Beyond the Monomer: Camphorquinone, TPO, and BAPO as Leachates of Light-Cured and 3D-Printed Dental Polymers—Radical Chemistry, Redox Signalling, and the Dose-Realism Gap

Research on the biological cost of resin-based dental materials has focused almost exclusively on residual methacrylate monomers. The photoinitiators—camphorquinone (CQ) with its amine co-initiator, and the acylphosphine oxides TPO and BAPO—remain partly unconsumed, can elute, and are seldom measured. Vat photopolymerisation, where acylphosphine oxides dominate, makes this omission material. We critically appraise the photochemistry, elution behaviour, molecular stress responses, and exposure data for CQ, TPO, and BAPO, weighted toward orthodontic appliances, and ask whether a margin of safety can be estimated. Within studies, TPO is 15- to 250-fold more cytotoxic than CQ, and neither TPO nor BAPO raised intracellular reactive oxygen species measurably in the two studies that measured it, so the CQ redox narrative cannot be assumed to transfer. Extractable TPO content of printed splint and orthodontic resins spans about a thousand-fold; the highest values exceed the TPO EC50 numerically, but exhaustive extraction is not an intraoral dose. No retrieved study has measured any photoinitiator in human saliva. Photoinitiator hazard is established in vitro; a clinically relevant, physiologically anchored margin of safety cannot currently be estimated reliably. TPO’s EU cosmetics prohibition alongside its continued intraoral use sharpens this asymmetry.