Background

One of the most widely used dental materials is dental amalgam, made up of mercury and alloy particles. A recognised limitation of dental amalgams compared to other dental materials such as composites,is that they cannot bond to the dental tissue. The gap between dental tissue and amalgam restoration risks attracting a buildup of the waste products of the dental amalgam.1 The microleakage from the tooth and restoration interface has been reported as a potential contributing factor towards some of the problematic symptoms experienced by patients following placement of amalgam restorations, for example postoperative sensitivity.2,3 Therefore, some dentists recommend using liners between the amalgam and dental tissue. In this review, we intend to evaluate the available evidence for the effectiveness of different dental liners placed under amalgam restorations.

Methods

First, a search for systematic reviews was conducted in PubMed and the Cochrane Library (Cochrane Database of Systematic Reviews (CDSR) and Database of Reviews of Effects (DARE)). The details of the search strategy for systematic reviews are available in Appendix 1. We did not find any systematic review that was directly relevant to the question of this rapid assessment. There was one review that was partially relevant and examined the question of whether bonded amalgams are better than non-bonded ones in restoring permanent teeth.1 The authors found one trial with 31 patients (113 restorations) that compared adhesively bonded amalgam restorations (Dycal (LD Caulk) liner; ED primer (Kuraray); Panavia 21TC (Kuraray); Oxyguard II gel (Kuraray); Dispersalloy amalgam (Dentsply)) versus non-bonded amalgam restorations (Dycal (LD Caulk) liner.

Table 2 Appendix 1 Search strategy for identifying systematic reviews.
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A search was then conducted in Medline (OVID) and the Cochrane Central Register of Controlled Trials (CENTRAL) to identify any randomised controlled trial (RCT) comparing use of a lining under amalgam restorations versus no lining or RCTs comparing differing lining materials under amalgam against each other. We excluded studies that were done on extracted teeth or done on primary teeth. Details of the search strategy are provided in Appendix 2.

Table 3 Appendix 2 Search strategy for identifying randomised controlled trials
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Findings

In the one identified systematic review, bonded amalgam restorations (amalgam restorations with dental adhesive) compared to non-bonded amalgam (amalgam restorations without dental adhesive) showed no significant differences in postoperative sensitivity. The authors concluded that the evidence is inadequate to conclusively judge whether bonded amalgam is better than non-bonded.

The searches for primary studies identified 94 results in Medline and 52 results in PubMed. After screening the title and abstracts, there were 20 potentially relevant studies. We excluded one study by Baratieri et al.,4 one study by Gordan et al.5 and those by Gupta et al.,6 Miller et al.7 and Wright et al.8 as it was not clear whether they were RCTs. We also excluded studies by Fanian et al.,9 Lim and McCabe,10 Sandoval et al.11 and Qvist et al.12 as they were conducted on extracted teeth. Finally, we also excluded studies by Shaddy et al.13 and Hucke et al.14 as they were conference abstracts and full access to data and study details was not possible. Eight studies were included. Characteristics of these studies are provided in Table 1.

Table 1 Characteristics of the included studies
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The studies compared a range of liners against each other or against a control group or bonded amalgam. The outcome measurements were very diverse. The studies were categorised based on the comparison group in Table 1. A summary of the results of the included studies are provided below.

Comparing amalgam restorations with liners and bases versus amalgam restorations with no liners

There were three studies that had relevant comparison groups.15,16,17 It seems that amalgam restoration with copal varnish have less postoperative sensitivity compared to amalgam restorations with no liners. The other comparisons were:

  1. 1

    Amalgam restoration with calcium hydroxide versus amalgam restoration with no liner

  2. 2

    Amalgam restoration with modified glass ionomer liner versus amalgam restoration with no liner

  3. 3

    Amalgam restoration with fluoridated desensitising agent versus no liner.

The other comparisons did not find consistent significant differences. The current data are inadequate to reach a definite conclusion.

Comparing amalgam restorations with different liners and bases against each other

There were six studies with relevant comparison groups.3,17,18,19 Data for the following comparisons were available:

  • Amalgam with glass ionomer liner versus amalgam with copal varnish (three studies)

  • Amalgam with copal varnish versus amalgam with a fluoridated desensitising agent (two studies)

  • Amalgam with glass ionomer and calcium hydroxide (Dycal) versus amalgam with zinc phosphate and calcium hydroxide (Dycal) – Dycal was only used if the cavity was deep (one study)

  • Amalgam with copal varnish versus amalgam restoration with calcium hydroxide (one study)

  • Amalgam with glass ionomer versus amalgam with calcium hydroxide (one study).

The data were limited or inconclusive and the studies are therefore not adequate to detect consistent significant differences.

Comparing amalgam restorations with liners and bases versus bonded amalgam (amalgam with adhesives)

There were five studies that included relevant comparisons.15,16,17,20,21 Data for the following comparisons were available:

  • Amalgam with copal varnish, glass ionomer and calcium hydroxide (Dycal) – Dycal was only used if the cavity was deep – versus bonded amalgam (amalgam with dental adhesive liner) (one study)

  • Amalgam with copal varnish, zinc phosphate cement and calcium hydroxide (Dycal) – Dycal was only used if the cavity was deep – versus bonded smalgam (amalgam with dental adhesive liner) and calcium hydroxide (Dycal) – Dycal was only used if the cavity was deep (one study)

  • Amalgam with copal varnish versus bonded amalgam (amalgam with dental adhesive liner) (three studies)

  • Amalgam with a fluoridated desensitising agent versus bonded amalgam (amalgam with dental adhesive liner) (one study)

  • Amalgam with glass ionomer liner versus bonded amalgam (amalgam with dental adhesive liner) (two studies)

  • Amalgam restoration with calcium hydroxide versus bonded amalgam (one study).

The data were limited or inconclusive and the studies were of low quality or were of small sample sizes, making it difficult to detect consistent significant differences. The current evidence can not demonstrate whether one of the dental liners is better than bonded amalgam in reducing postoperative sensitivity.

Summary

In conclusion, the current studies are inadequate to claim or refute a difference in postoperative sensitivity between different dental liners. The reasons are the low quality of studies, small sample sizes, and inadequate reporting of the outcome data. Further well-conducted RCTs are needed to answer this question. These RCTs would be preferably included and synthesised in a systematic review.