In this review, all the included studies involved molars. This might be due to the evolution of furcation pathosis, especially in mandibular molars, and the antibacterial effectiveness of root canal-filling materials due to the complex anatomy of their root canals.

In this review, nine different root canal-filling materials used in pulpectomy for primary teeth were compared to ZOE. The alternative root canal-filling materials can be classified into 4 groups:

The first group was ZOE with iodoform. Iodoform is a bactericidal, non-irritant and radiopaque material prepared from iodine. It presents greater resorbability, biocompatibility and antibacterial properties, compared to ZOE. The iodoform-based material was introduced into root canal therapy in primary teeth by Walkhoff in 1928. It is available in different preparations of root canal-filling materials, including Maisto paste, Walkhoff Paste, KRI paste, Guedes-Pinto paste, Endoflas®, Vitapex®/Metapex® and RC fill. ZOE with iodoform was reported to have a similar clinical success rate (100%), but a lower radiographic success rate (94%), compared to ZOE (97%) at 12 and 30 months, with no difference in the overfilled material resorption at the end of 30 months (0%). These results were in accordance with those found in the studies conducted by Ramar and Mungara (2010) and Moness et al. (2013).

The second group was calcium hydroxide with iodoform. It is available on the market as premixed syringe (30.3% calcium hydroxide, 40.4% iodoform, and 22.4% silicone oil), with two commercial formulations, namely Vitapex® (Tokyo, Japan) and Metapex® (Seoul, South Korea). In this review, calcium hydroxide with iodoform was reported to have lower success rates compared to ZOE, with significant differences (p < 0.005) at 12 and 18 months. The clinical success rate ranged from 71.4 to 100% in 18–30 months of follow-ups and the radiographic success rate ranged from 53.6 to 90% in 18–30 months. These results were consistent with those reported in the systematic reviews and meta-analyses conducted by Najjar et al. (2019) and Coll et al. (2020). The extruded particles of calcium hydroxide with iodoform beyond the apex were completely resorbed within 6 months. However, the extruded particles of ZOE remained after 30 months, which was consistent with the results reported in the systematic review conducted by Barja-Fidalgo et al. (2011) and the findings reported in the studies conducted by Bawazir et al. (2006) and Ozalp et al. (2005). The resorption rate of calcium hydroxide with iodoform was faster than the physiological root resorption of deciduous teeth in 56.3%–84% of cases after 12–18 months of follow-ups. The same result was reported by numerous authors (Trairatvorakul and Chunlasikaiwan 2008; Ramar and Mungara 2010; Ou-Yang et al. 2021). The resolution of furcation and peri-apical radiolucencies in teeth filled with calcium hydroxide with iodoform was better than in those filled with ZOE. Similar results were found in the study conducted by Trairatvorakul and Chunlasikaiwan (2008) and they were explained by the antibacterial properties of this material.

The third group was a combination of ZOE, calcium hydroxide and iodoform. It is available as Endoflas®, MPRCF and Endoflas chlorophenol free. Endoflas® is a root canal-filling material presented in powder liquid form. The powder is a mixture of 56.5% zinc oxide, 40.6% iodoform, 1.63% barium sulphate and 1.07% calcium hydroxide. The liquid consists of eugenol and paramonochlorophenol. This material became rapidly popular in paediatric dentistry because of its hydrophilic property that allows filling in humid canals, its great antibacterial efficacy and its ability to disinfect dentinal tubules and accessory canals. In this review, Endoflas® was reported to have higher success rates compared to ZOE in 12 months of follow-ups, but without significant difference. The clinical success rate ranged from 92 to 96.3%, while the radiographic success rate ranged from 72% to 88.9%. These results were in accordance with those reported in the systematic review/meta-analysis of Coll et al. (2020). In this review, the extruded particles of Endoflas® were completely resorbed in almost all teeth within 3 months. These results were similar to those found in the studies conducted by Rewal et al. (2014) and Mittal et al. (1995). In this review, resorption with Endoflas® started at the same time and rate as the physiological root resorption of deciduous teeth in 88%–100% of cases after 12–24 months. The same result was found by Rewal et al. (2014) and Brar et al. (2019). The maximum decrease (50%) in the size of pre-operative inter-radicular radiolucencies was noted with Endoflas® and the minimum (15%) was noted with ZOE at 12 months. This reduction is explained by the large spectrum of antimicrobial activity of Endoflas® compared to the limited antimicrobial action of ZOE and the high pH that induces the decrease of peri-apical inflammatory reaction and stimulates the healing process and bone regeneration.

Endoflas chlorophenol free was developed by Al-Ostwani et al. in 2016 by eliminating chlorophenol, which is responsible for the presence of radiolucent lesions following endodontic treatment with Endoflas® in primary teeth, from the liquid of Endoflas®. The study conducted by Al-Ostwani et al. was the only one in the literature that used and compared Endoflas chlorophenol free to ZOE. The success rates of Endoflas chlorophenol free were better than those of ZOE. The clinical and radiographic success rates were, respectively, 87.5% and 81.3% after 12 months. Resorption with Endoflas chlorophenol free was faster than physiological root resorption in 56.3% of the cases. Thus, its resorption was faster than ZOE and Endoflas® (with chlorophenol) in the other included studies.

MPRCF consists of 44.7% zinc oxide, 14.9% eugenol, 38.3% iodoform and 2.1% calcium hydroxide. The study conducted by Chen et al. was the only one in the literature that used this material and compared it with ZOE. MPRCF was also reported to have better success rates than ZOE. The clinical and radiographic success rates were, respectively, 96.2% and 92.2% at 18 months. There was a significant difference in the resorption of overfilled material between ZOE and MPRCF. Resorption of MPRCF was more corresponding with root resorption than ZOE, with a significant difference.

The fourth group was a mixture of zinc oxide. The powder of zinc oxide was mixed with propolis, aloe vera, 10% sodium fluoride and ozonated oil.

Given the interesting properties of propolis, it was mixed with zinc oxide and used as root canal-filling material in primary teeth. It consists of 50% zinc oxide powder with 50% propolis extracted from raw propolis. Propolis is a non-toxic natural coating produced by bees. Flavonoids and phenolics are the main constituents that provide this product with strong antibacterial, antifungal, and anti-inflammatory action. In this review, the mixture of zinc oxide–propolis was reported to have a better success rate than ZOE, with a significant difference (p < 0.05). Its overall success rate was 95% compared to 70% for ZOE at 24 months of follow-ups. These results were in accordance with those reported in the study of Gupta et al. (2019).>>

Thanks to the effective role of aloe vera in bone regeneration, and its antimicrobial, inflammatory and antifungal activities, it was mixed with zinc oxide and used for root canal filling. The mixture of zinc oxide and aloe vera was reported to have good clinical and radiographic success rates of 83.3% and 79.2%, respectively, in the study of Goel et al. (2018), and 93.75% and 50%, respectively, in the study of Goinka et al. (2020) at 12 months of follow-ups, without significant difference with ZOE. The active components of aloe vera, such as enzymes, amino acids, minerals and saponins, provide this material with strong antimicrobial activity, which was demonstrated by Bhardwaj et al. (2012). Furthermore, in the study of Khairwa et al. (2014), tenderness and pain were reduced, respectively, in 93.34% and 86.67% of teeth due to the analgesic properties of anthraquinones contained in aloe vera. In this review, 60% of the cases treated with zinc oxide–aloe vera showed decreased radiolucency compared to ZOE (15%), which was in accordance with the results found by Khairwa et al. (2014).

Goel et al. (2018) used zinc oxide 10% sodium fluoride mixture in their study. This material showed better success rates than ZOE. The clinical and radiographic success rates were, respectively, 92.9% and 85.7% at 12 months of follow-ups. These high success rates were due to the anti-inflammatory, antibacterial and antifungal activities of fluoride ions and the formation of ZnF2 that is responsible for the decrease in pain.

Finally, ozonated oil was mixed with zinc oxide due to the bactericidal and fungicidal effects, and the excellent healing properties of ozone peroxides. Zinc oxide–ozonated oil mixture was used in the study conducted by Chandra et al. (2014) and it showed better success rates compared to ZOE. The clinical and radiographic success rates were, respectively, 100% and 93.3% after 12 months. This result was in accordance with those reported in the study of Doneria et al. (2017). A more important decrease in radiolucencies and progressive bone regeneration wase noted with zinc oxide–ozonated oil, which was explained by the low effectiveness of ZOE in chronic pulpitis with extensive peri-apical bone loss.

“Resorbable bioceramics”, a new root canal-filling material, was used by Hachem et al. (2022) in primary teeth. This material was reported to have interesting antimicrobial, physiochemical, and bioactivity properties. However, compared to ZOE, the higher solubility and the lower antimicrobial action could be the main disadvantages of this material.

In this review, ZOE was used alone or in combination with other materials for root canal filling in primary teeth. Among the nine materials, ZOE with calcium hydroxide and iodoform showed better clinical and radiographic success rates, a resorption rate similar to that of the roots, faster resorption of the extruded particles and a maximum decrease in the size of pre-operative inter-radicular radiolucencies, compared to ZOE.

However, none of these materials could be the ideal root canal-filling material in primary teeth since they can cause harm to the peri-apical area or the developing tooth germ and they display no bioactivity for long-term success. Thus, more high-quality well-designed randomised clinical trials in children with a long follow-up periods are required to develop the ideal root canal-filling material for primary teeth, to generate evidence-based data and to elaborate endodontic guidelines for paediatric dentists.