Sealing margins occur through which mechanism?

• A. Buildup of corrosion products
• B. Polymer deposition
• C. Thermal expansion mismatch
• D. Fluid infiltration

Answer: (A)

The sealing of margins comes from the buildup of corrosion products that form a stable, adherent surface film at the metal–tooth interface. This protective layer, often an oxide, fills tiny gaps and acts as a barrier to fluid flow and bacterial ingress, helping to seal the margin and reduce further corrosion. In metal-based restorations, the spontaneous formation of this oxide layer is a key reason margins can become effectively sealed over time.

Polymer deposition would require applying a sealant and isn’t the natural mechanism at play. Thermal expansion mismatch tends to create or widen gaps with temperature changes, not seal them. Fluid infiltration describes leakage, which is the opposite of sealing.