This review, carried out within Cochrane Oral Health, describes the effects of tooth-coloured (composite resin) fillings compared with amalgam fillings when placed directly into cavities (holes) in permanent teeth in the back of the mouth.
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; however, due to unhappiness with their metallic look and concerns about the mercury they contain, they are being used less often, particularly in high-income countries. 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-colored 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.
We searched scientific databases until 16 February 2021 and found eight relevant studies. The studies evaluated 3285 composite fillings and 1955 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.
Participants in six studies received composite and amalgam fillings in different teeth (known as 'split-mouth design'), whilst participants in the other two studies received either composite or amalgam fillings ('parallel-group' design).
Our main analysis focused on the two parallel-group studies that treated 921 children (aged 6 to 12 years) who had their teeth restored with amalgam (1365) or composite resin (1645) 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 (known as 'secondary caries') 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.
Certainty of 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.
Implications of the evidence
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.
Low-certainty evidence suggests that composite resin restorations may have almost double the failure rate of amalgam restorations. The risk of restoration fracture does not seem to be higher with composite resin restorations, but there is a much higher risk of developing secondary caries. Very low-certainty evidence suggests that there may be no clinically important differences in the safety profile of amalgam compared with composite resin dental restorations.
This review supports the utility of amalgam restorations, and the results may be particularly useful in parts of the world where amalgam is still the material of choice to restore posterior teeth with proximal caries. Of note, however, is that composite resin materials have undergone important improvements in the years since the trials informing the primary analyses for this review were conducted. The global phase-down of dental amalgam via the Minamata Convention on Mercury is an important consideration when deciding between amalgam and composite resin dental materials. The choice of which dental material to use will depend on shared decision-making between dental providers and patients in the clinic setting, and local directives and protocols.
Traditionally, amalgam has been used for filling cavities in posterior teeth, and it continues to be the restorative material of choice in some low- and middle-income countries due to its effectiveness and relatively low cost. However, there are concerns over the use of amalgam restorations (fillings) with regard to mercury release in the body and the environmental impact of mercury disposal. Dental composite resin materials are an aesthetic alternative to amalgam, and their mechanical properties have developed sufficiently to make them suitable for restoring posterior teeth. Nevertheless, composite resin materials may have potential for toxicity to human health and the environment.
The United Nations Environment Programme has established the Minamata Convention on Mercury, which is an international treaty that aims "to protect the [sic] human health and the environment from anthropogenic emissions and releases of mercury and mercury compounds". It entered into force in August 2017, and as of February 2021 had been ratified by 127 governments. Ratification involves committing to the adoption of at least two of nine proposed measures to phase down the use of mercury, including amalgam in dentistry. In light of this, we have updated a review originally published in 2014, expanding the scope of the review by undertaking an additional search for harms outcomes. Our review synthesises the results of studies that evaluate the long-term effectiveness and safety of amalgam versus composite resin restorations, and evaluates the level of certainty we can have in that evidence.
To examine the effects (i.e. efficacy and safety) of direct composite resin fillings versus amalgam fillings.
An information specialist searched five bibliographic databases up to 16 February 2021 and used additional search methods to identify published, unpublished and ongoing studies
To assess efficacy, we included randomised controlled trials (RCTs) comparing dental composite resin with amalgam restorations in permanent posterior teeth that assessed restoration failure or survival at follow-up of at least three years.
To assess safety, we sought non-randomised studies in addition to RCTs that directly compared composite resin and amalgam restorative materials and measured toxicity, sensitivity, allergy, or injury.
We used standard methodological procedures expected by Cochrane.
We included a total of eight studies in this updated review, all of which were RCTs. Two studies used a parallel-group design, and six used a split-mouth design. We judged all of the included studies to be at high risk of bias due to lack of blinding and issues related to unit of analysis. We identified one new trial since the previous version of this review (2014), as well as eight additional papers that assessed safety, all of which related to the two parallel-group studies that were already included in the review.
For our primary meta-analyses, we combined data from the two parallel-group trials, which involved 1645 composite restorations and 1365 amalgam restorations in 921 children. We found low-certainty evidence that composite resin restorations had almost double the risk of failure compared to amalgam restorations (risk ratio (RR) 1.89, 95% confidence interval (CI) 1.52 to 2.35; P < 0.001), and were at much higher risk of secondary caries (RR 2.14, 95% CI 1.67 to 2.74; P < 0.001). We found low-certainty evidence that composite resin restorations were not more likely to result in restoration fracture (RR 0.87, 95% CI 0.46 to 1.64; P = 0.66).
Six trials used a split-mouth design. We considered these studies separately, as their reliability was compromised due to poor reporting, unit of analysis errors, and variability in methods and findings. Subgroup analysis showed that the findings were consistent with the results of the parallel-group studies.
Three trials investigated possible harms of dental restorations. Higher urinary mercury levels were reported amongst children with amalgam restorations in two trials, but the levels were lower than what is known to be toxic. Some differences between amalgam and composite resin groups were observed on certain measures of renal, neuropsychological, and psychosocial function, physical development, and postoperative sensitivity; however, no consistent or clinically important harms were found. We considered that the vast number of comparisons made false-positive results likely. There was no evidence of differences between the amalgam and composite resin groups in neurological symptoms, immune function, and urinary porphyrin excretion. The evidence is of very low certainty, with most harms outcomes reported in only one trial.