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.

Learn about dental amalgam, its advantages, disadvantages, indications, contraindications, classification of amalgam alloys, and cavity preparation techniques. Explore the importance of conservative outlines and resistance-retention form for amalgam restorations.