Changes in the Immunomodulatory Properties of Adipose Stromal Cells Due to Hemarthrosis After Knee Injury

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