The goal of this observational study is to gain insight in the natural course of pro-inflammatory factors and hemarthrosis in patients older than 18 with a recent knee trauma. The main question[s] it aims to answer are:

• What is the natural course of pro-inflammatory factors and hemarthrosis in knee trauma?
• What are the effects of inflammation and hemarthrosis on the anti-inflammatory potential of ASC's in order to better determine eligible patients and circumstances for ASC's therapy?

Participants will:

• undergo blood withdrawal
• undergo knee arthrocentesis for synovial fluid sample collection
• physical examination
• fill out a questionnaire on knee complaints

Recent studies have found an amelioration of post-traumatic osteoarthritis by the use of adipose-derived stromal cells (ASC's) in an experimental animal model. This amelioration was only seen under inflammatory conditions. The presence of hemarthrosis has not been taken into account in the before mentioned animal model, and might have cytotoxic effects on ASC's and aggravate the inflammatory response. The objective of this study is to gain insight in the natural course of pro-inflammatory factors and hemarthrosis and its effects on the anti-inflammatory potential of ASC's in order to better determine eligible patients and circumstances for ASC's therapy.

Therefore the investigators designed a pilot study with exploratory analyses in and with synovial fluid in a longitudinal cohort.

Our study population will consist of 20 patients, older than 18 years, who sustained a recent knee trauma and are presented to the emergency department.The investigators will gather synovial fluid and blood samples for laboratory analysis perform and physical examination and provide questionnaires for clinical follow-up.

The main study endpoint are the anti-inflammatory capacities of ASC's in synovial fluid that contains blood as a result of hemarthrosis after knee injury.

Inclusion Criteria:

• Recently suffered an injury of the index knee (rotational or hyperextension)
• A swollen joint
• Inability to mobilise

Exclusion Criteria:

• Patients with an active inflammatory or infectious comorbid disease (including rheumatic diseases)
• Patients using systemic immunosuppressant medication
• Patients with a contra-indication for undergoing a MRI-scan (non-compatible implants or claustrophobia for example)
• Patients with knee prosthesis
• Patients with coagulation disorders
• Patients with a history of a cruciate ligament or meniscal injury of the index knee
• Patients with a tibiofemoral fracture of the index knee due to current distortion
• In case the inclusion of the required amount of patient is reached in one the patient groups, additional patients belonging in the same patient group will not be included in the study

Adult patients who are presented at the emergency department of the CWZ hospital because of a recent distortion of the knee.