What shape should an amalgam preparation have for resistance?

• A. Cup-shaped
• B. Cylindrical-shaped
• C. Box-shaped
• D. Conical-shaped

Answer: (C)

When we talk about resistance form, the goal is to shape the cavity so that biting forces are spread over a broad, stable area and the tooth-restoration complex is protected from fracture. For an amalgam restoration, the box-shaped preparation is the most effective because it creates flat, wide walls and a flat pulpal floor. This geometry distributes occlusal stresses evenly across the material, minimizes stress concentrations at sharp corners, and helps resist cuspal forces that would otherwise cause cracks or fractures. The flat floor and straight or slightly converging walls provide enough bulk of amalgam to withstand chewing forces without deforming or chipping.

In contrast, a cup-shaped or conical shape concentrates stress into a smaller area and introduces curved or tapered walls that can flex under load, increasing the risk of fracture. A cylindrical form lacks the broad, supportive walls needed to resist lateral and occlusal forces effectively. So, the box-shaped design offers the best combination of resistance by providing a stable, well-supported restoration that can handle functional loads over time.