Scientists dive deep into joint tissue to unlock secrets of rare arthritis

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * For all participants: * Signed consent form * Patients over 18 years old * Affiliated with a social security scheme * For cases: * Referred for arthritis or bursitis occurring in the context of a rare systemic autoimmune disease (SAID) or drug-induced toxicity. * Presenting at least one clinical arthritis with synovial thickening confirmed by ultrasound (grade ≥2 in B-mode) and Doppler inflammation of grade ≥1, or synovial thickening ≥B2 associated with joint effusion. For bursitis: thickening ≥2mm of at least one periarticular bursa on ultrasound associated with Doppler inflammation of grade ≥1, or synovial thickening ≥2mm associated with joint effusion, with clinical evidence of inflammatory involvement. \*For the control group: For arthritis: * Presenting clinical arthritis with synovial thickening confirmed by ultrasound (grade ≥2 in B-mode (B2)) and Doppler inflammation of grade ≥1, or synovial thickening ≥B2 associated with joint effusion. * Diagnosed, according to disease-specific classification criteria, with either rheumatoid arthritis (according to ACR/EULAR 2010 criteria), axial or peripheral spondyloarthritis (according to ASAS 2009 criteria), psoriatic arthritis (according to CASPAR criteria), or reactive arthritis based on clinician assessment. For bursitis: * Thickening ≥2mm of at least one periarticular bursa on ultrasound associated with Doppler inflammation of grade ≥1, or synovial thickening ≥2mm associated with joint effusion. * Meeting the ACR/EULAR 2012 criteria for polymyalgia rheumatica. Exclusion Criteria: * For all participants: * Patients under protective measures or unable to consent * Pregnant or breastfeeding women * Anticoagulant treatment: vitamin K antagonists (Coumadin, fluindione), direct oral anticoagulants (Eliquis, Pradaxa, Rivaroxaban), heparins at curative doses * Contraindication to local procedure: lymphedema near the joint, chronic wound at the site of the joint, joint prosthesis on the affected joint, corticosteroid infiltration less than 3 months ago at the same site * History of treatment with biotherapy or targeted therapy (JAK inhibitors) within the last 3 months, or within the last 6 months if treated with Rituximab