Black staining (BS) can cause social problems. It can occur at any age, but it is most common in preschool-aged children[1]. BS is black and conspicuous, so children with BS have esthetic concerns when they smile or expose their teeth. It is hard to remove through tooth-brushing because of its sticky, hard and insoluble properties, and relatively frequent professional mechanical tooth cleaning (PMTC) or scaling by a dental professional is required because it easily recurs within few months.

The prevalence rate of BS is 6.17 - 14.8% and it varies by country and age[1]. It is extrinsic discoloration and occurs most frequently in prepubertal children. BS is different from intrinsic discoloration in that it accumulates on acquired pellicle. It appears mostly on the cervical third of the tooth’s smooth surface, parallel to the marginal gingiva and the proximal area[2]. It begins as black dots and becomes dotted lines or half-circles. Only rarely does it cover more than half of the tooth surface, but it always begins in the cervical area where dental plaque is easily accumulated.

The etiology of BS is still unclear, but iron consumption and bacterial colony from dental plaque are positively correlated with BS[3]. Iron in drinking water, medicines that contain ferric salts, and frequent consumption of iron-rich food are considered to be the main causes of BS[4]. Hydrogen sulfide from chromogenic gram-positive bacteria from dental plaque have an important etiological role in BS formation because they chemically react with iron in oral cavity[5]. Hypothesis that BS originates from dental plaque also can be accepted, because BS and dental plaque occur in the same areas.

The American Dental Association approved the use of essential oils to treat supragingival plaque[6]. Its anti-plaque effect was also proved through clinical study[7]. Thus, it was hypothesized that essential oils could be used to treat BS because BS forms on top of plaque. Assuming reduced plaque has positive relation with relief of symptoms, two patients with BS were prescribed with essential oil-containing mouth rinses to determine whether they could reduce BS. Past studies had focused on the etiology and microbial composition of BS rather than its solution. Its esthetic problems cause patient’s social problems, so there is a need for a simple in-home therapy.

BS is black due to the ferric sulfide that form when hydrogen sulfide reacts with metal salts in the mouth[8]. Some Actinomyces strains produce hydrogen sulfide, which can form ferric sulfide in the presence of metal salts in the saliva or gingival exudates[3]. Actinomyces belongs to the resident oral microbiota of supra-gingival plaque and Actinomyces is abundant in plaque of BS patient so, Actinomyces is expected to be the cause of BS. Actinomyces may not be the only microorganism involved in BS formation[3,9], because BS patients’ dental plaque also contains more Cardiobacterium, Haemophilus, Corynebacterium, Tannerella, Treponema than those without BS. On the other side, bacterial diversity of BS is lower than those without BS[10]. Therefore, richness of Actinomyces as well as this ecological imbalance which is named dysbiosis can be considered starting point for BS[11]. Some researchers reported that BS is weakly positively correlated with the frequency of caries and strongly positively correlated with gingivitis[4,12], likely as a result of reduced levels of Streptococcus mutans, one of the main pathogens of dental caries[13]. These relationships may also be a product of the biofilm’s higher calcium and phosphate content[8] and the higher salivary buffer capacity of BS patients[9].

In these case reports, the essential oil-containing mouth rinse played an important role in slowing BS formation and pigmentation maturation in the dental plaque mass. Essential oils are combinations of phenolic compounds including eucalyptol, menthol, methyl salicylate, and thymol. Essential oils have antibacterial properties and do not have any adverse effects in this context. They do not impair dental material when directly exposed to, and their alcoholic solvents are not to associated with xerostomia or oropharyngeal cancer[7]. The following mechanisms support the bactericidal effects of essential oils. Essential oils cause protein denaturation and alter cell membranes, resulting in the leakage of intracellular contents, which leads to cell death. Especially, thymol perforates the cell membrane. Essential oils can also alter bacterial enzyme activity and inhibit prostaglandin synthetase formation, giving them anti-inflammatory properties. Even their anti-inflammatory properties are seen at lower concentrations than their anti-bacterial properties[7]. As a result, apoptosis occurs and cells cannot reproduce, resulting in lower bacterial mass and toxicity[14,15].

Essential oils can improve oral hygiene when used in combination with tooth brushing. Essential oils can penetrate deeply into interproximal dental plaque mass, reducing gingivitis severity[16-18]. Much of the bacteria that survive initial exposure to essential oils are later killed after coming into contact with essential oils remaining in the mouth[15]. It is not clear whether essential oils are more effective than fluoride or chlorhexidine gluconate (CHG) as a mouth rinse[19], but essential oils have more advantages than fluoride or CHG[20]. Essential oils kill a wide variety of both aerobic and anaerobic bacteria, such as Aggregatibacter actinomycetemcomitans, Actinomyces viscosus, Streptococcus mutans, Streptococcus sanguis, and Bacteroides[7]. Essential oils can also kill gram-positive bacteria, which can grow up colonies by adhering to acquired pellicle[12,21]. Although CHG is the golden standard antimicrobial mouth rinse, it is not often used on a daily basis due to its side effects, such as causing calculus formation, altering patients’ sense of taste, causing discoloration, and allowing opportunistic infection[22]. Furthermore, CHG’s effectiveness is reduced when it interacts with blood serum, other antimicrobial agents, and toothpaste. In contrast, essential oils have a neutral electrical charge, so they do not interact with charged ions found in blood serum, other antimicrobial agents, or toothpaste[7]. Thus, essential oil-containing mouth rinses can be used immediately after tooth brushing, whereas CHG-containing mouth rinse may be done at least 30 minutes after tooth brushing has been done. It is possible to dilute essential oils by up to 50%, because some patients want to reduce its bitter taste. Such dilution does reduce their antibacterial effectiveness. Essential oils diluted to 25% do not have any effect but, more than 50% is effective[7]. As long as essential oils have a concentration of at least 50% they can reduce initial colony formation[15]. Using a mouth rinse of 50% essential oils at least twice per day for 30 seconds at a time can improve oral health[7]. This is why the patients in this study may use 50% essential oil mouth rinse twice per day for 30 seconds at a time.

Recently, researchers have been trying to find alternative treatments for BS. It is a way to interfere with accumulation of dental plaque or prevent ferric sulfide formation by removing metal salts. Probiotics with Streptococcus salivarius M18 counteract BS by inhibiting plaque formation producing dextranase and urease. It has the additional ability to reduce the number of Actinomyces, which relieves BS[5]. Cinnamon, cranberry, and other herbal mouth rinses are also effective anti-plaque agents. Its anti-adhesion ability prevent reformation of BS[22]. Lactoferrin, in a different way, help treat BS by absorbing iron. Lactoferrin can modulate iron homeostasis in oral cavity through sequestration of ferric salts. Lowered ferric salts concentration decreases BS formation. It is additional that low ferric salts concentrations reduce reproduction of bacteria[23]. None of these alternatives are proved to outperform essential oil- or CHG-containing mouth rinses. In addition to these alternatives, diet can also have an effect. Consuming food with high iron content and high-pH water are all risk factors of BS[10]. Drinking tap water aggravates BS more than purified water [10]. If accompanied by a diet, treatment for BS prevention will be more effective.

This case report has two limitations. One is about absence of strict variable control. Methods of oral hygiene management except for mouth rinsing were not controlled. Repeated tooth brushing instruction, characteristic of toothbrushes and diet therapy can be no less associated with relief of BS than essential oil mouth rinse. Comparison of effect of each factor (essential oil, characteristics of toothbrushes, diet) was not considered. The other limitation is dependence on patient’s cooperation. Suggested therapies in this report except for PMTC were not treatments conducted directly by dentists, but types of prescription or patient education. Dentists can recommend, but not force the patient. Therefore, the dentists’ intention is reflected in the results differently depending on the patient’s level of cooperation. If these limitations are considered in subsequent clinical studies, the effectiveness and importance of essential oil to BS patients will be more detailed.

Black staining is painless but can cause social problems. Conventional treatments like professional mechanical tooth cleaning and scaling must be repeated because black staining can recur. Antimicrobial medications and probiotics may be able to treat black staining but their side effects are unknown. This study proved the effectiveness of essential oil-containing mouth rinses in younger patients. Essential oils are derived from natural substances and they do not strongly react chemically with other medicines and intra-oral tissue, so they are thought to be a stable option. Essential oils disturb bacterial adherence to tooth surfaces, lengthen the bacterial reproduction cycle, and destroy bacterial cell walls. This study showed that accurate tooth brushing followed by the use of essential oil-containing mouth rinses is an effective way to prevent recurrence of black staining in children. It can be additive treatment option to patients with black staining.