Where teeth require a large filling- generally speaking, if more than about half the surface of the tooth needs fixing these are preferred.
When standard filling materials and filling procedures are used to do very large fillings the lifespan of the filling is not good and the remaining tooth is also weak which may lead to fracture.
A standard filling is not recommended since it can compromise the integrity of the tooth and make it vulnerable to fractures.
Composite Overlays are more conservative than crowns since they need less tooth reduction and can be done in one visit. They cover the whole biting surface and help to protect the teeth from fracturing.