dental amalgam
TRANSCRIPT
Presentation by:Dr. Piyush VermaDept of Pedodontics & Preventive Dentistry
Index Introduction
History
Classification
Indications & contraindications
Advantages/disadvantages
Composition of amalgam & Amalgamation reactions
Manufacturing process
Properties of amalgam
Manipulation of amalgam
Index Mercury toxicity & various health hazards
Recent advances
Repair of amalgam restorations
Clinical considerations
Amalgam wars
Conclusion
Introduction Dental amalgam is an alloy made by mixing mercury
with a silver tin alloy. Dental amalgam alloy is a silver tin alloy to which varying amount of copper and small amount of zinc has been added.
According to Skinner’s, amalgam is a special type of alloy in which one of its constituent is mercury. In dentistry, it is common to use the term amalgam to mean dental amalgam.
History Amalgam -- First used by Chinese. There is a mention
of silver mercury paste by Sukung (659AD)in the Chinese medic
1578-lshitichen used 100 parts if Hg, 45 parts of Ag and 100 parts of Sn
Liu Wen-Thai (1508) and Li Shih-Chen (1578)discussed its formulation; 100 parts of mercury to 45 parts of silver and 900 parts of tin, trituration of these ingredients produced a paste said to be as solid as silver.
.
Introduced in 1800’s in France alloy of bismuth, lead, tin and mercury plasticized at 100ºC poured directly into cavity
1819, Bell advocated the use of a room temperature mixed amalgam as a restorative material, in England
1826, M.Traveau is credited with advocating the first form of amalgam paste , in France.
1833
Crawcour brothers introducedamalgam to US
powdered silver coins mixed with mercury
expanded on setting
1895
To overcome expansion problems
G.V. Black developed a formula for modern amalgam alloy
67% silver, 27% tin, 5% copper, 1% zinc
Black’s formula was well accepted and not much changed for nearly sixty years.(1890-1963)
1946 - Skinner, added copper to the amalgam alloy composition in a small amount. This served to increase strength and decrease flow.
Traditional or conventional amalgam alloys predominated from 1900 to 1970.
1960’s - conventional low-copper lathe-cut alloy was introduced
1962 - A spherical particle dental alloy was introduced, by Demaree and Taylor
The work of Innes and Youdeis (1963) has led to the development of high copper alloys.
Had longer working time, less dimensional change, easy to finish, set faster, low residual mercury, low creep & higher early strength
Added spherical silver copper eutectic alloy(71.9wt% Ag and 28.1wt%Cu)particles to lathe cut low copper amalgam alloy particles.
These alloys are called admixed alloys
1971 – Johnson designed a spherical particle alloy having the composition 64% Ag, 26% Sn and 10% cu by weight, and exhibiting no Sn8Hg after amalgamation.
1973 - first single composition spherical alloy named Tytin (Kerr) a ternary system (silver/tin/copper) was discovered by Kamal Asgarof the University of Michigan
1980’s alloys similar to Dispersalloy and Tytin was introduced
Classification (Marzouk)I. According to number of alloy metals:
1. Binary alloys (Silver-Tin)
2. Ternary alloys (Silver-Tin-Copper)
3. Quaternary alloys (Silver-Tin-Copper-Indium).
II.According to whether the powder consist of unmixed or admixed alloys.
Certain amalgam powders are only made of one alloy. Others have one or more alloys or metals physically added (blended) to the basic alloy. E.g. Adding copper to a basic binary silver tin alloy
III. According to the shape of the powdered particles.
1. Spherical shape (smooth surfaced spheres).
2. Lathe cut (Irregular ranging from spindles to shavings).
3. Combination of spherical and lathe cut (admixed).
IV. According to Powder particle size.
1. Micro cut
2. Fine cut
3. Coarse cut
V. According to copper content of powder
1. Low copper content alloy - Less than 4%
2. High copper content alloy - more than 10%
VI. According to addition of Nobel metals
Platinum
Gold
Pallidum
VII. According to compositional changes of succeeding generations of amalgam.
First generation amalgam was that of G. V Black i.e. 3 parts silver one part tin (peritectic alloy).
Second generation amalgam alloys - 3 parts silver, 1 part tin, 4% copper to decrease the plasticity and to increase the hardness and strength. 1 % zinc, acts as a oxygen scavenger and to decrease the brittleness.
Third generation: First generation + Spherical amalgam –copper eutectic alloy.
Fourth generation: Adding copper upto 29% to original silver and tin powder to form ternary alloy. So that tin is bounded to copper.
Fifth generation. Quatemary alloy i.e. Silver, tin, copper and indium.
Sixth generation (consisting eutectic alloy).
According to Presence of zinc.
Zinc containing (more than 0.01%).
Non zinc containing (less than 0.01%).
INDICATIONS OF AMALGAM
Class I and class II cavities.-moderate to large restorations. As a core build up material. Can be used for cuspal restorations (with pins usually) In combination with composite resins for cavities in
posterior teeth. Resin veneer over amalgam. As a die a material. Restorations that have heavy occlusal contacts. Restorations that cannot be well isolated
In teeth that act as an abutment for removable appliances
INDICATIONS OF AMALGAM
Class 3 in unaesthetic areas eg.distal aspect of canine.especially if
Preparation is extensive with minimal facial involvement
Class 5 lesions in nonesthetic areas especially when access is limited and moisture control is difficult and for areas that are significantly deep gingivally.
CONTRA INDICATIONS OF AMALGAM
Anterior teeth where esthetics is a prime concern
Esthetically prominent areas of posterior teeth.
Small –to-moderate classes I and II restorations that can be well isolated.
Small class VI restorations
Advantages Ease of use, Easy to manipulate
Relatively inexpensive
Excellent wear resistance
Restoration is completed within one sitting without requiring much chair side time.
Well condensed and triturated amalgam has good compressive strength.
Advantages Sealing ability improves with age by formation
of corrosion products at tooth amalgam interface.
Relatively not technique sensitive. Bonded amalgams have “bonding benefits”.
Less microleakage Slightly increased strength of remaining tooth
structure. Minimal postoperative sensitivity.
Disadvantages Unnatural appearance (non esthetic)
Tarnish and corrosion
Metallic taste and galvanic shock Discoloration of tooth structure
Lack of chemical or mechanical adhesion to the tooth structure.
Mercury toxicity
Promotes plaque adhesion
Delayed expansion
Weakens tooth structure (unless bonded).
Composition of amalgamConventional Amalgam Alloys: (G.V. Black’s: Silver- tin alloy or Low copper alloy).
Low copper alloys are available as comminuted particles (Lathe -cut and Pulverized) and spherical particles.
Low copper composition:
Silver : 63-70%
Tin : 26-28%
Copper : 2- 5%
Zinc : 0-2%
Role of individual component
Silver:
Constitutes approximately 2/3rd of conventional amalgam alloy.
Contributes to strength of finished amalgam restoration.
Decreases flow and creep of amalgam.
Increases expansion on setting and offers resistance to tarnish.
To some extent it regulates the setting time.
Tin:
Second largest component and contributes ¼th of amalgam alloy.
Readily combines with mercury to form gama-2 phase, which is the weakest phase and contributes to failure of amalgam restoration.
Reduce the expansion but at the same time decreases the strength of amalgam.
Increase the flow.
Controls the reaction between silver and mercury.
Tin reduces both the rate of the reaction and the expansion to optimal values.
Copper: Contributes mainly hardness and strength. Tends to decrease the flow and increases the
setting expansion
Zinc: Acts as Scavenger of foreign substances such
as oxides. Helps in decreasing marginal failure. The most serious problem with zinc is delayed
expansion, because of which zinc free alloys are preferred now a days.
Indium/Palladium: They help to increase the plasticity and the resistance to deformation.
HIGH COPPER AMALGAM ALLOY (COPPER ENRICHED ALLOYS)
To overcome the inferior properties of low copper amalgam alloy -- shorter working time, more dimensional change, difficult to finish, set late, high residual mercury, high creep & lower early strength, low fracture resistant
Youdelis and Innes in 1963 introduced high copper content amalgam alloys. They increased the copper content from earlier used 5% to 12%.
Copper enriched alloys are of two types:
1) Admixed alloy powder.
2) Single composition alloy powder.
I. Admixed alloy powder:
Also called as blended alloys.
Contain 2 parts by weight of conventional composition lathe cut particles plus one part by weight of spheres of a silver copper eutectic alloy.
Made by mixing particles of silver and tin with particles of silver and copper.
The silver tin particle is usually formed by the lathe cut method, whereas the silver copper particle is usually spherical in shape.
I. Admixed alloy powder:Composition:
Silver-69 %
Copper-13 %
Tin-17 %
Zinc-1 %
I. Admixed alloy powder: Amalgam made from these powders are stronger than
amalgam made from lathe cut low copper alloys because of strength of Ag-Cu eutectic alloy particles.
Ag-Cu particles probably act as strong fillers strengthening the amalgam matrix.
Total copper content ranges from 9-20%.
II. Single composition alloy (Unicomposition):
It is so called as it contains particles of same composition.
Usually spherical single composition alloys are used.
As lathe cut, high copper alloys contain more than 23% copper.
II. Single composition alloy (Unicomposition):
1. Ternary alloy in spherical form, silver 60%, tin 25%, copper 15%.
2.Quaternary alloy in spheroidal form containing Silver: 59%, copper 13%, tin: 24%, indium 4%.
AMALGAMATION REACTION/ SETTING REACTION
Low copper conventional amalgam alloy
Dissolution and precipitation
Hg dissolves Ag and Snfrom alloy
Intermetallic compoundsformed
Ag3Sn + Hg Ag3Sn + Ag2Hg3 + Sn8Hg
Ag-Sn Alloy
Ag-Sn
Alloy
Ag-Sn
AlloyMercury
(Hg)
SnSn
Sn Ag
Hg Hg
Ag
Ag
1 2
Low copper conventional amalgam alloy
Gamma () = Ag3Sn
unreacted alloy
strongest phase and corrodes the least
forms 30% of volume of set amalgam
Ag-Sn Alloy
Ag-Sn
Alloy
Ag-Sn
AlloyMercury
Ag
SnSn
Sn Ag
Hg
Hg
Ag
Hg
Low copper conventional amalgam alloy
Gamma 1 (1) = Ag2Hg3
matrix for unreacted alloyand 2nd strongest phase
10 micron grainsbinding gamma ()
60% of volume
Ag-Sn Alloy
Ag-Sn
Alloy
Ag-Sn
Alloy
1
Low copper conventional amalgam alloy
Gamma 2 (2) = Sn8Hg
weakest and softest phase
corrodes fast, voids form
corrosion yields Hg which reacts with more gamma ()
10% of volume
volume decreases with time due to corrosion
2
Ag-Sn Alloy
Ag-Sn
Alloy
Ag-Sn
Alloy
Admixed High-Copper AlloysInitial reaction
Ag3Sn + Ag-Cu + Hg Ag3Sn + Ag2Hg3 + Sn8Hg + Ag-Cu
Ag-Sn
Alloy
Ag-Sn
AlloyMercury
Ag
AgAg
SnSn
Ag-Cu Alloy
AgHgHg
1 2
Final reactionAg-Cu Alloy
1
Ag-Sn
AlloyAg-Sn
Alloy
2
Sn8Hg + Ag-Cu Cu6Sn5 + Ag2Hg3 + Ag-Cu
1
Single Composition High-Copper Alloys
Ag-Sn Alloy
Ag-Sn Alloy
Ag-Sn Alloy
1
Ag3Sn + Cu3Sn + Hg Ag2Hg3 + Cu6Sn5 + Ag3Sn + Cu3Sn
1
Manufacturing Process
Lathe-cut alloys Ag & Sn melted together
alloy cooled
phases solidify
heat treat
400 ºC for 8 hours
grind, then mill to 25 - 50 microns
heat treat to release stresses of grinding
Manufacturing Process
Spherical alloys
Atomizing process produces these different shapes.
First liquefying the amalgam alloy, it is sprayed through a jet nozzle under high pressure in a cold atmosphere.
If particles are allowed to cool before they contact the surface of chamber, they are spherical in shape.
If they are allowed to cool on contact with the surface they are flake shaped.
PROPERTIES:
ADA specification No.1 for amalgam lists following physical properties as a measure of quality of the amalgam.
Creep
Compressive strength
Dimensional changes
Modulus of elasticity
StrengthCompressive strength
Amalgam is strongest in compression and weaker in tension and shear
The prepared cavity design and manipulation should allow for the restoration to receive compression forces and minimum tension and shear forces.
The compressive strength of a satisfactory amalgam restoration should be atleast 310 MPa.
Compressive Strengths of Low-Copper and High Copper Amalgam
Amalgam Compressive Strength
(MPa)
1 h 7 day
Low copper 145 343
Admix 137 431
Single
Composition
262 510
Tensile strength
Amalgam is much weaker in tension
Tensile strengths of amalgam are only a fraction of their compressive strengths
Cavity design should be constructed to reduce tensile stresses resulting from biting forces
High early tensile strengths are important – resist fracture by prematurely applied biting forces
Product Tensile strength (Mpa)
15min 7 days
LOW COPPER ALLOYSa) Lathe cutb) spherical
3.2 514.7 55
HIGH COPPER ALLOYSa) Admixedb) Unicompositional
3.0 438.5 56
Tensile strengths of amalgam
The factors affecting strength of amalgam are:
1) Temperature:
Amalgam looses 15% of its strength when its temperature is elevated from room temperature to mouth temperature
looses 50% of room temperature strength when temperature is elevated to 60OC e.g. hot coffee or soup.
2) Trituration:
Effect of trituration on strength depends on the type of amalgam alloy, the trituration time and the speed of the amalgamator.
Either, under trituration or over-trituration decreases the strength for both traditional and high copper amalgams.
More the trituration energy used, more evenly distributed are the matrix crystals over the amalgam mix and consequently more the strength pattern in the restoration.
Excess trituration after formation of matrix crystals will create cracks in the crystals, lead to drop in strength of set amalgam
3) Mercury Content:
Low mercury alloy content, contain stronger alloy particles and less of the weaker matrix phase, therefore more strength
Mercury is too less -- dry, granular mix, results in a rough, pitted surface that invites corrosion.
If mercury content of amalgam mix is more than 53-55%, causes drop of compressive strength by 50%.
4) Effect of condensation:
For lathe-cut alloys
Greater the condensation pressure, the higher the compressive strength
Higher condensation pressure is required to minimize porosity and to express mercury from lathe-cut amalgam.
For spherical alloys
Amalgams condensed with lighter pressure produce adequate strength.
5) Effect of Porosity:
Can be due to
Under trituration,
Particle shape,
Insertion of too large increments into the cavity,
Delayed insertion after trituration,
Non-plastic mass of amalgam.
Facilitate stress concentration, propagation of cracks, corrosion, and fatigue failure of amalgam restoration.
6) Effect of rate hardening
Patient may be dismissed from the dental chair within 20 min, rate of hardening of the amalgam is of considerable interset
At the end of 20 min, compressive strength – 6% of the 1 week strength
ADA specification stipulates minimum compressive strength of 80 Mpa at 1 hr
Clinical significance -- Patient should be cautioned not to subject the restoration for high biting force for 8 hrs after placement– 70% of its strength is gained
Modulus of elasticity
High copper alloys tend to be stiffer than low copper alloys
When rate of loading increased, values of approx 62 Gpa have been obtained
Knoop Hardness
110 kg/mm2
DIMENSIONAL CHANGES: When mercury is combined with amalgam it
undergoes three distinct dimensional changes.
Stage -1: Initial contraction, occurs for about 20minutes after beginning of trituration. Contractionresults as the alloy particles dissolve in mercury.Contraction, which occurs, is no greater than 4.5 µcm.
Stage -2: Expansion- this occurs due to formation andgrowth of the crystal matrix around the unconsumedalloy particles.
Stage -3: Limited delayed contraction.
Factors that affect the dimensional changes:
1) Particle size and shape:
More regular the particle shape, more smoother the surface area.
Faster and more effectively the mercury can wet the powder particles and faster amalgamation occurs in all stages with no apparent expansion.
2) Mercury:
More mercury , more will be the expansion, as more crystals will grow.
Low mercury: alloy ratio favors contraction
3) Manipulation:
During trituration, if more energy is used formanipulation, the smaller the particles will become ,mercury will be pushed between the particles,discouraging expansion.
More the condensation pressure used duringcondensation, closer the particles are broughttogether; more mercury is expressed out of mixinducing more contraction.
Moisture contamination (Delayed Expansion):
Certain zinc containing low copper or high copper amalgam alloys which get contaminated by moisture during manipulation results in delayed expansion or secondary expansion
Occur 3-5 days after insertion and continues for months.
Zinc reacts with water, forming zinc oxide and hydrogen gases.
Complications that may result due to delayed expansion are:
Protrusion of the entire restoration out of the cavity.
Increased micro leakage space around the restoration.
Restoration perforations.
Increased flow and creep.
Pulpal pressure pain.
Such pain may be experienced 10-12 days after the insertion of the restoration
Flow and Creep:
Time dependent plastic deformation
When a metal is placed under stress, it will undergo plastic deformation.
The high copper alloys, as compared with conventional silver tin alloys, usually tend to have lower creep values.
Factors influencing creep:
A) Phases of amalgam restorations
Creep rates increases with larger 1 volume fraction and decreases with larger 1 grain sizes.
2 is associated with high creep rates.
In absence of 2, low creep rates in single composition alloy may be due to phase which act as barrier to deformation of 1 phase.
B) Manipulations:
Greater compressive strength will minimize creep rates.
Low mercury: alloy ratio, greater the condensation pressure and time of trituration, will decrease the creep rate.
Corrosion
Excessive corrosion can lead to:
Increased porosity.
Reduced marginal integrity.
Loss of strength.
Release of metallic products in to the oral environment.
Phases in decreasing order of corrosion resistance
Ag2Hg3
Ag3Sn,
Ag-Cu
Cu3Sn
Cu6Sn5
Sn7-8Hg.
Low copper amalgam system:- Most corrodible phase is tin-mercury or 2 phase.
Neither the nor the 1 phase is corroded as easily.
The corrosion results in the formation of tin oxychloride, from the tin in 2 and also liberates Hg.
Sn7-8Hg + 1/202 + H2O + Cl- Sn4 (OH) 6 Cl2 + Hg
Tin oxychloride
Reaction of the liberated mercury with unreacted
can produce additional l and 2 (MercuroscopicExpansion).
Results in porosity and lower strength.
The high copper admixed and unicomposition alloy :-
Do not have any 2 phase in the final set mass
The η phase formed has better corrosion resistance.
However, is the least corrosion resistant phase in high copper amalgam
Corrosion product CuCl2.3Cu (OH)2 has been associated with storage of amalgams in synthetic saliva.
Cu6Sn5 + 1/202 +H2O + Cl- CuCl2.3Cu (OH)2 + SnO.
Types of Corrosion:1) Galvanic corrosion:
Dental amalgam is in direct contact with an adjacent metallic restoration such as gold crown
2) Crevice Corrosion: Local electrochemical cells may arise
whenever a portion of amalgam is covered by plaque on soft tissue.
The covered area has a lower oxygen and higher hydrogen ion concentration making it behave anodically and corrode.
Stress Corrosion:
Regions within the dental amalgam that are under stress display a greater probability for corrosion, thus resulting in stress corrosion.
For occlusal dental amalgam greatest combination of stress and corrosion occurs along the margins.
MANIPULATION OF DENTAL AMALGAM
PROPORTIONS OF ALLOY TO MERCURY
Correct proportioning of alloy and mercury-essential for forming a suitable mass of amalgam
Some alloys require mercury – alloy ratios in excess of 1:1 (Eames technique)
whereas others use ratios of less than 1:1 with the percentage of mercury varying from 43% to 54%.
Automatic mechanical dispensers for alloy & mercury have been used in the past
Capsules with pre proportioned amounts of alloy & mercury have been substituted
Cross section sketch of a disposable capsule containing amalgam alloy & mercury
SIZE OF MIX
Manufacturers commonly supply capsules containing 400, 600, or 800 mg of alloy and the appropriate amount of mercury.
For large size cavities - capsules containing 1200 mg of all0y are also available.
TRITURATION
Process of mixing the amalgam alloy particles with mercury
Originally, the alloy and mercury were mixed, and was triturated by hand with a mortar and pestle
Mechanical amalgamation saves time and standardizes the procedure.
Amalgamator
Mechanical amalgamators are available in the following speeds:
Low speed: 32-3400 cpm.
Medium speed: 37-3800 cpm.
High speed: 40-4400 cpm.
Spherical/irregular low-copper alloys – triturated at low speed
High copper alloys – high speed
Time of trituration on amalgamation ranges from 3-30 seconds. Variations in 2-3 seconds can also produce a under or over mixed mass.
Over-trituration: Alloy will be hot, hard to remove from the capsule, shiny wet and soft.
Under-trituration: Alloy will be dry, dull and crumbly; will crumble if dropped from approx 30 cm.
Normal Mix: Shiny appearance separates in a single mass from the capsule.
Under-trituration
Normal Mix
Objectives of Trituration are:
To achieve a workable mass of amalgam within a minimum time
To remove the oxide layer
To pulverize pellets into particles, that can be easily attacked by the mercury.
To reduce particle size
To keep the amount of 1 or 2 matrix crystal as minimal as possible, yet evenly distributed
Mixing variables
1) Working time & dimensional change
All types of amalgam, spherical or irregular –decreases with overtrituration
Overtrituration – slightly higher contraction for all types of alloys
2) Compressive & tensile strength
Irregular shaped alloys – increase by overtrituration
Spherical alloys -- greatest at normal trituration time
3) Creep
Overtrituration increases creep
Undertrituration lowers it
Condensation
Refers to the incremental placement of the amalgam into the prepared cavity and compression of each increment into the others
Amalgam should be condensed into the cavity within 3 min after trituration.
Aims of condensation Adapt amalgam to the margins, walls and line angles
of the cavity.
Minimize voids and layering between increments within the amalgam.
Develop maximum physical properties.
Remove excess mercury to leave an optimal alloy: mercury ratio.
Purpose of Condensation
To get a continuous homogenous mass that is well adapted to all margins, walls and line angles.
Best carried out using hand instruments.
Hand condenser : Should allows a operator to
readily grasp it & exert a force of condensation
Size of condenser tip & direction & magnitude of the force placed, depends on the type of amalgam alloy selected
Irregular shaped alloys –
Condensers with relatively small tip, 1 to 2 mm
High condensation forces in vertical direction
As much mercury-rich mass as possible should be removed
Spherical amalgam alloys
Condensers with large tips are used
Condensed in lateral direction
High copper spherical amalgams – vertical & lateral direction condensation with vibration
Condensation pressure – load of 15 lb is recommended to be applied to each increment
Mechanical Condensers: Useful for condensing irregular shaped alloys when
high condensation forces are required
Need was eliminated with the advent of spherical alloys
Tend to lead to unreliable condensation as well as generation of heat and mercury vapor, both of which are undesirable.
Ultrasonic Condensers: Not recommended
Causes the release of considerable quantities of mercury vapor in the dental office
SPEED OF PLACEMENT
Once amalgam is triturated, phase formation commences and the setting reaction is underway.
Amalgam must be placed in a plastic state
No amalgam should be placed more than 3 minutes after the start of mixing.
Attempting to condense a partly set amalgam into a cavity will result in
Poor adaptation,
Reduced marginal seal and
A weak restoration.
BurnishingFirst Burnish (Pre-carve Burnish)
Carried out using a large burnisherfor 15 seconds
Use light force and move from the center of the restoration outwards to the margins.
Objectives of precarve burnishing :
Continuation of condensation, further reduce the size and number of voids on the critical surface and marginal area of the amalgam.
Brings any excess mercury to the surface, to be discarded during carving.
Adapt the amalgam further to cavosurface anatomy.
Carving Using remaining enamel as a guide,
carve gently from enamel towards the center and recreate the lost anatomy of the tooth.
Amalgam should be hard enough to offer resistance to carving instrument
A scarping or "ringing" (amalgam crying) should he heard.
If carving is started too soon, amalgam will pull away from margins.
Objectives of carving :
To produce :
A restoration with no underhangs
A restoration with the proper physiological contours.
A restoration with minimal flash.
A restoration with adequate, compatible marginal ridges.
A restoration with proper size, location, extend and interrelationship of contact areas.
Final Burnish (Post carve burnishing)
Following carving, check the occlusion and carry out a brief final burnish.
Use a large burnisher at a low load and burnish outwards towards the margins
Improves smoothness
Heat generation should be avoided
If temp raises above 60C, causes release of mercury accelerates corrosion & fracture at margins
Finishing & Polishing Finishing can be defined as the process, which continues
the carving objectives, removes flash and overhangs and corrects minimal enamel underhangs.
Polishing is the process which creates a corrosion resistant layer by removing scratches and irregularities from the surface.
Can be done using descending grade abrasive, eg. rubber mounted stone or rubber cups.
A metallic lusture, is always done with a polishing agent (precipitated chalk, tin or zinc oxide).
.
Objective of finishing and polishing :
Removal of superficial scratches and irregularities
Advantages:
Minimizes fatigue failure of the amalgam under the cyclic loading of mastication
Minimizes concentration cell corrosion which could begin in the surface irregularities
Prevents the adherence of plaque
Usually, 24 hours should pass after amalgam insertion before any finishing and polishing commences.
However, some new alloys can be polished after 8-12 hours still others require only a 30-minute wait after insertion.
RESISTANCE & RETENTION FORMS
Primary retention form
Attained by:
Mechanical locking of inserted amalgam into surface irregularities to allow good adaptation
Preparation of vertical walls that converge occlusally
Primary resistance form For tooth :
Maintaining as much unprepared tooth structure as possible
Having pulpal & gingival walls perpendicular to occlusal forces
Having rounded internal prepartaion angles
Removing unsupported & weakened tooth structure
Placing pins into the tooth as a part of final stage of tooth preparation
Primary resistance form For amalgam :
Adequate thickness – 1.5 -2 mm in areas of occlusalcontact, 0.75 mm in axial areas
Marginal amalgam of 90 degrees or greater
Box like preparation form
Rounded axiopulpal line angles in class II preparations
Secondary resistance & retention form
When insufficient resistance/retention forms are present in tooth, additional preparation is indicated
Such features include :
Placement of grooves, locks, coves, pins, slots or amalgam pins
Larger the tooth preparation, greater the need of secondary resistance & retention forms
BIO-COMPATIBILITY –MERCURY TOXICITY
Amalgams have been used for 150 years
About 200 million amalgams are inserted each year in the United States and Europe
Concern -- mercury in dental amalgam may pose threats to the health of patients, to the health of dental care providers and to the environment.
Mercury is available in 3 forms:
Elemental mercury (liquid or vapor).
Inorganic compounds.
Organic compounds.
ELEMENTAL MERCURY
Liquid mercury:
Absorbed relatively poorly across skin or mucosa.
Most mercury becomes charged (ionized) before it reaches the blood.
Ionized mercury is excreted well through kidneys and urine.
There is no known risk to patients from liquid mercury.
ELEMENTAL MERCURY
Mercury vapor:
Less benign -- rapidly absorbed into the blood via the lungs , remains uncharged and therefore highly lipid soluble, for several minutes.
Can cross the blood-brain barrier where it becomes charged and exists in extra cellular fluid of the brain and returns into the blood much more slowly.
High tissue levels- can lead to impaired brain function, insanity and death may occur at 4000 g/kg.
Low tissue levels- can lead to restlessness, tremors, and loss of concentration.
Inorganic compounds of mercury
S0urce – Drinking water, food
Amalgam contains several different inorganic mercury compounds,
They are of low or very low toxicity and are apparently harmless when swallowed.
Poorly absorbed, do not accumulate in body tissues and are well excreted.
Organic compounds of mercury
Source -- Drinking water, food (sea food)
Some organic compounds of mercury are highly toxic at low concentrations
But none are known to form in the oral environment through dental amalgam use.
Source g Hg vapour g inorganic Hg g methyl Hg
Atmosphere 0.12 0.038 0.034
Drinking Water --- 0.05 ---
Food & Fish 0.94 --- 3.76
Food & Non-Fish --- 20.00 ---
ESTIMATED DAILY INTAKE OF MERCURY
CONCENTRATIONS OF MERCURY
The Occupational Safety & Health Administration (OSHA) has set a TLV of 0.05 mg/m3 as the maximum amount of mercury vapor allowed in the work place.
Average Daily dose of mercury from dental amalgam for patients with more than 12 restored surfaces has been estimated at up to 3 g.
CONCENTRATIONS OF MERCURYClarkson TW (1997) --
Lowest dose of mercury that elicits a toxic reaction –3to7 g/kg body weight
Paresthesia -- 500 g/kg body weight
Ataxia -- 1000 g/kg body weight
Joint pain -- 2000 g/kg body weight
Hearing loss & death -- 4000 g/kg body weight
CONCENTRATIONS OF MERCURY
Mercury release has been quantified for a number of procedures:
Trituration: 1-2g
Placement of amalgam restoration: 6-8 g.
Dry polishing: 44 g.
Wet polishing: 2-4 g.
Amalgam removal under water spray & high velocity suction: 15-20 g
CONCENTRATIONS OF MERCURY
The release of mercury is:
Greater for low-copper amalgams, because of corrosion related loss of tin and increased porosity.
Greater from Unpolished surfaces
Increased by tooth brushing, which removes a passivating surface oxide film-although this re-forms rapidly.
Mercury in urine Body cannot retain metallic mercury, but passes it
through urine
Skare I et al (1990) –
urine mercury level peak at 2.54 g/L 4 days after placing amalgam restorations, return to zero after 7 days
On removal of amalgam, urine mercury levels reach a
maximum value of 4g/L, return to zero after 7 days
Mercury in blood
Maximum allowable level of mercury in blood is 3 g/L
Chang SB et al(1992) showed that freshly placed amalgam restorations elevated blood mercury levels to 1 to 2 g/L
As with urine mercury levels, there is first an increase of around 1.5 g/L, which decreases in about 3 days
Ott KH et al (1996) monitored blood mercury levels for 1 year, showed that patients with amalgams had lower than average blood mercury level (0.6 g/L ) than patients without amalgams (0.8 g/L )
Mackert JR et al(1997) indicated higher blood mercury levels in dentists, stated that -
elevated blood mercury levels may relate to mercury spills in the office
Both blood & serum mercury levels seem to correlate best with occupational exposure, not with number of amalgam & length of time with amalgam in place
BIO-COMPATIBILITY –MERCURY TOXICITY
Sensitivity to amalgam restorations Skin lesions being more common than oral lesions.
An urticarial rash may appear on the face and limbs andthis may be followed by dermatitis.
Long- term response -- oral lichen planus or lichenoidreactions with erosive areas on the tongue or buccalmucosa adjacent to an amalgam restoration.
BIO-COMPATIBILITY –MERCURY TOXICITY
AMALGAM TATTOO
AMALGAM TATTOOPossible causes are:
Scraps of amalgam may fall into open surgical or extraction wounds.
Excess amalgam may be left in the tissues following sealing the apex of a root canal with a retrograde amalgam.
Pieces of amalgam may be forced into the mucosa.
Sources of Mercury Exposure in Dental Office:
Dental amalgam raw materials being stored for use.
Mixed but unhardened dental amalgam during triturations, insertion and intraoral setting.
Dental amalgam scrap that has insufficient alloy to completely consume the mercury present.
Dental amalgam undergoing finishing and polishing procedure.
Dental amalgam restoration being removed.
DENTAL MERCURY HYGIENERecommendations from the ADA include the following:
The work place should be well ventilated, with fresh air exchange and outside exhaust
Use only precapsulated alloy, discontinue use of Bulk mercury & bulk alloy
Avoid the need to remove excess mercury before or during packing by selecting an appropriate alloy: mercury ratio
Use an amalgamator with a completely enclosed arm.
Mercury and unset amalgam should not be touched by the bare hands.
Floor coverings should be non absorbent & easy to clean
Spilled mercury should be cleaned up using trap bottles, tape or freshly mixed amalgam to pick up droplets
Do not use a house hold vaccum cleaner to clean spilled mercury.
Skin accidentally contaminated by mercury should be washed thoroughly with soap and water.
If a mercury hygiene problem is suspected, personnel should undergo urine analysis to detect mercury levels
Remove professional clothing before leaving the work place
Scrap amalgam disposal In a tightly closed container
Under radiographic fixer solution
Dispose mercury contaminated items in sealed bags
Donot dispose mercury contaminated items in medical waste containers or bags or along with the waste that will be incenerated
CLINICAL TECHIQUES TO ENHANCE MARGINAL SEAL
1) Copal resin varnish:
Apply two thick coats to the cavity walls and margins before placing the amalgam and it will gradually dissolve, beginning at the cavosurface, over 2-3 months.
As the varnish dissolves out, the gap will be filled with corrosion products from the amalgam and dissolution of the varnish will cease.
CLINICAL TECHIQUES TO ENHANCE MARGINAL SEAL
2) Glass-ionomer linings
Placed under an amalgam will seal the dentinal tubules and release small quantities of fluoride
Will not affect enamel margins or enhance the seal at the margin.
CLINICAL TECHIQUES TO ENHANCE MARGINAL SEAL
3) Oxalate solutions :
Such as potassium oxalate, can be applied to the cavity surface to reduce the permeability of the tubules and possibly seal the dentine.
The crystals this deposited will not wash out but will allow deposition of corrosion products.
RECENT ADVANCES
1) BONDED AMALGAMS
During the 1990’s some clinicians began to routinely bond amalgam restorations to enamel and dentine
After preparation of the cavity, enamel and dentine etched using a conventional etchant, a chemically cured resin-bonding agent applied to the walls of the cavity.
Amalgam is immediately condensed into the cavity before the resin bond has cured
Advantages of Bonded-Amalgam :
Conservation of tooth structure.
Fracture strength was as high as for composites
Decreased marginal leakage in class 5 restorations compared with unbonded amalgams
Some operators claim elimination of post-insertion sensitivity.
Reduces incidence of marginal fracture and recurrent caries.
Can be done in single sitting.
Allows for amalgam repairs.
Disadvantages of Bonded-Amalgam :
Clinical difficulty of application of more viscous bonding agents
Lightly filled resin bonding agents tend to pool at the gingival margin resulting in a higher potential for micro leakage.
Carving is difficult.
Requires practitioner to adapt to new technique.
Increases cost of amalgam restorations.
2) Gallium alloys
Mercury free metallic restorative materials proposed assubstitute for mercury containing amalgam are galliumcontaining materials and pure silver and/or silver basedalloys
Puttkammer (1928), suggested the use of gallium indental restoration
Attempts to develop satisfactory gallium restorativematerials were unsuccessful until Smith et al in 1956,showed that improved Pd-Ga and Ag-Ga materials hasphysical and mechanical properties that were similar to oreven better than those of silver amalgam.
ADVANTAGES OF GALLIUM BASED ALLOYS:
Rapid solidification.
Good marginal seal by expanding on solidification.
Heat resistant.
The compressive and tensile strength increases with time comparable with silver amalgam
Creep value are as low as 0.09%
It sets early so polishing can be carried out the same day
They expand after setting therefore provides better marginal seal
REACTION :
Ag3Sn + Ga Ag3Ga + Sn.
REACTION :
After mixing, the alloy tends to adhere to the walls of capsule, thus difficult to handle.
Moreover, by adding few drops of alcohol, the problem of sticking can be minimized.
Biologic considerations of Gallium based alloys :
Surface roughness, marginal discoloration and fracture were reported. With improvement in composition, these defects were reduced but not eliminated
Could not be used in larger restorations as the considerable setting amount of expansion leads to fracture of cusps and post operative sensitivity.
Cleaning of instruments tips is also difficult
Less popular because it is costlier than amalgam.
3) Fluoride releasing amalgam
Have been shown to have anticaries properties sufficient to inhibit the development of caries in cavity walls.
Concentration of fluoride is sufficient to enhance remineralization
Tviet and Lindh (1980) -- greatest concentration of fluoride i.e. about 4000µg/mL in enamel surfaces exposed to fluoride-containing amalgams were found in the outer 0.05µm of the tissue.
In dentin, the greatest concentrations, i.e. about 9000µg/ml were found at a depth of 11.5µm.
However, this release of fluoride decreases to minor amounts after 1 week.
Forsten L (1976) -- fluoride released from amalgams loaded with soluble fluoride salts was detectable within the first month and thereafter fluoride was not released in measurable amounts.
Garcia Godoy et al( 1990) – fluoride release can continue as long as 2 years (but at a much lower rate than that for GIC).
Marginal fracture of amalgam
Referred to as “Marginal breakdown”, “ditching”, and “crevice formation”.
Regardless of the type of amalgam, marginal fracture increases with time
The rate of increase is greater for low-copper amalgams.
CLINICAL TECHNIQUES TO PREVENT MARGINAL FRACTURE
Excess amalgam, left lying over the occlusal or proximal surface should be carved correctly
The angle of the carvo-surface margin should be greater than 70º and the cavity should be designed to allow for this.
On completion of packing, burnish the margins both before and after carving to improve marginal adaptation.
Repair Of Amalgam Restorations
When an amalgam restoration fails, as from marginal fracture, it is repaired
A new mix of amalgam is condensed against the remaining part of the existing restoration
The strength of the bond between the new and the old amalgam is important
Factors contributing to strength of repair
Presence of porosity and phase at the junction.
Inadequate condensation.
Contamination of the surface of the existing amalgam.
Corrosion & contamination from saliva.
CLINICAL CONSIDERATIONS
Marginal Adaptation And Seal :
Lack of marginal adaptation in first few weeks
May be associated with marginal deterioration, accumulation of debris, recurrent caries, post-restoration sensitivity or pulpal reactions.
CLINICAL CONSIDERATIONSSelf-Sealing :
After 48 hours, “self sealing” occurs
Low-copper amalgam -- seal within 2-3 months
High-copper amalgams -- corrode less and therefore take 10-12 months to provide a comparable seal.
Amalgam wars In 1845, American Society of Dental Surgeons condemned
the use of all filling material other than gold as toxic, thereby igniting "first amalgam war'. The society went further and requested members to sign a pledge refusing to use amalgam.
In mid 1920's a German dentist, Professor A. Stock started the so called "second amalgam war". He claimed to have evidence showing that mercury could be absorbed from dental amalgam, which leads to serious health problems. He also expressed concerns over health of dentists, stating that nearly all dentists had excess mercury in their urine.
Amalgam wars "Third Amalgam War' began in 1980 primarily
through the seminars and writings of Dr.Huggins, a practicing dentist in Colorado.
He was convinced that mercury released from dental amalgam was responsible for human diseases affecting the cardiovascular system and nervous system
Also stated that patients claimed recoveries from multiple sclerosis, Alzheimer’s disease and other diseases as a result of removing their dental amalgam fillings.
CONCLUSION
There are certain advantages inherent with amalgamsuch as technique insensitive, excellent wearresistance, less time consuming, less expensive whichare not present in the newer materials, these factorswill continue to make amalgam the material of choicefor many more years to come.
References Stephen. C. Boyne, Duane. F. Taylor, “Dental materials”, The Art and
Science of operative Dentistry, Mosby 3rd Edition 1997:219-235. Kenneth J Anusavice, D.M.D., PhD., “Philip’s Science of Dental
materials”, W.B. Saunders Company, 10th Edition 1996: 361-410. M.A. Marzouk D.D.S. M.S.D. et al, “Operative Dentistry Modern theory
and Practice”, IEA inc 1997:105-120. Craig, “Science of Dental Materials”. Jagannathan, K, “Cruise for Gamma 2 Free Mercury”, “Materials in
Restorative Dentistry”, MADC & H, 1998 66-69. John F. McCabe, Angus W.G. Walls, “Dental Amalgam”, Applied Dental
Materials, Blackwell Science, 8th Edition, 1998:157-168 Satish Chandra, Shaleen Chandra, “Dental Amalgam”, A Text Book of
Dental materials with Multiple Choice Questions”, Jaypee Brothers; 1st
Edition 2000. Vimal. K. Sikri, “Silver Amalgam”, Text book of Operative Dentistry”
CBS publishers, 1st Edition 2002, 204-242.
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