To classify the type of transient synovitis and determine a treatment plan using MRI.

From March 1985 to October 1998, 37 hips in 33 children with clinical symptoms of transient synovitis were evaluated with MRI. The average age of the patients was 7.5 years (range, 3.5-15 years) . The mean follow-up period was 18 months (range, 12-36 months) . The 37 hips were classified as grade l, ll, lll, or lV according to the amount of accumulated joint fluid on MRI.

Of 37 hips with transient synovitis, 36 hips had no involvement of epiphysis and metaphysis and one hip had a cyst and bone marrow edema in the metaphysis on MRI scans. The amount of joint fluid was classified as seen on MRI scans: grade 1 in 2 hips, grade 2 in 14 hips, grade 3 in 5 hips, and grade 4 in 16 hips.

MRI is a very useful diagnostic tool to classify and determine the treatment plan for transient synovitis. In patients with grade 3 and 4 joint fluid, the risk of vascular compromise of the femoral epiphysis could be higher. These patients should be hospitalized and immobilized in flexed position of the hip. Patients with grade 1 and 2 joint fluid can be treated at home with immobilization.