Synthetic Hydroxyapatite as a Biomimetic Alternative to Fluoride in the Prevention of Dental Caries

• The effectiveness of hydroxyapatite has been evaluated in many studies focusing on the prevention of oral diseases. Unlike fluorides, accidental ingestion of HAP as an ingredient in toothpaste is not associated with any significant systemic health risk
• The use of biomimetic HAP as an active ingredient in oral hygiene products may be clinically useful and helpful in planning dental care for children and adults even in cases of high caries risk

The main active ingredient of consumer products preventing tooth decay (toothpastes, rinses) for almost 100 years has been fluoride. Evidence that fluoride toothpastes significantly reduce caries is well documented in the literature (3, 4, 5, 6). The cariostatic mechanism of fluoride action consists mainly in inhibiting demineralization and promoting remineralization of enamel with calcium and phosphates from saliva and limiting the growth of plaque bacteria (7). To achieve an anti-caries effect, fluoride preparations should be applied directly to the tooth surface regularly, preferably daily (8).

Fluorine

Despite the beneficial anti-caries effects of fluoride, excessive consumption may have toxic effects on the body (9). It has been calculated that the risk of dental fluorosis is minimized if the daily intake of fluoride from all sources does not exceed 50 μg/kg body weight (10). However, it has been proven that the risk of dental fluorosis occurs even in places where drinking water has a low fluoride content (below 0.25 mg/l) (11). Daily use of oral hygiene products may contribute to increasing the total daily fluoride intake (12). Children are particularly susceptible to excessive fluoride intake, because they can swallow up to 50% of the applied toothpaste during toothbrushing (9). The average fluoride intake from swallowing toothpaste during daily toothbrushing is estimated at about 1.4 μg/kg body weight for adults and 11.5 μg/kg body weight for children (13). At the same time, it should be noted that the preventive effectiveness of fluoride toothpastes depends on the fluoride concentration. For safety reasons, it is recommended to limit the amount of toothpaste applied to the toothbrush in children, but at the same time it is recommended to use higher fluoride concentrations. The guidelines of Polish experts provide for the use of a trace amount of 0.1% F (1000 ppm) toothpaste in children aged 6-36 months, a pea-sized amount of toothpaste in children aged 3-6 years, and 1-2 cm of 0.15% F (1450 ppm) toothpaste in children over 6 years of age (14). However , in vitro studies show that the amount of toothpaste applied may affect its ability to clean the enamel surface and a better cleaning effect is obtained with a larger volume of toothpaste (15).

The above-mentioned limitations in the use of toothpastes containing fluorides justify the need to use other effective compounds with potential anti-caries effects that can be safely used by patients of all ages, in any concentration, without the risk of overdose and causing side effects.

Hydroxyapatite

A promising agent that can be considered in the prevention of tooth decay is hydroxyapatite (HAP). HAP – a mineral with the chemical formula Ca ₅ (PO 4 ) ₃ (OH)) – is a calcium phosphate with micro- and nanocrystalline morphology, which is the basic mineral component of teeth and bones. It is commonly used as a biomaterial in medicine and dentistry due to its excellent bioactivity (potential for the deposition of calcium phosphate deposits) and osteoconductivity (ability to create a scaffold for bone tissue) (16). In addition, this mineral has additional beneficial properties: it is biomimetic (it exhibits physical and chemical features similar to those characterizing the hard tissues of the human body), non-toxic, non-immunogenic and does not cause significant inflammatory reactions in cells and tissues, which has been confirmed in numerous in vitro and in vivo experiments (16, 17, 18, 19).

The efficacy of HAP has been assessed in many studies focusing on the prevention of oral diseases (20, 21, 22, 22). In contrast to fluorides, accidental ingestion of HAP as a component of toothpaste is not associated with any significant systemic health risk, e.g. fluorosis, because hydroxyapatite is the main inorganic component of human hard tissues such as teeth and bones (24). HAP can be synthesized in various crystalline forms and particle sizes (from nano- to micrometers) (25). HAP in the nanoscale is of particular interest to scientists investigating the issue of material biocompatibility.

The literature review shows that HAP nanoparticles are non-toxic (24). Although the application of HAP nanoparticles to tissues may increase the concentration of calcium ions in the cytoplasm of cells, in a living organism the excess ions can be easily eliminated through the cell membrane. HAP nanoparticles, used as a bone substitute and implant coating, do not have a harmful effect because they are absorbed by osteoclasts and macrophages. Oral administration of HAP is safe because the particles are dissolved by gastric acid, releasing calcium and phosphate ions. HAP nanoparticles are also not absorbed by the oral mucosa. Furthermore, the morphology and size (nano) of calcium phosphate particles do not seem to have a significant effect on the biological response of cells. In conclusion, the risk associated with exposure to calcium phosphate, including hydroxyapatite, at doses commonly used in dentistry, health care products and cosmetics is very low and, according to the available data, is not clinically significant (24).

Cariostatic mechanism of action of hydroxyapatite

According to the latest reports, the cariostatic mechanism of action of hydroxyapatite is based on (26):

a. creating a protective hydroxyapatite layer on the surface of hard tooth tissues due to the strong affinity of HAP for enamel and dentin. After application to the tooth surface, HAP molecules create mineral-mineral bridges with enamel hydroxyapatite crystals. This results in the formation of a protective layer on the tooth surface, which has the ability to remineralize decalcified tissues. At the same time, it can protect the tooth from the harmful effects of acids of bacterial or dietary origin. Long-term use of oral care products (toothpastes, rinses) containing HAP causes thickening and constant renewal of the layer adjacent to the tooth surface, strengthening its protective effect;

b. releasing calcium ions and neutralizing acidic pH. HAP not only creates a protective layer on the tooth surface, but can also be incorporated into dental plaque. Acids of bacterial or dietary origin dissolve the molecules of the protective layer of HAP, which causes the release of calcium ions, which participate in buffering acids, increasing the pH of dental plaque. The following benefits result from this:

• tooth tissues remain intact because acids attack the protective HAP layer and not the enamel
• Calcium ions released from HAP remineralize demineralized enamel, both in its superficial and deep layers. Additionally, the increase in calcium concentration in dental plaque and on the tooth surface shifts the chemical balance from the enamel dissolution phase towards a more stable state
• an increase in pH protects tooth tissues from demineralization. Due to the fact that the critical pH value at which enamel hydroxyapatite dissolves is approximately 5.5, even a small increase in plaque pH can protect the tooth surface from demineralization;

c. inhibition of bacterial adhesion to the surface of tooth tissues. HAP particles adsorbed to tooth tissues prevent the adhesion of the first microorganisms (initial colonizers) to the enamel surface covered with the acquired pellicle. It has been shown that bacteria adhere to HAP particles instead of to the tooth surface, forming aggregates that are easily removed from the oral cavity with saliva during swallowing or mouthwash. This reduces the number of microorganisms capable of forming dental plaque. This reduces the deposition of dental plaque and thus reduces the risk of demineralization of tooth tissues by acids of bacterial origin;

d. deep remineralization of carious lesions. HAP molecules are a source of calcium ions for the remineralization process. Regardless of the HAP concentration in oral care products, remineralization of carious lesions with HAP is more uniform compared to fluoride. In the case of fluoride, remineralization is limited to the superficial layer of the carious lesion. Remineralization with HAP appears to be deeper and more homogeneous throughout the subsurface layer of the lesion, contributing to the integrity of hydroxyapatite crystals in the enamel and promoting their growth. This biomimetic mineralization process enables the regeneration of enamel and dentin.

Studies confirming the effectiveness of HAP

In vitro studies have shown that HAP remineralizes decalcified enamel and dentin and protects teeth from demineralization (27). However, for practitioners, the most important evidence is the one from in vivo studies , designed as randomized clinical trials. Five randomized studies conducted so far in patients have shown that HAP contained in toothpastes has an anti-caries effect (21, 23, 28, 29, 30).

The first study from 1989 assessed the effect of using a toothpaste containing 5% hydroxyapatite on the occurrence of dental caries in primary school children. After 3 years of using the toothpaste, a statistically lower increase in the DMFT index was demonstrated in the group of children brushing their teeth with toothpaste containing hydroxyapatite compared to the control group (placebo toothpaste) (28).

Two other independent studies analyzed the remineralizing effect of hydroxyapatite toothpaste on initial carious lesions of the buccal and interproximal surfaces of permanent teeth of children and young adults (23, 29). The ICDAS system (International Caries Detection and Assessment System), laser fluorescence method (DIAGNOdent device), computer software analyzing the extent of carious lesions on digital photographs and digital radiography were used to assess the changes. The simultaneous use of multiple research methods increases the reliability and objectivity of the studies. Interestingly, from a methodological point of view, Badiee et al. (29) additionally used fluoride toothpaste as a positive control and although both toothpastes (with HAP and fluoride) significantly reduced the extent of initial carious lesions in patients after orthodontic treatment, better results were obtained with the HAP toothpaste.

Grocholewicz et al. (23) in a long-term (2-year) study compared the efficacy of a gel containing 10% HAP (group 1), ozone therapy (group 2) and the combined action of gel and ozone (group 3) in remineralization of initial carious lesions on the proximal surfaces of posterior teeth. Digital bitewing radiographs were used for the assessment. After two years of observation, a reduction in the percentage of initial carious lesions was noted in all groups, but the best effects were observed with the combined action of gel with HAP and ozone therapy.

Two subsequent randomized clinical trials conducted by Schlagenhauf et al. (21) and Paszyńska et al. (30) provided further evidence for the anticaries efficacy of HAP toothpastes in permanent and primary teeth of patients at high risk of caries. Both the 6-month follow-up by Schlagenhauf et al. (21), who evaluated young orthodontically treated patients, and the one-year study by Paszyńska et al. (30), which included children with primary dentition, showed that daily use of HAP toothpaste was as effective in preventing caries progression as using fluoride toothpaste.

Based on the cited studies, it can be concluded that the use of biomimetic HAP as an active ingredient in oral hygiene products can be clinically useful and helpful in planning dental care for children and adults even in cases of high risk of caries. Unlike fluorides, HAP is biocompatible and safe in case of accidental ingestion.

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Authors: dr hab. n. med. Małgorzata Pawińska ¹ , dr hab. n. med. Elżbieta Paszyńska ²

¹ Department of Integrated Dentistry, Medical University of Białystok

² Department of Integrated Dentistry, Poznań University of Medical Sciences

Article published in issue 7/2022 of Nowy Gabinet Stomatologiczny magazine. See full table of contents.