Ultrasonographic evaluation of fibrous structure and mechanical characteristics of the infrapatellar fat pad in female patients with knee osteoarthritis: a cross-sectional study

Background Fibrosis of the infrapatellar fat pad (IFP) is common in knee osteoarthritis (KOA); however, a comprehensive evaluation is lacking. This study aimed to use ultrasonography to investigate the fibrous structure and mechanical characteristics of the IFP in patients with KOA. Methods This cross-sectional study included patients with KOA, age-matched controls, and healthy young adults. Fifty knees with KOA were analyzed for Kellgren–Lawrence grade differences, whereas 50 knees with KOA, 14 age-matched controls, and 15 knees from healthy young adults were analyzed for between-group comparisons. The primary outcomes were IFP shear wave stiffness, fiber orientation intensity, proportion of high-intensity area, and movement, assessed using shear wave elastography, ImageJ analysis, binarized US analysis, and particle image velocimetry, respectively. The association between magnetic resonance imaging (MRI) signal intensity and outcomes was evaluated in the KOA group. Results Compared with the controls, the KOA group demonstrated greater IFP stiffness (95% confidence interval [CI] [0.152, 0.469]) and proportion of high-intensity area (95% CI [2.737, 11.263]) with reduced movement of the superficial part (95% CI [-0.240, -0.104]) and the deep part (95% CI [-0.290, -0.131]). MRI signal intensity of the IFP was positively correlated with the proportion of high-intensity area among the 14 KOA knees with MRI data ( r = 0.720, p = 0.004). The proportion of high-intensity area in grade IV was significantly higher than that in grades I and II. Conclusion IFP in KOA exhibits distinct ultrasonographic characteristics, including increased stiffness and proportion of high-intensity area, along with restricted movement. Notably, the proportion of high-intensity area was significantly higher in KL grade IV than grades I and II, and correlated with MRI signal intensity, suggesting its potential utility as a quantitative ultrasound-derived imaging parameter for assessing IFP tissue alteration in KOA.