The Latest Advances in the Use of Nanoparticles in Endodontics

Abstract

Recent decades clearly demonstrate the growing use of nanomaterials in medical practice, and their effectiveness is systematically confirmed by the consequent scientific research. An example of the use of nanomaterials in dentistry is endodontic treatment, which, due to its specificity, is one of the most demanding procedures, fraught with numerous challenges, such as difficulties in reaching tooth roots and ineffective cleaning or insufficient sealing of root canals, which may lead to re-infection or damage to adjacent structures. The use of nanomaterials has a positive impact on solving these problems, and the combination of biomaterials with nanometric technology makes endodontic treatment more effective, precise and comfortable for patients, which contributes to improving the quality of dental care. Currently, nanomaterials with a high biocompatibility can be used in endodontics as components of irrigation solutions, for rinsing root canals and as drug carriers for intracanal use. Nanomaterials are also components of sealants filling root canals. However, the latest research shows that reducing the size of materials to the “nano” scale significantly affects their basic physicochemical properties, which leads to increased reactivity and the ability to interact at the molecular level. These unique physicochemical properties, which have contributed to the use of nanomaterials in numerous medical-related solutions, raise concerns and provoke discussions about the safety of their use in direct contact with tissues.

1. Introduction

The anatomical structure of a tooth is quite simple—it is divided into three basic parts: crown, neck and roots (Figure 1). However, the structure of the individual parts is much more complex. The crown, visible above the gum line, is covered with enamel–the outermost and hardest layer of the tooth, resistant to mechanical forces and temperature changes. It consists mostly of inorganic minerals, mainly calcium phosphate in the form of dihydroxyapatite crystals, which determine its hardness and strength. Tooth enamel is formed until it erupts; after that, it has no ability to regenerate—it is therefore a non-renewable tissue. Enamel protects the underlying sensitive and porous dentin, composed mainly of hydroxyapatite and collagen. Dentin contains numerous microscopic dentinal tubules, which contain blood vessels and nerve fibers responsible for conducting pain stimuli when the destroyed enamel layer exposes the entrances to the dentinal tubules. The pulp lies beneath the dentin and occupies the central part of the tooth—the chamber and root canals. It is formed from the same tissues as dentin, and together they form the endodontium. Tooth pulp is connective tissue; rich in blood vessels, nerves and cells, it provides nutrition and innervation to the tooth. The tooth roots are covered with periapical tissue, which provides adhesion to the structural fibers that secure the tooth in the periodontal tissue [1,2,3,4,5]. Through the anatomical opening, the ventricular–periodontal canals and the lateral canals, the pulp connects with the periodontium. The pulp is composed of mesenchymal cells and fibroblasts, and also contains cells involved in the body’s immune reactions—macrophages, lymphocytes, mast cells and plasma cells [6]. Their number depends on the functional state of the pulp, which means that it increases in cases of inflammation. The outer layer of the pulp is composed of a low-cellular bright layer—the so-called Weil’s zone, consisting of individual fibroblasts, collagen and elastic fibers; a single layer of dentin-forming cells (odonoblasts) located on the periphery of the pulp; and non-mineralized predentin [7,8].

Deep damage to dentin causes injury to odontoblast processes, and the defensive reaction is the formation of tertiary dentin, which protects the deeper tooth structures against irritating factors. However, this protection is most often temporary, and the deepening of damage caused by irritating factors may contribute to the development of inflammation. As a result of the release of inflammatory mediators, the permeability of blood vessels increases and the pressure of tubular fluid increases, which allows for the more effective flushing of harmful substances, but also leads to the irritation of nerve fibers and increased pain sensitivity. If the irritating factor is not eliminated, the pressure constantly increases, which leads to hypoxia and pulp necrosis [9,10,11].

The field of dentistry dealing with the diagnosis and treatment of diseases of the dental pulp and periapical tissues is endodontics, and the basic method of endodontic treatment is commonly called root canal treatment. Endodontic treatment is necessary when irreversible pulp inflammation occurs. Inflammation of the tooth pulp is most often caused by bacteria that enter the pulp chamber through carious lesions [12]. Progressing inflammation causes the living pulp to die, forming necrosis, which in the next stage undergoes putrefactive decay, forming gangrene. In order to protect the periapical tissues and the entire tooth, it is necessary to remove the inflamed or necrotic coronal and root pulp [13]. The basis of treatment is the drilling of the crown, exposure of the chamber and enucleation of the diseased pulp. The next stage is aseptic preparation of the canals by widening them—this way the remaining infected tissues are removed. During this procedure, the prepared tooth chamber and roots are rinsed abundantly with antibacterial agents to destroy all microorganisms. The tooth is dried and filled with a special substance that closes and seals the prepared structures, preventing the growth of bacteria. The final stage involves restoring the tooth crown. This is a very important stage that allows you to maintain the function of the dead tooth in the biting and chewing process [12,13,14,15].

Due to the very small treatment area, the diverse, individual configuration of the canal system for each patient and the virtually impossible visual control of the work inside the chamber and tooth roots, endodontic treatment is very complicated [16]. The above factors and problems related to traditional endodontic treatment methods, such as difficulties in completely removing bacteria from root canals or limitations in the effectiveness and durability of fillings, have become a motivation for scientists to look for innovative solutions, including the use of nanotechnology in dentistry [17,18,19]. Nanotechnology is a field of science and technology that deals with the manipulation and use of matter on the nanometer scale, i.e., at the level of individual atoms or molecules. The nanometer scale corresponds to dimensions of 1 to 100 nanometers. Due to their small size and high surface area to volume ratio, nanomaterials can be effectively used to improve the properties of dental materials such as composite resins, dental cements and filling materials [20]. Nanocomposites used in dentistry may be characterized by increased mechanical strength, better resistance to erosion and abrasion and improved adhesion to tooth tissues. Additionally, nanomaterials can be used to create biomaterials that are more biocompatible and can integrate with a patient’s tissues more naturally [20,21,22]. Other innovative applications of nanotechnology in dentistry include the creation of nanoparticle drug delivery systems that can be used to more effectively treat gum disease or prevent tooth decay [23]. In addition, nanomaterials can also be used to produce special coatings or surfaces that prevent bacterial adhesion, which reduce the risk of infection [24,25,26]. Finally, nanotechnology can also be used in dental diagnostics through the development of advanced imaging techniques that allow for the more accurate detection and diagnosis of oral diseases [27]. All of these innovative applications of nanotechnology in dentistry have the potential to improve the effectiveness of treatment, extend the durability of dental materials and improve the overall oral health of patients. The most frequently used nanomaterials include hydroxyapatite and bioactive glass [28] and some others—graphene, silver particles, chitosan and silicon compounds, calcium, magnesium, titanium oxides and zirconium [29]. All these nanomaterials have wide applications in various fields, which makes them an important area of research and technological development.

Although the use of nanotechnology in dentistry has great potential to improve the effectiveness of treatment, reduce the risk of complications and increase patient comfort, as in other fields it requires appropriate monitoring and research on the safety of these new technologies [30]. This paper will provide an overview of current research and experiments related to the use of selected nanomaterials in endodontics, presenting existing evidence on their effectiveness and potential and contributing to a better understanding of the potential of nanoparticles in endodontics.

2. Nanomaterials in Endodontics

The human oral cavity is a habitat for over seven hundred species of microorganisms that create complex ecosystems. Most of these bacteria form biofilms attached to tooth surfaces and mucous membranes. This biofilm, also known as dental plaque, plays a key role in both oral health and the pathogenesis of various dental diseases—it is a major factor in the development of tooth decay and periodontal disease [25,27]. Bacteria in the biofilm produce acids that demineralize tooth enamel, leading to tooth decay. In periodontal disease, bacterial toxins and enzymes degrade the supporting tissues of the tooth, leading to gingivitis and bone loss. Traditional antibacterial agents and fillers used in dentistry have numerous limitations that may affect their effectiveness in treating infections in this complex oral environment. Often, they do not effectively reach all areas of infection in the mouth and may not be effective enough against a wide range of bacteria, which can lead to the recurrence of infections, and what is more, many antibacterial agents (e.g., 2 or 5% sodium hypochlorite (NaOCl), 3% hydrogen peroxide, 40 or 50% citric acid (C₆H₈O₇), chlorhexidine (C₂₂H₃₀Cl₂N₁₀; CHX), 17% sodium edetate solution (C₁₀H₁₄N₂Na₂O₈ × 2H₂O; EDTA) degrade quickly or release active substances in a short time, which prevents a long-term antibacterial effect [31]. Nanoparticles, on the other hand, can significantly improve the effect of antibacterial agents and fillers in dentistry, eliminating many of the above-mentioned disadvantages, and they can penetrate deep into the bacterial biofilm and tissues, delivering drugs directly to the site of infection. Some nanoparticles also have strong antimicrobial properties and can fight a wide range of bacteria, even resistant strains. Nanoparticles can also be designed to control the gradual release of active substances, which ensures long-lasting antibacterial effects. This can reduce the risk of the recurrence of infection [32,33,34]. Moreover, nanocomposites can increase the mechanical strength of root sealants and fillers for pulp reconstruction (

Table 1. Characteristics of selected nanoparticles used in endodontic treatment [32,33,34].

Application	Nanoparticles	Features/Properties
sealing agents	silver nanoparticles (AgNPs)	provide long-lasting antibacterial protection, reducing the risk of root canal reinfection
	zinc oxide nanoparticles (ZnO-NPs)	exhibit antimicrobial properties, improve the mechanical and adhesive properties of sealants
filling materials	nanohydroxyapatite (nHAp)	added to filling materials improves their mechanical and biological properties and supports the regeneration of tooth hard tissues
	nanodiamonds	increase the mechanical strength of filling materials and improve their adhesive properties
intrathecal drugs	carbon nanoparticles (e.g., fullerenes, carbon nanotubes)	used as drug carriers, enabling precise delivery of active substances to infection sites.
	gold nanoparticles (AuNPs)	used for imaging, diagnostic and therapeutic purposes because of their optical and plasmonic properties, improving the effectiveness of intrathecal medications
irrigation solutions	titanium oxide nanoparticles (TiO2-NPs)	in combination with UV radiation, TiO2-NPs generate reactive oxygen species that effectively disinfect root canals
	silver nanoparticles (AgNPs)	adding to irrigation solutions increases their antimicrobial effectiveness, penetrate bacterial biofilm, used to disinfect root canals, effectively remove different strains of bacteria, including difficult-to-fight Gram-negative bacteria

).

To summarize, nanoparticles are intended to increase the strength of filling and sealing materials, which leads to more lasting treatment results; to act as carriers of drugs with extended biological activity, enabling their precise delivery to sites of infection and increasing the effectiveness of therapy; to reduce the need to use large doses of drugs, which may reduce the risk of side effects and toxicity; and above all, to improve the antimicrobial effect of substances used in endodontic treatment [23].

The oligodynamic effect of nanoparticles means that they can attack multiple target sites inside the cell, which distinguishes them from traditional antibiotics, which usually act selectively on specific structures or intracellular processes. However, due to this extraordinary ability to affect various cellular areas, it is necessary to thoroughly understand the mechanisms of action of nanoparticles in order to use them in an effective fight against bacteria resistant to conventional treatment [24]. Nanoparticles, especially those with a positive surface charge, can interact electrostatically with the negatively charged bacterial cell membrane, leading to destabilization and damage of the cell membrane, resulting in the leakage of cell contents. Some nanoparticles can also penetrate the cell membrane, creating pores or holes in it, leading to a loss of its integrity [26]. Many nanoparticles, especially metal oxides (e.g., zinc oxide, titanium oxide), can generate reactive oxygen species (ROS) through photocatalytic or redox activity. ROS are highly reactive and can cause the oxidation of lipids in the cell membrane, which leads to its peroxidation and damage. Moreover, they can damage enzymatic proteins, nucleic acids and other key bacterial proteins (which leads to oxidative stress and disturbs cellular functions), and induce DNA strand breaks or modifications of nitrogenous bases (which in turn leads to mutations and DNA replication disorders) [35]. Nanoparticles can disrupt various metabolic pathways in bacterial cells and inhibit the action of enzymes key to energy metabolism and biosynthesis. Under certain conditions, nanoparticles can aggregate on the surface of bacterial cells, forming layers that physically block access to nutrients and oxygen, or bind key nutrients such as trace metals that are essential for bacterial growth (Figure 2). All these mechanisms lead to bacterial cell death and make nanoparticles promising tools in the fight against bacteria, especially in difficult-to-treat cases such as biofilm-related infections. Their multifunctionality and ability to attack bacteria in different ways make them more effective than traditional antibacterial agents [36].

2.1. Nanohydroxyapatite

Hydroxyapatite (HA) in the form of calcium phosphate is a substance that occurs naturally in the human body. It is the main building component of enamel, dentin and bones [37]. Due to its biocompatible properties, it can be used as a drug carrier for bone tissue, e.g., to treat inflammation. Thanks to its structure and properties, hydroxyapatite releases drugs gradually, ensuring long-lasting therapeutic effects. One of the advantages of using hydroxyapatite as a drug carrier is its ability to mimic natural biological processes in the body, which minimizes the risk of adverse reactions and improves the body’s tolerance to therapy. Moreover, hydroxyapatite has the ability to interact with bone cells, which can support the healing and regeneration processes of bone tissue [38,39].

Due to the origin of HA, three types can be distinguished: mineralogical, biological and synthetic [40]. Mineralogical hydroxyapatites are a natural form of hydroxyapatite that occur in living organisms. This type of hydroxyapatite is the main mineral component of tooth enamel, dentin and bones. Biological hydroxyapatites occur in pathologically calcified tissues, forming kidney or tonsil stones [41]. Synthetic hydroxyapatite can be obtained by various chemical reactions, hydrolysis, pyrolysis or the sol–gel method. Obtaining nanohydroxyapatite is possible using the same technologies, which, however, require several modifications [42].

In order for a substance synthesized in this way to be effectively used in medical applications, the critical factors—the size and shape of the particles—must be as close as possible to the crystals that build natural bone. Nanoparticles of appropriate sizes and morphology can easily penetrate cells and influence their metabolic functions [43], and the molar ratio of calcium to phosphorus (Ca/P) in the structure of nanohydroxyapatite is important for its biological properties, including its stability, biological activity and ability to interact with tissues [44].

Nanohydroxyapatite is biocompatible with human tissues, which is why it is used in implantology, conservative dentistry, dental prophylaxis and endodontics. The high bioactivity of nanohydroxyapatite is due to many factors, including its similarity to natural bone apatite (it has a structure and chemical composition similar to natural bone apatite, which makes it well recognized by the body and it can easily integrate with bone tissues) and strong ion exchange affinity (this enables interaction with ions in the surrounding tissue, which is crucial for the processes of the regeneration and remineralization of bone tissue) [45,46]. It is well tolerated by the human body and does not cause undesirable immunological reactions, it does not create a contact layer of pulp necrosis, there are little or no inflammatory reactions in the pulp after using this preparation, it affects the regeneration of dentin and bone stimulation in the healing process—this means that research is constantly being conducted and new endodontic materials containing nHAp are being created [47]. They are mainly used for biological pulp treatment in direct and indirect covering or as sealers for root canal fillings [48].

Biological methods of treating pulp diseases include direct and indirect pulp capping. The goal of these procedures is to keep some or all of the pulp viable. Living pulp is the best protective barrier against local and general complications resulting from the presence of dead tissue in the body. As a result of the removal of pathogenic stimuli and the use of odontotropic preparations, defensive dentin is formed [49]. Currently, many materials are used to cover the pulp, but they do not always meet the expectations of clinicians. Therefore, in recent years, there have been many reports on the possibility of using bonding systems in preparations for direct pulp capping.

nHAp is experimentally added to adhesive resins. The available studies indicate good results when covering the pulp with such preparations. No layer of necrosis was observed in the vicinity of the applied nHAp, but a lower degree of pulp irritation was noted compared to other materials. Interestingly, nHAp stimulates the deposition of larger layers of reparative dentin compared to the calcium hydroxide material. It was found that the addition of 10% nHAp to adhesive resins caused a good pulp response, in the form of a large amount of reparative dentin deposited and the shortest healing period compared to the calcium hydroxide material [50]. Other studies have shown that adding nanohydroxyapatite even in an amount of 5% also causes almost the same phenomena, so this concentration of nHAp also seems to be sufficient [51].

Okamoto et al. studied the usefulness of the newly synthesized nanohydroxyapatite in direct pulp capping in rat teeth and compared it with Calvital (a material based on calcium hydroxide) and formocresol. After using nanohydroxysapatite, compared to other preparations, there was a significantly lower level of irritation of the tooth pulp after contact with the material, which was probably related to the pH in the place of the applied compound, which remained at a level close to neutral. Calvital with pH = 12 showed a high level of pulp irritation. Moreover, nHAp caused a much earlier resolution of local inflammation and showed no necrotic layer after its application [52].

Kiba et al. evaluated the effectiveness of polyphase calcium phosphate (Poly-CaP) in the direct pulp capping of rats. In this study, the researchers used blocks created by annealing raw nanohydroxyapatite that had two different surfaces—one of soluble calcium phosphate and the other containing insoluble nanohydroxyapatite—combined with a bonding resin that was applied after etching that surface. In this way, polyphase calcium phosphate (Poly-CaP) was obtained, which is a derivative of nanohydroxyapatite The sides prepared in this way were placed on the pulp of rat teeth, and after two and four weeks, the formation of a dentine bridge, the degree of pulp inflammation and the bonding of nohydroxyapatite with adhesive resins were observed. Studies have shown slightly better treatment results when using Poly-CaP blocks compared to nHAp. After 4 weeks, more dentin bridges were observed on the Poly-CaP surface than on the pure nHAp surface. Moreover, moderate pulp inflammation was observed with nHAp and only mild pulp inflammation with Poly-CaP. At the same time, in terms of combining preparations with adhesive resins, a favorable phenomenon of a strong combination of nHAp and resins was observed in all tested samples. Ionic bonds were formed between calcium ions from nanohydroxyapatite and the monomers contained in the adhesive resins [53]. It was found that this phenomenon may be important for increasing the strength of dental fillings by creating a strong chemical bond between resins and tooth tissues.

When analyzing the subsequent properties of the substance in question, it is worth mentioning research suggesting that nanohydroxyapatite may have a beneficial effect on nerve regeneration [54]. One of the mechanisms that may be involved in the induction of nerve regeneration by nanohydroxyapatite is the increase in mitochondrial activity in neurons. Mitochondria are cellular organelles responsible for the production of energy in the form of ATP and for the regulation of many cellular processes, including metabolic and signaling processes related to the regeneration of nervous tissue [55,56,57]. The interaction between the timing of neuroinflammation and the increase in mitochondrial activity may be crucial for stimulating regenerative processes. Nanohydroxyapatite, through its physicochemical and biological properties, can influence the metabolism of nerve cells, stimulating their activity and accelerating repair processes [58]. Currently, many studies are focusing on the potential applications of nanohydroxyapatite in the regeneration of nervous tissue due to its properties that stimulate cell growth and its ability to interact with nerve cells.

There are studies suggesting that nanohydroxyapatite can also be used to improve the bioactive properties of polymeric materials and strengthen their structure. The addition of nanohydroxyapatite to polymeric materials can affect their physical, chemical and biological properties, leading to improved usefulness in various applications. The increased abrasion resistance, mechanical strength and thermal stability of these polymeric materials have been proven, which makes them more durable and resistant to operating conditions [58]. In addition, it can increase their ability to stimulate bone tissue regeneration and integration with biological tissues [59].

Another example of the interesting properties of nanohydroxyapatite is stimulating the expression of genes related to the process of bone tissue regeneration. Liu et al. conducted research using a combination of chitosan (polysaccharide obtained from chitin) with nanohydroxyapatite, Runx2 subunits (a transcription factor regulating osteoblast differentiation), ALP (alkaline phosphatase), Smad1 (a transcription factor activated by BMP), BMP-2/4 (bone morphogenetic protein), collagen I and integrin, creating scaffolds that imitated the bone tissue microenvironment and provided appropriate signals to cells to stimulate their differentiation and proliferation towards osteoblasts and the production of new bone tissue and the production and mineralization of bone matrix [60,61,62].

Although hydroxyapatite has many advantages, its poor mechanical properties may limit its applications in load-bearing applications. To improve the mechanical properties of nanophase hydroxyapatite and increase its durability, various modification methods are used, including doping with metal ions [63,64,65,66]. The addition of elements such as magnesium, strontium or zinc can significantly affect the physical and chemical properties of hydroxyapatite. It was confirmed that doping with metal ions can lead to a reduction in the biodegradation rate, which is important for long-term applications, and chemical modification through surface functionalization can be used to improve its mechanical properties, stability and functionality in various biomedical applications [67,68].

Scientific research has shown that adding nanohydroxyapatite to the sealing materials used to fill root canals can provide benefits by improving their properties and effectiveness. Sobczak-Kupiec et al. tested the cytotoxicity of biomaterials based on calcium phosphates: laboratory-synthesized nanohydroxyapatite and two commercially available ones—Apatite Root Sealer Type I and II (Sankin Industry, Osaka, Japan) [69]. A good response and biological tolerance of cells was demonstrated in samples with pure materials—CHA and Apatite Root Sealer Type I—while Apatite Root Sealer Type II, probably due to iodoform content in its composition, showed increased cytotoxicity. nHA-based materials, in addition to biocompatibility, also have sealing properties comparable to other materials available on the market. Nanohydroxyapatite mixed with epoxy resin showed a higher tightness than preparations based on zinc oxide and calcium hydroxide [70]. Due to the very promising research results, a new sealing material, BioAggregate, containing hydroxyapatite [71] deserves special attention. An important feature of this preparation is the very high tightness of the fillings and much better biocompatibility compared to other available materials.

Moreover, research conducted by del Carpio-Perochena et al. on the use of chitosan-hydroxyapatite (CS-HA) nanocomplexes in endodontics suggests that they may be an effective method of modifying the dentin substrate before filling the root canal [72]. CS-HA nanocomplexes can help restore the wettability of the root canal dentin surface, which plays an important role in the adhesion of filling materials to the root canal walls. CS-HA may also help strengthen dentin collagen, which is crucial for its stability and mechanical strength [72]. Improving stability may contribute to increasing the durability of the root canal filling and reducing the risk of its rupture or damage. Using the principle of biomimetic mineralization, CS-HA nanocomplexes can influence the process of rebuilding the mineral structure of dentin, which in turn can contribute to strengthening its structure and preventing demineralization processes [72]. Moreover, chitosan–hydroxyapatite nanocomplexes, when used in combination with a tricalcium silicate-based root canal sealer, contribute to the chemical modification of the dentin surface and subsurface by creating an ion-rich layer. The use of these nanocomplexes during root canal treatment may facilitate increasing the interfacial integrity of the interface between sealant and gutta-percha that fills root canals [73,74,75].

2.2. Bioglass

Bioglass (BG) was first developed by Larry Hench and his colleagues in 1960. It is a material consisting mainly of silicon dioxide (SiO₂), sodium oxide (Na₂O), calcium oxide (CaO) and phosphorus pentoxide (P₂O₅) [76]. One of the characteristic features of bioglass is its ability to create a layer similar to hydroxyapatite in contact with living tissue and the unique ability to bond to both hard tissues such as bone and soft tissues [77]. That versatility gives it an advantage over synthetic hydroxyapatite and make bioglass an attractive material for biomedical applications such as dental fillings, bone implants and tissue regeneration materials [78].

Bioglass has the ability to release various ions such as sodium, phosphates, calcium and silicon into the environment. The release of silicon ions plays a particularly important role. Once released, silicon ions react to form silanols, which spontaneously polymerize to form a layer of silica gel. This layer acts as a seed for the formation of new bone tissue. Additionally, when bioglass is exposed to an aqueous environment, a layer of hydroxyapatite forms on its surface [79]. This phenomenon makes bioglass even more bioactive, which means it promotes the regeneration and integration of bone tissues [80]. Bioglass allows the formation of hydroxyapatite layers on its surface, which provide a physical connection with the bone. The process of dissolving bioglass causes the release of silica, the activation of genes that control osteogenesis, i.e., the process of forming new bone tissue, and the production of growth factors; bioglass thus stimulates osteoblasts to produce new bone through various mechanisms [81]. Equally important is that the osteoblasts can interact with collagen, which is the main component of bone tissue. This allows for a more comprehensive bonding of bioglass with bone tissue, which is very similar in structure to natural tissue [82].

Bioglass not only stimulates the regeneration of bone tissue, but also accelerates the bone healing process. Some scientists report that bioglass can also be used to stimulate the healing of soft tissue wounds and the regeneration of peripheral nerves. It is used to remineralize early carious lesions and to treat the excessive sensitivity of dentin, so it can help maintain oral health [83,84,85,86,87]. Additionally, products based on bioactive glass have the ability to stimulate angiogenesis, i.e., the process of creating new blood vessels [88,89,90,91]. This property can be beneficial in therapy where increased blood flow to damaged tissue is needed to speed up the healing process. These applications demonstrate the versatility and potential of bioglass as a medical material.

Direct pulp capping is an important procedure to preserve the vitality of the tooth pulp. The pulp covering preparation must be biocompatible and have the ability to induce the formation of a dentine bridge protecting the pulp [92]. In this treatment, bioactive glass works better than most traditional dental materials. At the same time, the material used for this purpose should ensure a tight closure of the root canals to prevent re-infection of the pulp, should not cause negative immunological reactions, and should have antibacterial properties and be easy to use, which will facilitate the root canal treatment process and ensure a precise application [93,94]. The ideal material should also support the process of creating a dentin bridge, which protects the pulp against bacteria and other external factors, while supporting the process of the regeneration of tooth tissues.

In experimental studies, the formation of dentin-like tissue was observed after bioglass was placed on the pulp stumps of mouse teeth. In subsequent studies carried out on the first molars of rats, after mechanical exposure of the pulp, bioglass material was used to directly cover the pulp. Four weeks after application of the preparation, a thin dentin bridge was formed and no local inflammation or only minor inflammation was observed in the vicinity of the biomaterial [95]. Research indicates that the ions released by bioglass particles not only do not inhibit the growth of human dental pulp stem cells, but also contribute to the formation of a dense and strong dentin bridge. This discovery is important in the context of regenerative dentistry, where the formation of durable and strong dentin structures is crucial to maintaining tooth health and function [96,97,98].

The use of Bio-Gutta, i.e., gutta-percha in combination with Bioglass 45S5, as an alternative to conventional gutta-percha in root canal treatment seems to be very beneficial. The combination of these two materials ensures the effective sealing of the root canals, which is crucial in preventing bacteria from leaking into the pulp cavity [99]. Additionally, Bio-Gutta has additional benefits, such as the ability to bond to the dentin walls without the need to use additional sealants, which improves the durability of the filling and simplifies the root canal treatment process [100,101]. Additionally, the fact that Bio-Gutta is a biocompatible material means it has the potential to minimize allergic reactions or other unwanted side effects in patients [102].

Sealing the root canal system with biocompatible and dimensionally stable filling materials plays a key role in the success of endodontic treatment. Correct sealing can be achieved through the appropriate technique and use of the appropriate filling material, which must meet certain physicochemical and biological properties, such as flowability, viscosity and a good adhesion to the substrate [103]. Gutta-percha and endodontic cements are considered necessary materials in the final phase of endodontic treatment, enabling the filling of the root canal system [104]. However, one of the biggest problems associated with the failure of these procedures remains the lack of an apical seal, which may lead to the leakage of fluids from the oral cavity into the root canals, which in turn is associated with the recurrence of infection or other complications [105]. Therefore, bioactive glass is now added to endodontic sealants. In in vitro tests, samples of material with the addition of bioglass were hardened in a simulated body fluid and a gradual decrease in its pH was observed, which stabilized at a level of approximately pH = 10. It was shown that this pH value is optimal for the formation of hydroxyapatites on the surface of bioglass [106].

Bioglass is often supplemented with additional substances that support the fight against microorganisms. Thanks to the alkaline reaction of the material and the addition of silver, preparations with antibacterial properties are obtained. They prove to be effective in combating bacteria found in the oral cavity, such as E. coli, P. aeruginosa, S. sanguinis, A. viscosus and S. mutans, responsible for the development of caries and periodontal diseases [107,108,109,110,111].

An interesting example here is the research of Correia et al., who created and then tested three different bioactive glass compositions produced using the sol–gel method [112]. These compositions differed in chemical composition and percentage of SiO₂, CaO, MgO and CuO. The researchers showed that two out of three bioglass nanoparticle compositions inhibit the growth of E. faecalis after 48 h of incubation, while for all compositions a significant reduction in the viability of C. albicans was noted [112].

An important requirement for materials sealing root canals is their biocompatibility, due to their direct contact with the hard tissues of the tooth and the soft tissues of the periapical area. In most studies, the biocompatibility of materials is assessed based on cytological tests [113]. This was also analyzed in in vivo studies after removal surgery of coronal and root pulp in rat teeth. After filling tooth root canals with bioglass preparations, the healing process of tissues in the periapical area was not inhibited, and tissue regeneration was even stimulated. These studies confirmed the significant biocompatibility of this material [114]. In vitro studies using mouse and human osteoblasts demonstrated the proliferation and migration of bone-forming cells in direct contact with bioglass. These studies also confirmed the lack of cytotoxicity of this material [115].

In the treatment of teeth with diseased pulp, the tight filling of the root canals is crucial to prevent the spread of infection to the periapical area of the tooth. Nanotechnology can also be used to develop new diagnostic and therapeutic methods that can improve the effectiveness of endodontic treatment and prevent complications. In addition, it allows the creation of materials that enable better penetration and a tight filling of root canals. Nanoparticles can be added to filling materials such as gutta-percha or endodontic cements to increase their sealing properties and resistance to infections [116].

2.3. Nanocompounds Based on Chitosan

One of the most frequently chosen substances that complement the properties of bioglass is chitosan—an organic chemical compound from the group of polysaccharides, which is obtained by the deacetylation of chitin in an alkaline environment. Chitosan is a homopolysaccharide, which means it consists of one type of sugar monomer—glucosamine. Zakrzewski et al. emphasized that chitosan contains deacetylated and acylated glucosamine units in its structure—D-gucosamine and N-acetyl-D-glucosamine [117]. Its source is the exoskeletons of arthropods, molluscs and insects. Due to its properties, such as biocompatibility, biodegradability and the ability to create matrices, chitosan is widely used as a carrier of biologically active substances, in tissue regeneration and in protection against infections [118]. The solubility of chitosan in an acidic environment plays a key role in its antibacterial effect. When chitosan is exposed to acids, its molecules become protonated, which creates positive charges on the amino groups. These positively charged amino groups are able to interact with cations formed by the protonation of C-2 amino groups [119]. As a result, these positively charged amino groups can disorganize the structures of microbial cell membranes by interacting with the negative charges of these membranes. This, in turn, may lead to damage to the cell membranes, which may result in the death of microorganisms [120]. The antibacterial effect of chitosan, especially in an acidic environment, may contribute to reducing the number of E. faecalis bacteria in root canals, which is crucial for effective endodontic treatment and the prevention of recurrent infections [121,122].

Caballero-Flores et al., as part of their research, created a cellular fibrin hydrogel scaffold that allowed odontoblast cells to develop and organize in the dentin pulp. The addition of chitosan to this scaffold contributed to providing additional antibacterial effectiveness, which may be particularly important in the case of infections within the pulp. It is also important that the addition of chitosan did not negatively affect the morphology and viability of the collagen matrix, which is crucial for the proper development of pulp tissue. Therefore, the cellular fibrin hydrogel with chitosan may be a promising tool in the regeneration of tooth tissues and in the fight against infections, contributing to the effective treatment and protection of the dentin pulp [123].

Sanap et al. also obtained interesting research results. They developed a chitosan hydrogel that delivered organic amelogenin to enamel defects. Amelogenin is a protein that plays a key role in the enamel mineralization process, and its presence in damaged areas can support natural repair mechanisms, while chitosan, as a biodegradable and biocompatible material, is an excellent scaffold for the transport and release of amelogenin in the place of enamel defects [124].

The goal of endodontic therapy focuses on eliminating the microbiological load in the root canal system. The intracanal medications and root canal rinsing agents used are designed to effectively combat bacteria, fungi and other microorganisms present in the root canal. Research on them often involves testing their effectiveness in reducing single-species and multi-species biofilms, which are the main cause of infection in tooth periapical tissues. This makes it possible to select the best and most effective agents that can be used in clinical practice to improve the results of endodontic therapy [125]. Research on the antibiofilm effect of chitosan nanoparticles against multi-species infections involving bacteria such as S. mutans, fungi such as C. albicans and Enterococcus faecalis bacteria was conducted, among others, by Malinowska et al. and Loyola-Rodríguez et al. They showed that chitosan nanoparticles have the potential to inhibit the formation of biofilms by these pathogens, and the addition of chitosan nanoparticles to the endodontic sealants used to fill root canals brings benefits in the form of the effective sealing of root canals, while inhibiting biofilm development and preventing re-infection [126,127].

2.4. Metallic Nanoparticles

Nanomaterials used in dentistry include both conventional and unconventional nanoparticles. The first category includes metallic nanoparticles and metal oxides in the form of nanoparticles. Metallic nanoparticles, such as silver or copper nanoparticles, are known for their antibacterial and antiseptic properties, which makes them useful in the production of dental materials to prevent infections. However, metal oxides can be used as fillers in dental materials, improving their mechanical and biological properties [128,129]. These are advanced nanomaterials that are used as fillers in modern dental materials. One of their advantages is the possibility of easy modification, which allows them to adapt their properties to a specific application [130]. The use of both conventional and unconventional nanoparticles in dentistry opens new possibilities in the design of dental materials with excellent mechanical, biological and antibacterial properties. At the same time, there is a need for further research on the safety and effectiveness of these materials in clinical practice [131].

2.4.1. Nanosilver

Silver ions have powerful antibacterial properties that have been thoroughly researched by scientists. The process of inactivating enzymes that are key to DNA replication is one of the mechanisms by which silver ions can kill microorganisms. The main mode of action of silver ions on microorganisms is to disrupt metabolic and structural processes inside bacterial cells. Silver ions can combine with enzymatic proteins, leading to their denaturation and loss of function. As a result, the bacterial cell loses its ability to replicate DNA and reproduce, which leads to its death. Due to its strong antibacterial properties, silver has been widely used in various fields of medicine, including dentistry. In dental materials, such as fillings, dressings or anesthetic impregnations, the addition of silver ions can provide additional protection against bacterial infections, supporting the treatment process and preventing the recurrence of infections [132,133,134].

Chávez-Andrade et al. examined the properties of silver nanoparticles, which they had previously coated with polyvinyl alcohol and farnesol. The aim of the study was to verify their antimicrobial properties against E. faecalis, C. albicans and Pseudomonas aeruginosa. The results they obtained confirmed that silver nanoparticles support the disinfection of root canals and inhibit the formation of biofilm [135]. Juan M. Martinez-Andrade et al. added nanosilver to edetic acid, used for rinsing root canals. As a result, in addition to the antibacterial effect, they also observed an increased effectiveness in removing the smear layer from the canals [135].

The effectiveness of silver nanoparticles was also observed in studies conducted by Ozdemir et al. In this case, a preparation containing nanosilver at a concentration of 0.02% was applied to a biofilm of E. faecalis created in laboratory conditions [136]. These observations were used by Wu et al., who compared a preparation of the same concentration with calcium hydroxide at a concentration of 0.1%. Both substances had a gel formula and were used for seven days. After this time, an almost complete destruction of the biofilm was observed on samples covered with the preparation containing nanosilver [137]. Although silver nanoparticles have undeniable advantages, they also have side effects—they stain the surrounding tissues. Looking for a solution to this problem, scientists proposed combining silver nanoparticles with potassium iodide. However, this method has been questioned due to the short-term effect and reduced bond strength of the resin composite with dentin [138].

Metallic nanoparticles used in dentistry also include gold and copper nanoparticles. Their unique properties, such as osteoinductivity and antibacterial properties, are used to improve the effectiveness and stability of titanium implants [139].

2.4.2. Metal Oxide Nanoparticles

Research on nanoparticles of stable metal oxides, such as zinc oxide, titanium oxide or zirconium oxide, has indeed yielded many promising results in recent years. One of the main areas of their use is dental fillings. Metal oxide nanoparticles are mainly added to composite resins to improve their mechanical properties, durability and biocompatibility. Additionally, they have antibacterial properties, which is very important in the context of preventing infections and maintaining oral hygiene [140,141].

The antibacterial properties of zinc oxide nanoparticles (ZnO NP) are significantly effective in eliminating microorganisms. There are several mechanisms that contribute to this effectiveness. Firstly, the presence of zinc ions leads to damage to the bacterial cell membrane, which causes the loss of integrity of the cell structure and, consequently, leads to its death. This mechanism is associated with the increased permeability of the cell membrane in the presence of zinc ions [142]. Secondly, zinc nanoparticles can also increase the production of reactive oxygen species (oxygen radicals), which have a bactericidal effect by damaging the cell membrane and other elements of the bacterial cell. Additionally, zinc ions (Zn²⁺) are toxic to bacteria, disturbing their metabolic processes and inhibiting their growth [143]. The total combination of these mechanisms, i.e., damage to the cell membrane, increased production of reactive oxygen species and the toxicity of zinc ions, causes zinc nanoparticles to have a strong antibacterial effect—desirable in endodontics [144].

The hydrolysis process of tetrabutyl titanate and its kinetics can be regulated, which allows for the modification of the titanium ion content in the +3 oxidation state and the number of oxygen defects (OV) in the TiO₂ structure—this in turn allows for the adjustment of visible radiation-induced photocatalytic efficiency and antibacterial properties [145]. One of the key features of TiO₂ is its ability to generate reactive oxygen species, such as hydroxyl radicals (OH) and hydrogen peroxide (H₂O₂), when exposed to UV radiation. These reactive oxygen species are strong oxidants and can damage bacterial cell membranes through the oxidation process. This damage leads to the lysis of bacterial cells, which ultimately eliminates the microorganisms. In addition, TiO₂ also exhibits other properties that may be beneficial in dentistry, such as a high mechanical strength and corrosion resistance, which makes it an ideal material for use in dental implants and surgical instruments or as a coating on dental instruments. Additionally, the excellent biocompatibility of TiO₂ makes it safe for body tissues, which is a key factor in dentistry, where contact with the soft and hard tissues of the oral cavity is inevitable [146,147,148].

In terms of stiffness, performance, fatigue resistance and wear resistance, zirconium oxide is indeed a material that may be superior to other substances used in dentistry. The use of laser vaporization techniques to produce zirconium oxide nanoparticles of very small sizes, such as 20–50 nm, opens up new possibilities for the use of this material [149,150]. Zirconium oxide nanoparticles of such small sizes have increased surface activity, which affects their properties, including chemical reactivity, biocompatibility, osteoconductivity and the tendency to reduce the accumulation of dental plaque [151,152]. Chęcińska et al. examined the use of zirconium oxides in dentistry by doping PMMA with ZrO₂ nanoparticles. In comparative tests, zirconium oxide ceramics showed a higher strength and bending resistance than aluminum oxide ceramics [153]. In turn, Souza et al. showed that ZrO₂ nanoparticles combined with glass ionomer cement were characterized by a higher compressive strength and crack resistance [154].

The biofilm that can accumulate on implants is a complex structure of microorganisms, including bacteria, that can lead to the infection and inflammation of the tissues around the implant. Iron oxide nanoparticles (FeO, Fe₂O₃), due to their antibacterial properties, can be an effective tool for disinfecting implant surfaces [155]. They have the ability to destroy the biofilm structure and eliminate bacteria, which may contribute to maintaining the cleanliness and hygiene of dental implants. Moreover, iron oxide nanoparticles can be applied in the form of solutions or nanometric coatings to the surface of implants, which enables their effective penetration and antibacterial effect even in hard-to-reach places [156]. Due to their antibacterial properties, iron oxide nanoparticles have the potential to improve the long-term durability and clinical success of dental implants by reducing the risk of infection and inflammation of peridental tissues [157].

MgO (magnesium oxide) and CaO (calcium oxide) nanoparticles have antibacterial properties against both Gram-positive and Gram-negative microorganisms. These nanoparticles have the ability to damage their cell membranes, which prevents them from surviving and multiplying. This mechanism is widely accepted and used in the fight against bacterial infections, mainly in dental materials, aimed at preventing infections and improving oral hygiene [158,159]. The potential use of these nanoparticles in endodontics was proposed by Takhar et al. They designed an experiment to compare the long-term effectiveness of NaOCl, MgO and chitosan nanoparticles in the eradication of E. faecalis [160]. Based on the conducted expert opinions, scientists showed that all these substances were more effective than sodium hypochlorite, commonly used to disinfect root canals during endodontics [160].

CuO nanoparticles have antibacterial potential against both Gram-positive and Gram-negative bacteria by penetrating the bacterial cell membrane and damaging important intracellular enzymes, which leads to the inhibition of bacterial growth and reproduction. Additionally, CuO nanoparticles may also have some antifungal properties, making them a promising candidate for use in various fields, including endodontics. However, the number of published studies on the use of CuO nanoparticles in endodontics is limited, which indicates the need for further long-term studies to assess their effectiveness, safety and potential side effects [161,162].

2.5. Graphene Oxide Nanoplatelets

Nanographene also has similar antibacterial properties. In studies conducted by Shrestha et al., the authors demonstrated the high effectiveness of products with the addition of graphene in preventing bacterial colonization on root canals and dental implants [163]. Similar research was conducted by Rygas et al., using graphene oxide nanoparticles to assess their antimicrobial effectiveness against S. mutans. The results they achieved exceeded the effectiveness of traditional bactericidal substances used in endodontics [164]. Moreover, due to their large, developed surface area and susceptibility to functionalization, graphene nanoparticles have a promising application direction related to their use to transport therapeutic agents directly to tissues that are inflamed [165]. Dubey et al. used nanographene as a substrate for a dental composite and bone cement, showing an increase in the mechanical properties of both of these complexes compared to products without nanoparticles. Moreover, the manufactured products were characterized by a higher resistance to biofilm development, which can be used in the process of the regeneration of tooth pulp and periapical tissues [166].

2.6. Nanodiamond

Almost half of the human population suffers from dental caries (which is caused by acid-forming bacteria, the most dangerous of which is the Streptococcus mutans bacterium), while the sixth disease most often affecting people in the world is periodontal disease (11.2%), especially of the gums, which are attacked by the Gram-negative bacterium Porphyromonas gingivalis. The communities of microorganisms found in the oral cavity form a very complex structure—the biofilm—which is the main cause of infection. The analysis of phenomena related to the formation and ultimately the removal of biofilm from the oral cavity is very important, considering that the bacteria causing tooth and gum diseases are associated with various systemic diseases. It is suspected that biofilm influences the development of Alzheimer’s disease, as well as cardiovascular diseases and obesity. In other words, fighting biofilm in the oral cavity also means fighting the most serious diseases that plague humanity [167]. Biofilm, due to its complexity, is characterized by a high resistance to conventional antibiotics, which is why intensive work is being carried out to search for alternative therapies.

Promising research results were presented in the groundbreaking work of Prof. Dean Ho et al. [168]. Their research concerned the standard of endodontic treatment, which is filling a mechanically and chemically prepared root canal with gutta-percha. Gutta-percha is a plastic material, volume-stable in cold techniques, biocompatible, non-resorbable, impermeable to bacteria and non-staining of the teeth. Moreover, the presence of zinc oxide in gutta-percha points gives them some antibacterial properties. However, the main disadvantage of gutta-percha points is the lack of adhesion to the dentin, which does not ensure the tightness of the filled canal and requires the use of a sealing material [169]. Faced with these problems, the team of Prof. Ho proposed modifying gutta-percha with amoxicillin-coated nanodiamond particles. The effect of these studies was, first of all, the confirmed inhibition of biofilm growth, and in addition, tomographic imaging showed that the degree of the filling of the canal with new material was better than in the case of conventional gutta-percha [168].

Moreover, the team led by Dr. Chu and Dr. Neelakantan has shown in experiments that diamond nanoparticles have a strong antibacterial effect on planktonic cells and interfere with the deposition of these cells on tissue surfaces, hindering the growth and formation of biofilm [170].

Recent publications by a team of scientists from Taiwan have revealed the potential of nanodiamonds as ingredients in irrigation fluids. After the mechanical cleaning of the tooth canals, it is necessary to remove the smear layer. For this purpose, sodium hypochlorite and ethylenediaminetetraacetic acid are most often used, but the disadvantage of this solution is the weakening of the dentin and the low susceptibility of organic substances in the smear layer. Scientists proposed the use of nano and submicron diamond particles in irrigation solutions, the effect of which was additionally enhanced by sonic and ultrasonic oscillations. The high effectiveness of removing the smear layer from the walls of dental canals and canal apices was confirmed by scanning microscopy [171].

2.7. Regenerative Endodontic Treatment at Nanoscale

Regenerative endodontic treatment (RET) is an advanced field of dentistry that aims to restore the living pulp in the tooth, which allows it to continue to develop and function. This procedure is particularly important for young patients whose teeth have not yet reached full maturity [172,173]. Regenerative endodontics focuses on the use of advanced biological methods and innovative materials that are designed to stimulate the regenerative processes occurring inside the tooth. It is based on three key elements of regenerative engineering: stem cells (

Table 2. Stem cells in regenerative endodontic treatment [174,177,178].

Stem Cells	Role in RET
Dental Pulp Stem Cells (DPSCs)	capable of differentiating into many cell types, including odontoblasts, which are responsible for dentin formation
Stem Cells from Human Exfoliated Deciduous Teeth (SHEDs)	differentiate into odontoblasts and other cell types that support pulp regeneration and differentiate into endothelial cells that contribute to the formation of blood vessels in the pulp
Immature Dental Pulp Stem Cells (IDPSs)
Dental Follicle Stem Cells (DFSCs)	capable of differentiating into cells that can support the regeneration of periodontal tissues
Periodontal Ligament Stem Cells (PLSCs)	the potential to regenerate periodontal tissues, including root cement, periodontal ligament and alveolar bone
Stem Cells from Apical Papilla (SCAPs)	the ability to differentiate into odontoblasts, which form dentin, and into pulp-like cells that can contribute to the regeneration of vascular tissue
Supernumerary Tooth Stem Cells (SNTSCs)	research into their regenerative potential is still ongoing

), bioactive molecules, and scaffolds. The stem cells used in RET can come from various sources, such as dental pulp, alveolar bone or peripheral blood. They are pluripotent cells, meaning they have the ability to differentiate into various cell types, including dental pulp cells [174]. Bioactive molecules, such as growth factors (e.g., BMP, TGF-β, FGF), are essential to stimulate stem cell activity. They support the processes of cell proliferation, differentiation and migration. Other bioactive mediators, such as bone morphogenetic proteins, act synergistically with growth factors, promoting the complex development and regeneration of dental tissues [175]. Scaffolds, on the other hand, are biocompatible structures that provide a spatial architecture for stem cells, enabling them to settle, proliferate and differentiate. They provide mechanical support and an appropriate microenvironment for regenerating tissues. They can be made of various materials, such as collagen, hydrogels, synthetic polymers or natural biopolymers—biodegradable and bioactive [176].

The most commonly used method for producing nanofibers is electrospinning. This process involves creating fibers by drawing out a polymer solution using an electric field. The resulting fibers have diameters in the nanometer range and can be formed into three-dimensional structures that mimic the natural extracellular matrix (ECM), creating a suitable microenvironment for stem cell growth and differentiation. They are made from a variety of materials, including synthetic and natural polymers, which allows their properties to be tailored to specific regenerative needs [179].

Remaining on the subject of regenerative endodontics, it is worth mentioning resolvins, especially RvE1; these are natural lipids derived from omega-3 fatty acids, which play a key role in anti-inflammatory and regenerative processes. Resolvins support tissue repair processes by stimulating cells to proliferate and differentiate, as well as by inhibiting apoptotic processes (cell death) [180]. In the context of regenerative endodontics, research on the use of resolvins focuses on their potential in the regeneration of infected and damaged dental pulp. Experiments conducted on both laboratory models and animals have shown that resolvin RvE1 can effectively reduce inflammation and support regenerative processes in dental pulp. In clinical cases of infected and damaged pulp, the use of RvE1 shows promising results in terms of pain reduction, the reduction of swelling and the regeneration of damaged tissues [181].

Melanocortin peptides, in particular α-MSH (α-melanotropin, α-melanocyte-stimulating hormone), have significant potential in endodontic regeneration due to their anti-inflammatory, antioxidant and cell proliferation-stimulating properties. α-MSH exhibits strong anti-inflammatory effects by inhibiting the production of pro-inflammatory cytokines and chemokines and reducing the infiltration of inflammatory cells into the site of damage [117]. Studies on the use of these peptides in dental pulp treatment indicate their effectiveness in reducing inflammation and supporting tissue regeneration. Innovative approaches, such as the use of nanostructured scaffolds functionalized with α-MSH, may contribute to the further development of regenerative therapies in dentistry [182]. In a study conducted by Fioretti et al., α-MSH was shown to have strong anti-inflammatory properties and to stimulate the proliferation of pulp fibroblasts. The authors showed that melanocortin peptides can be effectively used in endodontic treatment to support the regeneration of dental pulp [183]. The use of such biomaterials may contribute to improving treatment outcomes by delivering α-MSH directly to the lesion site. Further research on melanocortin peptides may lead to the development of new regenerative therapies that will be more effective and minimally invasive. The possibility of functionalizing nanofibrous scaffolds with α-MSH and the controlled release of melanocortin peptides open new perspectives in endodontic treatment [184].

3. Discussion

There are two main limitations to this work. The first one—due to the authors’ interests—is the focus on the use of nanoparticles in endodontics, whereas these materials are used in the whole spectrum of dentistry. The second limitation, resulting from the attempt to systematize the latest achievements, was the inclusion in the review mainly of articles published in the last five years. In this case, however, a few exceptions were made, because several of the cited works are considered milestones in the discussed subject.

In the field of dentistry, modern nanomaterials play an increasingly important role. Thanks to their nanometric structure, they provide a high durability, aesthetics and an abrasion resistance higher than traditional materials, which is why they are used as fillers in modern dental composites. Due to their fine-particle structure, these substances contribute to the better penetration and sealing of root canals, which is crucial for the success of endodontic treatment [185]. Moreover, dynamic research on the use of nanomaterials for tissue regeneration in dentistry has resulted in the development of new antibacterial coatings on dental implants and prosthetic tools, which significantly minimizes the risk of infection. What is worth emphasizing is the fact that a broad spectrum of antibacterial activity and an increased ability to induce the processes of differentiation and mineralization of pulp cells have been proven [186,187].

Nanoparticles can also be used for the controlled release of growth factors, which accelerates the process of wound healing and tissue regeneration [188,189]. The nanoparticles described in this work are intended for use in endodontics, but they do not exhaust the catalog of materials and solutions that may be implemented in dental procedures in the near future. An example is very sensitive and precise sensors made of quantum dots, which can be used to diagnose caries and periodontal diseases [190]. Another solution, still in the experimental phase, is nanorobots programmed to precisely perform minor surgical operations in the oral cavity, thereby minimizing the risk of tissue damage and shortening the recovery time.

Nanoparticles are designed to combine different functions, such as drug delivery and imaging. Thanks to this, synergistic therapeutic and diagnostic effects can be achieved. These particles can be modified in such a way as to respond to various stimuli, such as changes in pH, temperature, or the presence of biomolecules. This allows for the controlled release of drugs in response to biological environmental conditions [191,192,193].

However, remaining in the circle of considerations regarding materials used in the treatment and reconstruction of dental pulp and roots, it is necessary to emphasize once again their high bactericidal potential and the need to limit the development of biofilm, which causes superinfection and forces repeated cleaning procedures of infected tissue structures [25,27]. Bacteria often penetrate so deeply that tissue resection is necessary, the reconstruction of which requires more serious treatments and complex medical procedures. Therefore, a very important aspect of endodontic treatment is to thoroughly clean the canals and protect them so that they are not colonized by microorganisms [49,50]. And nanoparticles fulfill this task in an unquestionable way, which has been proven by many researchers. Moreover, the remineralization potential of some of these nanomaterials and the inhibition of demineralization should be emphasized. The results indicating the reconstruction of the hydroxyapatite layer became the basis for further research analyzing the molecular phenomena occurring at the tissue–biomaterial interface [52,194,195,196,197].

An important application of nanocomposites from the point of view of endodontics is their inclusion in the composition of sealing and restorative materials. Nanoparticles show an increased hydration rate compared to conventional-sized particles of the same substances, which in turn results in a shorter setting time and a higher microhardness value. This is particularly important because teeth are naturally exposed to various types of pressure and load; therefore, the supplementary material must have properties as close as possible to natural tissue, so that it is possible to reproduce the biomechanical conditions in the mandible and maxilla [58,63,65,147,151].

Despite so many indisputable advantages and the almost unlimited possibilities of using nanoparticles in the scientific world, as well as supporters of such innovative solutions, there are also opponents whose opposition is based on specific arguments. The scale of research currently being conducted is limited to the observation and assessment of the properties of materials in the form of samples conditioned in a simulation environment, the analysis of results obtained on the basis of cell cultures in laboratory conditions, and research using laboratory animals. These studies are short-term and do not allow us to formulate clear conclusions about how the use of nanomaterials affects tissues and the body in the long term. The interactions between nanomaterials and living organisms are very complex, which is why they are still the subject of intensive scientific research. The main concerns relate primarily to potential toxicity, especially with long-term exposure to high concentrations. This is a very important aspect because the mechanisms of toxicity may include inflammatory reactions, oxidative stress and also DNA damage. Once released into the bloodstream, nanoparticles can spread throughout various organs and tissues, affecting their functioning in unexpected ways; they may therefore disturb cell proliferation, differentiation or functions. Moreover, the introduction of nanomaterials into the body may lead to the activation of the immune system. The immune response to nanomaterials can be complex and depend on many factors, including the physicochemical properties of nanomaterials. Even nanomaterials from the same material may exhibit different behavioral properties in relation to individual cellular tissues. This may be due to differences in size, shape, surface or degree of functionalization [156,164,190].

It is important to remember that materials used in dentistry operate in an environment that promotes degradation, so in addition to being effective in treatment, they should demonstrate chemical stability. However, some research actually suggests that some materials—including dental fillings—may release chemical compounds that may potentially affect patients’ health [82,169,183]. Conducting scientific research that thoroughly assesses the impact of nanomaterials on living organisms, including the oral environment, the immune system and potential long-term effects, is essential to understand their impact on surrounding tissues. Such research is key to developing guidelines and regulations for the use of nanomaterials in dentistry, and providing sound scientific data will also enable the development of innovative nanotechnology-based dental products that can be used with greater confidence in their safety and effectiveness.

4. Conclusions

The promising research results on nanomaterials suggest that their use can significantly improve the effectiveness and durability of dental procedures, as well as opening new possibilities for the treatment and regeneration of oral tissues. One of the main uses of nanomaterials in dentistry is to strengthen the polymer composites used for dental fillings. Adding nanoparticles to these composites can improve their strength, abrasion resistance and aesthetics, ultimately leading to more durable and effective fillings. Nanoparticles, which are an addition to scaffolding materials, increase the cell adhesion surface and stimulate the regeneration of bone and periodontal tissues. Moreover, nanotechnology has been used in tissue engineering, especially in the creation of scaffolds that are used to regenerate oral tissues. All these innovations open the door to more precise, effective and lasting dental procedures, which can significantly improve the quality of life of patients. However, before nanomaterials become widely used in dental practice, further clinical research and the development of safety standards and regulations for their use are essential to maximize the potential of nanomaterials in the field of dentistry, while ensuring the safety and effectiveness of dental procedures.