Commercially available desensitizing agents are thriving to meet all ideal requirments; mainly the long-term effect, resistance to acidic and mechanical challenges and the remineralizing ability. On the other hand, research has never gave up on developing an agent that enjoys such properties and earn the title of the “gold standard” in management of dentin hypersensitivity. Recently, a paradigm shift has paved the way for natural products to compete with other chemical agents in offering a reliable agent to treat such condition.
In the current study, the different desensitizing toothpastes were brushed onto EDTA etched dentin surface for 2-min two times per day to mimic a daily oral hygiene routine for 7-d. Then, a 4-d citric acid cycling with pH 5 was established. Citric acid was selected for being one of the major organic acids found in fruits juices, as well as soft drinks. This acid challenge was performed as a post-treatment to simulate the consumption of acidic fruits, juices and soft drinks in the oral environment and to evaluate the resistance of the tested toothpastes to the acidic attack (Chen et al. 2015).
The null hypothesis of the current study was rejected, as the results showed that; NovaMin-containing toothpaste was the only toothpaste that was able to increase dentin surface microhardness following a week of twice daily tooth brushing in comparison with the other tested toothpastes. This finding might be related to the chemistry of NovaMin which consists of calcium sodium phospho-silicate which might lead to the deposition of such minerals onto the dentin surface which consequently will cause hardening of the dentin surface.
These results agreed with the study of Parkinson and Willson (2011) who found that; there was a positive change in dentin surface microhardness of the specimens brushed with NovaMin-containing toothpaste after 4 days of brushing. They related the hardening of the dentin surface to the chemical composition of NovaMin and the subsequent formation of mineral deposits in the dentinal tubules, which are harder than the dentin itself. Furthermore, they claimed that; the increase in dentin microhardness might be also due to the presence of residual unreacted calcium sodium phospho-silicate particles attached to the dentin surface. Therefore, they suggested that; the increase in microhardness is due to a combination of dentin surface mineralization and the presence of partially reacted NovaMin particles on dentin surface.
Correspondingly, Jones et al. (2015) conducted an in situ randomized clinical study to investigate the remineralizing ability of NovaMin. They stated that; under the in situ conditions of the study, NovaMin-containing toothpaste exhibited an increase in dentin microhardness after 5 and 10 days of twice daily brushing. They attributed the dentin hardening to the chemical composition of NovaMin in association with the presence of fluoride in the toothpaste formulation.
On the other hand, a contradicting study conducted by Sauro et al. (2011) revealed that; no change occurred in dentin surface microhardness neither after immediate application of a prophy paste containing NovaMin onto demineralized dentin specimens, nor following storage in a remineralizing solution. The authors suggested that; single application of NovaMin-containing paste is not enough to induce dentin remineralization.
The present study showed different results than Agiuar et al. (2017) who reported that; the extended use of a dentifrice containing NovaMin on dentin by simulating 2 years of tooth brushing did not promote an increase in the surface microhardness. They attributed the discrepancies of their results with the other studies results to the different methodological design.
In the current study, brushing dentin specimens for a week with Propolis- and Moringa-containing toothpastes and distilled water did not increase the dentin surface microhardness. These findings might be related to the chemical formulation of these toothpastes.
A study conducted by Bhagawat et al. (2016) compared the effect of 4% Propolis, 17% EDTA, 2% chlorhexidine and 18% etidronic acid as irrigants on root dentin microhardness. They concluded that; 4% Propolis produced a slight decrease in dentin microhardness in comparison with EDTA and chlorhexidine, and they recommended its use as an irrigating solution in root canals.
On the other hand, no data were available in the literature till this very day concerning the effect of Moringa extract on dentin surface micorhardness.
In the present study, NovaMin-containing toothpaste showed a partial recovery of dentin microhardness occurred at pH 5, these results might be related to the partial re-opening of the dentinal tubules and the incomplete washing out of the calcium sodium phospho-silicate particles from the dentin surface following the citric acid cycling.
The results of the present study were supported by the results obtained by Diamanti et al. (2010, 2011). The authors stated that; after subjecting root dentin specimens brushed with NovaMin toothpaste to a 50 h acid resistance testing, microhardness values were like those of the control toothpaste. Moreover, microhardness values obtained by NovaMin toothpaste were significantly lower in comparison with the other tested fluoridated toothpastes.
Similarly, the data obtained from Jones et al. (2015) study corroborated the current study. When a 20-d in situ study was conducted to evaluate the mineralizing ability of a NovaMin-containing toothpaste, the authors concluded that; after the introduction of an intra-oral acid challenge with orange juice from day 11 to 20 of the study, dentin surface showed decreased surface microhardness values; however, the results obtained were greater than the placebo control group. The authors explained that; NovaMin maybe providing a degree of protection from demineralization due to the presence of an acidic medium that may accelerate the chemical decomposition of NovaMin which would allow a local buffering effect and prevent more demineralization of the dentin surface.
The results of the current study disagreed with the results obtained by Burwell et al. (2009). The authors claimed that; after subjecting the bovine root dentin specimens to a 10-day pH cycling model; which included immersion of specimens in a demineralizing solution twice daily followed by brushing with 7.5% NovaMin, a partial recovery of dentin surface microhardness occurred, which might be due to the formation of a tenacious surface layer protecting dentin from acidic challenge.
On the other hand, the results of the current study were in contradiction with the data revealed by Parkinson and Willson (2011). They found that; the dentin surfaces treated with NovaMin-containing toothpaste exhibited a relative resistance to twice daily citric acid challenge with pH 3.2 for 2 days in a 4-d study. They related their results to the chemistry of NovaMin in acidic medium, where the frequency of calcium and phosphate release increases.
The data obtained from the current study showed a significant decrease in dentin surface microhardness values of the Propolis and the Moringa groups after citric acid cycling in comparison with the NovaMin group. On the other hand, the group that received no treatment at all (A2) presented the least microhardness values following the citric acid challenge. This decrease in the microhardness values might be related to the chemical composition of Propolis and Moringa extracts. However, no data were available in the literature till the day concerning the effect of acid challenge on dentin surface microhardness following the application of neither Propolis- nor Moringa-containing products.
Several studies demonstrated the high efficiency of a NovaMin-containing toothpaste in occluding dentinal tubules and maintaining its occlusive effect following acid challenge, in addition to its capability to increase dentin surface microhardness. However, other studies revealed contradicting results. The different results are related to the diversity in methodological designs used for evaluation.
On the other hand, natural products were introduced as a clinically safe and low-cost alternative treatment to compete with the chemical agents in treating dentin hypersensitivity. Some investigations proved the ability of Propolis to obliterate the dentinal tubules and to resist acid attack, while other investigations found the opposite. The contradicted results are related to the different botanical origins, extraction, preparations and delivery methods of Propolis.