Composites in contact with fibroblasts are cytotoxic due to which factor?

• A. Fully cured resin matrix
• B. Low pH environment
• C. Unpolymerized components
• D. Water sorption

Answer: (C)

Cytotoxicity of composites toward fibroblasts mainly comes from residual, unpolymerized components that can leach out from the resin. Even after curing, not all monomers react into the polymer network; some free monomers such as common dental resin components remain. These low‑molecular‑weight monomers readily diffuse into nearby cells and can disrupt cellular processes, impair mitochondrial function, damage DNA, and promote cell death or reduced viability. The degree of conversion (how completely the material polymerizes) strongly influences how much monomer remains and is released; deeper layers, inadequate curing light, or variation in shade can leave more unpolymerized material and increase cytotoxic potential. A fully cured matrix minimizes leaching, making it less cytotoxic. While a low pH environment and water sorption can contribute to irritation or long‑term effects, the immediate cytotoxic effect on fibroblasts in vitro is most directly due to unpolymerized components migrating from the resin.