Dental caries deserves attention because it has become a chronic disease affecting children worldwide. ^{1, 2}

Dental caries is a multifactorial disease caused by the simultaneous interaction of various factors— sugar and dental biofilms —in the oral cavity.

The introduction of fluoride into dental practice over seventy years ago is widely regarded as the principal reason for the global decline in dental caries. Despite this public health success, the sole confirmed adverse effect of fluoride use in dentistry is dental fluorosis, a condition resulting from excessive fluoride ingestion during tooth development. Recent epidemiological trends indicate a concurrent rise in the prevalence of dental fluorosis and a decline in caries rates. ³

The most common varnish used in dentistry to prevent tooth caries is sodium fluoride (NaF). ⁴

Topical application of NaF on teeth mitigates enamel demineralization while enhancing the remineralization process. The primary antimicrobial mechanism of fluoride is not direct bacterial destruction, but rather an indirect action through the inhibition of bacterial acid production, which results in a higher pH at the tooth surface. ^{5, 6}

The combined use of laser irradiation and topical fluoride represents a novel approach to caries prevention by increasing the enamel’s affinity for and incorporation of fluoride. Zancope et al. were the first to report that CO2 lasers cause morphological changes on the surface of tooth enamel, such as fusion and dissolution. ⁷

On the other hand, studies were conducted to evaluate the effect of carbon dioxide laser on the structure of tooth enamel and dentin, and showed that its absorption by dental tissue was high. ⁸

The 10600 nm CO2 laser has an absorption coefficient in hydroxyapatite ten times lower than that of 9600 nm and this allows deeper penetration and higher flow by the 10600 nm CO2 laser to produce the desired thermal effects on caries inhibition compared to the CO2 laser at other wavelengths. ⁹

Calcium, phosphorus and fluoride ions play a crucial role in remineralizing tooth enamel. ⁸

A high level of calcium in the remineralization solution may enhance the rate of mineral deposition in the caries lesion and the calcium level greatly affects the defense mechanism of the hard tissues of the teeth and as the number of caries increases the calcium level decreases. ¹⁰

This study aimed to evaluate and compare the efficacy of CO ₂ laser irradiation versus a CO ₂ application in enhancing fluoride uptake by the surfaces of permanent teeth following treatment with NaF varnish.

The results were evaluated according to the surface shape in electron microscope images and according to the concentration of fluoride, calcium, phosphorus and carbon.

In this study, SEM images showed that CO2 laser irradiation fuses the surface, creating a smooth, recrystallized side. Fusion between enamel surface crystals was also observed after carbon dioxide laser treatment ( Figure 5, 6).

Comparing the apparent differences in the mean of the measured items between the two groups ( Table 2) and the descriptive statistics for fluoride, calcium, phosphorus, and carbon ratios in both study groups ( Table 3, 4, 5, 6).

The results of the Mann-Whitney U test showed a significant statistical difference in fluoride ratios between the two groups (P value = 0.026) smaller than 0.05, indicating the effectiveness of using CO ₂ lasers in enhancing fluoride absorption within the tooth structure. In contrast, the remaining elements (calcium, phosphorus, carbon) did not show significant differences, although there were apparent changes in the arithmetic mean, in the rest of the descriptive statistics measures, and in the ranks of the averages ( Table 7).

This study evaluated the efficacy of a CO ₂ laser for enhancing fluoride uptake in dental enamel. The combined application of a CO ₂ laser with fluoride varnishes or gels, which can quadruple fluoride absorption, may significantly improve caries prevention. This synergistic effect could potentially reduce the required fluoride dosage and frequency of clinical applications. The mechanism is attributed to an increased formation of fluorapatite within the enamel, thereby elevating its resistance to acid demineralization. ¹³

In order to obtain a protective effect, a laser was used to raise the temperature of the tooth enamel to the melting point of its inorganic components or to the decomposition point of its organic components. On the other hand, non-thermal laser radiation was used after tooth fluoridation to achieve a slow fluoride release effect. This type of reaction is called laser-activated fluoride treatment. ¹¹

Carbon dioxide laser irradiation of tooth enamel, within certain parameters, results in a specific temperature increase above the irreversible pulpitis threshold of 5.5 °C, ¹⁴ which changes the chemical composition of the tooth enamel surface and the morphological structure ¹⁵ and the increase may lead to permanent damage to the tooth pulp.

Excessive temperature elevation during laser application poses a risk of injury to the dental pulp. A study addressing this risk compared the thermal effects of two laser systems: a CO ₂ laser (at 1 and 2 W) and a diode laser (at 5 and 7 W). The results indicated that the mean temperature rise remained below the 5.5 °C critical threshold for pulp damage. Based on this evidence, the authors determined that both lasers could be used safely without compromising pulp health. ¹⁶

The absorption depth of port and ivory is 12 μm, compared to the absorption depth of water of 15 μm. ¹⁷

The absorption effects of tooth enamel and dentin at these depths depend on how the laser energy affects them and the laser energy depends on laser parameters such as power, mode of operation (continuous or pulsed wave mode), flow (energy density) and dose.

The continuous wave mode resulted in carbonization, dissolution and cracking of the enamel and the development of the pulse mode allowed variations by frequency and pulse duration in milliseconds (ms) and microseconds (μs). This improved the thermal control of the laser in tissue. The dose depends on the radiation method: spot radiation or sweeping motion, the number of repeated radiations. ⁹

Several hypotheses have been advanced to explain the mechanisms by which combined laser and fluoride treatment enhances resistance to enamel demineralization. A principal theory posits that laser irradiation creates micro-spaces within the enamel structure, which subsequently serve to retain fluoride ions more effectively. These structural alterations are also believed to contribute directly to reducing enamel solubility. The observed morphological changes on the enamel surface following the sequential application of topical fluoride and CO ₂ laser irradiation provide a rationale for the superior protective effect compared to fluoride treatment alone. This synergistic effect is further attributed to the laser-induced conversion of hydroxyapatite to the more acid-resistant fluorapatite, a process accompanied by the melting and recrystallization of hydroxyapatite crystals. ¹⁸

The results of previous studies showed that treating the surface of tooth enamel using a carbon dioxide laser reduces demineralization of tooth enamel compared to the control group. Studies have shown that wavelengths from 9 to 11 micrometers of CO2 lasers are effectively absorbed by dental hydroxyapatite, causing loss of carbonate mineral, which in turn reduces acid reactivity. ^{19, 20}

The integration of lasers in preventive dentistry reflects a modern paradigm shift within the field, moving away from a traditional focus on restorative treatment and toward a philosophy of disease prevention. This approach is consistent with the recommendations of major professional bodies and is part of a wider scientific consensus advocating for less invasive caries management strategies, ²¹ Which aims to preserve tissues to the maximum extent, and in this new approach, prevention has acquired an important role that it has not played before in the past. ¹¹

All experiments combining CO2 and fluoride laser radiation have shown better results in preventing tooth caries when compared to a single treatment. Therefore, this ‘combination therapy’ may be clinically effective, and at the same time, it may only involve moderate daily doses of fluoride and low levels of laser radiation energy, which is consistent with our current study.

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Irradiation of tooth enamel at specific wavelengths and CO2 laser energy densities alters hydroxyapatite crystals, making the surface more resistant.