Sonographic characterization of Hoffa’s fat pad. A pilot study

This study addresses a topic that was previously unaddressed in the literature: the normal sonographic appearance of the Hoffa’s fat pad (HFP) in young, asymptomatic subjects. The aim of the study is to describe the sonoanatomical features of HFP, including its echostructure, echogenicity, elasticity, and vascularization, in subjects without knee pathology. Knees of healthy subjects were examined with grayscale ultrasound (US) to determine the sonographic characteristics of the HFP. Echogenicity was assessed by pixel intensity quantification. Vascularity was evaluated by color Doppler (CD) US. Elasticity of the adipose tissue was examined by sonoelastography. The absence of HFP pathology was confirmed clinically in all participants and by magnetic resonance imaging in randomly selected participants. Seventy-two knees from 36 subjects were assessed. The HFP presented a characteristic well-defined two-layered echostructure: the superficial adipose tissue was hypoechoic with respect to the nearby patellar tendon and contained septae, whereas the deeper layer was homogeneously hypoechoic and lacked connective tissue septae. Echogenicity differed between the superficial and deeper layers (the deeper layer was brighter), suggesting that the degree of acoustic impedance differed between layers. CD detected blood flow in 55.5% of HFPs. The superficial HFP layer presented greater elasticity than the deeper HFP layer (88.8 vs. 75.3 Kpa; p < .001). Our study provides a comprehensive description of the normal sonographic features of the HFP, which is useful to increase the potential of US for the discrimination of pathological findings involving this anatomical area.