Dental Amalgam Restorations: Classification and Cavity Preparation

Amalgam Restoration

• Rounding of the axiopulpal line angle and all line angles is essential.
• If a cusp is undermined and needs to be capped with amalgam, it must be reduced to a minimum of 2 mm.
• Every portion of compound cavities should have adequate self-retention; auxiliary portions should not depend on principle portions for retention.

Manipulation of Amalgam

• Selection of alloy and mercury is crucial.
• Proportioning, trituration, condensation, carving, and finishing and polishing of the restoration are essential steps.

Selection of Alloy

• Spherical amalgam provides soft consistency, requiring light condensation force, and is recommended for restoring pulp-capped teeth and adapting around pins.
• High copper amalgam provides high strength and is used for extensive restorations.
• Moisture contamination requires the use of Zinc-free alloys to prevent delayed expansion.
• Alloys are supplied in the form of tablets, powder, and capsules.

Proportioning of Alloy and Mercury

• The amount of alloy and mercury to be used depends on the amount required to slightly overfill the cavity, the amount that can be condensed within a limited period of 3-5 minutes, and gauged by weight.
• If the cavity requires an amount in excess of what can be condensed in 3-5 minutes, more than one mix should be utilized.

Alloy/Mercury Ratio

• The amount of mercury should be enough to wet every alloy particle.
• If less mercury is used, some particles will remain uncoated, resulting in a non-coherent, friable mix that is difficult to condense.
• If excess mercury is used, more Hg will be left in the final restoration, resulting in increased formation of γ2 phase, reduced bond between the non-consumed particles and the different phases, and reduced strength, increased expansion, decreased hardness, and increased flow and creep.

Trituration

• The amount of trituration is a function of time, speed, and pressure, and must be standardized.
• Undertrituration results in some particles remaining covered with oxides, uncoated with or bound to the structure, with excess of residual Hg remaining, resulting in a weak, corrodible, and excessively expanding amalgam.
• Overtrituration results in a less plastic, yet homogeneous and coherent mix of amalgam, setting fast as a result of rapid Hg consumption and crystallization of produced phases, resulting in a fast-setting and contracting amalgam with higher early strength and smoother surface.

Methods of Trituration

• Manual trituration involves the use of a mortar and pestle.
• Mechanical trituration involves the use of amalgamators or amalgamizers.

Condensation

• Definition: The process of forcible packing of fresh triturated mix into details of prepared cavity with objectives including elimination of excess mercury and minimizing voids.

Cavity Preparation

• Amalgam is a brittle material, and cavity preparations should have:
  • Conservative outlines, excluding centric holding areas, if possible.
  • Adequate bulk for strength, which depends on an increase in cross-sectional areas.
  • A resistance-retention form.
• Amalgam cavities should have specific features, including:
  • Cavo-surface angle of 90 degrees.
  • Cavity walls parallel or perpendicular to the direction of occlusal force of mastication.
  • A definite gingival floor, 1.5 mm deep from the surface and 0.2 mm deep in dentin in compound cavities.
  • An isthmus area of minimum width of one quarter the bucco-lingual distance between cusp tips and an increased depth.