Dental amalgam properties
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Microleakage, Corrosion, Dimensional changes, thermal and biological Properties of dental Amalgam have been discussed
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Dental amalgam properties
- 1. 20/04/2020 1 Dental Amalgam Properties Professor Dr Tahir Ali Khan
- 2. Objective At the end the student will be able to know The properties of amalgam 20/04/2020 2
- 3. 20/04/2020 3
- 4. 20/04/2020 4 Material-Related Variables Leakage Dimensional change Strength Corrosion Creep Thermal Properties Biological Properties
- 5. MICRO or Nano LEAKAGE OCCURS DUE TO PENETRATION OF FLUIDS OR DEBRIS AROUND THE MARGINS THAT CAN LEAD TO RECUURENT/SECONDARY CARIES AMALGAM HAS GOT A SELF SEALING PROPERTY – CORROSION PRODUCT WILL FILL THE TOOTH RESTORATION INTERFACE & PREVENT LEAKAGE 20/04/2020 5
- 6. 20/04/2020 6 MODERN AMALGAM ALWAYS SHOWS CONTRACTION OLDER AMALGAM SHOWS EXPANSION DIMENSIONAL CHANGES CONTRACTION EXPANSION
- 7. 20/04/2020 7 Dimensional Changes Contraction or expansion depends on Its manipulation Ideally it should be small Severe contraction can lead to Micro leakage, plaque accumulation and recurrent/secondary caries
- 8. Dimensional Changes Excessive expansion Pressure on the pulp and post-operative sensitivity, and protrusion of a restoration ADA/ANSI Specification No.1 requires ± 20µm/cm measured at 37Cº b/w 5 min and 24 hr after beginning of Trituration with a device that is accurate to at least 0.5µm 20/04/2020 8
- 9. 20/04/2020 9 Mixing of Alloy and Mercury Results in Contraction The dissolution of particles begins and the Gamma 1 grows Contraction continues as long as the growth of Gamma 1 phase continues As gamma 1 crystal grow, they impinge against one another If condition appropriate, this impingement of Gamma 1 can produce an outward pressure, tending to oppose the contraction
- 10. 20/04/2020 10 Plastic matrix in case of sufficient mercury Gamma 1 crystals impingement causes expansion Once the matrix rigid matrix, growth of gamma 1 crystals cannot force the matrix to expand If sufficient mercury is present, net expansion will occur
- 11. 20/04/2020 11 Net contraction Type of alloy used spherical alloys have more contraction Less mercury Zinc (Delayed Expansion) Zn+H2O → Zno+H2 Hydrogen causes considerable delayed expansion (40 micrometer, during trituration and condensation process) Condensation technique Greater condensation = higher contraction Trituration time Over trituration causes higher contraction Phillip’s Science of Dental Materials 2003
- 12. 20/04/2020 12 Strength Develops slowly • 1 hr: 40 to 60% of maximum (50MPa) • 24 hrs: 90% of maximum (300MPa) Spherical alloys and Cu enriched alloy develop strength more rapidly than conventional lathe cut alloy Fine grain lathe cute alloys develop more rapidly than coarse grain prodcts • Require less mercury (Optimum Properties = 44 – 48% Hg) Phillip’s Science of Dental Materials 2003
- 13. 20/04/2020 13 Greater the condensation pressure the higher the strength (lathe cut) Condensation with lighter pressure produces adequate strength (spherical alloy) Higher compressive vs. tensile strength
- 14. 20/04/2020 14 AMALGAM IS SRONGEST IN COMPRESSION & MUCH WEAKER IN TENSION & SHEAR THE PREPARD CAVITY DESIGN SHOULD MAXIMIZE THE COMPRESSION FORCES IN SERVICE & MINIMIZE TENSION SHEAR FORCES
- 15. 20/04/2020 15 Weak in thin sections • Unsupported edges fracture • Voids and porosity are possible factors influencing the compressive strength • In case of delayed condensation and under tritutation The porosity will more and strength will be reduced
- 16. 20/04/2020 16 Property Enamel Dentine Amalgam Modulus of elasticity GPa 50 12 30 Compressive strength at 7 day 250 280 350 Tensile strength at 7 day 35 40 – 260 60 Vickers Hardness 350 60 100
- 17. 20/04/2020 17 Hardness of amalgam is lower than enamel Surface hardness is 3 times lower than enamel Appears to have adequate resistance to intra-oral abrasion and rarely fails by this mechanism
- 18. 20/04/2020 18 Corrosion Should not be confused with tarnish. Reduces strength Seals margins low copper 6 months SnO2, SnCl gamma-2 phase high copper 6 - 24 months SnO2 , SnCl, CuCl eta-phase (Cu6Sn5)
- 19. 20/04/2020 19 Why Dental amalgam behaves as a self sealing restoration because SnO helps to seal the space against microleakage (Along the margins) Electrochemical corrosion of high-copper amalgams produce both Cu & Sn oxides & oxychloride
- 20. 20/04/2020 20 The most electrochemically reactive is 2 phase, readily forms the anode in an electrolytic cell The 2 phase breaks down Hg which may be able to combine with unreacted alloy ( phase). Small quantity is inevitably ingested which is of concern
- 21. 20/04/2020 21 Creep or plastic deformation DEFINED AS A TIME DEPENDENT PLASTIC DEFORMATION UNDER CONSTANT STRESS. ACCORDING TO ADA SPECIFICATION NO 1 CREEP SHOULD BE BELOW 3%. CREEP OF LOW Cu AMALGAM IS 0.8-8% & HIGH Cu IS 0.4- 1%.
- 22. 20/04/2020 22 Amalgam undergoes a certain amount of creep when subjected to dynamic intra-oral stresses Creep causes the amalgam to flow, such that unsupported amalgam protrudes from the margin of the cavity These unsupported edges are weak and may be further weakened by corrosion
- 23. 20/04/2020 23 Fracture cause the formation of ditch around the margin of amalgam restorations Gamma 2 phase in case of low copper alloys is responsible for the relatively high values of creep High copper alloys got low creep values
- 24. 20/04/2020 24 Material type Creep % Convention amalgam alloy 2.5 Admixed 0.2 Copper enriched 0.6 containing 0.5 % palladium
- 25. 20/04/2020 25 Creep High-copper amalgams have creep resistance prevention of gamma-2 phase requires >12% Cu total single composition spherical eta (Cu6Sn5) embedded in gamma-1 grains interlock admixture eta (Cu6Sn5) around Ag-Cu particles improves bonding to gamma 1
- 26. 20/04/2020 26 Thermal Properties Relatively high value of thermal diffusivity. A cavity liner or base is recommended to be used under amalgam restoration. The coefficient of thermal expansion is 3 times greater than that for dentine
- 27. 20/04/2020 27 Biological Properties 2 aspects: on Patient and on Dentist & Auxiliary staff. Neurotoxicity Renal dysfunction Birth defects Reduce immunocompitence Compromised general health.
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Editor's Notes
- Excessive expansion can produce
- Mixing of Alloy and Mercury Results in Contraction
- If sufficient mercury present to provide a plastic matrix, Expansion will occur when gamma 1 crystals impinge upon one another After a rigid matrix formation, growth of gamma 1 crystals cannot force the matrix to expand If sufficient mercury is present, expansion will occur otherwise contraction will occur
- 3-5 days then for maximum upto 6 months, 40micrometer delayed expansion. This is due to moisture contamination during trituration and condensation process.
- (Amalgam readily tarnish due to the formation of sulphide layer on surface) TARNISH & CORROSION Undergoes Chemical & Electrochemical corrosion Chemical corrosion results in formation of surface Silver sulfide layer Electrochemical corrosion – Galvanic, Crevice & Stress Corrosion The most common corrosion products of traditional amalgam alloys are oxides & oxychlorides of tin
- The 2 phase of conventional amalgam is the most electrochemically reactive and readily forms the anode in an electrolytic cell. The 2 phase breaks down to give tin containing corrosion products and Hg which may be able to combine with unreacted alloy ( phase). Not all Hg is formed during corrosion is able to combine rapidly with unreacted alloy and small quantity is inevitably ingested which is of concern.