JADA Continuing Education
EFFICACY OF PREFORMED METAL CROWNS VS. AMALGAM RESTORATIONS
IN PRIMARY MOLARS: A SYSTEMATIC REVIEW
ROS C. RANDALL, B.CH.D., M.PHIL.,
MATTHIAS M. A. VRIJHOEF, PH.D. and
NAIRN H. F. WILSON, PH.D., M.SC., B.D.S., F.D.S., D.R.D.
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ABSTRACT
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Background. The authors evaluated the treatment efficacy of preformed metal crowns, or PMCs, vs. amalgam restorations in primary molars by means of a literature review and meta-analysis.
Types of Studies Reviewed. From a literature search, the authors selected clinical studies that evaluated treatment with PMCs vs. amalgam control restorations in primary molars and provided data against which treatment outcomes could be compared.
Results. Ten studies with durations ranging from 1.6 to 10 years fulfilled the selection criteria. Their failure rates, based on need for subsequent treatment or retention of the restoration at final evaluation, ranged from 1.9 to 30.3 percent for PMCs and 11.6 to 88.7 percent for amalgam restorations. Overall, PMCs demonstrated greater longevity and reduced retreatment need compared with amalgam control restorations. The odds ratio for all studies fell within the boundary favoring treatment with PMCs.
Clinical Implications. Analysis of the literature, though mainly retrospective studies, demonstrated evidence of a more favorable outcome for PMCs than for amalgam restorations in primary molars requiring multisurface restorations.
The challenges involved in using amalgam to restore multisurface caries in primary molars are well-documented.1–3 The close proximity of the pulp to the outer mesial surface of the first primary molars makes it difficult to obtain adequate retention for an amalgam restoration.3 The broad contact area between primary molars can lead to flared proximal box preparations in Class II situations, weakening the tooth and reducing support for an amalgam restoration.2 A number of authors have cited the preformed metal crown, or PMC, as the preferred treatment for multisurface caries on primary molars4–6 and as the restoration of choice after endodontic therapy for primary molars.3,6–8 Unlike amalgam, which requires retention features to be incorporated into the cavity design, the preformed crown obtains its retention from the flexibility of the thin, precontoured crown margins. This allows it to spring into and be retained by the undercut area apical to the cementoenamel junction in a primary molar.
In anticipation of an increasing requirement to provide evidence-based treatment in dentistry, there is a need for a number of widely adopted procedures in restorative dentistry to be fully reviewed for evidence of efficacy. Our objective was to test the hypothesis that, based on a qualitative review of the literature, the clinical success rate of PMCs in the restoration of primary molars is similar to that of amalgam restorations and to further analyze the data obtained by means of meta-analysis.
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MATERIALS AND METHODS
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Literature search.
We performed a MEDLINE literature search dating back to 1972. The key words we used were pre-formed metal crowns, stainless steel crowns, primary molar, deciduous molar, clinical and in vivo. We also retrieved articles from relevant references cited in literature obtained.
We adopted two selection criteria for inclusion of articles into the study. The first was that they included a clinical comparison of the longevity or success rate of PMCs in primary molars vs. that of an amalgam control in primary molars. The second criterion was that data from the study would be available for use in comparing treatment outcomes. Specifically, we intended to construct such a comparison in a tabular format.
For the first criterion, when differentiation was possible we selected data on Class II amalgam restorations only; when differentiation was not possible, but amalgam had been used as the control, we used combined data for both Class I and II restorations. We excluded composite and glass ionomer restorative materials.
We anticipated that the likely sources of heterogeneity across the studies would be patient variables—such as age at placement, caries risk and whether the study tooth received concurrent endodontic therapy—and operator variables—such as extent of pediatric dental specialization. These points were included in the qualitative analysis of the literature obtained.
Review criteria.
We reviewed the selected articles in the following categories:
- – retrospective or prospective study;
- – if a prospective study, whether treatments were randomized to teeth;
- – type of control restoration (Class II or Classes I and II);
- – operators who did placements;
- – patient source and treatment location;
- – patients’ caries susceptibility;
- – sample size;
- – duration of study;
- – patient age at placement;
- – endodontic treatment status of study teeth;
- – failure rate of preformed crown vs. amalgam control;
- – main reasons for restoration failure;
- – whether true and false failure rates were reported (false failures were restoration failures resulting from a deficiency unrelated to the restoration itself; we excluded false failures from the review where it was possible).
Statistical analysis.
We evaluated the results of each study quantitatively by means of meta-analysis, again excluding false failure data. We constructed a table of treatment outcomes, comparing successful and failed restorations, for each of the reviewed studies (Table 1
). From these data, we calculated the odds ratio, or OR,9 and 95 percent confidence interval, or CI,10 for each study outcome. The Mantel-Haenszel summary odds ratio, ORMH,11 and 95 percent CI of the ORMH11 were calculated to give an overall numerical estimate of the treatment effect of PMC vs. amalgam (Box11,12).
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RESULTS
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The literature search produced 35 articles, 10 of which fulfilled the selection criteria.4,5,13–20 Two of these articles reported the results of the same study at six months13 and seven years15; we counted them as one study. In addition to these 10 studies, two additional articles,21,22 which did not meet the study criteria individually but did when combined, reported complementary results and were treated as one article. Thus, a total of 10 studies were available for qualitative analysis; results are given in Table 2. All except two of the studies were retrospective evaluations of patient records. Of these two studies, one was a prospective, nonrandomized clinical trial,15 and the other was a 10-year prospective evaluation of restorations and fissure sealants in 732 patients,16 as subsequently revealed by Roberts.23 Another study was a prospective evaluation of pulpotomies in primary molars; however, differences in success rates between PMC and amalgam restorations were evaluated retrospectively by the authors.20
The majority of the patient treatments were carried out in a dental school or hospital pediatric clinic. In two studies,17,20 the patients were children who had been referred for treatment because of behavioral problems. In general, however, detailed documentation of patient characteristics was not given.
Control and PMC restorations were placed in the same patients in four studies4,14,15,19; in the remaining articles, both isolated and multiple restorations were placed in patients or no details were given. In one trial,15 control teeth were reported as being the tooth corresponding to the crowned tooth under evaluation on the opposite side of the same arch. As a number of these control contralateral teeth were sound and unrestored at the start of the study, we only included data for restored control teeth in the present review.
The true and false failure rates were discussed in six articles.4,5,16,18,19,21 False failures for both PMC and amalgam were indicated as orthodontic extractions,4,18,19 extractions due to pulp pathology apparently unrelated to the restoration4,5,16,18 and caries occurring in a restored tooth remote from the restoration.16,18,22 The main reason given for true restoration failure for PMC was loss of a crown leading to the need for recementation, and secondary caries and fracture in the case of amalgam. Across all the studies reviewed, we found a consistently lower failure rate for PMC compared with amalgam, varying between 1.5 and nine failed amalgams for every failed PMC.
Heterogeneity may have resulted from differences in severity of caries risk, in levels of operator skill, in treatment methods or materials used, in study duration or various combinations of these.
Results for the meta-analysis are shown in the figure. The OR and 95 percent CI for the evaluated studies fell almost entirely below 1.0, indicating efficacy of treatment with PMC. The ORMH for pooled data from all the studies and 95 percent CIMH were calculated as follows:
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DISCUSSION
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Heterogeneity among the studies evaluated is to be expected. Articles obtained for this review spanned from 1975 to 1997—22 years—and involved patients from different countries and of differing ages at restoration placement. Heterogeneity may have resulted from differences in severity of caries risk, in levels of operator skill, in treatment methods or materials used, in study duration or various combinations of these. Despite the dissimilarities between studies, we saw the same positive effect of PMC compared with amalgam across the data.
In at least two of the studies, the larger, multisurface preparations tended to be restored with PMC,15,16 while the amalgam restorations were used for smaller carious lesions. In the remaining studies, the decision process by which an amalgam or a PMC restoration was chosen for a specific tooth was not discussed; the majority of PMCs, however, most likely were placed in areas of larger or multisurface decay. We suggest that this creates a bias against the PMC treatment groups, lending strength to an interpretation of the results as demonstrating a more favorable outcome for PMC than for amalgam restorations.
Of the articles we reviewed, the study reporting the largest sample size and greatest longevity of restorations recorded the lowest failure rate for both PMC and amalgam.16 The study’s authors stated that use of amalgam had been restricted to cavities of minimal classical design; the remainder received stainless steel crowns. This, together with the specialist practice setting of the study, may explain the reported low failure rate of 11.6 percent for amalgam and 1.9 percent for PMC restorations over 10 years.
The study by Braff 4 reported the highest failure rates for both PMC and amalgam restorations. Braff probably was using a type of preformed crown with rather extended walls,3,19 which likely needed considerably more adjustment to obtain a retentive fit than the pre-belled, more anatomically correct design subsequently made available.24 The average patient age of 4.2 years at restoration placement also may have been a factor in the high failure rate.4 Patient age at restoration placement has been reported to be a determinant in restoration longevity in primary molars.22,25–27 Unfortunately, changes in caries rates and materials over the time span of the present survey, as well as the differences in study duration and patient population, make it difficult to assess an age effect from the review data.
Four studies excluded failures due to pulp inflammation that were seemingly not directly associated with the restoration.4,5,16,18 In other studies, all failures due to pulp inflammation were counted as failure of the restoration.15,20,21 False failure of a restoration may occur when a pulpotomy is not carried out when it should have been, or where a pulpotomy is performed but fails due to operator error. An example of true failure of a restoration resulting from pulp pathology would be where endodontic therapy was carried out but failure occurred as a result of restoration leakage leading to a recurrence of pulp inflammation. There can be difficulties in establishing the correct cause of failure when pulp pathology is involved, and it may often be multifactorial in origin. In the studies in which false failures were categorized, it is equally possible that some of these were true failures that should have been scored against the restorations being evaluated.
It is clear that there were differences in methodologies and variability in the data among the studies we evaluated for this review. Meta-analysis is used to assess whether treatment effects in different studies are of the same general magnitude.28 The tendency of the results of this meta-analysis to fall almost entirely on the same side of the "no difference" line favoring treatment with PMC suggests an overall advantageous treatment outcome for PMC vs. amalgam restorations. The majority of articles we evaluated were retrospective studies. The disadvantage of retrospective studies is the reliance on patient records and their accuracy, as well as an inability to allow for changes in treatment practice over time. Any review of retrospective studies must take into account the inevitable bias present from different unmeasured confounders, as must interpretations of meta-analysis of such data, since it also will be biased.
Despite the inherent bias in the data, however, a treatment effect is demonstrable. As some of the bias goes against PMC, it is interesting to speculate whether the difference between PMC and control restorations is such that it can be measured easily, perhaps resulting in a reduced likelihood of obtaining a type I or type II error.
The best basis for evidence would be a well-designed and -executed clinical trial. We doubt, however, that it would be ethically justified to involve children in a clinical trial of a dental procedure—such as placement of PMCs—that has been in use in dentistry for 48 years or more and in which treatments would be randomized to teeth rather than selected by the operator. Further study is needed.
Work to evaluate the relative ratio of benefit to harm in tooth tissue removal for PMC vs. a multisurface amalgam restoration would be of value. In addition, a comparison of contemporary resin-modified glass ionomer cements with the conventional glass-ionomer or zinc phosphate cements in regard to retention over the long-term would be helpful. Likewise, few studies evaluating the long-term effect of PMC on the permanent successor tooth have been published to date and more are needed.
It is generally accepted among pediatric dentists that PMCs are the preferred treatment for primary molars with multisurface cavities, whether or not these teeth received endodontic therapy.
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CONCLUSION
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The qualitative and quantitative results from this systematic review did not support the hypothesis of a similar success rate for PMCs and amalgam restorations in primary molars. The results demonstrated evidence of enhanced clinical effectiveness of treatment with PMCs vs. amalgam for the restoration of multisurface cavities in primary molars.
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FOOTNOTES
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Dr. Randall is clinical research manager, 3M Dental, Laboratoires 3M Santé, France, Boulevard de l’Oise, 95029 Cergy Pontoise Cedex, France. Address reprint requests to Dr. Randall.
Dr. Vrijhoef is senior technical manager, 3M Dental, Laboratoires 3M Santé, Cergy, France.
Dr. Wilson is professor of restorative dentistry, University Dental Hospital of Manchester, England.
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ABSTRACT
MATERIALS AND METHODS
