Reaction of Silver Diammine Fluoride with Hydroxyapatite and Collagen

Objectives: Silver diammine fluoride (SDF) is used as an anticaries agent; however, its mode of action is uncertain, whether chemical, physical or mechanical. As a preliminary study, the effect of SDF on hydroxyapatite (HAp) and gelatine (as a chemical equivalent of collagen) was examined.

Methods: 2.5 mg HAp powder specimens and 0.5 mL 10% gelatine (Riedel-de Haën) (initially as a sol at ~37 °C), were mixed with 0.5 mL of 38% SDF (J. Morita), 4% NaF (Sigma) or 40% AgNO3 (Sigma) and tumbled in 1.5 mL polypropylene tubes (Sarstedt) for 48 h at ~23 °C, in two series: exposed to laboratory lighting, and kept dark at all times. The HAp specimens were separated by centrifugation and decanting, then these and one set of gelatine specimens were dried at 60 °C in situ; a second parallel set of gelatine specimens were dried at ~23 °C. Each was washed with 1 mL deionized water for 1 min, 3 times. Treated materials were observed, before and after washing, with Scanning and Transmission Electron Microscopy (SEM, TEM); Energy-Dispersive X-ray analysis (EDX), and X-Ray Diffraction (XRD).

Results: SDF appeared to produce globular particles of CaF2 on the surface of the HAp, but these disappeared on washing, while with AgNO3 yellow cubic crystals of Ag3PO4 formed, not dissolved on washing, which darkened, converting gradually to metallic silver, on exposure to light. NaF had no effect on gelatine, while with SDF and AgNO3 particles of silver were produced which were resistant to washing.

Conclusions: Both principal components of tooth tissue react with SDF; the solubility of the putative CaF2 formed weakens the case for it exerting a caries-protective effect. The importance of the persistent silver needs further study.