Secondary Objectives:~Search for poor prognostic factors in native septic arthritis of the knee.~To look for an association between initial sonographic factors and the use of joint lavage.~Look for associations between sonographic signs and clinico-radio-biologic factors at each visit (D10, 6 weeks, 3 months and 6 months)
Primary objective: Description of ultrasound abnormalities seen in native septic arthritis of the knee during each visit.~Primary endpoint: Describe the ultrasound abnormalities observed on Day 0, Day 10, 6 weeks, 3 months, 6 months, during native septic arthritis of the knee.