Australian Dental Council (ADC) Practice Test

For an amalgam onlay, achieving good resistance form is essential to ensure that the restoration can withstand the occlusal forces and remain adequately supported by the tooth structure. Reducing a cusp by 2 mm provides sufficient space for the amalgam material to create a strong bond with the remaining tooth structure while also enabling the restoration to resist forces that may cause dislodgment or fracture.

The reduction depth of 2 mm allows for an appropriate thickness of the amalgam, which is critical for its durability and resistance to wear over time. Adequate reduction ensures that the restoration can properly engage with the tooth, minimizing the risk of loosening or failure. This depth balances the need for a sufficient amount of material for strength and the preservation of remaining tooth structure.

Options suggesting a reduction of 1 mm may not provide enough space for the material to effectively resist forces, potentially compromising the longevity of the restoration. A reduction of 3 mm may unnecessarily sacrifice more tooth structure than is required for effective restoration, which can negatively impact tooth vitality and integrity. Thus, a reduction of 2 mm strikes the right balance for good resistance form in an amalgam onlay.