Synovial biopsies are now routinely performed both in research and in clinical practice. The developments of ultrasound (US) and of US-guided needle biopsies devices have facilitated their use and tolerance by the patients. However, their interest in clinical practice in the context of an undifferentiated arthritis remains debated.

In this study, the investigators will collect all the synovial biopsies performed in clinical practice in 18 Rheumatology Unit throughout France. All the participants received a standardized training on US-guided synovial biopsies. For each biopsy, the indications, clinical presentation, procedure (length, tolerance), biopsy results (Pathology, bacteriology, etc ...) will be collected in a Clinical Research Form. The investigator will determine the number of cases where the synovial biopsy allows drawing a definite diagnosis.

Inclusion Criteria:

All patients undergoing an US-guided synovial biopsy in clinical practice in the context of an arthritis of unknown origin

Exclusion Criteria:

Age under 18 year old, pregnancy, breastfeeding