Potential for Hyaluronic Acid in Periodontal Research

Hyaluronic acid, known also as hyaluronan, is the most abundant glycosaminoglycan in the extracellular matrix. It is produced by most of the cells in the human body, possibly contributing to various fundamental biological processes. This prompts interest in its therapeutic value. Hyaluronic acid (HA) has been used for various treatments owing to its broad range of properties in medical and dental field specially in periodontology. The review gives an overview on the effects of HA on fibroblasts, bone, wound healing, periodontal regeneration, aphthous ulcer/stomatitis and stem cells. Outcomes of including Hyaluronic acid in non-surgical periodontal therapy, drug delivery system and surgical procedures such as root coverage, papilla regeneration, root conditioning, intrabony defects, guided tissue regeneration are rather optimistic. HA has shown astounding potential in treating periimplantitis. Newer frontiers have to be explored to comprehend the other promising properties of HA. Keyword – Hyaluronic Acid, Periodontology.


Introduction
Hyaluronic acid, known also as hyaluronan, is the most abundant glycosaminoglycan in the extracellular matrix(1). This linear polysaccharide has multiple repeating disaccharide units [(1→3)-β-dGlcNAc-(1→4)-β-d-GlcA-] (2). It is estimated that almost 15 grams of hyaluronic acid are present in the body of a normal adult weighing 70 kgs (3). It is produced by most of the cells in the human body, possibly contributing to various fundamental biological processes.
This prompts interest in its therapeutic value (4).
Hyaluronic acid has been used for various treatments owing to its broad range of properties. It is widely used in medical fields such as orthopaedics, ophthalmology, dermatology, cardiology, radiology. It has shown promise as a delivery system for vaccines and other medicines.
Moisture retention is one role of HA that has been exploited well resulting in the availability of several pharmacological preparations. It is an attractive therapeutic option even in the field of dentistry owing to its pseudo-elasticity, viscoelasticity, biocompatibility, anti-inflammatory and bacteriostatic nature (5). HA forms a crucial element in the extracellular matrix of the soft tissues such as gingiva and periodontal ligament, while it is present in lesser quantities in the mineralized periodontal tissues of cementum and alveolar bone (3). The range of applications for HA in periodontal research are manifold and are summarized in this review.
There is consensus that wound healing is accelerated with HA. It acts via modulating specific HA receptors to regulate inflammation, cellular migration and angiogenesis, which are the main phases of wound healing. Sudhakar et al 2020 stated that proteins that bind to HA cause activation of CD44 surface receptors which facilitate cell adhesion, migration and differentiation (6). 3 A retrospective study on healthy adults with minor recurrent aphthous stomatitis using a barrierforming HA mouthwash (GUM® AftaClear® rinse) and topical gel (GUM® AftaClear® gel) showed reduction of pain, and lesion dimensions during treatment. Complete lesion closure was obtained in 60 and 56% of the cases, respectively(7).
According to a systematic review by Casale et al in 2017, patients receiving HA for treatment of aphthous ulcers experienced faster and prolonged relief of symptoms accompanied by a reduction in the number of ulcers and inflammatory signs. Reasons for this improvement include suppression of immune response by HA and its antioxidant effect. Being hygroscopic and viscoelastic, it promotes re-epithelialization and reduces scarring. Further, the frequency of ulcer recurrence is also reduced, which is related to the inability of viral and bacterial penetration through the HA layer (8).
Hence, the wound healing property may be extrapolated to vesiculobullous lesions commonly seen on the gingiva. The topical form application of 0.2% hyaluronic acid, Hydroxychloroquine sulphate is suggested to be effective in oral erosive Lichen planus (9). Attempts at periodontal regeneration have also derived from this wound healing property of HA.

Effects on Stem cells
Umemura et al, 2016 conducted a study to assess the differentiation of odontoblasts in DPSCs (Dental Pulp Stem Cells) with the application of 1% HA (Nacalai Tesque Co., Kyoto, Japan).
The flow cytometry analysis suggested that the use of HA can lead to an upsurge of the cells that are responsible for the expression of CD44 cells. It increases by about 10% 5 minutes posttreatment which showed a reduction to 54%, 30 minutes post-treatment. HA treatment for a week showed an enhancing effect on the alkaline phosphatase protein levels and therefore on DPSC mineralization. The mRNA levels of DSPP and DMP-1 (odontoblastic differentiation 4 markers) increased 6.7 and 7.7 times respectively thereby increasing the protein levels as well without having any effect on the odontogenic differentiation markers. This action takes place due to HA's property of enhancing CD44 signalling(10).

Schmidt et al 2020 assessed the impact of different concentrations of Low Molecular Weight
Hyaluronic Acid (LMW-HA) (800 Da, 1600Da, 15000 Da) on the proliferation activity, cell size, and phenotypic surface markers of DPSCs. The cell viability was seen to be greater than 94% in all the test groups and significantly lesser in the control group (80.2%-97.2%). The cell size did not show a significant difference between the groups. The proliferation was found to be statistically significant in all the test groups when compared to the control (C vs T1-0.0015, C vs T2-0.0143, C vs T3-0.0864). The test groups showed significantly higher numbers of surface markers compared to the control group(5). osteogenic differentiation medium in all the groups but more at 1:10 dilution of both non-crosslinked and cross-linked HA (12).

Effects on bone
HA was also shown to enhance the osteoblast production and differentiation leading to more weeks and compared to SRP alone. No significant difference was seen between the groups in Plaque index at both intervals. Gingival index (GI), PPD, CAL improvement was seen in both the groups but more with SRP+HA. Where statistically significant p-value was ≤ 0.05, GI at 6 weeks was 0.0485, GI at 12 weeks was 0.0457, PPD at 6 weeks was 0.0130, PPD at 12 weeks was 0.0407, CAL at 6 weeks was 0.0477 and CAL at 12 weeks was 0.0232 (18).   (33).

Nanosystem for drug delivery
HA has proven to be an ideal drug carrier providing an adequate encapsulation and sustainable release. Turcsanyi et al 2020 conducted a study on various types of chitosan (Chit) modified HA as a nanosized (100-300 nm) drug carriers for vitamin D3 and vitamin E. It was concluded that chit: HA:: 1:1 would be unstable as it is influenced by the acidity and the purity of Chit hence Tripolyphosphate (TCP) was required. TCP treated Chit-HA gave shape stability and encapsulation as a nanoparticle carrier. Chit+TCP+HA shell was used in the 3rd group which 13 showed the best results with encapsulation, shape stability, homogeneity and drug carriage for both vitamin D3 and vitamin E. The results of the study proved that HA can be used as a nanodrug delivery system along with Chitosan and TCP as it provides a proper encapsulation and a sustainable drug release system (34).

Newer frontiers
Research by far has proven HA to be widely advantageous while being biocompatible and non-toxic. Further investigations are required in some fields to identify the true potential of HA. Research is required on the compatibility of HA with various drugs and their forms of delivery. HA has been used for targeted drug delivery in breast cancer (35). The role of HA in dentinal hypersensitivity is not known. Hence, the effects of HA on the exposed dentinal tubules and dentinal hypersensitivity are warranted.

Conclusion
HA is a vital part of the physiology in humans and other beings. Apart from the naturally occurring hyaluronan, it can be used as a supplement at therapeutic levels for various treatments in different fields. The wide application of HA makes the future perspective of research quite attractive.