The Remineralizing Effect of Toothpaste Containing Hydroxyapatite on Initial Interproximal Caries Adjacent to Different Restorative Materials: An In Vitro Study

The mouth is a dynamic battlefield where two forces constantly vie for control over the surface of our teeth. On one side, bacteria in dental plaque produce acids that dissolve the hard mineral structure of enamel, a process called demineralization. On the other side, natural defenses like saliva, along with protective agents in our daily care, work to rebuild that lost mineral, a process known as remineralization. When the acid wins, a cavity forms; when the repair mechanisms win, the tooth heals itself. This healing is most effective in the earliest stages of decay, when the damage is still microscopic and the tooth surface remains intact. For decades, fluoride has been the gold standard for tipping the balance in favor of repair, strengthening the enamel and helping it resist future acid attacks. Recently, however, a new contender has entered the arena: synthetic hydroxyapatite. This material is a microscopic version of the mineral that makes up our natural teeth, designed to fill in tiny surface defects and provide a scaffold for new mineral growth. While both fluoride and hydroxyapatite show promise, it remains unclear how they perform when placed next to the different materials dentists use to fill cavities, and whether they can effectively heal the hard-to-reach areas between teeth where decay often begins.

To answer these questions, researchers at Chulalongkorn University in Thailand conducted a controlled experiment to see how different toothpastes interact with early decay next to two common types of dental fillings. They created eighty small blocks of human tooth enamel, each featuring a tiny, standardized area of artificial decay designed to mimic the very beginning of a cavity. These blocks were then paired with two different filling materials: a resin composite, which is the tooth-colored plastic material used in most modern fillings, and a glass ionomer cement, a material known for releasing fluoride ions over time. The researchers attached these enamel blocks to the fillings to simulate the tight contact point found between teeth in a real mouth. They then divided the specimens into eight groups, exposing them to different daily treatments: a standard fluoride toothpaste, a toothpaste combining fluoride and hydroxyapatite, a toothpaste containing only hydroxyapatite, or no toothpaste at all as a control. Over a period of fourteen days, the specimens were subjected to a rigorous cycle of acid exposure and mineral rebuilding, mimicking the harsh environment of a mouth prone to cavities, before being carefully measured to see what had changed.

The results revealed a clear distinction between the two filling materials. The glass ionomer cement consistently created a more favorable environment for healing than the resin composite. In the groups paired with glass ionomer, the enamel blocks showed a net gain in mineral density, meaning the tooth structure actually became stronger and the depth of the artificial decay decreased. This happened even in the group that received no toothpaste, suggesting that the glass ionomer itself released enough minerals to aid the repair process. In contrast, the enamel blocks next to the resin composite generally lost mineral density, indicating that the plastic material did not offer the same protective benefit. The resin groups saw their decay deepen regardless of the toothpaste used, though the rate of damage varied.

When looking at the toothpastes themselves, the findings were nuanced and depended heavily on which filling material was present. In the glass ionomer groups, all three types of toothpaste helped reduce the depth of the decay compared to the control, but the combination of fluoride and hydroxyapatite did not show a clear advantage over the others in terms of rebuilding the internal mineral structure. However, the hydroxyapatite-only toothpaste produced the smoothest surface on the enamel, reducing the roughness that bacteria love to cling to. In the resin composite groups, the story was different. The standard fluoride toothpaste was the only one that managed to keep the mineral density stable, preventing further loss. The hydroxyapatite-only toothpaste, while it could not stop the overall loss of minerals, was the only formulation that successfully reduced the depth of the decay compared to doing nothing. It also produced the smoothest surface of all, creating a finish that was significantly less rough than the other treatments.

Microscopic analysis provided a closer look at why these differences occurred. The surfaces of the enamel blocks treated with hydroxyapatite, either alone or with fluoride, appeared much smoother and more uniform under high magnification, with fewer visible pores and irregularities compared to the rough, pitted surfaces of the untreated controls. Chemical analysis showed that the glass ionomer groups had detectable levels of fluorine on the enamel surface even without toothpaste, confirming that the filling material was actively contributing fluoride to the environment. In the resin groups, fluorine was only found where fluoride toothpaste had been applied. Interestingly, the group with glass ionomer and hydroxyapatite toothpaste showed the highest levels of calcium and the best ratio of calcium to phosphorus, suggesting a rich environment for mineral deposition.

The study concludes that while the choice of filling material plays a major role in whether early decay can heal, the type of toothpaste used also matters significantly. Glass ionomer cement appears to offer a superior environment for remineralization compared to resin composite. For patients with resin fillings, standard fluoride toothpaste remains the most reliable option for preventing further mineral loss. However, toothpastes containing hydroxyapatite offer a distinct benefit by smoothing the enamel surface and reducing the depth of early decay, particularly when used alongside glass ionomer restorations. This smoothing effect could be clinically valuable, as a smoother surface is less likely to trap plaque and bacteria. While these findings are based on a laboratory simulation and not a clinical trial on living patients, they suggest that the interaction between daily oral care products and dental restorations is a complex relationship that dentists and patients should consider when managing early tooth decay.