This case-control study will enroll 176 patients age- and sex- matched at the Oral Medicine Unit, Fondazione Policlinico Universitario A. Gemelli IRCCS, Rome. There will be two groups: The control group will consist of 88 healthy patients enrolled among those routinely attending the dental outpatient clinic. An additional 88 patients diagnosed with OLP will form the case group. Both groups will undergo standard dental examinations, including periodontal probing. The primary objective is to evaluate whether patients with OLP present a higher prevalence and severity of periodontitis compared to healthy subjects. Secondary objectives include the assessment of gingivitis prevalence and RT1 gingival recessions in OLP patients compared with controls. Periodontal status will be assessed using major clinical indices and classified according to the 2017 AAP/EFP case definition. The study aims to contribute evidence on the shared immune-inflammatory mechanisms underlying both conditions, with potential clinical implications for the integrated periodontal management of OLP patients.

Inclusion Criteria:

• OLP group (case)

Age ≥ 18 years Clinical and histopathological diagnosis of OLP according to WHO criteria (2021)

Control group

Age ≥ 18 years Absence of clinical signs of OLP

Exclusion Criteria:

• Both groups

Oral lichenoid lesions (OLL): oral lichenoid contact reactions (OLCR), lichenoid drug reactions (LDR), lesions associated with food/substances (e.g. cinnamon) Oral lesions associated with graft-versus-host disease (GVHD) History of haematopoietic stem cell transplantation (HSCT) Pregnancy Oncologic patients (including oral cancers) Smokers Diabetes mellitus Obesity Metabolic syndrome Immunosuppressive therapy Medications causing gingival hyperplasia

Control group only

Immune-mediated inflammatory diseases with oral involvement (e.g. psoriasis, systemic or discoid lupus erythematosus) Autoimmune oral diseases other than OLP

OLP group only

Immunosuppressive therapy for conditions other than OLP Autoimmune oral diseases other than OLP

Patients with oral lichen planus (OLP) attending the Oral Medicine Unit at Fondazione Policlinico Universitario A. Gemelli IRCCS, Rome, and healthy subjects presenting to the same outpatient clinic for routine dental care, without clinical evidence or history of oral mucosal lichen planus. Both groups will be matched 1:1 for age and sex. Patients with major systemic confounders known to affect periodontal status, including diabetes mellitus, obesity, metabolic syndrome, and smoking, will be excluded from both groups to increase internal validity.