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J Am Dent Assoc, Vol 131, No 5, 607-611.
© 2000 American Dental Association |  |
RESEARCH |
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ABSTRACT |
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 TOP ABSTRACT METHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Methods. For 42 months, the authors followed two groups of subjects who had amalgam restorations placed in a previous study. In this double-blind study, these subjects had been randomly assigned to have amalgam restorations placed with an adhesive liner or with a copal varnish placed under all restorations and a bulk base of zinc phosphate cement for deeper lesions. The authors evaluated anatomical form, marginal adaptation, retention and the presence of secondary caries at six, 18, 30 and 42 months.
Results. At 42 months, the authors found that all restorations in both groups still were retained, were free of secondary caries and were rated clinically acceptable. No difference between the groups was found for any category (P > .05; analysis).
Conclusions. Placement of amalgam restorations with adhesive liners was found to produce results equivalent to that of traditional methods over a 42-month period.
Clinical Implications. Practitioners wary of using new methods that have not undergone thorough clinical testing can feel comfortable placing adhesive liners under amalgam restorations.
The use of an adhesive dentinal liner under amalgam restorations popularly is presumed to reduce or eliminate microleakage and, thus, postoperative sensitivity; minimize cuspal flexure and, thus, fatigue fracture of cusps; and eliminate the need for macromechanical retentive features in preparation design. Lured by the promise of these potential improvements to the traditional placement of amalgam restorations, 64.2 percent of 8,143 practitioners reported that they use adhesive liners under amalgam restorations.1 These perceived enhancements, however, are the result of anecdotal reports and inferences based on the results of laboratory studies. Since there are a dearth of data based on clinical trials, it is uncertain to what extent these improvements are illusory or real.
Results of three laboratory microleakage studies comparing a copal varnish and a 4-methacryloxyethyl trimellitate anhydride, or 4-META, liner are mixed.2–4 While one study indicated a reduction of microleakage with the adhesive liner even after one year,2 the other two studies reported different results.3,4 The second study initially found a significant difference between the adhesive liner and the copal varnish but found no difference in microleakage at one year.3 These results are similar to those of the third study, which initially demonstrated significantly less microleakage in specimens with an adhesive liner but saw a significant increase in microleak-age when specimens were aged for 30 days.4 These investigators also identified a breakdown of the interface between the amalgam and the resin liner as the source of the leakage.4 Instability of the resin liner over the long term would be a concern in the clinical setting as it would lead to poor marginal seal—a result opposite that intended by its use.
Similarly, clinical studies on postoperative sensitivity are mixed. A study comparing sensitivity to cold after placement of an amalgam restoration with a 4-META liner found a significant reduction in postoperative sensitivity,5 while another study using a bisphenol A glycidyl dimethacrylate liner found no significant difference.6
Fracture resistance has been shown to be unaffected by the use of an adhesive liner under an amalgam restoration,7,8 while the compressive strength of amalgam has been shown to be reduced by the use of a resin liner.9 Again, laboratory results conflict. Other studies have shown an improvement in resistance to cuspal fracture.10–12 Bonilla and White13 compared amalgam restorations placed with an adhesive liner with those placed with a copal varnish. They found that the adhesive liner demonstrated an initial advantage in improving resistance to cuspal fracture, but that this advantage disappeared with longer storage time and cyclic loading of the sample before cus-pal fracture occurred. With these findings, the liner’s long-term stability is questionable.
Improvement in retention seems to be a clear benefit of bonded amalgam. Bonding has been shown in vitro to enhance the retention of amalgam in Class II preparations using a proximal box only, a proximal box with retention grooves, and a proximal box and the more traditional occlusal extension.14 These in vitro results are supported by a clinical study comparing retention rates of large, complex amalgam restorations.15 The study found no significant difference between restorations retained with an adhesive liner and those retained with pins. All restorations in both groups were retained for the two-year study period.
In light of the uncertainty created by concerns about the long-term stability of the resin liner and conflicting in vitro results, we conduct this study to investigate the long-term performance of adhesive amalgam restorations. During 42 months, we observed two groups of restorations. One group’s restorations were placed using conventional lining and basing techniques, and the other group’s were placed with an adhesive liner. Using the clinical criteria developed by Cvar and Ryge,16 we evaluated three categories: secondary caries, anatomical form and marginal adaptation. We recorded the number of restorations retained for both groups and tested the hypothesis that restorations placed with an adhesive liner are superior to those placed with conventional liners and bases against the hypothesis that there is no difference at a significance level of P < .05.
While all materials were handled according to the manufacturers’ instructions, it is of interest to note that the directions for the use of the adhesive resin liner were atypical. They called for light-curing the dual-cure resin for 30 seconds before placing the amalgam; most other products have the amalgam placed over the uncured resin.
In the previous study, we measured sensitivity by a patient’s timed response to a standardized stimulus of cold water and self-reported response using a visual analog scale. At one week, there were no statistically significant differences in postoperative sensitivity between the subjects in the two groups for either measure. For the majority of subjects, postoperative pain had subsided significantly after one week. There were no further reports of postoperative pain from subjects at any of the subsequent recalls. A more complete description of the demographics of the two groups and the protocol used to base the restorations have been described elsewhere.6
We contacted subjects from that study and asked them to participate in this follow-up study. Thirty-nine of the 60 subjects agreed: 19 subjects from Group 1 and 20 from Group 2. After a thorough explanation of the project, subjects gave written consent to participate. The consent form, as well as the project protocol, were approved by the institutional review board of The Health Sciences Center, University of Tennessee at Memphis.
Next, we evaluated the subjects’ restorations using a modified version of the criteria developed by Cvar and Ryge (Table 1
METHODS AND MATERIALS
TOP
ABSTRACT
METHODS AND MATERIALS
RESULTS
DISCUSSION
CONCLUSIONS
REFERENCES
This article presents a follow-up investigation to an earlier study.6 In the previous study, we investigated postoperative sensitivity after treatment for moderate Class I or Class II lesions in a random sample drawn from a large metropolitan area. That study consisted of two groups. Group 1 had an adhesive resin liner (Optibond, Kerr) under all restorations; Group 2 had restorations placed with conventional basing techniques, copal varnish (Copalite, Harry J. Bosworth) under all restorations and a bulk base material (Fleck’s, Mizzy) in larger lesions. Group 1 had 36 percent Class I lesions and 64 percent Class II lesions. Group 2 had 44 percent Class I lesions and 56 percent Class II lesions. All restorations were restored with admix amalgam (Contour, Kerr). 
).16 Two independent evaluators (W.W.J. and P.N.G.) rated each restoration for anatomical form, marginal adaptation and the presence of secondary caries. When there were rating disagreements between the evaluators, they discussed the situation and arrived at a common rating. At each examination, we took color photographs and evaluated the need for radiographic evaluation; we took radiographs only when in the judgment of the evaluators they were needed to ensure the best possible care for the subject. We performed examinations at six, 18, 30 and 42 months after placement of the restorations.
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We compared data on secondary caries, anatomical form and marginal adaptation using a 
2 test. We set the significance level at P < .05 and recorded the number of restorations retained in both groups.
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RESULTS |
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TOPABSTRACTMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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We found that all restorations were free of secondary caries, and that all restorations were retained at 42 months. In addition, no restorations were rated as clinically unacceptable—Charlie—in the category of marginal adaptation or anatomical form. The percentage of Alfa ratings at 42 months for all three categories evaluated are presented in Table 2. We conducted a 2 analysis comparing the percentage of Alfa ratings for all three categories and found no significant difference between the two groups, P > .05.
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DISCUSSION |
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TOPABSTRACTMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Because the present study used conventional preparations modeled after accepted principles for retention and resistance form, we did not evaluate the ability of the adhesive liner to provide an alternative form of retention or to provide support for weakened tooth structure.
In a study by Belcher and Stewart,15 compromised tooth structure in the bonded amalgam group was reinforced with an adhesive liner, while the control group’s compromised tooth structure was removed. The authors found that both amalgam and tooth structure still were retained for all restorations after two years. Unfortunately, the value of this information is minimized by the fact that they did not leave any compromised tooth structure in the control group. As a result, it is uncertain whether the fact that no teeth were seen to fracture during the two-year observation period can be attributed to the adhesive liner or to chance. Our clinical experience leads us to believe that unsupported tooth structure often does not fracture in the near term, and a two-year-old amalgam restoration is relatively young.
Our results indicate that amalgam restorations placed with an adhesive liner performed as well as those placed using more traditional materials: copal varnish and, where appropriate, a bulk base of zinc phosphate cement. This finding is supported by the other clinical trials.15,17 Two concerns remain, however. First, while 42 months is longer than the typical observation period for a study using an experimental design, it is not long relative to the life span of an amalgam restoration. The second is that we found that amalgam restorations bonded with the adhesive liner performed as well as, not better than, conventional techniques. Dentistry would be well-advised to abandon a proven technique for an alternative technique only when the new procedure is shown to be superior. That superiority may be demonstrated in terms of a better clinical result, easier application or lower cost.
Bonded amalgam restorations seem to offer two advantages: a substantial ability to retain amalgam and a reduction of initial postoperative sensitivity. Laboratory and clinical evidence supports both these claims.5,14,15,17
The present study and the other clinical studies cited provide a measure of confidence that the placement of bonded amalgam restorations leads to a clinically acceptable result. A decision to establish bonded amalgam, however, as the standard of care requires more. The profession needs to examine the degree to which the patient benefits from these potential enhancements to conventional amalgam restorations and, especially in the present climate, the additional cost that will be incurred. Future research should include a cost-benefit analysis since the use of an adhesive liner adds substantially to the cost of materials and to the time required to place the restoration.
The possibility that these advantages may be very important for a particular subgroup of restorations, but not for every amalgam restoration, also should be considered. While the addition of an adhesive liner has been shown to improve retention in vitro,15 the best retention was gained by use of macromechanical retention and an adhesive liner.15 For large, complex amalgam restorations, additional retention may be essential, but it is difficult, if not impossible, to predict clinically exactly how much retention may be required. As several factors, including the mechanical properties of amalgam, dictate the internal and external form of the cavity preparation, making radical changes to conventional preparation design to take advantage of the retention offered by adhesive liners may be problematic. Thus, the tooth structure conserved by substituting an adhesive liner for macromechanical retention may be inconsequential. The theory that bonded amalgam restorations provide sufficient cuspal reinforcement and that traditional guidelines for resistance form can be modified remains clinically untested, and laboratory results are conflicting. Considering all these factors, it is possible that the additional retention provided by an adhesive liner may offer little additional benefit except in the case of large, complex amalgam restorations.
Similarly, in previous studies covering a one-week period,5,6 we found subjects’ reports of postoperative pain to be less than had been reported previously in the literature.5,6 Forty-two percent of 118 subjects experienced no sensitivity to cold, 30 percent reported minor pain, and the remaining subjects reported only mild pain.18 These subjects had been treated using one of several techniques commonly used for lining and basing. As noted previously, other researchers have found no difference in postoperative sensitivity with use of an adhesive liners at two weeks17 and at one and two years.15 So, the benefit derived from placing an adhesive liner to reduce postoperative pain, like the benefit gained from additional retention, is not dramatic.
In our opinion, the evidence from this clinical study and the other clinical trials cited support the routine use of a liner in large, complex amalgam restorations. The routine use of an adhesive liner under other amalgam restorations as the standard of care in dentistry is a separate question. Practitioners may feel it imperative to provide their patients with every conceivable advantage and consider cost a secondary factor. While that dedication to patient care is admirable, increasing the cost of such a basic procedure as the placement of an amalgam restoration has consequences. Such a change would make it even more difficult to provide care for those who already are underserved and would increase the pressure on the dental profession and third-party payers to find ways to decrease the cost of delivering dental care. It is important that the profession be sure that the additional costs of using an adhesive liner with an amalgam restoration are clearly justified by additional benefit to the patient rather than assume that they are.
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CONCLUSIONS |
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TOPABSTRACTMETHODS AND MATERIALSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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When we compared restorations for secondary caries, marginal adaptation and anatomical form over a 42-month period, we found no significant difference between restorations placed with an adhesive liner and those placed with copal varnish and bulk bases.
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FOOTNOTES |
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