The research study aims to evaluate the synovial fluid to highlight the presence of possible micropolymers (micro/nanoplastics) in order to better understand the pathophysiology of joint degenerative processes. For patients with liquid under tension at the joint level, the symptomatology often requires evacuation by puncture under ultrasound guidance. The intervention will be performed only for patients for whom this therapeutic indication exists anyway by their physician.

The investigators want to use this liquid for Microscopic and Spectrophotometric evaluation.

Inclusion Criteria:

• arthritis of the knee with liquid aspiration recommendation
• age >18 years

Exclusion Criteria:

--refusal of the patient to participate

-age<18 years

Inpatients on a Rehab Department with arthritis of the knee that require evacuation of the knee joint