Which base is favored for its thermal insulation and rigidity under amalgam?

• A. Glass ionomer cement base.
• B. Zinc phosphate base.
• C. Zinc oxide eugenol base.
• D. Polycarboxylate base.

Answer: (B)

When preparing a cavity for an amalgam restoration, the base chosen under the amalgam must both insulate the pulp from temperature changes and provide a rigid foundation to resist occlusal forces. Zinc phosphate cement fits this well: after it sets, it becomes a hard, dense layer with good compressive strength, giving the necessary rigidity to support the overlying amalgam and distribute biting forces. It also acts as a fairly effective thermal barrier, helping limit heat transfer from hot or cold foods to the dentin and pulp. Other bases either lack sufficient stiffness (for example, zinc oxide eugenol is soft and offers little structural support), or emphasize bonding to dentin and fluoride release at the expense of rigidity (glass ionomer and polycarboxylate bases), or have other drawbacks that make them less ideal for a rigid, insulating foundation under amalgam.