjusted system) or the hybrid systems.35 A lingual appliance can be ‘off the shelf’, custom-made or semi-customized, made from metal or ceramic material, whilst the ligation may be conventional or self-ligation.36,37 The innovative ‘Incognito’ system is currently the most popular lingual appliance system with arguably fewer complications than other systems.38,39,40,41 It is a fully custom-made bracket and wire system with a thin profile leading to reduced discomfort.35 The second-order bend (vertical height and angulation) and the third-order bend (torque prescription) are pre-set into each bracket slot, while the first-order guidance (in-out bend) is delivered via the custom-manufactured archwire.42 Indirect bonding is used for the initial bond-up; extractions are usually carried out after initial bond-up, as any change in the position of the teeth either side of the extraction site could prevent the transfer tray carrying the brackets from seating correctly.35,42,43,44
The disadvantages include limited accessibility for patient cleansing and operator adjustment,45 speech changes and tongue discomfort.38,39,46 The variable morphologies of the lingual surfaces of the teeth make bracket fitness problematic which may then require wire bending and adjustment for fine control of tooth position. Although the wires' dimensions in lingual appliances are smaller than for labial appliances, the inherent smaller inter-bracket span reduces the archwire flexibility, which has been claimed to be associated with an increased root resorption. Finishing and torque control is another weakness within the lingual appliance system, unless a customized bracket system is used. As the force of application within the lingual bracket lies nearer to the centre of resistance of the tooth than labially-placed brackets, any positioning error is reflected in an exaggerated in-out or torque error.41
These are invisible custom-fabricated devices used for straightening teeth and are intended to replace conventional appliances in mild cases.47 Aligners are generally made from poly-vinyl siloxane (PVS)48 and should be worn full-time, except when eating, and replaced every two weeks, with the aim of moving the teeth by 0.25 mm with each align.49,50 The simplified idea of clear aligners was originally described by Schwartz and Sheridan,51 which was then developed and popularized by Align Technology (Santa Clara, Calif) in 1988 under the trade name ‘Invisalign’. It is now the most popular orthodontic aligner. The fabrication of Invisalign aligner utilizes CAD/CAM stereo-lithographic technology (STL) to predict the steps of teeth movements and creates a custom-made aligner for each planned step.52 Align Technology has recently refined and expanded the types of product it offers, to include ’Invisalign Lite’ which is aimed at patients only wishing to address alignment of the anterior segments.53
Invisalign aligners are used to treat mild crowding (1–5 mm), spacing (1–5 mm), deep overbite problems and narrow arches but are not suitable for open bite correction, severe rotations or teeth with short clinical crowns.53 Evidence shows that Invisalign produces less discomfort and less periodontal damage than conventional appliances owing to the feature of being a removable and cleansable appliance.54,55 However, it has been reported that the majority of patients require refinement and/or detailing with fixed appliances. The mean accuracy of Invisalign for all tooth movements was estimated at 41% in a recent prospective clinical study which was significantly lower than that of the conventional appliance.56 Some authors claim that Invisalign appliance treatments may be quicker than traditional orthodontic appliances, with almost 50% reduction in the treatment duration. The theory is that the teeth move according to digitally predetermined steps from their original position to the final destination with no ‘round tripping’.57,58 Djeu et al, in a retrospective study, compared the outcomes of Invisalign and Tip-Edge fixed appliance treatments measured against the American Board of Orthodontics (ABO) rating.59 They concluded that there was a significant difference in the exam pass rate of the candidates who used Invisalign when compared to Tip-Edge treatment (20.8% to 47.9%, respectively) but with shorter treatment duration (1.4 years to 1.7 years, respectively). However, Lagravere and Flores-Mir, in a systematic review, were unable to draw conclusions about the effectiveness of the Invisalign system compared to other orthodontic appliance systems.60
Another advance in orthodontics to address adults' aesthetic concerns is the resin-coated metal archwire which has been introduced to use in conjunction with ceramic brackets. As with any other archwires, they are not ideal and carry some pitfalls among which are the high cost, high friction, force degradation and tendency of the coating to peel-off during use. 61,62 It was concluded that 25% of the coating peeled after one month of use and the coating occupies more of the bracket slot space than the supposed delivered wire size, which affects the interaction between the wire and the bracket.63
OrmcoLTD introduced another archwire under a trade name ‘Optiflex’, which is an aesthetic, semi-transparent, non-metallic wire, containing a silica core, a silicone resin middle layer, and a stain-resistant nylon outer layer.7 Subsequently, another aesthetic wire has been developed containing glass fibres embedded in a polymeric matrix called fibre-reinforced polymer composite (FRPC) archwire.64,65,66,67
A recent study showed that FRPC and NiTi wire have comparable frictional wear when used with conventional and self-ligation brackets.68 Research is currently being undertaken to manufacture an aesthetic wire with shape-memory feature similar to the heat-activated NiTi wires,69 but there is still a significant financial issue in the use of aesthetic wires.70
Aesthetic ligature wires are similar in their composition to aesthetic archwires and are used with ceramic brackets for aesthetic purposes. However, the problems include peeling of the coating, self-unravelling and increased frictional forces during sliding.71
There is a high correlation in the growth of adult orthodontic treatment and the increased demand for aesthetic appliances and modalities. It is important to be familiar with the advances in orthodontic technology that could address the aesthetic problem of metal or visible braces.
