Higher condensation pressure results in higher compressive strength (only for lathe- cut alloys).
Reason A good condensation technique will minimize porosity and remove excess mercury from lathe-cut amalgams. If heavy pressures are used in spherical amalgams, the condensor will punch through. However, spherical amalgams condensed with lighter pressures produce adequate strength.
Effect of Porosity
Voids and porosities reduce strength.
Porosity is caused by:
— Decreased plasticity of the mix (caused by too low Hg/ alloy ratio, under trituration and over trituration)
— Inadequate condensation pressure
— Irregularly shaped particles of alloy powder — Insertion of too large increments.
Increased condensation pressure improves adaptation at the margins and decreases the number of voids. Fortunately, voids are not a problem with spherical alloys.
Effect of Rate of Hardening
Amalgams do not gain strength as rapidly as might be desired. After 20 minutes, compressive strength may be only 6% of the one week strength.
The ADA stipulates a minimum of 80 MPa at one hour.
Since the initial strength of amalgam is low, patients should be cautioned not to bite too hard for a least 8 hours after placement, the time at which at least 70% of its strength is gained. The one hour compressive strength of high-copper single- composition amalgams is exceptionally high (262 MPa), so the chances of accidental fracture is less.
Even after six months, some amalgams may still be increasing in strength, suggesting that the reactions between the matrix phases and the alloy particles may continue indefinitely.
Effect of Cavity Design
• The cavity should be designed to reduce tensile stresses
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Creep
It is defined as a time dependent plastic deformation. Creep of dental amalgam is a slow progressive permanent deformation of set amalgam which occurs under constant stress (static creep) or intermittent stress (dynamic creep).
The Significance of Creep to Amalgam Performance
Creep is related to marginal breakdown of low-copper amalgams. That is, the higher the creep, the greater is the degree of marginal deterioration (ditching).
According to ADA Sp. No.1 creep should be below 3%. Creep Values
Low-copper amalgam — 0.8 to 8.0% High-copper amalgam— 0.4 to 0.1% Factors Affecting Creep
Microstructure The γ1 (Ag-Hg) phase has a big effect on low-copper amalgam creep rates. Increased creep rate is shown by larger γ1 volume fractions. Decreased creep rate is shown by larger γ1 grain sizes. The γ2 phase is associated with higher creep rates.
Single-composition high-copper amalgams have very low creep rates, due to absence of γ2 phase and due to the presence of n Cu6Sn5 rods, which acts as barrier to deformation of the γ1 phase. An increase in zinc content gives less creep.
Effect of manipulative variables For increased strength and low creep values. — Mercury : alloy ratio should be minimum.
— Condensation pressure should be maximum for lathe-cut or admixed alloys. — Careful attention should be paid to timing of trituration and condensation. Either under or over-trituration or delayed condensation tend to increase the creep r a t e .
Retention of Amalgam
Amalgam does not adhere to tooth structure. Rather retention of the amalgam filling is obtained through mechanical locking. This is achieved by proper cavity design (see cavity design in technical considerations). Additional retention if needed can be obtained by placing pins within the cavity.
Tarnish and Corrosion
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Factors related to excess tarnish and corrosion
• High residual mercury
• Surface texture—small scratches and exposed voids • Contact of dissimilar metals, e.g. gold, and amalgam • Patients on a high sulfur diet
• Moisture contamination during condensation
• Type of alloy—low copper amalgam is more susceptible to corrosion (due to greater γ2 content) than high copper. Also n (Cu6Sn5) phase of high copper is less susceptible to corrosion
• A high copper amalgam is cathodic in respect to a low-copper amalgam. So, mixed high copper and low copper restorations should be avoided.
Corrosion of Amalgam can be Reduced by • Smoothing and polishing the restoration.
• Correct Hg/alloy ratio and proper manipulation.
• Avoid dissimilar metals including mixing of high, and low copper amalgams. TECHNICAL CONSIDERATIONS
MANIPULATION OF AMALGAM
The clinical success of most amalgam restorations is highly dependent on the correct selection, manipulation of the alloy and cavity design. If a restoration is defective, it is usually the fault of the operator and, not the material (Fig. 10.6).
FIGURE 10.6: An amalgam restoration. The cavity design should be designed for mechanical retention (cavity walls diverge as it approaches the pulp creating an undercut design). The base is placed to protect the pulp from thermal shock
CAVITY DESIGN
Providing retention Since amalgam does not adhere to tooth structure, proper design of the cavity is very important. The amalgam cavity is designed to provide
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maximum mechanical locking of the amalgam. This is achieved by creating a cavity that converges towards the outer surface. This results in a cavity mouth that is narrower, effectively locking the amalgam within the cavity. Additional retention if needed can be obtained by placing pins within the cavity.
Four wall support For effective condensation, the cavity should have four walls and a floor. If one or more of the walls of the cavity is absent, a stainless steel matrix can compensate for the missing walls. Failure to have a four wall support can result in inadequate condensation which can weaken the amalgam.
Preventing tensile fracture Since amalgam has poor tensile strength, the cavity should have sufficient depth and width in order to provide sufficient bulk to the amalgam, especially those in high stress areas.
Cavosurface angle The junction of the cavity with the external surface should be as close to a right angles as possible. Beveling is not indicated for amalgam as it can cause fracture of the amalgam at the margins.
SELECTION OF MATERIALS Alloy
The alloy is selected based on clinical need:
• For restorations subjected to occlusal forces, an amalgam with high resistance to marginal fracture is desirable.
• If strength is needed quickly the best choice is spherical or high copper alloys, but they require a fast operator.
• A non-zinc alloy is selected in cases where it is clinically difficult to control moisture. • Indium containing alloys: Indium performs the same functions as zinc and in
addition, it decreases the γ2 phase. Mercury
There is only one requisite for dental mercury and that is its purity. Common contaminating elements such as arsenic, can lead to pulpal damage. A lack of purity may also adversely affect physical properties. High purity mercury is labelled as ‘triple distilled’.
• Freezing point : -38.87°C • Boiling point : 356.90°C
ADA Sp. No. 6 for dental mercury requires that the mercury should possess no surface contamination and less than 0.02% nonvolatile residue.
Dispensers
Because proportioning is important, manufacturers have developed some simple dispensers for alloy and mercury. Dispensing by volume is unreliable because it