Why should zinc oxide and eugenol not be used under a composite filling?

Zinc oxide and eugenol is not recommended for use under composite fillings primarily because it can negatively affect the bond and adhesive properties of the composite material. This combination can create a chemical environment that interferes with the bonding agents typically used with composites, leading to reduced strength and potential failure of the restoration. The acidic nature of some components and the ability of eugenol to inhibit the polymerization of resin materials can compromise the integrity of the bond, which is crucial for the long-term success of a composite filling.

While discolouration of the filling is a concern with some materials and might be influenced by other factors, the impact on bond strength is more critical when considering the durability and effectiveness of the composite restoration. Sensitivity and the thickness of the material can also be issues, but the primary reason for avoiding zinc oxide and eugenol underneath composites is their detrimental effect on adhesion and bonding.