Root Canal Filling Materials in Primary Teeth – Review

Inadequate treatment of endodontic infections in primary teeth results in a loss of а deciduous tooth or abnormality in the development of hard dental tissues of the permanent successor. The main goal is to maintain the integrity and health of the primary tooth until their physiological exfoliation. The success of the endodontic treatment depends on the antimicrobial activity of the root canal filling material that contributes to the elimination or reduction of the microbial infection of the root canal system. The aim of this article is to systematically review the contemporary scientific literature concerning the root canal filling materials used in primary teeth. A critical evaluation of the antimicrobial activity, as well as the advantages and disadvantages of the investigated materials, has been made. Knowledge of the composition and characteristics of the available obturating materials is a useful advantage to dentists to address the functional problems associated with endodontic infections in very young patients. Future studies should also seek and compare the long-term effects of the use of traditional and alternative intracanal materials.


INTRODUCTION
Endodontic infection of the primary teeth is an infection of the root canal system and as such it can involve the pulp and the periapical tissues surrounding the apex of the tooth root. The best treatment option in such cases is pulpectomy. This treatment modality consists in complete removal of the necrotic and irreversibly infected pulp tissue of the pulp chamber and root canals of a tooth affected by extensive caries lesions, traumatic injuries or other causes. 1,2 The main goal is to maintain the integrity and health of the primary tooth until their physiological exfoliation. 3 As the tooth remains asymptomatic, in anatomical and functional condition, it performs an extremely important role in the masticatory process and has phonetic, aesthetic, and morphological functions. 1,3 The inadequate or inappropriate treatment or lack of treatment at all of the endodontic infection results in a loss of the deciduous tooth or abnormality in the development of hard dental tissues of the permanent successor known as Turner's hypoplasia. The loss of teeth leads to many problems such as loss of the arch length, inadequate space for permanent teeth, ectopic eruption, disturbance in the eruption sequence, development of the tongue thrusting habit, embarrassment, speech disorders, and eating and masticatory disorders. 4,5 The success of the endodontic treatment depends on the elimination or reduction of the microbial infection of the root canal systems. 6 Rocha et al. found that microorganisms with a predominance of anaerobic bacteria were disseminated within the whole root canal system (dentinal tubules, accessory canals, secondary canals, apical foramen, apical root cementum surface, etc.) as well as on the physiological resorptive areas on the external apical tooth surface. 7  three important and essential elements in pulp therapy are required -adequate root canal debridement, antimicrobial irrigants, and antibacterial filling materials. 8 The complex anatomy of the root canal system (sharp curvatures and lateral canals) and the physiological resorption (no apical limit, occurs irregularly, not seen radiographically oftentimes) make the access and the instrumentation hard. 3,9,10 In addition, Faria et al. found that several resistant microorganisms could persist after biomechanical preparation and use of intracanal irrigants. 11 Observing the importance of this aspect of the endodontic infections in primary dentition, a significant number of researchers emphasized the antimicrobial activity of the root canal filling material. The major goal of the filling materials is to eliminate the microorganisms from the root canal system thus neutralizing their toxic products, preventing reinfections, and supporting the healing process. [12][13][14]

AIM
The aim of this article is to systematically review the contemporary scientific literature concerning the root canal filling materials used in deciduous dentition. A critical evaluation of the antimicrobial activity and the advantages and disadvantages of the investigated materials has been made. The expository analysis provides insight into and summarizes the succession in the right therapeutic approach.
The ideal characteristics of a root canal filling material in primary teeth include biocompatibility, broad antimicrobial activity, to be a bactericidal material or not to encourage microbial growth, degradation similar to the physiological tooth root resorption, easy filling and removal of the material from the root canal system, resorbable in cases of periapical extrusion, no aggression to the periapical tissues and tooth germ of the permanent tooth, radiopacity, and no discoloration of the tooth. 9,15,16 Considering the limitations of the root canal instrumentation and the obstacles to the intracanal irrigants, the use of root canal filling material with antimicrobial activity is of great importance for the success of the endodontic treatment in primary dentition. 10,13 Numerous researchers investigated the antimicrobial spectrum of action of the materials used and the results obtained demonstrated significant divergence. [17][18][19][20] The agar diffusion test has been widely used for this purpose. 3,6,15,16,21,22 As an in vitro method, it requires the investigated material to diffuse into the agar, and the inhibitory effects are a combination of diffusion potential and antimicrobial activity. 23 The root canal filling materials used in deciduous dentition are divided into several groups according to their active ingredients (Table 1). They could be used alone or in combinations with other active substances (formocresol, chlorhexidine, tetracycline, chloramphenicol, zinc acetate, metronidazole, etc.), be available in ready-to-use paste form or for ex tempore preparation.

Zinc oxide eugenol
Zinc oxide eugenol (ZOE) is a mixture of zinc oxide (powder) and eugenol (liquid). It is the first recommended obturating material for primary teeth and is still the most commonly and traditionally used material in the USA for treatment of endoinfections in deciduous teeth and the second choice in Brazilia. 10,21 The disadvantages of this material include a slower rate of resorption than the physiological one of the root of the primary teeth, absence of biocompatibility, transitory to severe irritation of the periapical tissues, necrosis of the alveolar bone and cementum, but it does not show cytotoxicity. 10,23,24 Antoniazzi et al. showed a risk of causing a deviation of eruption trajectory of the permanent successor tooth if the paste was forced through the radicular apex. 3 The same authors also found more rapid absorption of ZOE when mixed with blood, fluids, and debris. Quieroz et al. and Kriplani et al. found a strong antimicrobial activity against E. faecalis, E. coli, S. aureus, Str. mutans, and Kocuria rhizophila, with the largest zones of bacterial growth of inhibition formed around E. coli. 6,13 The results of most studies investigating its antibacterial efficiency demonstrated that ZOE inhibited more effectively gram-negative than gram-positive bacteria. 13,25 Several authors attributed the antimicrobial effects of ZOE to eugenol. 13,14,16 Navit et al. observed that eugenol-based root canal filling materials showed the highest antimicrobial activity against E. faecalis at 24 hours time intervals. 21 In contrast, in an in-vitro evaluation of antimicrobial efficiency, ZOE demonstrated no inhibition of microbial growth against the most prevalent organisms in infected canals of primary teeth. 10 Concerns about the surroundings of ZOE led to a search of alternative root canal filling materials for deciduous teeth and it has been combined with various materials such as cresol and formocresol which increase its success rate but do not make it more resorbable than ZOE alone. 4

Iodoform-based pastes (IBP)
Iodoform is a bactericidal, non-irritant radiopaque material and with high rates of clinical success in pulpotomies and pulpectomies. 3,16 Iodoform pastes have better resorbability, biocompatibility, and disinfectant properties than ZOE. 4,20 Gomes et al. considered that due to the incorporation of iodoform as a bactericidal agent, the iodoform-based pastes showed better antimicrobial activity. 16 Its antiseptic action is due to the slow release of iodine as an oxidizing agent, causing irreversible oxidization and inactivation of essential metabolic compounds such as proteins, nucleotides, and fatty acids. 16 21 The antimicrobial activity of eugenol-based root canal filling materials such as Endoflas and ZOE is due to the action of eugenol, causing protein denaturation rendering the microorganisms non-functional. 21 In addition, the authors found that ZOE and materials containing zinc oxide were more effective against microorganisms compared to materials without zinc oxide. 15 IBPs cause yellow-brownish discoloration of the tooth crown. Nowadays, this type of pastes is no longer available for use in Europe and alternative components are researched.

Calcium hydroxide-based pastes (Ca(OH) 2 )-based pastes)
Calcium hydroxide has a moderate to weak antimicrobial action as a result of its ionic dissociation into Ca 2+ and OHions. 15 This process depends on the vehicle used in the formulation of the root canal filling material -aque-ous, oily and viscous. 20 Ozalp et al. reported a high degree of solubility when an aqueous vehicle is used that resulted in complete absorption of the paste from the root canal before the end of the physiological resorption, requiring new endodontic obturation. 26 The authors also found that the solubility of the pastes with a viscous vehicle was low, while those with oily vehicles had the lowest one. When used alone, calcium hydroxide does not inhibit E. faecalis and E. coli, which could be explained by their tolerance of high pH values, varying from 9 to 11. 20,22 The neutralization of the high pH by blood or buffers in culture media is a possible reason. Rezende  The possible reason could be the antibacterial and antifungal efficiency of the used broad-spectrum antiseptic. 21

Calcium hydroxide + iodoformbased pastes
The combination of calcium hydroxide pastes with an oily vehicle and iodoform as an antibacterial substance showed good results in the endodontic treatment in primary teeth. 15,30 When combining pure iodoform-based pastes with calcium hydroxide-based pastes, excellent results were also obtained based on clinical, radiographic and histological evaluation. 3 One study found that Vitapex was more effective than ZOE and no extruded material was observed in the periapical tissues, while particles of ZOE of extruded material had not changed in size at the end of the experiment. 20 However, Ozalp et al. emphasize that the complete resorption of the root canal filling is a disadvantage, requiring retreatment of the tooth until the physiological change of the tooth occurs. 26 Mortazavi et al. considered it a good alternative to ZOE due to the more convenient application of the paste as well as resorption similar to the rate of the physiological resorption of deciduous teeth. 30 Concerning the antimicrobial activity of Vitapex, the authors found also that it was statistically significantly better than the root canal filling material containing Ca(OH) 2 alone. 20,26 In several studies, the combined use of Ca(OH) 2 + iodoform exhibited no antimicrobial action against pure cultures of microorganisms in the diffusion agar test, probably due to the reaction between the calcium hydroxide and the diacid combinations of endodontic substances. 4,6,15,18 Other root canal filling materials In India, the use of antibacterial mixture including ciprofloxacin, metronidazole, and minocycline demonstrated considerable clinical and radiographic success. 5 In 2020, Brazilian researchers investigated an experimental phytotherapeutic paste containing terpineol and cinnamaldehyde for endodontic treatment of primary teeth and reported antimicrobial activity against E. faecalis similar to the ZOE paste. 9 Kirplani et al. found a superior antimicrobial efficiency of Aloe vera and sterile water as an obturating material in primary teeth compared to ZOE, Metapex, and calcium hydroxide. 6 In 2008, Fabiane et al. determined that MTA had high levels of antimicrobial activity in comparison with ZOE and a combination of calcium hydroxide and iodoform. In contrast, ten years later, MTA showed no antibacterial activity in an in vitro study in Iran. 10

CONCLUSIONS
Numerous root canal filling materials for deciduous teeth are described in dental literature. 20,26,31 However, there is little agreement between paediatric dental practitioners for the best available treatment option. 20,32 Future studies should also seek the long-term effects of the treatment. The antimicrobial activity is the first step in the investigation of the filling materials. The next step includes tests associated with genotoxicity, cytotoxicity, carcinogenicity. 3 A balance needs to be achieved. Knowledge of the indications for the use, composition, and characteristics of the available root filling materials carries a huge advantage for the dentists when they have to address the functional problems associated with an endodontic infection in very young patients.