The aim of this trial is to study the interest of ultrasonography among patients with septic arthritis.

Currently, ultrasonography is useful for detecting small fluid effusions, for examining otherwise inaccessible joints, such as the hip and for helping the joint aspiration.

Accurate assessment of disease activity and joint damage in septic arthritis is important for monitoring treatment efficiency and for prediction of the outcome of the disease.

Nowadays, magnetic resonance imaging (MRI) provides better resolution than radiography or tomography for the detection of joint effusion and for differentiation between bone and soft tissue infectious. The sensibility is reported to be nearly 100% with a specificity of more than 75%.

However, MRI is expensive and not rapidly accessible.

Therefore, ultrasonography, a non invasive, painless, inexpensive, and non radiating exam can be used to assess the presence and extent of inflammation, destruction, and tissue response.

The objective is to describe, using ultrasonography, the abnormalities joint structure influenced the septic arthritis evolution and prognosis.

The investigators hope, at the end of this study, to evaluate ultrasonography interest in septic arthritis and to establish ultrasonography prognosis factors to predict treatment efficiency and functional outcome.

Inclusion Criteria:

• Major patients hospitalized in Rheumatology, Infectiology, Orthopedy Department (hospital of Nantes, of La Roche sur Yon and of Saint-Nazaire) with a septic arthritis of shoulder, or elbow, or wrist , or hip, or knee, or ankle on native joint, with positive microbiologic culture joint fluid or positive hemoculture; the diagnosis was based on positive histology or imagery with an inflammatory joint fluid if the microbiologic cultures (hemoculture or joint effusion) were negative.

Exclusion Criteria:

• Presence of material on the targeted joint
• Age < 18 years old
• Patients under guardianship
• Pregnancy