Which ingredient in composite resin materials acts as the photoinitiator to start the polymerization process?

The correct answer is Camphorquinone.

- In composite resin materials used in dentistry, the polymerization process is initiated by a photoinitiator.
- Camphorquinone is the most commonly used photoinitiator because it effectively absorbs blue light (wavelength around 468 nm) and generates free radicals necessary to start the polymerization of resin monomers.
- Upon exposure to the curing light, camphorquinone reacts with a co-initiator (usually an amine compound), which leads to the formation of free radicals that trigger the polymerization process, turning the resin from a pliable state into a hardened solid.

Other options:
- Benzoyl peroxide is a chemical initiator but is mainly used in self-curing (auto-polymerizing) composite resins, not in light-cured systems.
- Hydroquinone is an inhibitor used to prevent premature polymerization during storage.
- Dimethylaminoethyl methacrylate is a co-initiator (amine) that works alongside camphorquinone but is not the primary photoinitiator itself.

Therefore, the presence of camphorquinone in light-cured composites is essential to harness the curing light for effective and controlled polymerization.

Reference: Sturdevant's Art and Science of Operative Dentistry, 6th Edition, Chapter 11 / Page 406