Introduction

Studies have found that teeth have a tendency to relapse to their pre-treatment positions in around 70% of orthodontic treatment cases.1–3 The aetiology of this phenomenon is not completely understood but is probably related to growth, the periodontium, soft tissue pressures or the occlusion3 and less likely to be linked to the degree of tooth movement,4–7 number of extractions or pre-treatment tooth positions.6,8 Therefore patients will invariably need to wear retainers after orthodontic treatment to maintain teeth in their new positions. There is no accepted duration for this retention phase but on average, in relation to the periodontium, it takes a minimum of 232 days for the periodontal fibres to become accustomed to the new tooth positions.9 A retention period of 12 months is commonly recommended because of this10 but relapse can still occur after this time because of skeletal growth and maturation of the soft tissue, so indefinite retention has been advocated to minimise this.11,12 Long term retention is commonly achieved with fixed retainers, especially for the lower anterior teeth.13,14

Credit: ©PIKSEL/iStockphoto/Thinkstock

Fixed retainers

The first fixed retainer consisted of a length of stainless steel wire fitted to the lingual surface of the anterior teeth and soldered to bands on the canines or premolars. The first retainer to be bonded directly to the teeth was reported by Rupert Kneirim15 and consisted of a length of 0.028” (0.7mm) stainless steel wire, adapted to the lingual surfaces of the lower anterior teeth and bonded to the canines. Bonded retainers have the same advantages as banded retainers but also offer greater aesthetics.15,16

Numerous wire diameters and materials have since been proposed but the multi-stranded wire, introduced by Björn Zachrisson,17,18 is now the gold standard.13 Multi-stranded wire is round or rectangular in cross-section and formed from strands that are twisted (Fig. 1), arranged coaxially (five wires of equal size wrapped around a single, core of wire of the same-size) (Fig. 2) or braided (Fig. 3).13,19–21 Multi-stranded wire for retainers is most commonly round in cross-section, made from stainless steel and is widely available in imperial (0.015” to 0.0215”) and metric (0.38mm to 0.5mm) sizes from various orthodontic supply companies. Gold (0.5mm) and titanium (0.44mm and 0.5mm) multi-stranded wire is also available (Dentaurum, Ispringen, Germany).

Figure 1
figure 1

Three stranded twisted round wire

Full size image
Figure 2
figure 2

Six stranded coaxial round wire

Full size image
Figure 3
figure 3

Eight stranded braided rectangular wire

Full size image

Advantages and disadvantages

Bonded retainers require less patient compliance, improved aesthetics and predictable long-term retention compared with removable retainers.13

Multi-stranded wire has been a successful material for bonded retainers because its flexibility allows for physiological tooth movement,22 meaning that teeth can move independently of one another, making bond failure less likely.13,21 The irregular surface of this material means that it has a greater surface area than regular wire, which should give increased mechanical retention for the composite.18

It is generally accepted that bonded retainers complicate oral hygiene procedures23 but while the presence of a bonded retainer has been associated with an increased incidence of gingival recession, increased plaque accumulation and increased bleeding on probing,24 it does not necessarily lead to caries,23,24 affect periodontal health22,25 or lead to long-term tissue damage.26,27

As with all bonded retainers, care needs to be taken to ensure that the wire or composite does not obstruct the occlusion and torque may be introduced if the wire is not entirely passive, which can result in unwanted tooth movement.28

The advantages and disadvantages of bonded retainers are summarised in Table 1.

Table 1 Advantages and disadvantages of bonded retainers
Full size table

Fabrication

The retainer will typically consist of a length of stainless steel multi-stranded wire bonded to the mid third of the palatal or lingual surfaces of each tooth in the anterior segment (Fig. 4 and 5) with composite. They can involve fewer teeth or may be extended around the second premolar to prevent space reopening in extraction cases.15 The retainer extends two thirds of the width of the last tooth involved to allow for composite encapsulation of the wire ends. The retainer may be fabricated directly on the teeth at the chairside or on a model. If constructing on a model, upper and lower impressions and a bite registration are necessary, even in cases where only an upper or lower retainer is required to confirm that the wire does not interfere with the occlusion (Fig. 6).

Figure 4
figure 4

A 0.0195” stainless steel three stranded wire retainer for the upper anterior teeth

Full size image
Figure 5
figure 5

A 0.0195” stainless steel three stranded wire retainer for the lower anterior teeth

Full size image
Figure 6
figure 6

Upper and lower retainers on models to ensure the wire does not impede the occlusion

Full size image

Fitting

Bonded retainer positioning can be assisted with dental floss, elastic separators29 or ligature wires. The use of an acrylic transfer tray,30,31 molar bands,32 ceramic locking elements33 and magnets34,35 have also been reported. Using a transfer stent has been found to be statistically significantly quicker than direct bonding.36

The enamel surface is cleaned and may be sandblasted to ensure that there is no moisture contamination prior to bonding. Light-cured composites are the favoured material for bonding the retainer.37 A 1mm thickness of composite is used, any more can result in plaque retention and gingival inflammation.38 Glass ionomer cements are another option and these will bond to wet enamel surfaces and release fluoride to reduce the decalcification risk. Glass ionomers have a lower bond strength than composites but this should be sufficient for bonded retainers.39,40

Long term maintenance

While a bonded retainer may offer a ‘permanent’ retention solution, long term monitoring is required, as with any other appliance. Failure rates have been the focus of many studies13,27,41–46 and vary between 11%13 and 50%.27 Failures are more commonly seen during the first six months after bonding.43,44 Early failure is likely to be due to insufficient saliva isolation during bonding or insufficient composite. Failure after this period will probably be due to composite abrasion, occlusal forces or wire fatigue.43,47

A thicker wire is likely to have a higher detachment rate because of increased rigidity47,48 but thinner wires can be distorted and will fracture more easily.21 Bonded retainers in the upper arch tend to have a higher failure rate than the lower arch, especially if extended to the canines21,43 – this may be because upper fixed retainers are exposed to greater occlusal forces.

Bonded retainers can fail due to a stress fracture of the wire or debond at the adhesive-enamel or wire-composite interface.26 The wire-composite interface has been reported as the most frequent site of failure, usually because of composite abrasion.37 This bond can be improved by sandblasting the wire ends prior to bonding. Sandblasting the enamel and avoiding wire movement during bonding should avoid bond failure at the adhesive-enamel interface.26

Conclusion

While no retention system is without its drawbacks, fixed retainers offer a significant benefit in that they need little patient compliance beyond keeping them clean. They are usually bonded to the palatal or lingual surfaces of the teeth, so are generally considered more aesthetic than removable retainers. The multi-stranded wire retainer has been found to deliver an effective method of providing long term retention because its flexibility permits physiological tooth movement and its greater surface area should give better mechanical retention for the composite.