## 1 Introduction  
Oral lichen planus (OLP) affects around 2% of the adult population, with a higher prevalence in women [1]. Diagnosis is based on clinical appearance and histopathology [2]. OLP is considered an inflammatory mucoctuneous disease; however, the mechanisms are not entirely clarified. A genetic (HLA-DR2) background and infection as a predisposing or provoking factor are considered to be involved in the pathogenesis of OLP [3]. Patients often suffer from pain or burning sensations, and from psychosocial impairment linked to anxiety and the burden of a chronic, and potentially malignant, disease, even if the malignant transformation is only in a small subset of OLP patients (around 1.1%) [4]. Especially erosive oral mucosal conditions radically affect oral health-related quality of life, and are accompanied by a high frequency of psychological problems [2]. Pathognonomic reticular lesions (Wickham striae) are usually discovered incidentally during routine intraoral examination, and are mostly asymptomatic. However, erosive lesions may cause considerable discomfort, affecting quality of life and leading patients to seek medical care [5]. Depending on the severity of the clinical symptoms, various therapeutic options have been described: the first line, mainly symptomatic treatment targeting the alleviation of pain and erythema, is topical corticosteroids (TCS), which can be recommended as safe, efficacious, and cost-effective. Although TCS is usually successful, it has side-effects, including candidiasis, burning, or stinging sensation, mucosal atrophy, bad taste, nausea, sore throat, and dry or swollen mouth. However, a superior effectiveness of topical corticosteroids, compared to other therapeutics, has not been corroborated in systematic reviews [6]. For steroid-refractory OLP, there is a plethora of alternative topical treatment approaches. Evidence-based local treatments, as supported by randomized controlled trials (RCTs), comprise macrolide immunosuppressants (tacrolimus, pimecrolimus, cyclosporine A), retinoids, and low-level laser therapy [7; 8]. In addition to standard protocols, systemic strategies with immunosuppressive or immunostimulants are well supported by research [9; 10; 11; 12; 13; 14]. To minimize side effects, such as burning sensations, fungal infections [15; 16], or risk of potentially malignant transformation [17; 18; 19], nutraceuticalals with anti-inflammatory/antioxidative properties (e.g., aloe vera, honey, curcumin) have been investigated in RCTs with promising, but inconclusive, results [7; 8; 20; 21]. Hyaluronic acid (HA), a naturally occurring glycosaminoglycan with high molecular weight, is a promising novel alternative agent to immunosuppressiveants. It is a major component of the extracellular matrix, and present in various body fluids. Its major features are a high hygroscopicity and viscoelasticity. Due to its anti-edematous, anti-inflammatory, bacteriostatic, fungistatic, and pro-angiogenic properties, it has been shown to promote wound healing in several tissues, and, therefore, could be topically used in OLP [22; 23; 24]. Beneficial effects of HA in OLP have already been reported in recent studies [24] focusing mainly on subjective outcome parameters, such as the visual analogue scale (VAS) and clinical appearance [24; 25; 26; 27]. Therapeutic interventions could also be monitored by the determination of various biomarkers possibly related to the pathogenesis of OLP. The pro-inflammatory cytokine interleukin-6 (IL-6) is thought to be of major pathobiological importance in OLP; therefore, it was shown that salivary IL-6 might represent a useful diagnostic and therapeutic biomarker in OLP [28]. Calprotectin, a protein released during inflammatory conditions by activated polymorphonuclear leukocytes, monocytes, and macrophages, was shown to be increased in the saliva of OLP patients, and might be also used as a monitoring marker [29; 30].