Amalgam in Dentistry: Composition, Advantages, and Disadvantages

1. e g1 Ag-Cu Dental Amalgam g2 h Hg g Prepared by: Dental Materials Department Yenepoya Dental College Yenepoya University, Mangalore. Ag-Sn

2. Objectives • Know the advantages and disadvantages of amalgam • Know the classification and understand the amalgamation process of each type of amalgam • Know the corrosion process of amalgam • Know how to control the quality of amalgam • Know the recommendations in mercury hygiene Ref: Phillips' Science of Dental Materials by Anusavice, K. J. (Editor), 10th edition

3. Definitions • Amalgam = an alloy of mercury and one or more other metals • Dental amalgam is produced by mixing liquid mercury with solid particles of an alloy of silver, tin, copper, and sometimes zinc, palladium, indium and selenium.

4. Trituration • Amalgam alloy is mixed with mercury. “Amalgamation Process” • Mercury dissolves the surface of alloy particles  a composite plastic mass (some new phases form). • Setting and hardening occur as the liquid mercury is consumed in the formation of new solid phases.

5. Dental Caries 1 2 4 3

6. 6 5 7 8

7. Use • Direct, permanent, posterior restorations • Large foundation restorations • Cores for crown or fixed partial denture restorations

8. Advantages • Easy to insert • Not overly technique sensitive • Maintain anatomical form • Have adequate resistance to fracture • Prevent marginal leakage after a period of time in the mouth • Can be used in stress bearing areas • Have a relatively long service life

9. Disadvantages • Silver color does not match tooth structure • Somewhat brittle • Subject to corrosion and galvanic action • May demonstrate a degree of marginal breakdown • Do not help retain weakened tooth structure • Regulatory concerns regarding amalgam being disposed in the wastewater

10. Fracture

11. Amalgam Alloy Composition • ANSI/ADA Spec. #1 (ISO 1559) requires that amalgam alloys be predominantly silver (Ag) and tin (Sn). • containing Zn > 0.01%  “Zinc Containing” • containing Zn < 0.01%  “nonzinc”

12. Classification • Low-copper (Conventional) amalgam alloy (at least 65 wt% Ag, 29 wt% Sn, < 6 wt% Cu) • lathe-cut (irregular) powder or spherical particles or mixed • Ag-Sn • High-copper amalgam alloy (6-60 wt% Cu) • (1) Admixed (Ag-Sn + Ag-Cu) • A mixture of irregular and spherical particles of different or same composition • (2) Unicomposition or Single composition (Ag-Sn-Cu) • All spherical particles  90% of the dental amalgams currently placed

13. Admixed amalgams containing 10% to 15% indium (In) in the mercury • Newly developed • Decreases the amount of Hg needed  decreases the Hg vapor during and after setting, increases wetting • Low creep and lower early-compressive strengths, but higher final strengths

14. Composition

15. Lathe-cut v.s. Spherical Irregular or Lathe-cut Spherical

16. Symbols of Phases

17. Silver-Tin (Ag-Sn) System (1) • Most commercial alloys fall within the limited composition range of B to C. • (b + g) and g • If Sn > 26.8 wt%  g + Sn-rich phase is formed.  Sn-Hg phase (g2) in the final product which lacks corrosion resistance and is the weakest component.

18. Silver-Tin (Ag-Sn) System (2) • 4 wt% to 5 wt% Cu is substituted for Ag. • Hardens and strengthens the Ag-Sn alloy • Zinc = Deoxidizer: a scavenger (O2) during melting  formation of oxides • Can cause an abnormal expansion if the amalgam is condensed in the presence of moisture.

19. Manufacture of Alloy Powder Lathe-cut (Irregular) powder Atomized (Spherical) powder Particle size v.s. Properties

20. Lathe-cut (Irregular) Powder • Metal ingredients heated poured into a mold  ingot (Ag3Sn (g) + someb, e, h) • Ingot  homogenizing anneal • An annealed ingot of alloy is place in a machine and is fed into a cutting tool. • 60-120 µm in length, 10-70 µm in width • Some aging of the alloy is desirable.

21. Atomized (Spherical) Powder • Made by melting the desired elements together • The liquid metal is atomized into fine spherical droplets of metal by being sprayed under high pressure of an inert gas. •  “Spherical powders” 2- 43 µm • Also have heat treatment and are usually washed with acid.

22. Homogenizing Anneal • The ingot is placed in an oven and heated at a temperature below the solidus (at 400°C) for sufficient time (6-8 hours) to allow diffusion of the atoms to occur and the phases to reach equilibrium.

23. Particle Treatments • Acid-washed amalgam powders • tend to be more reactive. • Stress-relief process after particle cutting • annealing cycle at ~ 100°C for several hours • “aging process”

24. Lathe-cut v.s. Atomized alloys • Lathe-cut or admixed powders resist condensation better than spherical powders. • Spherical alloys require less mercury than typical lathe-cut alloys because of the smaller surface area per volume. • Amalgams with a low mercury content generally have better properties.

25. Particle Size • A powder containing tiny particles requires a greater amount of mercury to form an acceptable amalgam. • A small-to-average particle size  a more rapid hardening and a greater early strength • Particle size distribution can affect the character of the finished surface. • The larger particles may be pulled out during carving  a rough surface  corrosion

26. Amalgamation Process “amalgamator” Low-copper alloys High-copper alloys admixed alloys single composition alloys

27. Low-copper Alloys (1) • When a powder (Ag3Sn, g) is triturated, the Ag and Sn in the outer portion of the particles dissolve into Hg (mercury). • Hg also diffuses into the alloy particles. • Solubility for Ag = 0.035 wt%, for Sn = 0.6 wt%

28. As the remaining mercury dissolves the alloy particles, g1 and g2 crystals grow. Low-copper Alloys (2) • When the solubility is exceeded, crystals of two binary metallic compounds precipitate into the mercury. • Ag2Hg3 compound (g1) precipitates first. • Sn7-8Hg compound (g2) precipitates later. trituration condensable, carvable

29. Unconsumed particles (smaller after being partly dissolved) are surrounded and bound together by solid g1 and g2 phases. Low-copper Alloys (3) • As the mercury disappears, the amalgam hardens. Particles become covered with newly formed crystals, mostly g1. trituration condensable, carvable harden, no longer workable

30. P = b and g Ag-Sn E = e (Cu3Sn) G1 = g1 (Ag2Hg3) G2 = g2 (Sn7-8Hg) Low-copper Alloys (4) • A typical low-copper amalgam is a composite in which the unconsumed particles are embedded in g1 and g2 phases.

31. Low-copper Alloys (5) • In summary, Alloy particles (b + g) + Hg  g1 + g2 + unconsumed alloy particles (b + g) • Physical properties • g-phase  strongest, g2 phase  weakest • Hardness: g > g1 >>> g2 • g2  poor corrosion resistance

32. High-Cu: Admixed Alloys (1) • Spherical silver-copper (Ag-Cu) eutectic alloy particles are added to lathe-cut low-copper amalgam alloy particles (Ag-Sn or g +b). • The final powder is composed of two kinds of particles.  “admixed” • Ag-Cu particles act as strong fillers, strengthening the amalgam matrix.

33. High-Cu: Admixed Alloys (2) • Silver-Copper eutectic alloy • 71.9 wt% Ag, 28.1 wt% Cu Temperature Eutectic composition

34. High-Cu: Admixed Alloys (3) • Ag dissolves into the Hg from the Ag-Cu alloy particles. • Both Ag and Sn dissolve into Hg from the Ag-Sn alloy particles. (same as in low-Cu alloy) • Sn in solution diffuses to the surface of Ag-Cu alloy particles and reacts with the Cu to form the h phase (Cu6Sn5) (therefore, the Sn7-8Hg or g2 is eliminated)

35. High-Cu: Admixed Alloys (4) • A layer of h forms around unconsumed Ag-Cu particles. • g1 phase is the matrix. • The final structure composes of the g phase, Ag-Cu particles, e particles, g1 matrix, and h reaction layers.

36. High-Cu: Admixed Alloys (5) • In summary: Alloy particles (b + g) + Ag-Cu eutectic + Hg  g1 + h + unconsumed alloy of both types of particles • g2 has been eliminated in this reaction, being replaced by h. • The effectiveness in eliminating g2 depends on % of copper-containing particles. (net copper concentration of > 12% in alloy powder)

37. High-Cu: Single-composition Alloys (1) • Each particle has the same chemical composition. • Major components: Ag-Cu-Sn

38. g1 h P High-Cu: Single-composition Alloys (2) • Phases found in each single-composition alloy particle are b (Ag-Sn), g (Ag3Sn), and e (Cu3Sn). • h crystals are found as meshes of rod crystals at the surfaces of alloy particles (P), as well as dispersed in the matrix.

39. High-Cu: Single-composition Alloys (3) • In summary: Alloy particles (Ag-Sn-Cu) + Hg  g1 + h + unconsumed alloy particles • Little or none of g2 phase can form.

40. Properties of Amalgam Dimensional Change Corrosion Strength Creep

41. Dimensional Change ADA spec. #1: amalgam neither contract nor expand more than 20 µm/cm between 5 min and 24 hours after beginning of trituration.

42. Mechanism • Contraction results as the particles dissolve and the g1 grows. • If there is sufficient liquid mercury present to provide a plastic matrix, expansion will occur when g1 crystals impinge against one another. High-Cu, admixed High-Cu, single comp. Low-Cu

43. Other factors (1) • Lower Hg:alloy ratios and higher condensation pressures  less Hg in the mix  contraction • Longer trituration times, used of smaller particle size alloys  accelerate setting  contraction • Modern amalgams  contraction • Hg:alloy ratio • Speed of trituration

44. Other factors (2) • Zinc-containing amalgam contaminated by moisture during trituration or condensation  delayed or secondary expansion. • Zn  water  H2 collected within the restoration  creep

45. Dimensional Change v.s. Clinical Success • “?” • Contraction of amalgam  marginal leakage • If amalgam expanded during hardening, leakage around the margins of restorations would be eliminated. • Evidently the detrimental effect of shrinkage occurs only when the amalgam mass shrinks > 50 µm. (ADA allows 20 µm/cm shrinkage)

47. Corrosion

48. Corrosion (1) • Order of corrosion resistance (in pure phases) • Ag2Hg3 (g1) > Ag3Sn (g) > Ag3Cu2 > Cu3Sn (e) > Cu6Sn5 () > Sn7-8Hg (g2) • In the low-Cu amalgam, the most corrodible phase is the Sn7-8Hg (g2) (11-13% of amalgam mass). • Corrosion  liberated Hg + Tin oxide or Tin Chloride •  porosity and lower strength

49. Corrosion (2) • High Hg:alloy ratio  formation of g2 phase  promoting corrosion • Gold v.s. amalgam restorations • Galvanic corrosion • Free mercury can weaken the gold restoration.

50. Corrosion (3) • The space between the alloy and the tooth permits the microleakage of electrolyte, and a classic concentration cell process results.