Clinical, Microbiological, and Biochemical Outcomes of Hyaluronic Acid in Non-Surgical Periodontal Therapy: A Systematic Review and Meta-Analysis

Abstract

Aim: This systematic review and meta-analysis evaluated the adjunctive use of hyaluronic acid (HA) in non-surgical periodontal therapy to improve clinical outcomes in patients with moderate-to-severe chronic periodontitis. The aim was to assess its effectiveness in reducing probing pocket depth (PPD), improving clinical attachment level (CAL), and decreasing bleeding on probing (BOP). Methods: Four electronic databases were searched (PubMed, Cochrane Library, Scopus, Web of Science, Grey Literature) for randomized controlled trials (RCTs) published until January 2025 and involving otherwise healthy patients treated with scaling and root planing (SRP) plus HA after at least 12 weeks of follow-up. Risk of bias was assessed using RoB 2.0; data were collected on clinical assessment, microbiological, and biochemical analysis and then analyzed using a fixed-effects or random-effects model considering the high heterogeneity of the included studies, which calls for caution when interpreting the results. Results: Fourteen of the 21 RCTs reviewed underwent a meta-analysis. Compared with SRP alone, the addition of adjunctive HA ≥ 0.8% led to an improvement in PPD, but the reduction in BOP was less consistent. Biochemical markers indicated less inflammation, oxidative stress, and selective antimicrobial activity. Although studies on 0.2% HA are still limited, improvements have been observed in clinical parameters, along with better biochemical and microbiological outcomes in the experimental group compared to the control group. Conclusions: Despite moderate heterogeneity and methodological limitations, the evidence supports the use of HA combined with SRP in periodontal treatment.

1. Introduction

Periodontitis is a complex chronic inflammatory disease that damages dental structures [1]. Clinical hallmarks are progressive destruction of the gums, the periodontal ligament, and the alveolar bone due to an exaggerated inflammatory response to bacterial accumulation [2]. When untreated, periodontitis can lead to tooth instability and potential tooth loss, as well as potential systemic repercussions throughout the body, as evidenced in the literature. Individuals with moderate to severe periodontitis have an established higher risk of developing diabetes and cardiovascular disease, while the effect of periodontitis is related to many other systemic diseases, which, however, require further studies to be ascertained [3]. Although scaling and root planing (SRP) remain the primary non-surgical treatment approach, their effectiveness may vary depending on disease severity and patient factors [3].

The use of additional agents to improve outcome after non-surgical periodontal treatment has gained wider interest [4]. Hyaluronic acid (HA) has emerged as a promising candidate due to its natural occurrence in the human body and its well-documented anti-inflammatory, healing, and moisturizing properties [5]. It has found various applications in dentistry, for example, in the treatment of lichen planus [6], a lesion that can undergo malignant transformation [7], in maxillofacial district surgery [8], in periodontal regenerative surgery [9], and in the non-surgical treatment of periodontal disease [10].

In the management of periodontitis, several trials have evaluated the efficacy of HA as an adjunct to scaling and root planing (SRP). These studies indicate that HA contributes to significant reductions in probing pocket depth and improvements in clinical attachment levels, particularly in moderate to deep periodontal pockets [11].

The clinical benefits are attributed to HA’s ability to modulate inflammation, promote fibroblast proliferation, stimulate angiogenesis, and enhance extracellular matrix remodeling in the periodontal tissues. Its viscoelastic and hygroscopic properties help maintain a moist wound environment, thereby accelerating soft tissue repair and stabilizing the healing site.

HA is currently available in multiple formulations—such as gels, rinses, and injectable solutions—with varying molecular weights and application protocols. High-molecular-weight HA, in particular, has demonstrated superior anti-inflammatory effects, while lower molecular weights may better promote cellular proliferation.

Clinically, HA is often applied subgingivally post-SRP either as a single dose or through repeated applications over several weeks, depending on the formulation. Furthermore, its combination with other antimicrobial or oxygen-releasing agents has shown synergistic effects in disrupting subgingival biofilms, enhancing both its antimicrobial action and the overall periodontal healing response. [12]

With the present systematic review and meta-analysis, we wanted to analyze the efficacy of HA as an adjunct to non-surgical periodontal therapy for reducing probing depth, clinical attachment level, and bleeding on probing in periodontitis patients. The review describes the mechanisms of action of HA, evaluates the current clinical, microbiological and biochemical evidence, and discusses future prospects for periodontal treatment.

2. Materials and Methods

The present meta-analysis and systematic review were conducted according to PRISMA 2020 guidelines [13]. The review was registered in PROSPERO, the international registry for systematic reviews, under the identifier number CRD42024629547.

2.1. Research Question

Does the adjuvant use of HA (hereafter, the intervention) have a positive effect on clinical, microbiological, and biochemical parameters (hereafter, the outcome) compared with conventional scaling and root planing (hereafter, the control) in patients with periodontitis (hereafter, the patient)?

2.2. Search Strategy

Four electronic databases (PubMed, Cochrane Library, Scopus, Web of Science, Grey Literature) were searched for randomized clinical trials (RCTs) and clinical trials published up to January 2025 and using the search terms and Boolean operators (AND, OR, NOT) (

Table 1. Research string for databases.

Database	Search Terms	Boolean Operators	Notes
PubMed, Cochrane Library, Scopus, Web of Science	(“Periodontitis”[MeSH] OR non-surgical periodontal therapy OR periodontal disease) AND (“Hyaluronic Acid”[MeSH] OR hyaluronan OR hyaluronate)	AND, OR	Up to January 2025
PubMed, Cochrane Library, Scopus, Web of Science	(“Hyaluronic Acid”[MeSH] OR hyaluro*) AND (“Periodontal Diseases”[MeSH] OR periodont*) AND non-surgical	AND, OR	Up to January 2025
PubMed, Cochrane Library, Scopus, Web of Science	(“Hyaluronic Acid” OR hyaluro*) AND (Periodontitis OR periodont*) AND non-surgical	AND, OR	Up to January 2025

).

If the selected studies also included microbiological and biochemical analysis, those data were included in the systematic review.

2.3. Screening and Selection

Two independent reviewers (VM and AP) screened study titles, abstracts, and full texts. Disagreement over eligibility was resolved through discussion. The search was restricted to RCTs and clinical trials in humans. In detail, the inclusion criteria are as follows: publication date from January 2004 to the present; must be English language; full-text article; split- or full-mouth design; participants with ASA (Physical Status Classification System) I health status and periodontitis; in good health condition; without systemic, organic, or psychiatric diseases; administration of HA in monotherapy as an adjunct to non-surgical periodontal treatment; HA formulations from 0.2% to 1.6% were considered (but only studies from 0.2% to 0.3% and those from 0.8% to 0.9% were included in the meta-analysis in order not to compromise heterogeneity too much); minimum 3-month follow-up. The primary outcome was changes in pocket probing depth (PPD); the secondary outcomes were changes in clinical attachment level (CAL), bleeding on probing (BOP), microbiological, and biochemical parameters.

The PICO elements considered in this review were as follows:

• P (Population): patients diagnosed with periodontal disease;
• I (Intervention): adjunctive use of HA gel in support of SRP;
• C (Comparison): SRP alone;
• O (Outcome): clinical, microbiological, and biochemical parameters.

The exclusion criteria were as follows: patients with systemic disease and/or substantial functional limitations with one or more moderate to severe diseases (ASA II, III) and patients with systematic disease (ASA II, III) who had received the following: surgical therapy for periodontitis; dental implants; antibiotic prophylaxis; and topical agents active on biofilm (systemic administration of drugs, antibiotics, probiotics, laser, ozone therapy). The following types of studies were excluded: reviews, editorials, systematic reviews, meta-analyses, PhD theses, case reports, in vitro trials, and studies that did not report clinical outcomes.

2.4. Risk of Bias Assessment

Two reviewers (VM and AP) assessed the studies for risk of bias using the RoB 2 tool developed by Sterne et al. (2019) [14]. The RoB 2 tool (a revised Cochrane risk-of-bias tool for randomized trials) can be applied to evaluate various domains and classify them as having an unclear risk, a low risk, or a high risk of bias. The risk of bias was assessed in accordance with the Cochrane Risk of Bias 2.0 tool. Judgements for each domain were made for each included study according to the official recommendations [14]. The domains are as follows: random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, and other relevant factors. A study was classified as having a high risk of bias if a domain was rated as high risk, an unclear risk of bias if a domain was unclear and none were high risk, and a low risk of bias if all domains were rated as low risk.

2.5. Data Extraction

The data were entered on an Excel spreadsheet (Microsoft 365, Version 2503) including the following: main author, year of publication, study design, country, sample size, patient mean age and sex, test and control interventions, follow-up, outcomes. Where available, means and standard deviations were recorded. For missing standard deviations, the methods outlined in the Cochrane Handbook were applied [15]. No contact was made with the authors of the included studies. In case of missing data, they were excluded from the quantitative summary and reported in the characteristics table. If a study provided the mean and confidence interval, the standard deviation was calculated.

2.6. Data Analysis

Of the twenty-one articles reviewed, fourteen were selected for inclusion in the meta-analysis, and their inclusion in the statistical analysis was based on the availability of data regarding clinical parameters, the percentage of HA used, the application of HA in non-surgical periodontal therapy, and the presence of follow-up data. A meta-analysis was performed using Review Manager Web [15] and the inverse variance method and a fixed-effects or random-effects model to compute 95% confidence intervals (CI). The level of heterogeneity was assessed according to the Cochrane Handbook guidelines: low (0–40%), moderate (30–60%), considerable (50–90%), and high (75–100%) [16].

2.7. Grading the Body of Evidence

Quality of evidence was assessed using GRADEpro GDT (Grading of Recommendations, Assessment, Development and Evaluation and the Guideline Development Tool) [17]. Two reviewers (VM and AP) evaluated the evidence according to risk of bias, inconsistency, indirectness, imprecision, publication bias, effect size, potential confounding, and dose–response relationship. Disagreement between the reviewers was resolved by further discussion.

3. Results

3.1. Study Flow Chart

The search retrieved 186 records. After filtering for article type (articles only), 134 records were deleted; 11 duplicates were deleted, yielding 21 records for further screening. Ultimately, 14 records were deemed eligible for review by both reviewers. Thirteen studies were RCTs and one was a retrospective analysis.

The study selection process is summarized in Figure 1.

3.2. Study Characteristics

The studies originated from fifteen countries: Kingdom of Saudi Arabia, India, Turkey, Vietnam, Poland, Lebanon, Sweden, Malaysia, Egypt, China, Africa, Switzerland, Lithuania, Italy, and the United States of America (USA). The RCTs compared changes in probing depth (PD) after the use of topical agents as an adjunct to biofilm disruption in non-surgical periodontal treatment. Twelve studies used patient assignment and nine had a split-mouth design.

HA gel 0.2% was used in five studies, HA gel 0.8% in eight, and HA gel 0.3% in one study; two studies compared the application of HA gel 0.2% and 0.8%, one study compared HA gel 0.2% and 0.9% NaCl rinses with only SRP and NaCL rinses, one compared HA gel 1.6% and 0.2%, and one used a crosslinked HA gel baseline. They did not specify the HA concentration (

Table 2. Study characteristics.

First Author and Year	Study Design	Study Design	Country	Sample Size and Sex	Mean Age/Rang (Years)	Test and Control Interventions	Follow-Up	Outcome
Xu 2004 [18]	RCT	Split-mouth	China	20 (11 M, 9 F)	48.6	1 SRP 2 SRP + 1 mL 0.2% HA gel	6 and 12 wks	BOP, CAL, PD
Johannsen 2009 [19]	RCT	Split-mouth	Sweden	12 (sex not stated)	42–63	1 SRP 2 SRP + 0.2 mL + 0.8% HA gel	12 wks	BOP, PI, CAL, PD
Koshal 2012 [20]	RCT	Random assignment	Africa	52 (sex not stated)	/	1 SRP + placebo 2 SRP + HA	12 wks	GI, PD
Chauhan 2013 [21]	RCT with biochemical analysis	Random assignment	India	11 (sex not stated)	30–65	1 SRP 2 SRP + HA gel 3 SRP + HA gel + CHX gel	6 and 12 wks	PD, CAL, PI
Eick 2013 [22]	RCT with microbiological analysis	Random assignment	Switzerland	34 (sex not stated)	41–72	1 SRP 2 SRP + 0.8% HA gel in office + 0.2% HA gel at home	12, 24, 36 wks	BOP, PI, CAL, PD
Rajan 2014 [23]	RCT	Split-mouth	India	33 (15 M, 18 F)	NI	1 SRP 2 SRP + 0.2% HA gel	4 and 12 wks	GI, PI, BOP, PPD, CAL
Polepalle 2015 [24]	RCT	Split-mouth	India	18 (11 M, 7 F)	30–60 45	1 SRP 2 SRP + 0.2 mL 0.8% HA gel	4 and 12 wks	PI, BOP, PPD, CAL, CFU
Shah 2016 [25]	RCT	Split-mouth	India	9 (sex not stated)	30–60 45	1 SRP 2 SRP + 0.8% HA gel + periodontal dressing	4 and 12 wks	GI, PI, PPD, RAL
Sharma 2016 [26]	RCT	Split-mouth	India	24 (sex not stated)	25–55 40	1 SRP + d coenzyme Q gel 2 SRP + 0.8% HA gel 3 SRP alone	1, 2, 6 wks	PI, EIBI, GCCI, PD, CAL
Al-Shammari 2018 [27]	RCT with biochemical analysis	Split-mouth	Kingdom of Saudi Arabia	24 (14 F, 10 M)	24–57 40.5	1 SRP alone 2 SRP + 0.8% HA	6 and 12 wks	PI, GI, PBI, PPD, CAL, human beta defensin-2 (hBD-2) expression
Lobato 2019 [28]	RCT	Split-mouth	India	16	41–68 54.5	1 SRP 2 SRP + 0.8% HA	6 and 12 wks	FMPS, GI, BOP, PD CAL
Aydinyurt 2020 [29]	RCT with biochemical analysis	Random assignment	Turkey	96 (40 M, 56 F)	22–55 38.5	1 SRP + saline irrigation 2 SRP + 0.2% HA gel 3 SRP + 0.2% HA hydrogel mouth rinse 4 SRP + 0.2% HA hydrogel mouth rinse + 0.2% HA gel	1, 2, 4 wks	PI, GI, BOP, PD, GRH, CAL, biochemicals (ADA, CAT, GSH)
Nguyen 2021 [30]	RCT with microbiological analysis	Split-mouth	Vietnam	34	ni	1 SRP + 0.9% NaCl solution rinse 2 SRP + 0.9% NaCl solution rinse + 1 mL 0.2% HA gel	1, 2, 3, 6 wks	PI, GI, PPD, CAL, BOP
Olszewska 2021 [31]	RCT	Random assignment	Poland	100 (51 F, 49 M)	25–65 45	1 SRP + HA gel (1.6% reticular HA + 0.2% HA natural) 2 SRP alone	12 wks	API, BOP, PPD, CAL
Vajawat 2022 [32]	RCT with microbiological analysis	Split-mouth	India	24	35–65 50	1: SRP + placebo 2: SRP + 0.8% HA gel	4, 12 wks	CAL, PD, BI, GI, PI
Mazloum 2023 [33]	RCT	Random assignment	Lebanon	75	20–60 40	1 SRP + 0.8% HA gel 2 SRP + with red i-PRF 3 SRP alone	4, 8, 12 wks	CAL, PD, PI, GI, BOP
Bertl 2024 [34]	RCT	Random assignment	Sweden	56 (37 F, 19 M)	35–75 55	1 SRP + 0.3% HA gel 2 SRP + saline solution	12, 24, 36, 48 wks	PPD, PI, GI, BOP, CAL
Jaafar 2024 [35]	RCT	Random assignment	Malaysia	36 (25 F, 11 M)	30–60 45	1 SRP + 0.8% HA gel 2 SRP alone	12 wks	PI, PD, CAL, BOP, REC
El Emam 2024 [36]	RCT	Random assignment	Egypt	28 (14 M, 14 F)	28–40 34	1 SRP + local 0.2% HA gel 2 SRP alone	12 wks	PI, BI, CAL, MSBI, PD
Ramanaus kaite 2023 [37]	RCT	Random assignment	Lithuania	21	30–72	1 SRP alone 2 SRP+ crosslinked HA gel baseline	12 and 24 wks	PD, CAL, BOP, PI
Iorio-Siciliano 2025 [38]	RCT	Random assignment	Italy	38 (14 M, 24 F)	Test 49.3 ±11.6 Control 50.8 ± 10.8	1 SRP 2 SRP + HA	4, 12, 24 wks	PD, FMPS, FMBS, CAL

Abbreviations: wk(s), week(s); RCT, randomized clinical trial; NI, no information; PD, probing depth; PPD, pocket probing depth; CAL, clinical attachment level; BOP, bleeding on probing; SRP, scaling and root planing; HA, hyaluronic acid; PI, Plaque Index; RAL, relative attachment level; GCCI, Gingival Color Change Index; EIBI, Eastman Interdental Bleeding Index; PBI, Papillary Bleeding Index; GI, Gingival Index; FMPS, Full Mouth Plaque Score; GRH, gingival recession level; API, Approximal Plaque Index; REC, recession; CFUs, colony-forming units; MSBI, Mean Sulcus Bleeding Index; FMPS, Full Mouth Plaque Score; FMBS, Full Mouth Bleeding Index.

).

3.3. Microbiological Analysis

Eick et al. [22] analyzed six periodontopathogenic bacterial species. At 6 months after root polishing (SRP), A. actinomycetemcomitans (p = 0.027) and C. rectus (p = 0.008) were more prevalent in the control group. Treponema denticola was decreased in the test (p = 0.043) and in the control group at 3 months (p = 0.043), while C. rectus was decreased in the test group (p = 0.028). P. intermedia (p = 0.043) and P. gingivalis (p = 0.016) were increased in the control group. No significant changes in T. forsythia were noted.

Vajawat et al. [32] found a statistical difference at 12 weeks only for A. actinomycetemcomitans and P. gingivalis, with a greater reduction in the test than in the control site. Polepalle et al. [24] reported a significant reduction in colony-forming units (CFUs) in the test compared with the control site at the 12-week follow-up (p < 0.001). The CFU/mL value decreased from 5.21 × 10⁶ to 2.10 × 10⁵ in the test and from 5.12 × 10⁶ to 3.55 × 10⁵ in the control site.

Nguyen et al. [30] observed a significant reduction in T. forsythia after the use of 0.2% HA compared with SRP alone, but no differences in other bacterial species at the 6-week follow-up. In contrast, El Emam et al. [36] and Xu et al. [18] reported no statistical difference between the use of 0.2% HA and SRP alone at the 12-week follow-up.

Biochemical Analysis

Al-Shammari et al. [27] measured human beta defensin-2 levels using an enzyme-linked immunosorbent assay. At the test site, human beta defensin-2 (hBD-2) levels were significantly increased over baseline at 6 and 12 weeks (p = 0 for both comparisons). At baseline, there were no significant differences between the test and the control sites (p > 0.05). At 6 weeks, however, the test site showed significantly higher levels of hBD-2 than the control site (p = 0.012). The mean hBD-2 in the test site at 6 and 12 weeks (29.85 and 28.77) was higher than in the control site.

In contrast, the study by Chauhan et al. [21] measured the total leukocyte count (TLC), differential leukocyte count (DLC), and C-reactive protein (CRP). Only one patient tested positive for CRP at baseline and was not included in the final analysis. Systemic and hematologic parameters showed an increase in white blood cell (WBC) count at 24 h and a decrease at 1 month and 3 months in all groups (p < 0.05), with a greater reduction in the test group. Neutrophil, eosinophil, and monocyte counts peaked at 24 h and were decreased at 3 months, with greater reduction in the test group. WBC count decreased initially and then increased at subsequent follow-ups.

Aydinyurt et al. [29] reported a statistically significant decrease in adenosine deaminase (ADA) levels in gingival crevicular fluid (GCF) in the control and the test group. An increase in catalase (CAT) and glutathione (GSH) levels in GCF was observed in both groups at follow-up (p < 0.05). Differences between baseline and week 1, 2, and 4 were statistically significant for ADA, CAT, and GSH levels in GCF in the control and the test group already at week 1. No statistically significant differences in ADA, CAT, and GSH levels between the groups were observed at the other follow-ups.

3.4. Risk of Bias Assessment

A risk of bias assessment of the RCTs revealed an overall low risk of bias. A total of 15 of the 21 RCTs were classified as low-risk, and 4 had some concerns, while the remaining 2 were high-risk. Concerns arose mainly in the areas of random sequence generation, allocation concealment, and whether the protocol was properly recorded and aligned. The risk of bias assessment is shown in Figure 2.

3.5. Meta-Analysis

Fourteen studies were analyzed quantitatively because they shared similar characteristics such as HA concentration and 12-week follow-up. A meta-analysis was conducted on two subgroups, which were divided into those that used a HA concentration > 0.8% (not including the 1.6%) and those that used a HA concentration < 0.3%. This was performed to compare the effect of HA on reducing probing pocket depth (∆PPD), clinical attachment level (∆CAL), and bleeding on probing (∆BOP) from baseline to 12 week follow-up. The means and standard deviations of the change in PPD, CAL, and BOP from baseline are presented in Figure 3, Figure 4, and Figure 5, respectively.

Baseline scores generally showed no significant differences. The overall analysis yielded the following: [WMD −0.68; 95% CI (−1.14, −0.21) with heterogeneity I² 95%, and p < 0.00001]. The first subgroup analysis (SRP plus HA > 0.8%) showed the following: [WMD −0.73; 95% CI (−1.35, −0.10 with heterogeneity I² 95%, and p < 0.0001]. The second subgroup analysis (SRP plus HA < 0.3%) showed the following: [WMD −0.59; 95% CI (−1.68, 0.50) with heterogeneity I² 95%, and p < 0.0001]. The PPD analysis indicates that the addition of HA 0.8% yields slightly better results, although HA < 0.3% also produces a positive outcome compared to traditional therapy, with a minimal difference between the two concentrations (Figure 3).

For CAL analysis, the baseline scores generally showed no significant differences. Overall analysis yielded [WMD—−124; 95% CI (−1.37, −1.11) with heterogeneity I² 94%, and p < 0.00001]. The first subgroup analysis (SRP plus HA > 0.8%) showed [WMD −132; 95% CI (−1.46, −1.18) with heterogeneity I² 95%, and p < 0.0001]. The second subgroup analysis (SRP plus HA < 0.3%,) showed [WMD −0.69; 95% CI (−1.04, −0.33) with heterogeneity I² 87%, and p = 0.004]

BoP analysis on SRP with HA 0,8% versus SRP showed statistically significant differences with p < 0.00001. Overall analysis showed the following: [WMD −041; 95% CI (−0.44–0.37) with heterogeneity I² 99%, and p < 0.00001] (Figure 5).

The meta-analysis revealed substantial or considerable heterogeneity among the included studies (I² ranging from 87% to 95%), which considerably limits the reliability and generalizability of the aggregate estimates. Potential sources of heterogeneity may include differences in study design, population characteristics, sample size under investigation, HA concentrations or formulations, outcome measures, and duration of follow-up. Although a random-effects model was applied to account for this variability, the results should be interpreted with caution.

3.6. Grading the Body of Evidence

The certainty of evidence and strength of recommendations were assessed using GRADEpro GDT [17]. The risk of bias ranged from low to serious, and the reporting bias showed some concerns. The analysis suggested moderate certainty in the evidence regarding the additional benefit of HA beyond its application in non-surgical periodontal therapy alone. Its use for enhancing the reduction in probing pocket depth over time may be recommended(Figure 6).

4. Discussion

4.1. Clinical Parameters

The present systematic review discusses the efficacy of topical HA at different concentrations in addition to SRP in non-surgical periodontal therapy for periodontal disease. Non-surgical periodontal therapy and adjunctive therapies have gained increasing attention, with several studies reporting a correlation between periodontitis and systemic disease [39,40]. Non-surgical periodontal treatment seeks to eliminate biofilm through the use of manual, ultrasonic, or sonic devices [3]. Nonetheless, relying on mechanical removal alone may not be sufficient in some clinical scenarios, and the continued presence of periodontopathogens can result in persistent periodontal pockets [41].

Previous research indicates that additional therapy with HA can reduce the need for surgical procedures and improve outcomes for probing pocket depth (PPD) [42]. Three recent systematic reviews [11,43,44] demonstrated the effectiveness of the addition of HA in improving PPD at the 12-week follow-up. A meta-analysis of the PPD parameter, measuring the difference (Δ) between baseline and follow-up, found a statistically significant improvement with the application of HA gel. Nonetheless, due to the high risk of bias and heterogeneity, the need remains for further well-designed RCTs.

In their review, Bertl et al. [45] examined highly heterogeneous studies on HA, its mode of application, and high risk of bias, thus precluding a meta-analysis. For this reason, the application of HA as an adjunct to non-surgical periodontal treatment could not be recommended. There was a statistically significant improvement in PD, CAL, and BOP at test sites in the HA treatment group at both < 0.3% HA and > 0.8% HA. An evaluation of the clinical parameters showed no statistically significant differences in PD between the two concentrations; however, a slightly higher efficacy was noted for treatment with HA > 0.8% than for HA < 0.3%. There was a statistically significant improvement in BOP at the test sites compared with the control sites. The reason for the difference may have been more for the effect of SRP than for the bacteriostatic and non-bactericidal activity of HA, however. A statistically significant difference in efficacy was found for CAL after treatment with HA > 0.8%.

Iorio-Siciliano et al. [38] reported a statistically significant difference in PD and CAL and an acceleration of healing after treatment with HA. Their data suggested a more conspicuous difference between the test and the control group at 12 weeks than at the second follow-up at 24 weeks. Similarly, Polepalle et al. [24] reported improvement in PD, CAL, and BOP, underscoring the beneficial properties of HA in reducing inflammation and accelerating healing. Finally, Olszewska-Czyz et al. [31] reported an improvement in CAL and PD, suggesting the anti-inflammatory action and periodontal regeneration of 1.6% HA.

4.2. Microbiological and Biochemical Analysis

The effect of HA on pathogenic bacteria and the persistence of some species after periodontal treatment led to a decrease in some cases and an increase in others, suggesting the influence of multiple clinical factors. Since its effect appears to be primarily inhibitory rather than eliminative, its use should be reserved as an adjunct to conventional treatment, as demonstrated in the study by Pirnazar et al. [46], where a bacteriostatic but not bactericidal effect of HA was reported. For example, A. actinomycetemcomitans was sensitive to medium and high concentrations, with consistent growth inhibition, and P. gingivalis proliferation was reduced, suggesting the potential of HA to limit the spread of pathogenic bacteria. Furthermore, slight growth inhibition of S. mutans was noted, indicating that HA may modulate the microbial environment in the periodontal setting.

Biomarkers of inflammation, immune response (increase in beta-defensin 2 expression in treated sites), systemic parameters (e.g., leukocyte count), and C-reactive protein (progressive reduction in inflammation) were measured. Aydinyurt et al. [29] analyzed the levels of enzymes and antioxidants in gingival crevicular fluid and observed a significant reduction in adenosine deaminase (ADA), suggesting a potential decrease in inflammation. In addition, catalase (CAT) and glutathione (GSH) levels were increased, indicating improved antioxidant response. The greater improvement initially noted in the test group than in the controls diminished over time, however.

4.3. Formulation of HA and Its Clinical Impact

Different formulations of hyaluronic acid (HA), such as cross-linked versus linear forms, may influence clinical outcomes due to variations in bioavailability, viscosity, and degradation rates.

The studies included in this review often lacked detailed reporting on the type of HA used, limiting our ability to draw firm conclusions regarding formulation-specific effects [4,5,6].

From a clinical perspective, HA could be considered as a supplementary agent following scaling and root planing (SRP), particularly in patients with moderate to severe periodontitis or where enhanced soft tissue healing is desired. However, clear clinical guidelines are still lacking, and treatment protocols remain heterogeneous across studies.

4.4. Study Limitations

One of the main limitations of the meta-analysis was the short-term follow-up. A long follow-up period of 6 or 12 months would have facilitated a more accurate assessment of the effects of HA treatment over time and to compare its efficacy with SRP alone. Furthermore, there was considerable heterogeneity (I² > 90%) across the studies, which impacted the reliability of the results. Such heterogeneity could result from differences in how periodontal cases are defined, as well as in the diverse stages and grades of periodontal disease. This review did not include formal sensitivity analyses to explore sources of heterogeneity, which is a limitation. It is recommended that future systematic reviews, with more homogeneous data and greater availability of study-level variables, include such analyses. A further limitation concerns the BOP parameter, for which a comprehensive analysis could not be performed owing to the few studies that used a HA concentration < 0.3%. Consequently, the meta-analysis included only studies with a HA concentration > 0.8% per cent and precluded the assessment of efficacy in relation to the percentage used and limiting itself to an absolute analysis of the treatment effect.

An important limitation of our meta-analysis is the variability in the classification of periodontal disease among the included studies. Different classification criteria (e.g., mild, moderate, and severe periodontitis) were used, which may have affected the aggregated results. Despite the review and inclusion of studies published by different sources, no funnel plots or Egger tests were conducted to assess publication bias. This might represent a limitation in our meta-analysis, as a publication bias might have influenced the results. Future analyses should include these tests to explore more critically the possibility of publication bias and its influence on the overall results.

4.5. Implications for Future Research

Future studies should aim to standardize the type, concentration, and application protocols of HA to facilitate more robust comparisons. Moreover, longer follow-up durations, better reporting of HA formulations, and subgroup analyses based on disease severity are recommended to clarify the clinical relevance of HA in periodontal therapy [25,26].

5. Conclusions

Despite the limitations of the current review, it can be concluded that, based on the evidence from a 12-week follow-up, the local application of 0.8% HA gel combined with scaling and root planing (SRP) positively impacts periodontal health in patients with moderate-to-severe chronic periodontitis. Although studies on 0.2% HA are still limited in the literature, its use has shown benefits even at smaller dosages. Indeed, clinical parameters such as PD, BOP, and CAL, along with biochemical and microbiological parameters, improved in the experimental group compared to the control group. Considering the available evidence and the methodological limitations of the meta-analysis, the statistical analysis indicates that adjunctive HA may aid in achieving and maintaining periodontal health.