However, if the dentin in the cavity floor is thin (< 0.1 mm), there is evidence that HEMA is ______ in vivo.

• A. Cytotoxic
• B. Non-toxic
• C. Benign
• D. Unknown

Answer: (A)

The main idea is that resin monomers like HEMA can diffuse through dentin and reach the pulp, so the thickness of the dentin barrier determines the level of exposure the pulp experiences. When the dentin on the cavity floor is very thin, less than about 0.1 mm, the barrier is too weak to dilute or block the monomer, allowing HEMA to reach pulpal tissues at concentrations that can harm cells. In vivo studies show that such exposure reduces pulp cell viability and elicits inflammatory responses, which is what we mean by cytotoxic. If the dentin were thicker, the barrier would better limit diffusion, reducing toxicity risk. Benign or non-toxic isn’t supported by the observed cellular damage, and unknown isn’t accurate given the evidence. So the in vivo effect under this thin-dentin condition is cytotoxic.