Introduction

Recurrent aphthous stomatitis (RAS) is an oral mucosal ulcerative disease characterized by recurring episodes of small ulcers, affecting mainly the non-keratinized mucosa [1, 2]. These ulcers are usually associated with severe pain and discomfort that interfere with oral functions such as eating, drinking, and speaking, thus adversely affecting the patients’ quality of life [3]. RAS is a highly prevalent disease afflicting up to 25% of the general population, with no gender predilection [2, 4]. Typically, it is a disease of adolescents and young adults, although the disease can affect any age group[4]. By and large, the etiopathogenesis of RAS is not yet clear [1, 2]. Some predisposing factors have been suggested including, but not limited to, immunological dysfunction, hematologic factors, stress, trauma, hormonal changes, genetic factors, and minerals and/or vitamin deficiencies [5,6,7,8,9,10]. However, recent evidence suggests that immunological mechanisms (both humoral and cellular) have an essential role in the etiopathogenesis of RAS [7, 8, 11].

Given the obscure etiopathogensis, there is no effective therapy available thus far [1]. Hence, the current management strategy aims primarily at alleviating pain, shortening the healing time, and reducing the frequency rates of new episodes [1, 12, 13]. In context of the latter, various topical medicaments have been used for management of RAS: corticosteroids, salicylic acid, antiseptic mouthwashes, analgesics, anesthetics, antibiotics, antioxidants like N-acetylcysteine, and various herbal remedies, with limited success [12, 14,15,16,17,18,19]. In severe RAS cases like more frequent attacks (commonly known as called complex aphthosis) and/or refractory major RAS, more potent systemic medications such as systemic corticosteroids, colchicine, pentoxyfilline, and thalidomide are used [20, 21]; however, these medications are associated with serious side effects, a matter that limits their use [20]. In principle, topical corticosteroids are the most widely prescribed medication for RAS patients, although they have limited efficacy [18], especially in reducing the healing time, and are associated with numerous side effects such as opportunistic fungal infections, thinning of the mucosa in addition to patients’ incompliance [12, 14].

Hyaluronic acid (HA), also known as Hyaluronan, has recently been introduced for the management of various oral and systemic inflammatory conditions with very promising results [22, 23]. HA is a carbohydrate component of the extracellular matrix that is available naturally in many tissues and body fluids [23]. It has been reported to have strong wound healing properties, probably through moderation of the inflammatory responses, promoting cell proliferation, and promoting re-epithelization via the proliferation of basal keratinocytes [23,24,25,26]. Additionally, many studies ascertained the analgesic and potent anti-inflammatory effects of HA [27, 28]. Such properties rendered HA a good candidate for management of various systemic and oral inflammatory conditions such as osteoarthritis, temporomandibular joint disorders (TMJ), dry socket, skin disorders, leg ulcers, lichen planus, and recurrent oral ulcers [24, 27, 29,30,31,32]. In this regard, a number of clinical trials have tried topical HA for management of RAS, and reported conflicting results, although promising to a large extent compared to the current medications [33,34,35,36,37,38]. Hence, the present systematic review sought to assess the available evidence regarding the efficacy of topical HA for reducing signs and symptoms of RAS.

Methods

Study protocol and focused question

The protocol of the present systematic review was registered by PROSPERO (Reg. #: CRD42021259970), and was performed in full adherence with the Preferred Reporting Items for Systematic Review and Meta-analyses (PRISMA) guidelines [39]. The addressed PICOS (Participants, Intervention, Control, Outcomes and Study design) question was: “Is topical hyaluronic acid (HA) efficient in the management of recurrent aphthous stomatitis (RAS)?”.

Eligibility criteria

The PICOS eligibility criteria applied in this review were as follows:

Participants (P): healthy individuals diagnosed with RAS; Intervention (I): topical HA; Comparator (C): any medical intervention or placebo controls; Outcomes (O): pain, healing time and/or size of the ulcers were studied as the primary outcomes, whereas side effects of the intervention were considered as additional outcomes; and Study design (S): randomized (RCT) and non-randomized controlled clinical trials (nRCT). Retrospective and prospective observational studies, case series, case reports, animal studies, review papers, editorials, letters to the editor, commentary, and monographs were excluded.

Literature search strategy

Two authors (NA and RH) performed an independent and thorough search in four databases (MEDLINE/PubMed, Scopus, Web of Science, and Google Scholar) and the gray literature (through Proquest) on 25 June, 2021 for all relevant published studies. The search was neither date- nor language-restricted. Different combinations of the following keywords/terms were used: hyaluronic acid; hyaluronan; aphthous stomatitis; recurrent aphthous stomatitis; recurrent aphthous ulcers; recurrent oral ulcers; and canker sores (Table 1). All identified articles were retrieved to an endnote program, and duplicates were removed. These two authors screened the articles independently through reading the titles and abstracts; the irrelevant studies were excluded. The full-texts of all potentially eligible studies were screened for inclusion. The reference lists of the retrieved studies were also hand-searched for any additional studies. In case of any disagreements, a third reviewer was consulted. Authors of the included studies were contacted in case of missing data or for any clarification.

Table 1 List of search keywords used in each database
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Quality assessment

Assessment of risk of bias was carried out independently by two reviewers (NA and SA) using the Cochrane risk of bias assessment tool [40]. Disagreements, if any, were resolved by discussion and/or by consulting a third reviewer. According to the above mentioned tool, seven domains were evaluated: sequence generation; allocation concealment; blinding of participants and personnel; blinding of outcome assessment; incomplete outcome data; selective outcome reporting; and other sources of bias. Accordingly, each study was graded as low, all items were of low risk; unclear, at least one item was evaluated to be of unclear risk but no item of high risk; or high, at least one item with high risk of bias [40].

Data extraction

Two reviewers (RH and SA) independently extracted all relevant data: study details (author, year of publication, and country of the study), study design, comparison groups, demographics of the participants (sample size, age, and gender), formulation and dosage of HA and the comparative interventions, primary and secondary outcomes measures (i.e., pain, ulcer size, healing time, and side effects), and the main findings.

Statistical analysis

The initial aim was to pool the results and quantify the effect size using the meta-analysis approach. However, the substantial heterogeneity among the included studies along with missing of numerical data in some of these studies precluded us from conducting the meta-analysis. Hence, the included studies were qualitatively analyzed.

Results

Search strategy results

Figure 1 depicts the results of the online search. Around 75 studies were found of which 33 were duplicates and thus removed. The titles and abstracts of the remaining 42 studies were screened by two independent reviewers and 20 studies were excluded as irrelevant. The full-texts of the remaining 24 articles were reviewed by the two independent reviewers, and 15 studies were excluded due to various reasons (Supplementary Table 1). The remaining nine studies [33,34,35,36,37,38, 41,42,43] fulfilled the eligibility criteria and thus were included in the subsequent qualitative analysis, and the relevant data were extracted.

Fig. 1
figure 1

Flowchart of the search strategy

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General characteristics of the included studies

Table 2 presents the general characteristics of the included studies. Eight RCT [33, 35,36,37,38, 41,42,43] and one non-randomized clinical trial [34] comprising 538 RAS patients (259 in HA group and 279 in the control group) were included. Three studies were conducted in Egypt [33, 37, 38], two in Iraq [34, 36], one in China [43], one in the UK [42], one in USA [41], and one in Turkey[35]. Number of RAS patients ranged from 25 [34] to 116 individuals [42], with a range of mean of age from 4 ± 6.8 to 45.5 years. All studies except one [37] reported gender of the included participants; around half of the subjects were females. With regards to type of RAS, five studies [33, 34, 37, 38, 41] recruited minor RAS, one study [36] recruited both minor and major RAS cases, while three studies didn’t report the type of RAS [35, 42, 43]. All studies reported diagnosis of RAS based on clinical features and history of the same, and excluded patients with systemic diseases that may cause RAS-like lesions (Table 2).

Table 2 General characteristics of the included studies
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Outcome measures

All studies assessed the efficacy HA in reducing pain as one of the main outcomes; seven studies [34,35,36, 38, 41,42,43] used the visual analogue scale (VAS) for pain assessment, one study used Wong- Baker faces rating Scale [37], while one study [33] did not provide any information in this regard. Five studies [34, 36,37,38, 41] assessed the efficacy of HA on ulcer size. Two studies [33, 38] measured the healing time in days. Two studies [42, 43] measured the number of ulcers, incidence of new ulcers and ulcer-free patients (Table 2).

Intervention and comparison groups

HA was used as gel in seven studies [33, 35,36,37, 41,42,43], as mouthwash (10 ml Hyaloronan sodium 25 mg/100 ml) in one study [38], and as spray (0.01% HA) in one study [34]. With regard to HA gel, only five studies [33, 36, 37, 41, 42] reported HA concentration, that ranged from 0.2 to 2.5%: 0.2% in three studies [36, 37, 42], 2% in one study [33]; 2.5% in one study [41].

The comparative groups varied greatly across the studies: triamcinolone in three studies [34,35,36], chlorhexidine mouthwash [38], lidocaine gel [41], placebo gel [42], iodine glycerin gel [43], Diclofenac in 2% HA base gel [33], and single application of Diode laser therapy [37]. Except for Saxen study [41], which did not provide any information, the reported duration of HA application varies from 5–11 days, with the most frequent duration was 7 days (Table 2).

Main qualitative results

As shown in Table 3, all studies reported comparable-to-superior pain-reduction efficacy in favor of HA compared to the different interventions assessed, except for one study [37], which reported inferior pain-reduction effect of HA in comparison to Diode laser. With regard to the gold standard comparative intervention (Triamcinolone acetonide), Mustafa et al. [36] reported better efficacy in reducing pain in favor of HA, while two studies by Koray et al. [35] and Hamed [34] reported comparable efficacy of both interventions except on the fourth and seventh days in Koray study [35] and second day in Hamed study [35] where HA was more efficacious in reducing pain.

Table 3 Reported side effects and the main outcomes of the included studies
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With regard to HA effects on ulcer size, five studies reported on this outcome and found variable results: one study [36] found superior efficacy in favor of HA as compared to Triamciolone acetonide; one study [37] reported inferior results in HA group compared to the control group ( single session of Diode laser application); and three studies [34, 38, 41] reported comparable results (Table 2). Concerning the efficacy of HA on ulcer healing, one study[38] reported better efficacy in HA group compared to control group, while one study found a comparable efficacy [33]. Number of ulcer/ulcer-free patients and occurrence of new ulcers were found to be significantly lower with application of HA compared to placebo [42, 43] (Table 3).

Side effects

Six studies [33, 35,36,37,38, 41] asserted that HA is safe as they did not find any side effects secondary to its use, while three studies [34, 42, 43] did not provide any information about the side effects (Table 3).

Quality of the included studies

Table 4 summarizes the results of quality appraisal of the included studies. Five studies were graded as high risk of bias [34,35,36, 38, 43], one study was graded as low risk of bias [41], while three studies [33, 37, 42] were of unclear risk of bias. The most frequent methodological flaws were related to the criteria of “Blinding of participants” and “Blinding of outcome” (Table 4).

Table 4 Risk of bias assessment results of the included studies
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Discussion

RAS is associated with significant pain and discomfort that negatively impact the patients’ quality of life [1, 3]. Unfortunately, irrespective of the high prevalence of, and the huge research conducted on RAS, its management is still quite challenging with no definitive cure [1]. HA is gaining ground as a treatment modality for RAS and other oral and systemic inflammatory conditions [24, 25, 27, 31]. In confirmation of the above, the results of the current systematic review revealed good efficacy for HA in reducing pain and speeding the healing time in RAS patients. Additionally, the results revealed that topical application HA is safe with a good patient’s compliance. Nevertheless, apart from the positive results reported in this review, they should be interpreted with caution due to the substantial heterogeneity among the studies and some methodological shortcomings in some of the included studies.

The main finding of the present systematic review is the positive effects of HA in reducing RAS-associated pain. The clinical efficacy of HA in reliving RAS symptoms can be attributed to its analgesic and potent anti-inflammatory properties [27, 28]. To elaborate, HA inhibits inflammation through regulating the inflammatory mediators associated with nociceptive pain such as prostaglandin E2, cyclooxygenase-2, and adenosine 5-triphosphate, a fact that may explain the potent and immediate analgesic effects of HA [28]. Actually, this result is in line with the findings of many previous studies that reported positive effects of topical HA application in alleviating pain and other inflammation-associated symptoms in a number of oral and systemic disorders such as, disorders of temporomandibular joint, arthritis, oral lichen planus, and radiation-induced oral mucositis [22, 27, 30, 31, 44]. Additionally, many case series and retrospective studies (not included in the present review) showed a good efficacy of HA in reducing signs and symptoms of RAS [45,46,47], which further substantiate the findings of the present review.

Another main finding of the present review is a good efficacy of HA in reducing the healing time of RAS: It was found to be as efficacious as or even better than triamcinolone. This can be ascribed to the strong wound-healing properties of HA [25, 26]. In fact, the hygroscopic and viscoelastic properties of HA play an important role in the wound healing process [23, 26, 30]. Further, HA has been reported to promote wound healing and re-epithelization through proliferation of basal keratinocytes and reduction of collagen disposition and scarring [22, 23].

Two important aspects of the management of RAS are the safety and patient compliance. Although it is the case with any disease, it must be emphasized more specifically with RAS given the recurrence nature of the disease and the need for long-term use of various therapies in some cases. The secondary outcome assessed in this review was the side effects associated with HA. The results of the current review showed that topical application of HA is safe and well-tolerated, rendering HA a feasible alternative therapeutic option for RAS. Another important advantage of topical HA is the fact that it is available over-the counter and can be used safely by all individuals including small children and pregnant women without any complications or drugs interactions [23, 42]. Customarily, topical corticosteroids— the most widely used medicaments for RAS—are associated with many local and systemic adverse effects limiting their use [42]. The results of the present systematic review corroborate previous studies that reported HA to be safe and well tolerated [23, 24, 32, 44, 48]. Another concern of the current RAS therapeutics is the cost of treatment, considering the chronic and recurrent nature of RAS, which necessitates long term treatment, resulting in a terrible financial impact on patients, especially in low-income countries [14]. Hence, a safe, efficacious, and cost-effective medicament like topical HA might be a viable alternative option for the management of RAS [48, 49].

The present systematic review has some weaknesses that limit its results. The key limitation is the low quality of some of the included studies as evident by the high risk of bias, a matter that weakens the evidence obtained from this review. Another key limitation is the marked heterogeneity among the included studies in different parameters such as comparison groups, outcome measures, formulation and dose of the intervention, duration of therapy, type of RAS, and age and gender of the participants. The heterogeneity in the comparison groups in particular made the inter-studies comparability very impossible, and thus no firm conclusion can be drawn. A further limitation was related to the discrepancy in the reported outcomes along with missing numerical data in some of the included studies, a matter that hindered us from pooling the data and thus no meta-analysis was conducted. However, despite these limitations, this review has some strengths that should be recognized. First, this is the first systematic review that evaluated the evidence regarding HA efficacy for RAS. Second, the study extensively searched the literature without any language restriction, and thus no potential studies might have been missed. Third, the review included a relatively good number of studies with a fairly good sample size (9 clinical studies involving 538 RAS patients) from different geographical regions and that somewhat substantiates the concluded evidence.

In conclusion, the available evidence suggests the potentially positive efficacy of HA in reducing signs and symptoms associated with RAS. Further, well-designed studies with large sample sizes and standardized methodologies are needed to confirm the efficacy of HA.