US assessment of hip joint synovitis in rheumatic diseases: A comparison with MR imaging

PURPOSE: To assess the significance of ultrasonography (US) in detecting hip joint synovitis in patients with rheumatic diseases. MATERIAL AND METHODS: Forty patients with rheumatic disease and suspected hip joint synovitis underwent MRI and US of the hip joint. In addition to the throughout MRI evaluation, the anterior collum-capsule distance (CCD) was determined by both MRI and US. Thirteen healthy volunteers were examined with MRI to establish the criteria for normal findings in MRI when classifying hip joints to those with synovitis and those without. MRI was used as a gold standard. RESULTS: Synovitis was found using MRI in 31 hips of 22 patients (9 patients had bilateral synovitis). The intraclass correlation was 0.61 between MRI and US in measuring CCD. In classifying hip joint synovitis with US, the sensitivity of the method was 87% and specificity 42%, when the CCD criterion for synovitis was determined to be > or = 7 mm. If the cut-off point was raised to 9 mm, the sensitivity decreased to 61% while specificity increased to 94%. A difference in CCD of > or = 1 mm between the hips as an additional criterion for synovitis increased the number of false-positive findings. CONCLUSION: Measurement of CCD with US proved to be a rather inaccurate method to point out synovitis in rheumatic patients when using MRI as a reference. The main reason for this result was the thickened capsule, which US could not differentiate from a thickened synovium.