Bonded amalgams: the economical restoration
The technique of bonding direct restorations such as amalgam to damaged tooth structure can provide the patient with an economical and minimally invasive solution to the problem of a damaged tooth.
In the current financial climate many patients may defer or decline dental treatment which they perceive as expensive. The restoration of damaged teeth using indirect restorations such as crowns or onlays may be one such example. These techniques incur laboratory fees and require considerable chairside time, resulting in their higher cost compared to direct restorations.
A further disadvantage of indirect restorations is the need for tooth preparation. This may increase the risk of structural or pulpal damage in the tooth being restored and incur further costs for the patient.
The aim of minimally invasive dentistry is that restoration of the tooth should be accomplished by the least damaging means possible. Using adhesive dentistry, the concept of bonding a direct restoration such as amalgam to a damaged tooth may help achieve these aims, while providing a more affordable alternative to a cast restoration.
Amalgam has been used in restorative dentistry since the nineteenth century1,11 and continues to be one of the most widely used restorative materials in dentistry today2 mainly due to its ease of handling, cost effectiveness and clinical serviceability3.
Dental amalgam is a mixture of metals, consisting of liquid mercury and a powdered alloy composed of silver, tin, copper and other trace materials. Conventional low-copper amalgam compositions resulted in creep and low corrosion resistance largely due to the γ2 tin-mercury phase4. This γ2 phase was considered to be the least strong phase of hardened amalgam with these voids having a drastic effect on the strength and corrosion resistance5.
Nonetheless, the conventional amalgam γ2 phase did have one benefit as it was thought that the corrosion products sealed the potential gap between the tooth-material interface4-7 and decrease micro-leakage. The high-copper amalgams increased the copper content from six per cent to 10-30 per cent4 in an attempt to reduce and eliminate the weak γ2 phase and with it increase the compressive strength, reduce creep and the susceptibility of corrosion.
Despite the well-documented clinical success of amalgam it is still far from ideal8, the loss of the γ2 phase has eliminated the previous tooth-material seal and therefore the use of another material is needed to overcome this problem.
Figure one shows a conventional amalgam restoration exhibiting creep and recurrent caries around the margins due to microleakage of the material. These shortcomings of amalgam including poor appearance, lack of adhesion to tooth structure, the need to remove sound tooth tissue to achieve mechanical retention and the high incidence of tooth fracture are widely recognized8.
The first documented attempts at increasing retention of an amalgam restoration was in 1897 when a technique using a thin coat of zinc phosphate cement on cavity walls and condensing amalgam immediately onto the wet cement was first described9. However, it wasn’t until the 1980s when two Japanese manufacturers began to develop resin composite adhesives, which enhanced bonding to metal surfaces following air abrading,10 that interest focused on bonding amalgam restorations.
The use of either Panavia F 2.0 (Kuraray) or Rely X ARC (3M Espe) as the resin bonding agent has been recommended by several authors for the bonding of amalgam restorations4,11.
Technique
Tooth preparation is clean, dry and caries free (Fig 2). A lining, such as Vitrebond (3M ESPE), should be placed where indicated.
The tooth should be isolated with rubber dam to ensure excellent moisture control required when working with any dental resin.
If a matrix band is required around the tooth, a thin layer of petroleum jelly should be applied to the inside of the band with a micro-brush to facilitate easy removal once the bonded amalgam has been placed (Fig 3).
The cavity is then etched with 35 per cent phosphoric acid for 15 seconds (Fig 4) and washed for 30 seconds to ensure that all the acid and the calcium phosphate precipitates created by the etching are washed away. The tooth is then dried with an oil-free air stream.
The dentine is then re-hydrated with a dampened micro-brush to facilitate wet dentine bonding. If not, the drying of the dentine can cause collapse of the unsupported collagen architecture inhibiting adequate wetting and penetration of the primer.
This is followed by the application of two layers of bonding agent, such as Scotchbond (3M ESPE), to the wet dentine (Fig 5). The first layer removes any residual water and begins to infiltrate the adhesive monomers into the etched dentine so when the second layer is applied, the fresh monomers re-dissolve the resin globules leaving a homogenous penetrative film12. This is gently air-dried for two seconds before being light cured for 20 seconds according to the manufacturer’s guidelines.
The adhesive cement, in this case RelyX ARC (3M ESPE), is mixed as per the manufacturer’s instructions and thinly applied to the cavity using a micro-brush (Fig 6). The resin is not light cured and the mixed amalgam is then packed immediately against the un-set resin (Fig 7).
Once the cavity has been bulk filled the matrix band can be removed and the restoration should then be burnished and carved using the same techniques as for a conventional amalgam. Time should be taken to carve and contour the amalgam to ensure that the cuspal anatomy is kept.
The rubber dam is then removed and the patient’s occlusion checked. It is important to give the patient an opportunity to rinse their mouth and relax their muscles. Swallowing reintroduces the patient back into the intercuspal position (ICP) after keeping their mouth open for long periods of time. The occlusion should not only be checked in ICP but also in lateral excursive movements to ensure there are no sliding movements or guidance is on the new restoration to prevent excessive forces during functional loading.
Occlusion is best checked holding the articulating paper between Miller forceps and allowing the patient to close into ICP. A thin layer of petroleum jelly on the articulating paper helps the contacts to be clearly visible on the tooth and restoration surfaces. I would use one colour of articulating paper for ICP and a contrasting colour of articulating paper for lateral excursive movements. Any adjustments should be made accordingly.
Finally, the restoration as seen in figure eight can be polished with green and brown stones or a prophy air jet.
Advantages of bonded amalgams
- Reduced costs compared to cast restorations.
- Simple technique.
- Restorations can be placed in a single visit.
- More conservative compared to cast restorations.
- More conservative compared to conventional amalgam restorations as mechanical retention features such as grooves or pins are not required4,21.
- Reduced microleakage and risks of secondary caries2,4,13-18.
- Reduced post-operative sensitivity19,20,22.
- Bonding can support weakened tooth structure.
Conclusion
There is strong evidence to support that bonding an amalgam improves the seal at the tooth-material interface compared to conventional techniques. It has been reported that reducing microleakage at the restoration margins helps to prevent recurrent caries at this tooth-material interface. Furthermore, the bonded amalgam restoration has also been shown to support weaker teeth with substantial tooth loss and is a useful technique for restoring unrete
ntive cavities.
Bonded amalgam restorations, when compared with conventionally placed amalgam restorations, fulfil the principles of minimally invasive dentistry and provide the patient with an economical alternative to cast restorations by saving chairside time and laboratory fees.
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About the author
Jessica Docherty qualified from Dundee Dental School in 2010. She is an LDFT at Loanhead Dental Practice in Midlothian and has an interest in adhesive dentistry.
References
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