Over 90% of hip joint effusions can be attributed to transient synovitis. 2
It has a male gender predilection and more frequently affects the right hip joint.
Etiology of transient synovitis remains unknown. Various theories include response to infection (URTIs), trauma, allergen / antigen etc. Clinical, laboratory and radiological findings help in differentiating Transient Synovitis from Septic Arthritis. The latter is usually associated with fever, elevated ESR & CRP.
X-Ray may reveal mild joint space widening due to effusion. Ultrasound is often the initial modality as it can reliably assess presence of joint effusion in the anterior recess.
MRI findings of Transient Synovitis include:
1. Joint effusion.
2. Synovial thickening & enhancement.
3. Adjacent soft tissue edema, with characteristic absent bone marrow edema.
4. Contralateral effusion (lesser volume usually).
Below MR images are of a 10yr old girl child with limping showing features of transient synovitis in right hip joint. No bone marrow edema or contra-lateral effusion is seen in this child. Grade 3 joint effusion is present in right hip joint.
Very rarely Transient Synovitis can cause osteonecrosis. But most often with analgesics and other symptomatic treatment, it resolves gradually, without any known long term sequelae. Eventhough it is most commonly seen in children, tranisent synovitis can also develop in adults.
Hip joint effusion is graded according to Mitchell Classification. 0 : None, 1 Minimal, 2 - Enough to surround the femoral neck and 3 distension of joint recesses.
References:
1. MR imaging of transient synovitis: Differentiation from septic arthritis. Pediatric Radiology, 2006.
2. Radiopedia.
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