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.

Secondary Objectives:

1. Search for poor prognostic factors in native septic arthritis of the knee.
2. To look for an association between initial sonographic factors and the use of joint lavage.
3. Look for associations between sonographic signs and clinico-radio-biologic factors at each visit (D10, 6 weeks, 3 months and 6 months)

Inclusion Criteria:

• Male or female of legal age
• Bacteriologically proven native septic arthritis
• Meets the definition of septic arthritis: Bacteriological documentation of joint puncture fluid-Native articulation
• Free subject, without guardianship or trusteeship, or subordination
• Non-opposition of the patient after clear and fair information about the study

Exclusion Criteria:

• Presence of material in the affected joint
• Arthritis with Mycobacterium Tuberculosis.
• Patients benefiting from a reinforced protection, namely: minors, pregnant and/or breast-feeding women, persons deprived of liberty by a judicial or administrative decision, adults under legal protection

Patients with native septic arthritis of the knee