Restorative & Materials · single question
What is the ideal cavosurface margin angle for an amalgam preparation?
Show answer and explanation
Correct answer: C. 90 degrees
- A. A 45° bevel produces exactly the thin, weak margin that amalgam cannot tolerate — bevels belong to CAST restorations.
- B. An obtuse cavosurface angle leaves the amalgam at the margin as a thin wedge that fractures readily.
- C. CORRECT. Amalgam requires a 90° cavosurface angle — a BUTT JOINT. This gives the amalgam at the margin its maximum bulk and therefore maximum edge strength. Amalgam is strong in compression but weak in tension and shear, so any thin feather edge of material at the margin will chip under load.
- D. 135° would create an extremely thin, unsupportable margin of amalgam.
Note: The contrast worth holding: amalgam takes a 90° BUTT JOINT; cast gold takes a BEVEL producing a 30–40° marginal metal angle, because gold is ductile and can be burnished. Never bevel enamel margins for amalgam.
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