Transient Synovitis-Plain Film & MRI

12 year old male presents with acute hip pain of 10 days duration. X-ray suggests mild widening of the joint space with positive "Waldenstorm"  sign and negative obturator sign. MRI shows free fluid of grade II variety with synovial capsular distention  with no significant marrow edema of the bony margins or soft tissues abnormality - Transient Synovitis is likely.

Teaching Points by Dr MGK Murthy :

1. Most common cause of hip pain children (usually 3 to 10 years). Etiology not understood. However trauma, viral, post vaccineal  and drug mediated responses could play a role.
2. Usually self limiting and resolves in about 12 to 14 days. Exact Incidence not known.
3. Boys more effected (2 to 1). Recent history of upper respiratory infection / Others present in more than 50 %.
4. Accentuated pericapsular shadow / Increase in tear drop distance  (medial acetabulum to the femoral head), Waldenstorm sign (lateral displacement of the femoral epiphysis with surface flattening), obturator  sign ( soft tissue shadow over the interpelvic aspect of acetabulum),  are plain X-ray findings suggested.
5. MRI suggests increased joint fluid with no marrow edema.  Contrast study suggests synovial enhancement and no significant soft tissue enhancement unlike the differential of septic arthritis. 
6. Bone scintigraphy is non-contributory with increase in uptake and decrease in uptake groups.
7. Fluid aspiration with negative gramstain will go in favour of Transient Synovitis.
8. A Review X-ray after 6 months would help exclude Recurrence and  possible follow up Perthes disease.