To achieve a good resistance form, how much would you reduce a cusp being replaced with amalgam onlay?

In restorative dentistry, particularly when working with a cusp that is being replaced with an amalgam onlay, achieving proper resistance form is crucial. Resistance form refers to the geometric design of a cavity that helps prevent the restoration from moving or dislodging under occlusal forces during function.

Reducing the cusp by 2 mm is recommended because this depth effectively provides a stable base for the amalgam onlay. A 2 mm reduction creates sufficient bulk of material, ensuring that the restoration can withstand the compressive forces it will encounter in a functional bite. Furthermore, this reduction also allows for proper adaptation and bonding of the amalgam, facilitating enhanced retention and minimizing the likelihood of fracturing or dislodging.

While maximum support and retention are important, the specific choice emphasizes resistance form, which is essential for the longevity of the restoration. It's important for practitioners to understand that too little reduction may compromise the mechanical properties of the restoration, whereas excessive reduction can weaken the tooth structure itself.

The focus on 2 mm aligns with established guidelines in restorative dentistry. Hence, this depth is a standard practice to ensure effective resistance form in the context of an onlay restoration.