Some caries excavation methods lead to leaving caries-affected dentin behind to serve as the bonding substrate, mostly in indirect pulp capping.
It is reported that the immediate bond strengths to caries-affected dentin are 20-50% lower than to sound dentin, and even lower with caries-infected dentin. How does caries progression correlates with this? First, it reduces mineral content, increases porosity and changes the dentinal collagen structure and its distribution too. These changes can cause a significant reduction in the mechanical properties in dentin e.g. hardness, stiffness, tensile strength, modulus of elasticity, and shrinkage during drying, which makes dentin in and under hybrid layer more prone to cohesive failures under occlusal forces.
Lower mineral content of the caries-affected dentin will allow phosphoric acid or acidic monomers to demineralize matrix more deeply than in normal dentin, which results in even more residual water in exposed collagen matrix.