Which shape of cavity causes rotation of the restoration and a wedging effect on the supporting dentin bridge?

• A. Cup
• B. Box
• C. Circle
• D. Oval

Answer: (A)

The shape tested is about how cavity geometry affects how the restoration sits and how it compresses the surrounding tooth structure. A cup-shaped preparation has an inwardly curved floor and walls that create an undercut around the cavity. When amalgam is condensed into this space, the material must curl into the rounded bottom, which often causes it to rotate slightly as it seats. At the same time, the condensed mass pushes against the surrounding dentin bridge, producing a wedging effect that loads and compresses the supporting dentin. This combination helps lock the restoration in place more effectively than shapes that lack that pronounced undercut and curved bottom.