Microbiome Changes After Laser and Chlorhexidine Disinfection in Caries Removal

Prevention and treatment of dental caries, one of the most common and oldest diseases, is closely related to human health. Dental hard tissue undergoes remineralization and demineralization processes in a cycle. Caries occur when this balance is disturbed in favor of demineralization. Dental caries is the loss of minerals caused by the acid produced by microorganisms.

In conventional caries removal, high-speed rotating hand instruments are generally used to reach the lesion. In contrast, low-speed rotating hand instruments (tungsten carbide burs) remove the caries. This method involves rapid and efficient caries removal but may result in the removal of healthy tissue or affected dentin capable of remineralization. During minimally invasive procedures, more sparing and selective methods of caries removal are widely used in operative dentistry as an alternative to the conventional method. One of these methods is polymer burs produced as an alternative to conventional tungsten carbide burs. They are made of soft polymer material. It removes only infected dentin. Polymer burs are harder than carious dentin and softer than healthy dentin. Therefore, when removing carious tissue with a polymer bur, the bur remains intact, but when it comes to healthy tissue, the bur wears out. It is designed to remove only carious dentin by selectively maintaining its cutting efficiency until it encounters caries-affected dentin. In a previous study, microbiological Streptococcus mutans bacteria count reduction before and after occlusal caries removal was statistically significant with polymer burs compared to tungsten carbide burs.

In restorative dentistry, complete removal of infected caries tissue before the restoration process plays a key role in the success of the treatment. Due to the risk of removing healthy dentin tissue while removing infected caries tissue, protective cavity preparation techniques have been developed. Criteria such as color and hardness, which are considered in the decision to remove caries from dental tissue, are subjective and depend on the senses of sight and touch. Although researchers recommend the use of caries indicators based on objective data, studies have shown that indicators are only 15-40% successful in detecting the presence of microorganisms. Despite the removal of the stained tissues, the presence of microorganisms was observed in the area close to the pulp, especially in the dentinal tubules. These microorganisms remaining in the tissue may cause secondary caries formation, postoperative sensitivity, and pulpal inflammation. Studies have shown that the occlusive and sealing properties of the restoration material cannot prevent the residual bacteria in the smear layer and dentinal canals from diffusing into the pulp tissue and causing infection. Therefore, the use of antibacterial cavity disinfectants has come to the fore. Today, chlorhexidine digluconate (CHX), benzalkonium chloride, sodium hypochlorite, hydrogen peroxide, iodine solutions, phosphoric acid, fluoride, propolis, ozone, light-activated disinfection systems, and laser systems are used as cavity disinfectants.

The microbiology of dental caries is a subject that has been studied for many years. Previous studies have used traditional culture-based methods to identify bacteria associated with dental caries. The culture-based method has allowed a basic understanding of the composition of dental plaque microbiota in dental caries. Microorganisms have been isolated from carious lesions or dental plaque samples collected from a cross-sectional or longitudinal study using culture-based techniques. However, research on the oral microbiome is still incomplete as artificial media cannot fully mimic the natural environment in which oral bacteria are found, because about half of the oral microbiome cannot be cultured. Therefore, it is not possible to detect all bacteria in the sample in studies using the culture method. Molecular genetic techniques can detect and identify the microbiome without culture using the analysis of 16s rRNA found in all these prokaryotes. The 16s rRNA gene is used as a standard for the classification and identification of the microbiome because 16s rRNA gene sequences contain hypervariable regions that can provide species-specific sequences for the identification of bacteria. Although initially used to identify bacteria, 16s rRNA sequencing was later used to reclassify bacteria into new species or genera. It has also been used to identify new species that have not yet been successfully cultured. Recently, high-throughput pyrosequencing has revealed unexpectedly high diversity within the human oral microbiome. Molecular genetic techniques have been used to detect and identify microorganisms in permanent teeth, necrotic pulp and periapical lesions, and clinical samples without direct culture.

The understanding of caries microbiology has been greatly enriched in recent years by the advancement of sequencing-based technology and bioinformatic analysis. However, there are no studies in the literature evaluating microbiome analysis in combination with different caries removal techniques and cavity disinfection methods. Therefore, this study aims to review the classical concepts of microbiological aspects of dental caries and to investigate the effects of cavity disinfection with laser and chlorhexidine on the microbiome in cavities prepared using different caries removal methods.