H. Letzel

Controlled clinical study of amalgam restorations: survival, failures, and causes of failure

The survival and modes of failure of amalgam restorations were investigated retrospectively. 2660 Class I or II lesions were restored and evaluated yearly or half-yearly for failures during the 30- to 84-month follow-up. Restorations with unacceptable margins were not counted as failures if no traces of secondary caries could be seen. 8% of the restorations were lost because of patient drop-out. Of the remaining restorations, 1% was replaced due to primary caries. Of the remaining number (2431), 9% failed because of all other reasons. The leading mode of failure was bulk fracture (4.6%), followed by tooth fracture (1.9%), and marginal ridge fracture (1.3%). For all other reasons, 0.8% of the restorations failed. Only two restorations were replaced because of secondary caries. The alloy selection in both conventional and high-copper categories significantly influenced the survival of the restorations for reasons directly related to the restoration.

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

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.

An Estimation of the Size Distribution of Amalgam Particles in Dental Treatment Waste

Regulations have been adopted in several countries of the European Union which prescribe that dental treatment waste water must be discharged via an amalgam separator device. Since the effectivity of the device strongly depends on the size of the amalgam waste particles, this size was evaluated in waste water samples from eight dental offices. Per sample, all solid particles were separated from the waste water by pressure filtration, then dried and divided into six fractions by being sieved over five sieves with decreasing mesh width. Of the particles in each fraction, the density was determined by picnometry, the mass by weighing, and the area and width by image analysis. For this analysis, width was defined as the dimension perpendicular to the length of the particles. By combining the density, area, and width determinations of all fractions, we obtained mass distributions per waste particle width of the samples. The proportional amalgam mass of the distributions was estimated with the measured density of the particle fractions and with a number of assumptions for the density of amalgam particles only and of other waste particles only. Each waste sample has its own characteristics with respect to the mass and density of the particle fractions. The size distribution of waste particles has a bimodal shape and consists of a distribution of small (width, 2 to 90 pm) and large particles (width, 160 to 5500 μm). For small particles with a width up to 60 μm, the influence of the assumptions on the estimation of the proportion amalgam of the waste mass distribution is minor when compared with the sampling error. By averaging the estimations over the samples, one can estimate the weight of amalgam particles with a width < 10 μm and < 50 μm, respectively, between 4 and 15% and between 15 and 30%. The smallest particles comprising 5% of the amalgam mass have an estimated width of up to 5 to 15 pm.

Loss of substance of dental composite restorations

Abstract A method for the direct in vivo measuring of loss of substance of Class I and II restorations was tested. On models of the teeth involved, silver caps were made. The occlusal surface was relieved by a piece of tinfoil. With the silver cap, silicone impressions were made for baseline registration and also at 3 recall periods, 6, 12 and 18 months after baseline registration respectively. The average thickness of the loss of substance was computed by the weight, the density and the thickness of both the enclosed impression material and the piece of tinfoil. The results show the in vivo repeatability of the test method to be in good agreement with data obtained in a previous in vitro study. The error of the test method accounts only for a relatively small part of the total experimental variance (7, 9 and 12%). The wear values show that the method registers, with adequate accuracy, wear rates of approximately 20 μm/year. Because of its simplicity, inexpensiveness and accuracy, the method is highly productive and suitable for clinical screening tests of the wear behaviour of restorations, especially for the registration of losses of substance of posterior composites.

A rapid scanning electron microscopic replication technique for clinical studies of dental restorations

Abstract A technique is described for the rapid and reliable production of replicas of dental restorations for examination with the scanning electron microscope. The technique makes use of the silicone rubber Xantopren blue and the epoxy resin Stycast 1266. A negative replica is made with the silicone rubber and from this negative a positive is made using the epoxy resin. The technique appears to be suited for high resolution (0.3 μm) analysis of the surface structure of tooth restorations on the condition that the surface to be replicated is sufficiently clean and dry.

Gamma-1 to beta-1 phase transformation in retrieved clinical amalgam restorations

A number of important reactions occur in clinical amalgam restorations. This investigation examined the extent of the gamma-1 to beta-1 transformation in a variety of amalgam restorations retrieved from clinical trials. Sixteen restorations of five brands of low Cu amalgams, 28 restorations of four brands of high Cu blended amalgams, and eight restorations of three brands of high Cu single particle amalgams were studied. The extent of the gamma-1 to beta-1 transformation was determined by the ratio of the integrated intensity of the 002 beta-1 peak to the 330 gamma-1 peak in x-ray diffraction patterns. These values were correlated with age, marginal deterioration grade, and residual Hg content. Restorations consistently contained more beta-1 than comparable laboratory controls. Blended high Cu amalgams generally contained more beta-1, which was correlated with better marginal grades. Beta-1 content was inversely related to residual Hg content. No other associations could be established. A model of the complex interactions between corrosion and transformations was developed.

Influences on the bulk fracture incidence of amalgam restorations: a 7-year controlled clinical trial

Abstract In a clinical study 360 posterior amalgam restorations were made in 57 adult patients. Each patient received 6 restorations of 6 different commercial alloys (5 conventional and 1 high copper alloy system). The restorations were equally distributed over mandibular and maxillary premolars and molars. Most of the restorations were of the Class 2 type. From annually taken black and white photographs of the occlusal surface of the restorations over a period of 7 years the incidence of occlusal and approximal bulk fractures was evaluated. The tooth and jaw type and the amalgam alloy each have a significant influence on the incidence of both types of bulk fractures.

Authors'reply to Letter to the Editor from J Dent Res 77:340, 1998

W e thank Dr. Osborne and Dr. Summitt for their valuable remarks on our article on the survival of Vamalgam restorations (Letzel et al., 1997), and for putting forward the topic that premature removal of amalgam restorations in clinical practice may be of greater importance than all the other factors discussed in the article. To comment further, it is necessary to go back 25 years to the time when clinicians were frequently disappointed when properly placed amalgams failed within a few years of placement. By todays standards, the amalgams of that time were prone to failure as we have shown, but amalgam was often, as Hollenback (1937) called it, abused in general practice. In addition, there were many instructions regarding the placement of amalgam that made little sense, and manufacturers often provided information of questionable clinical relevance. The development of clinical research in restorative dentistry in the 1960s led to our understanding that the best way to obtain reliable information on the clinical effects of amalgam alloys and application methods was with a program of long-term, randomized, controlled clinical trials. This approach has become the keystone of the current evidence-based approach. During the design of our clinical trials program, we understood that if the confounding of experimental variables was to be minimized, the experimental restorations had to be placed in well-prepared cavities of selected teeth in patients willing to undergo long-term recalls, by a group of trained operators following a carefully prescribed procedure, and using sufficient time to make restorations that minimized possible operator-related failures. As such, we produced amalgam restorations in an institutional setting, which is quite different from the circumstances in general practice. In the design phase, we formulated hypotheses based on variables thought to be relevant at the time and tested these in the trials. Luckily, we chose one variable, the amalgam alloy, which proved to be extremely meaningful. Now there are other problems with amalgam. We now know, from our work and those of many others including the authors of the letter, many of the important and unimportant variables associated with amalgam restoration survival. The premature removal of amalgam restorations is one of these major problems. Not using amalgam when indicated is another. We agree with Osborne and Summitt that the premature removal of amalgams based on suspicion is unnecessary and is a form of overtreatment that has often been taught in dental schools, but we hope is declining, and is often the norm in general practice. We did not compare our results with those based on analysis of failures in clinical practice, as done by Osborne and Summitt, since these were beyond the scope of our work, and the overall characteristics of the settings and methods of analysis may not be directly comparable. However, it is possible to compare the reasons for failure of amalgam restorations that were found in our studies with the results of studies on the reasons for replacement of failed amalgam restorations in general practice. Previously, we used the results from seven such studies to obtain a metaanalysis (Letzel et al., 1989). We have now recalculated the percentage replacement due to primary and recurrent caries based on the studies from the meta-analysis and compared them with the results from the secondary analysis as summarized in Table 7 of our article. The comparison is shown in the table below.