Restoring the back teeth: amalgam versus resin composite fillings

Traditionally, metal fillings made of a silver‐coloured material known as amalgam have been used to treat tooth decay in the back permanent teeth effectively and cheaply. But there have been concerns about the way they look, and concerns about the mercury they contain. The Minamata Convention on Mercury is a global agreement that has promoted a worldwide reduction in the use of mercury (including amalgam fillings) in order to reduce the impact of mercury on the environment. Tooth‐coloured fillings made of a composite resin material have been used as an alternative to amalgam fillings. Initially, they were used only in the front teeth, but as their quality has improved, they have been used in permanent teeth at the back of the mouth.

What was the research?

A systematic review of the effectiveness of tooth‐coloured (composite resin) fillings compared with amalgam fillings when placed directly into cavities in permanent teeth in the back of the mouth

Who conducted the research?

The research was conducted by a team led by Helen V. Worthington from the University of Manchester, on behalf of Cochrane Oral Health. Sara Khangura, Kelsey Seal, Monika Mierzwinski-Urban, Analia Veitz-Keenan, Philipp Sahrmann, Patrick Roger Schmidlin, Dell Davis, Zipporah Iheozor-Ejiofor and María Graciela Rasines Alcaraz were also on the team.

What evidence was included in the review?

We searched scientific databases until 16 February 2021 and found eight relevant studies. The studies evaluated 3,285 composite fillings and 1,955 amalgam fillings; however, it is unclear how many participants received these fillings. The exact age of participants was unclear in some studies, but the studies included both children and adults. The studies took place in the UK, the USA, Portugal, Sweden, the Netherlands, Belgium, Germany, and Turkey.

What did the evidence say?

Our main analysis focused on two studies that treated 921 children (aged 6 to 12 years) who had their teeth restored with amalgam (1,365) or composite resin (1,645) fillings. We found that composite resin fillings were significantly more likely to fail than amalgam fillings when used to fill cavities in permanent teeth at the back of the mouth. Tooth decay after a filling was placed occurred more frequently with composite resin compared to amalgam fillings. There was no suggestion of a difference between the materials in the likelihood of filling breaking.

Six of the trials used a ‘split‐mouth’ design, which means that each participant had both types of fillings in different teeth. These studies were less reliable, as they did not explain fully how they conducted the studies, and it was unclear how many people received the fillings. We analysed the split‐mouth studies separately from the parallel‐group studies, and undertook a statistical approach known as ‘subgroup analysis’. This showed that the findings of these studies were compatible with the results from the two parallel‐group studies.

Three studies reported negative side effects. Although we found that there were some possible side effects with each material used, this information is unreliable because the study authors carried out so many analyses that ‘false positive’ results were likely to be found. Overall, it seems that the materials may differ in terms of how safe they are, but the level of the differences identified in the studies may not be important.

To summarise, we found that composite resin fillings may be almost twice as likely to fail compared with amalgam fillings when used for filling permanent teeth at the back of the mouth. Composite fillings do not seem more likely to break, but do seem more likely than amalgam fillings to develop further tooth decay. The current evidence suggests there are no important differences in the safety of amalgam compared with composite resin dental fillings.

How good was the evidence?

We judge the available evidence to be ‘low certainty’, which means that the results may change with future research. As the colour of the amalgam and composite resin fillings differed, it would not have been possible to ‘blind’ those involved in the study from knowing the treatment administered, so there was a high risk of bias in all of the included studies. In addition, the findings were imprecise and sometimes inconsistent, so we cannot be sure that the evidence is reliable. 

What are the implications for dentists and the general public?

Overall, the evidence suggests that amalgam restorations are effective, enduring, and safe, while composite resin restorations are more likely to fail and lead to secondary caries. However, the studies in this review were quite old, and composite resin materials have likely improved since the included studies were conducted. Patients and dental providers can discuss together which material they want to use when permanent teeth in the back of the mouth require fillings in the dental clinic. Governments around the world are trying to reduce the use of dental amalgam (according to the Minamata Convention on Mercury), and so each local area will have their own regulations and guidance. 

What should researchers look at in the future?

There is a need for new research to demonstrate long‐term effectiveness and safety of the latest improved composite resin materials, and the techniques and instruments for placing them. If split‐mouth trial design is to be used in future studies, data should be analysed and reported appropriately

Link

Worthington  HV, Khangura  S, Seal  K, Mierzwinski-Urban  M, Veitz-Keenan  A, Sahrmann  P, Schmidlin  PR, Davis  D, Iheozor-Ejiofor  Z, Rasines Alcaraz  MG. Direct composite resin fillings versus amalgam fillings for permanent posterior teeth. Cochrane Database of Systematic Reviews 2021, Issue 8. Art. No.: CD005620. DOI: 10.1002/14651858.CD005620.pub3.