Introduction To Amalgam Restorations PDF

INTRODUCTION TO AMALGAM RESTORATIONS (Sturdevant, ch. 19, pg. 339-358). DENTAL AMALGAM • Alloy made by mixing mercury (Hg)with silver tin dental amalgam alloy (Ag-Sn) • Amalgam alloy is a silver-tin alloy to which varying amounts of copper (Cu) and Zinc (Zn) are added. • alloy particles mixed wi...

INTRODUCTION TO AMALGAM RESTORATIONS (Sturdevant, ch. 19, pg. 339-358). DENTAL AMALGAM • Alloy made by mixing mercury (Hg)with silver tin dental amalgam alloy (Ag-Sn) • Amalgam alloy is a silver-tin alloy to which varying amounts of copper (Cu) and Zinc (Zn) are added. • alloy particles mixed with mercury that produce a matrix of mercury reaction products surrounding residual dental amalgam alloy particles BEFORE REACTION Alloy Mercury AFTER REACTION Alloy Reaction Products Classification I) According to the cupper content: • A) Low-Copper Amalgam • B) High-Copper Amalgam: Silver (Ag) Tin (Sn) Copper (Cu) Zinc (Zn) Low Cu 70% 25% 4% 1% High Cu 60% 27% 13% 0% II) ACCORDING TO PARTICLE GEOMETRY: • Alloy is produced predominantly as irregular-shaped particles, spherical particles, or mixture of the two types A) Spherical Amalgam B) Lathe-cut particles Amalgam C) Admixed Amalgam • Irregular particles are generally “lathe-cut,” meaning that the ingot of alloy material is pulverized by filing it to pieces on a lathe • Spherical particles are created by atomizing or spraying liquid dental amalgam alloy through a nozzle into a vertical column of cooled nitrogen gas. This solidifies the spheres and protects them from oxidation during the cooling steps • Melting / Casting / Comminution  IRREGULAR Particles • Melting / Spray Atomization  SPHERICAL PARTICLES III) ACCORDING TO ZINC CONTENT • Zn was usually added to conventional amalgam s as a processing aid to suppress oxidation during manufacturing. • A side effect of the residual zinc was that moisture contamination before setting convert the zinc to zinc oxide and produce hydrogen gas that could expand the amalgam Zn + H2O ZnO + H2 A. Zinc Free Amalgam B. Zinc Containing Amalgam: usually less than 1% MERCURY/ ALLOY RATIO • The principal considerations for any amalgam are the amount of mercury in the final restoration and the type of reactions products formed • Originally, dental amalgams had relatively high Hg/alloy ratios • During the 1900s, the excess was removed by squeezing the mass with a squeeze cloth before placement and by compressing the placed mass within the cavity preparation. This reduced the final Hg/alloy ratio well below 50:50 by weight. Compressing (condensing) extrudes excess matrix from the setting mass and improves both strength and corrosion resistance. • Most contemporary mixtures (pre-capsulated) are now made with 45-55% initial mercury and end up with about 42% after the reaction and condensation steps are complete. • Proportioned alloy and mercury in prepackaged capsulres (Precapsulated) for mixing amalgam constituents using an amalgamtor • Mercury and powder separad by septum that must be perforated before mixing SETTING REACTION Ag: Silver Sn: Tin Cu: Copper Primary reactant Creates solubility, fluidity Reacts with Sn ( Increase strength and hardness) Zn: Zinc Alloy processing aid (oxygen scavenger ,Oxidation) Hg: Mercury Reactant with Ag (and Sn) Copper: • Improves the strength • Increases hardness of mix • Improves resistance to corrosion Silver: • Gives amalgam strength • Increases set expansion Tin: • Makes the mixture more workable and decreases the expansion Low-Copper Dental Amalgam: Ag3Sn gamma g + Hg  + Hg  Ag3Sn gamma g + + Ag2Hg3 + gamma-1 g 1 + Sn7-8Hg gamma-2 g-2 High-Copper Dental Amalgam Ag3Sn+ Hg  Ag3Sn g g + Ag2Hg3 + g -1 Sn7-8Hg SnHg (gamma-2) + Cu  CuSn + Ag2Hg3 Cupper-Tin replace Tin-Mercury Phase g -2 GAMMA – TWO PHASE (TIN-MERCURY) • Although (Sn-Hg) is less than 10% of the final composition, it is highly prone to corrosion (surface and subsurface deterioration). • In high copper amalgam, the (gamma – 2) phase is minimized or eliminated when excess copper preferentially reacts with the Sn forming Cu-Sn. • The resulting copper reaction (cupper-tin) Cu-Sn is much less corrosion prone than tin-mercury phase they replace. • Sn-Hg (g-2) crystals are elongated and look like blades. While the Sn-Hg crystals represent relatively little total volume of the mixtures, they tend to be touching (connected) throughout the entire dental amalgam (allowing “penetrating corrosion” to take place • Penetrating corrosion generates a porous and spongy amalgam with minimal mechanical resistance. LOW-CUPPER AMALGAM + Hg  + g + g1 g2 Penetrating Corrosion High Cu Amalgam + Hg  g 60 Ag 27 Sn 13 Cu 0 Zn DISPERSALLOY + g + CuSn g1 no Sn-Hg (gamma2) remains in the set amalgam as shown. DIMENSIONAL CHANGES • 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 EXP (+) ± 20 mm CONT (--) TIME Dimensional changes on depend on reaction variables: Particle size, Hg/alloy ratio, trituration time, condensation, ... • Dissolving the amalgam alloy into Hg and formation of Ag-Hg crystals leads to a slight decrease in volume (contraction). Most of this occurs during the mixing process. • Growing crystals push on each other and create expansion. Most of this occurs during placement and condensation. • Depending on the time of condensation and extent of reaction, the volume change may vary anywhere from -20 to +20 microns per cm over the setting reaction. Over long periods of time (weeks to a month) the reaction will slowly continue and can produce small amounts of solid state creep as well. Physical Properties 1. Thermal conductivity = [High] 2. Electrical conductivity = [High] 3. Coefficient of thermal expansion = (tooth :10 ppm) 25 ppm/ºC MECHANIC AL PROPERTIES • High Compressive Strength : range from • • • • (380 to 550 MPa Dental enamel has a compressive strength of 400 MPa. Low tensile Strength : 45 MPa (brittle) Amalgams that have inadequate bulk to distribute stress may fracture. At margins, where amalgam margins are less than 90 degree, marginal fracture could occur. High wear Resistance STRENGTH: • Resistance to compression forces is the most favorable strength characteristic of amalgam. • Amalgam cannot withstand high tensile or bending stresses. • Mercury content ( Hg Strength) Excess Hg left  reduction in strength • Trituration • Either undertrituration or overtrituration effects the strength • Overtrituration  increases strengths in lathe-cut alloys • Both over- and undertrituration  decrease strengths in spherical alloys and admixed high-Cu alloy Trituration • Process of mixing the silver alloy with the mercury ― ― Mechanical device (Amalgamator) Proper trituration is essential for clinical success • Undertrituration ― ― ― Dull, crumbly appearance Insufficient mass formation Poor compressive and tensile strength • Overtrituration ― ― ― Shiny and soupy, adhere to the inside of the capsule Overformation of the matrix Poor strength and creep, poor corrosion properties Under Proper Over • Amalgam strength decreased when: 1. There is excess mercury in the mix 2. corrosion has occurred 3. inadequate bulk 4. improper manipulation PROPERTIES OF AMALGAM II) CHEMICAL PROPERTIES: CORROSION • Dental amalgam restorations undergo both chemical and electrochemical corrosion • Chemical corrosion (Tarnish): occurs most notably on the occlusal surface and produces a black Ag-S tarnish film. This reaction is limited to the surface and does not compromise any properties, except for esthetics (Discoloration). CHEMICAL CORROSION: • Electrochemical corrosion: Occurs whenever chemically different sites act as an anode and cathode. This required that the sites be connected by an electrical circuit in the presence of an electrolytes (saliva). The anode corrodes, producing soluble and insoluble reaction products. If an amalgam is in direct contact with an adjacent metallic restoration such as a gold crown, the amalgam is the anode in the circuit. This type of electrochemical corrosion is called galvanic corrosion • During electrochemical corrosion of low-copper amalgams, the Sn-Hg phase is oxidized into Sn-O. Sn-O helps seal the space against the micro-leakage • Electrochemical corrosion of Sn-Hg does not appear to release free mercury into the oral cavity CREEP • Amalgam creep is plastic deformation principally due to very slow metallurgic phase transformations and produce volume increases. The associated expansion makes the amalgam protrude from the tooth preparation. Such secondary expansion can occur throughout the clinical life of a restoration. • On non-occlusal surfaces, the entire amalgam restoration may appear extruded, and this can produce unwanted esthetic problems or over hangs in some areas. On occlusal surfaces, abrasion and attrition tend to limit the overall extrusion CREEP OR MERCUROSCOPIC EXPANSION • Over long terms there is a slow continuation of the amalgam reaction that produces expansion and causes creep. Since corrosion may be occurring at the same time, as Sn is consumed, local Hg becomes available that reacts with residual Ag-Sn particles and produce more Ag-Hg. • This leads to further expansion (and creep) – and extrudes the amalgam margins above the normal anatomic contours allowing local stresses to fracture the margins • Marginal fracture of amalgam is known as “Ditching” Biological Properties Mercury Toxicity: The mercury content in amalgam restorations is much lower than the toxic levels of mercury Mercury Hypersensitivity: A very small number of people (< 1 in 100,000,000 patients) may be allergic to Hg. The reactions are low level (not life threatening). • Biological effect on the pulp: • Biocompatible but pulp irritation comes from: - Electrical conductivity - Thermal conductivity - Micro-leakage - Masticatory and condensation forces in deep cavities. So, pulp should be protected under amalgam and the dentinal tubules should be sealed OTHER PROPERTIES • Optical: Non-esthetic • Adhesion: Non-adhesive but amalgam bonding systems are available. • Micro-leakage: it requires initial sealing by either varnish or bonding agents till corrosion products Sn-O seals the restoration/cavity interface. MERCURY CONTROVERSY • Mercury has the potential to be hazardous if not managed properly • In amalgam restoration, Once the reaction is complete, only extremely minute levels of mercury can be released, and those are far below the current health standard. • All published research demonstrates clearly that there is no cause-and-effect relationship between amalgam restorations and other health problems. • Fears of amalgam are not a basis for amalgam removal • The health risk from amalgam use is clearly greater for members of the dental office team that for a patient. SOURCES OF MERCURY IN THE CLINIC: 1. Amalgam raw materials being stored for use 2. Mixed but unhardened amalgam during trituration, insertion, and intra-oral hardening 3. Amalgam scrap 4. Amalgam undergoing finishing and polishing 5. Amalgam restorations being removed • Storage locations should be near a vent that exhausts air out of the building. • In set dental amalgam, the Hg which was mixed with alloy resides only in the matrix phase (predominantly Ag-Hg).. The matrix melts at 127 C and so just above the boiling point of water, you can melting dental amalgam and release Hg from the matrix phase (e.g. polishing, amalgam removal) • Rubber dam, high volume evacuation and water cooling is essential during amalgam polishing or removal • Masks are not able to filter mercury vapor from the air. Amalgam Melting Piont = 127 C • Once Hg is released as vapor it can be breathed and absorbed into your system... • TLV: Threshold limit value for exposure to mercury The level is extremely small (5 ppb) but still it is relatively easy to stay 3 below the vapor for a 40-h work week = 50 mg/m level • A. B. C. Hg exists in three basic forms: Elemental (pure Hg) Inorganic: found in nature e.g. HgS in mines Organic mercury (primarily as dimethyl mercury for its use in farming as a fungicide and pesticide). It absorbed by many organisms and concentrated in food . SOURCES OF MERCURY IN NATURE: • Volcanic eruptions • This vapor gradually is deposited in the world’s oceans. • Concentrated in large fish e.g. swordfish and tuna (top of the ocean food chain). Within them, the concentration of mercury is 1000 mg/kg of mass. • Fungicides and herbicides to coat seeds used to plant farm fields • Industrial Pollution: e.g coal burning, Minamata Bay incident in Japan 1952 • Mercury Mine Mercury mine workers 300mg/m3  SYMPTOMS of chronic Hg poisoning: 1. Ataxic gait. 2. Convulsions. 3. Numbness in mouth and limbs. 4. Constriction of visual field. 5. Difficulty in speaking. • Absorption into the human body occurs via: A. The skin, B. Lungs, and/or C. Gastrointestinal tract. • As a vapor, metallic mercury can be inhaled and absorbed through the alveoli in the lungs at 80% efficiency. This is clearly the major route of entry into the human body. Metallic mercury is poorly absorbed through the skin or via the GIT. • Organic mercury is not practically absorbed in the lungs but is readily absorbed in the GI tract. • Mercury does not collect irreversibly in human tissues. • On average, the half-life Hg circulating in the body is 55 days. • Thus, mercury that came into the body years ago, is not longer present in the body. DENTAL MERCURY HYGIENE 1.Be awareRECOMMENDATIONS of the potential sources of mercury vapor 2.Because mercury vaporizes at room temperature, work in wellventilated spaces with fresh air exchanges and outside exhaust 3.Periodically check the dental operatory atmosphere for mercury vapor 4.Floor covering should be non absorbent 5.Use only pre-capsulated alloys 6.Avoid skin contact 7. Use high-volume evacuation when finishing or removing amalgam 8. Store all scrap amalgam (i.e. amalgam remaining after a procedure) in tightly closed container containing radiographic fixer solution. Recycle amalgam scrap 9. Do not place mercury-contaminated materials in medical waste bags because these will be burned and mercury will be vaporized.