## 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.