Validity of Radiograph-Based Infrapatellar Fat Pad Opacity Grading for Assessing Knee Synovitis: Correlation With Contrast-Enhanced MRI

Abstract

OBJECTIVE. The purpose of this study was to determine the validity of infrapatellar fat pad (IPFP) opacity grading based on lateral knee radiography for assessing knee synovitis using correlation with contrast-enhanced (CE) MRI. MATERIALS AND METHODS. Retrospective reviews of radiographs and CE knee MR images from 79 patients were independently performed by two radiologists. They evaluated IPFP opacity alteration (IPFP_CR) and joint effusion grades on lateral knee conventional radiographs, IPFP signal intensity alteration (IPFP_MR) and joint effusion grades on CE MR images, and synovitis (SYN_MR) grade in nine divided regions, three compartments (parapatellar, periligamentous, perimeniscal), and the whole knee on CE MR images. Correlations between radiographic grades and MRI assessments were evaluated using Spearman correlation tests, and the correlation coefficients (ρ) were compared. Interobserver agreement was evaluated using weighted kappa values. RESULTS. The IPFP_CR grade was very highly correlated with the IPFP_MR grade (ρ = 0.906, p < 0.001) and highly correlated with SYN_MR grades from four regions (suprapatellar, infrapatellar, intercondylar, lateral parapatellar recess), the parapatellar compartment, and the whole knee (ρ = 0.614–0.740, all p < 0.001). The IPFP_CR grade was moderately correlated with the SYN_MR grades of the remaining five regions and two compartments (ρ = 0.457–0.547, all p < 0.001). The differences between correlation coefficients for SYN_MR grades and the IPFP_CR and IPFP_MR grades were not statistically significant (p = 0.290–1.0). Interobserver reliabilities were excellent or good for IPFP_CR, IPFP_MR, and SYN_MR grades (κ = 0.661–1.000). CONCLUSION. IPFP_CR grade assessment enables valid evaluation and reporting of knee synovitis, especially in the parapatellar compartment and the whole knee.