Dental amalgams
1-Introduction:
Dental amalgams are metallic biomaterials for restoration, still widely used in conservative dentistry, intended for the filling of simple or complex coronal substance losses located at the level of premolars and molars.
2-Definitions:
According to SKINNER “an amalgam is a special type of alloy in which one of the constituents is mercury.”
According to ZIMMERMANN, “a conventional dental amalgam is formed from an alloy of two amalgams, one rich in tin, the other rich in silver.”
According to the ADA: “Dental amalgams are metal alloys, they are either binary, ternary or quaternary mercury alloys.”
3/Composition and role of the different constituents of silver amalgam
-A dental amalgam is prepared by grinding mercury (liquid) with an alloy (amalgam powder)
– the fundamental constituents of the alloy are silver and tin, to these two basic constituents copper and zinc are generally added
Modern amalgams are prepared from three types of alloy:
Conventional ones with a low copper content, generally ≤ 6% (alloys from the Black formula),
High copper single-phase ternaries, 12% ≤ Cu ≤ 30% (HCSC)
“Phase dispersions” or other alloy with a high copper content, the content of which is 7% ≤ Cu ≤ 40% this type of amalgam is the most appreciated thanks to the absence of release of mercury during its amalgamation and also called “non gamma2”.
Type 4: alloy of original composition or containing other components such as gold, platinum, fluorine, etc.
4- Role of the different constituents:
Silver: is the base metal of the alloy, it increases the resistance of the amalgam and allows reactivity with mercury.
Tin: improves the plasticity of the amalgam.
Copper: increases the resistance of the amalgam to creep. (This is the permanent plastic deformation of a material under a static load)
Zinc: with its deoxidizing power thus preventing other metals from oxidizing during melting and casting operations.
Mercury: is the only liquid metal at room temperature, it facilitates amalgamation .
6/PACKAGING AND HANDLING
Currently, silver amalgam comes in the form of pre-dosed capsules that contain one dose of powder and one dose of mercury.
Amalgamation (with an amalgamator or with a pestle and mortar) of these two components gives a ready-to-use amalgam in plastic form.
Pre-dosed capsule amalgamator mortar and pestle
7/ PROPERTIES OF SILVER AMALGAMS:
7-1 Biological properties
a/ Harmful actions
Toxicity of degradation products:
Cellular toxicity studies highlight the cytotoxic power of corrosion products. Components that can migrate towards the pulp can cause a pulpal immune reaction.
Mercury release:
It now seems accepted that the release and absorption of mercury from dental amalgams remains limited to tolerable values, both for patients and for healthcare personnel.
b- Beneficial actions
Some manufacturers added fluorine to the alloy in order to give the amalgam a certain anti-cariogenic power.
7-3-Thermal Properties:
A- thermal expansion: amalgam has a thermal expansion coefficient of 22 to 28, i.e. more than double that of the tooth, which is 8.3 for dentine and 11.4 for enamel, which is not without drawbacks and could cause problems with sealing and percolation.
B- thermal conductivity: dental amalgam has a coefficient of 0.055, so it is necessary to insert a thermal insulator between the filling and the dental tissues.
7-4/chemical and electrochemical properties:
Amalgam fillings, like all other metal fillings placed in the mouth in contact with salivary electrolyte, are subject to electrochemical degradation phenomena (corrosion).
8/ ADVANTAGES AND DISADVANTAGES:
Benefits :
Easy handling.
Fast setting time.
Rigid material.
Marginal adaptation acceptable.
Take and keep the polish.
Insoluble in oral fluids.
Well tolerated in the oral environment.
Does not cause thermal rise when taken.
Disadvantages:
May be toxic or brittle.
Corrodible.
Tarnishing phenomenon.
Thermal conductor.
High coefficient of expansion.
Unsightly.
Difficult to unclog.
9/CLINICAL MANIPULATION:
1/Preparation of the cavity
The cavity must be carefully prepared beforehand:
2/Cavity size: is done according to Black’s principles.
3/Cleaning the cavity Before cleaning the cavity, it will be necessary to isolate it from its salivary environment.
4/Bottom of the cavity
The practitioner will choose between modified zinc oxide eugenol or calcium hydroxide.
5/Possible placement of the matrix
Installation of a formwork system (metal matrix)
5-Preparation of the silver amalgam
A/Amalgamation
The amalgamation, mixing or grinding of the alloy with mercury aims to prepare an amalgam paste with a judiciously calculated plasticity.
B/Insertion and condensation of silver amalgam
Once the mixture has been made, the amalgam should not wait long for its condensation in the prepared cavity. An amalgam that is 3 and a half minutes old should be discarded and a new mixture made. The amalgam pellet is either placed in the chamois leather or in a soft cloth, then transported by an amalgam holder and expelled into the oral cavity.
manual condensation: which uses flat or grooved end amalgam pluggers and round end pluggers in different dimensions. The cavity must be filled with a large number of small portions.
*mechanical condensation: is done using small rammers operated by the lathe using a contra-angle
C/Finishing of silver amalgams
1/Roughing
It consists of possibly removing the matrix, removing excess amalgam from the filling and carrying out an initial leveling of the surface.
2 /Pre sculpture
The correct morphology of the cusps will be reproduced by digging the furrows and dimples and ensuring the exact inclination of the slopes which must be convex, special instruments are used for this.
3/Checking the occlusion
The patient will be allowed to bring the jaws together without biting down hard to check the height of the filling.
4/Burning or surfacing
Burnishing consists of exerting a certain amount of pressure on the edges of the filling using suitable burnishers .
5/Polishing
Polishing must be carried out at least 24 hours after the obturation and better a week later, avoiding any heating because the mercury is released at around 65°C. consists of two phases:
-Morphological correction if the structure has not been correctly carried out during the first session, which is done using multi-blade cutters and fine-grain paper discs.
– Polishing with soft brushes and a polishing paste (zinc oxide powder most often gives a perfect shine to the amalgam).
Dental amalgams
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