The Influence of the Amalgam Alloy on the Survival of Amalgam Restorations: A Secondary Analysis of Multiple Controlled Clinical Trials

Abstract

Data from 14 independent controlled clinical trials on the oral behavior of Classes 1 and 2 amalgam restorations, with a follow-up between five and 15 years, were re-evaluated by secondary analysis for the influence of alloy composition on the survival of amalgam restorations. For the analysis, 3119 restorations were available, which were made from 24 different alloys by a group of seven operators. The alloys were divided into four groups according to their zinc content (zinc-containing and zinc-free) and their copper content (conventional and high-copper). During the follow-up of the trials, the restorations were annually assessed for failures, which were classified as to (1) restoration-, (2) restorative process-, and (3) patient-related reasons. With the restoration-related failures, survival functions of the restorations were estimated by alloy and alloy group. The total number of failed restorations was 481, of which 77% were restoration-related and 14% process-related. Eighty percent of the restoration-related failures were due to some form of fracture of the amalgam. Restorations of conventional zinc-free alloys had the shortest survival. After 13 years, only 25% survived. Zinc and a high copper content had an equally favorable influence on the survival rate, which was 70% after 13 years when either was present. The highest survival rates were of restorations of zinc-containing high-copper alloys: 85% after 13 years. The zinc and copper contents of the alloy contributed to the corrosion resistance of the amalgams, which in turn influenced the survival of the restoration. The current ISO Standard 1559 on alloys for dental amalgam should be modified to account for these factors that influence the survival of amalgam restorations.

Penetration of amalgam constituents into dentine

“…Corrosion of Zn is supposed to delay corrosion of Sn 8 Hg (γ 2 phase) and Cu 6 Sn 5 (η' phase). 52,53 Zn has been identified as the major corrosion product released in saliva and ZnSn(OH) 6 has been detected on Zn-containing amalgams. 49,50,51,54 On both Zn-free conventional and Zn-free Cu-rich amalgams, CuSn(OH) 6 was found.…”

The tunnel restoration

“…38 However, when randomised clinical trails of restorations are carried out, secondary caries is rarely a problem. 39 It is impossible to know whether these differences between routine practice and clinical trials reflect an actual difference in the caries risk status of the patients in the two settings, or poor diagnosis of secondary caries, or poor restorative technique encouraging this failure. In one study listed in Table 1 caries active patients were specifically selected 23 and in two others the effect of caries rate on survival times was examined.…”

How long do routine dental restorations last? A systematic review

“…Only eight studies achieved at least six 'yes' scores and thus satisfied the criteria of being of 'adequate' quality. Of these, six were of longitudinal retrospective design, 3,10,11,18,26,47 one was a longitudinal prospective study, 21 and one was a nonrandomised clinical trial. 56 None of the randomised controlled trials or cross-sectional studies originally selected achieved this score.…”

“…This was used in six studies (Bentley and Drake, 3 Drake, 10,11 Hawthorne and Smales, 18 Jokstad and Mjor, 21 Robbins and Summitt, 47 ). Only two reported survival/failure rate as the sole outcome measure (Letzel et al, 26 Wilson and Norman, 56 ) while four used both MST and survival/failure rate. 3,10,11,21. Findings on longevity Table 3 shows the high degree of variability reported between the validated studies which made it difficult to determine an average period of time over which restorations would last.…”

“…The preferred criteria were those of Robinson, 48 and the USPHS system used by Wilson and Norman. 56 Letzel et al 26 and Robbins and Summitt, 47 defined their own criteria for replacement. Some influence from effect modifiers was suggested by the validated studies.…”