Amalgam vs. Composites

Dental Information

Indications for amalgam vs. composite

Most general practitioners continue to place amalgam restorations, and think amalgam is safe and poses no health risk to patients. They are, however, responding to widespread patient concerns and inquiries, according to recent survey results.

By Dr. Gary Unterbrink

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Profile: Dr. Gary Unterbrink http://www.dentalproducts.net/index.asp?c_page=/profiles/guest_authors/unterbrink.html
Dr. Unterbrink maintains a private practice in Triesen, Liechtenstein. Widely published in scientific journals and an internationally recognized lecturer on composite materials and adhesive systems, he was Director of Clinical Research for Ivoclar Vivadent for 7 years. He is a member of the International Association of Dental Research (IADR) and the Liechtenstein Dental Society.

Dear Dr. Unterbrink,

Each month in our practice we see a number of patients with fractured cusps. Many of these teeth had only Class I or small Class II amalgam restorations. Are these fractures related to the restorative material? Would a composite have been better? What is your opinion concerning the indications for amalgam vs. composite? - Dr. G.M. Warsaw, Poland

When evaluating the causes for failures, it is always difficult to separate material and technique factors. Your clinical observations are reflected in many in vitro studies concerning fracture resistance of the teeth with amalgam or bonded restorations. A clinical study concerning cusp fractures with endodontically treated teeth also showed significantly more fractures in the amalgam restoration group than in those teeth restored with bonded composites.1,2 So which properties of amalgam could contribute to cusp fractures? The two main factors are probably thermal effects and corrosion.

Thermal expansion and thermal diffusion

The coefficient of thermal expansion (CTE) of amalgam is similar to that of hybrid composites, which is approximately twice the CTE of human enamel and dentin. Another major difference is seen in the coefficient of thermal diffusion (CTD). Amalgam transmits temperature approximately 15 to 20 times faster than do composite resins.3

Considering the temperature and duration of thermal effects determined with "in vivo thermocycling" studies,4-7 it can be assumed that amalgam restorations clinically expand and contract considerably more than composite restorations. This cyclic dimensional change may contribute to the fatigue of the tooth structure and to cusp fractures.

Corrosion

Amalgam is not a stable material in the corrosive environment of the mouth; it continues to react for many years. Although the corrosion-prone Gamma-2 phase is reduced or eliminated in modern high-copper amalgams, the main Gamma-1 phase is itself a weakly bound compound. Slow but continuous phase shifts to Beta-1 with release of mercury occur, and in this sense, corrosion and dimensional changes never stop.8 It has been proposed that corrosion may produce additional lateral stress on cavity walls because by-products of corrosion occupy a larger volume.9

High-copper amalgams containing zinc seem to provide the best clinical performance. It is proposed that zinc helps protect the Gamma-1 phase.10 However, these amalgams also expand significantly if moisture contamination is present during placement.

The corrosion of amalgam restorations is considered to provide a "protective" function, that is, in essence, closing the microgaps. While the corrosion by-products and associated metal ion release probably does have some inhibitory effect on bacterial growth, a clinical study indicates a high incidence of bacterial penetration into dentin under apparently intact amalgam fillings.11 Bacterial microleakage under amalgam restorations also has been determined in vitro.12 In fact, amalgam restorations nearly always show 100% leakage and could be considered as an excellent negative control group in microleakage studies.13

Figure 1: Corrosion stress This premolar shows a fairly small Class I amalgam and a fracture of

Additional factors

Creep and flow: Amalgam permanently deforms under pressure. With larger amalgams, occlusal loading and flow could possibly create additional lateral stress at the buccal or lingual enamel.

Cusp deformation: The condensation of amalgam bends the remaining cusps outwards,14 and the setting reaction of filling-type amalgams produces a slight expansion, which may add additional stress.

Bonded Amalgams

It is known that caries and cavity preparations weaken teeth. Because it is well established that bonded restorations can restore at least some of the original fracture resistance, some studies have looked at the potential of bonding amalgam. We have three interfaces:

	Resin-dentin: The problem of dentin bond stability, as mentioned in an earlier column concerning composites, remains. Microleakage still occurs with bonded amalgam restorations and increases over time.15
	Resin-enamel: The resin bond to etched enamel is fairly stable, provided the preparation technique crosses the prisms.
	Resin-amalgam: The bond of the amalgam to the resin is primarily a mechanical intermixing of components and will, therefore, always be somewhat unpredictable. Placing amalgam restorations with a resin bonding agent could improve fracture resistance, but it is not surprising that investigations have shown mixed results.16-23

Indications for amalgam vs. composite

Class I: With Class I amalgams, one frequently sees vertical fractures at the proximal marginal ridges. An occlusal cavity is "non-compliant"; the cusps can't move unless something breaks. Even slight expansion of the restorative material can create high stresses. These fractures seem to correlate more with the width of the restoration than with the depth, but I see these fractures and subsequent proximal caries very frequently (Figs. 1-3). We do not see this problem with our composite restorations, many of which have been in function for more than 10 years.

I personally consider amalgam to be contraindicated for Class I restorations. Restoring Class I cavities with composites is a better choice. Isolation for the bonding procedure is simple; the time required for placement of a small Class I composite is not significantly different than for amalgam. A fast-setting radiopaque glass ionomer base, for example Ketac-Molar (3M ESPE) or Fuji IX (GC Europe), can be placed in deeper lesions to improve predictability by reducing the volume of composite.

Class II: It is easier to argue about the best material for Class II restorations. Initial small restorations with all margins in enamel are not difficult with composite resin. As the preparations become larger, in particular when proximal margins are in dentin and proximal contacts are open, the direct composite becomes challenging. Neither is a properly placed amalgam necessarily easy, but it is at least "easier".

Clinical studies, especially long-term studies, show somewhat higher failure rates with composites.24-31 In particular, the incidence of secondary caries tends to increase with composites after five to eight years.32, 33 Gaps created by the shrinkage of the resin is probably the main reason for this.

Additional considerations

The physical properties of amalgam (creep, flow, CTE/CTD, corrosion, etc.) contribute to the typical fracturing of the margins, generally known as "ditching". The marginal quality of amalgam restorations decreases more rapidly than direct composites.34 and then one of the major advantages of amalgam becomes apparent-marginal staining is invisible around a black filling! On a serious note, the esthetic disadvantages of amalgam are obvious, and this is the reason most of our patients request an alternative.

Diagnosis of secondary caries is more difficult with amalgam restorations than with composites. We have all seen the gray staining of adjacent dentin around older amalgam restorations, which makes visual evaluation more difficult. The extremely high radiopacity of amalgam also is a disadvantage; the diagnosis of secondary caries can be more accurate with composites.

Another controversy is the effect of mercury or other components of amalgam on human health. In my opinion, the available data indicate that amalgam should be avoided whenever possible; but at the same time, the possible risks with other direct restorative materials also are inadequately assessed. I would refer readers to an excellent literature review for more information.35

Conclusions

Although controlled studies are lacking, my greatest concern is the vertical enamel fractures seen with Class I amalgams. Let us assume that the fracture of the proximal enamel requires five years, and the development of caries along this crack an additional five years. The occlusal amalgam would be considered successful, and the subsequent Class II restoration "normal" by most dentists, i.e. due to primary proximal caries and not directly caused by the amalgam.

Amalgam has served dentistry well, and continues to do so. Overall, amalgam restorations demonstrate an excellent life expectancy and are considered to be the least technique-sensitive direct restoration. The often cited tolerance of amalgam in relation to technique may in part be explained by dental education. Dentists learn how to place amalgam correctly in school.

I wonder if this fact isn't the source of our problems with composites still today. Composite advertising frequently references similarities with amalgam techniques. However, a preparation for amalgam should be filled with amalgam, not composite! If moisture control is a problem, amalgam might not be good, but a composite will certainly be worse! A red flag should wave in your brain if the word "amalgam" appears in any advertisement for a composite material. nEDPR

Figure 2: Vertical Fractures Upon removal of a Class I amalgam due to sensitivity, vertical fractures are apparent in both marginal ridges and the lingual and buccal fissures. Note the propagation of these cracks into the dentin.

Figure 3: Horizontal Fractures Both lingual cusps demonstrate horizontal fractures at the base of the cavity. Since these cusps were not sensitive to pressure pre-operatively, they were maintained.

References

1. Hansen EK. In vivo cusp fracture of endodontically treated premolars restored with MOD amalgam or MOD resin fillings. Dent Mater 1988;5:169-73.

2. Hansen EK, Asmussen E. Cusp fracture of endodontically treated posterior teeth restored with amalgam. Teeth restored in Denmark before 1975 versus after 1979. Acta Odontol Scand 1993;51:73-7.

3. Restorative Dental Materials. 7th Edition. C.V.Mosby Company 1985. Editor RG Craig. 45-52.

4. Kappert HF, Schueren M, Fritsch F. Dauerfestigkeitsprüfung des Metall-Kunststoff-Verbundes unter dem Einfluss von Temperaturwechseln. Quintessenz Zahntech 1996;22(3):293-307.

5. Ernst C-F, Euler T, Willershausen B. Approximal temperature increase and decrease during thermocycling in vivo. J Dent Res 1997; Special Issue, Abstr 231:42.

6. Youngson CC, Glyn Jones JC, Smith IS, Fox K. In vivo temperature changes during a standarized thermal challenge. J Dent Res 1998; Special Issue, Abstr 2585;955.

7. Rossomando KJ, Wendt, SL. Thermocycling and dwell times in microleakage evaluation for bonded restorations. Dent Mater 1995:11:47-51.

8. Marek M. The effect of tin in the Ag-Hg phase of dental amalgam on dissolution of mercury. Dent Mat 1997;13:353-359.

9. Malmstrom C. Oral Miljöforurening. Tidsskrift för tandlaeger 1988:2:53-59.

10. Letzel H, Van 't Hof MA, Marshall GW, Marshall SJ. The Influence of the Amalgam Alloy on the Survival of Amalgam Restorations: A Secondary Analysis of Multiple Controlled Clinical Trials. J Dent Res 1997;76(11):1787-1798.

11. Boston DW, Graver HT. Histobacteriological analysis of acid red-dye stainable dentin found beneath intact amalgam restorations. Operative Dentistry. 19(2):65-9, 1994.

12. Zivkovic S, Bojovic S, Pavlica D. Bacterial penetration of restored cavities. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2001;91:353-358.

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22. Cannon ML, Tykla JA, Sandrick J. Three Year Clinical Study of Adhesive and Conventional Amalgam Restorations. J Dent Res 1999 (Special Issue);Abst 995:230.

23. Browning WD, Johnson WW, Gregory PN. Postoperative Pain following Bonded Amalgam Restorations. Operative Dent 1997;22:66-71.

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