Quantitative measurement in knee joint effusion: Correlation between plain radiographs and MRI

Abstract

Background: Knee joint effusion is a common manifestation of synovial disease resulting from traumatic injury, infection, inflammation and degenerative disease. Plain radiographs and magnetic resonance imaging (MRI) play an important role in diagnosing knee effusion. However, there is scant data demonstrating the correlation of the two studies.

Objective: The purposes of our study were to investigate the correlation of knee effusion between thickness of radiodense area within the suprapatellar pouch on lateral radiographs and volume measurement on MRI studies, and to assess the distribution of effusion in various compartments.

Methods: Quantitative measurement of effusion volume on sagittal fat-saturated T2 weighted images was compared to thickness of radiodense area within suprapatellar recess on lateral radiographs performed within 2-week interval. The correlation between the thickness and volume was assessed by using Pearson test.

Results: Eighty-four studies, performed both MRI scan and lateral knee radiographs during 2-week intervening time, were retrospectively identified. The median thickness of joint effusions on lateral knee radiographs was 7.5 mm (interquartile range (IQR), 2.6 - 16.3 mm). The median volume of suprapatellar fluid on sagittal fatsaturated T2 weighted images was 19.5 mL (IQR, 2.6 - 85.7 mL). Most effusions were found in the central portion (98.8%) and less frequently seen in Baker’s cyst (19.0%).

Conclusion: The relationship of knee effusion between thickness of soft tissue density within the suprapatellar pouch on lateral radiographs and the volume on MRI studies was demonstrated as a linear regression model. We have assumed that the volume of MRI is about 1.9 folds of the thickness on lateral radiographs plus 7.0. The accuracy of the equation was 20.9%. To apply for clinical use, we recommend a concise formula that the volume of fluid equals to 2 folds of the thickness plus 7.