The review, authored by a team from Peking University Third Hospital led by Chong Zhang and Xinxin Xie, argues that ultrasound deserves a far larger role in evaluating the fat pad than it currently enjoys. Magnetic resonance imaging has long been the default tool for assessing inflammation, fibrosis, and scarring in this tissue, and it remains the reference standard for comprehensive soft tissue evaluation. But MRI comes with well-known drawbacks: high cost, long examination times, and, crucially, an inability to capture the fat pad in motion. Ultrasound, by contrast, is portable, inexpensive, and clinically accessible, and it offers something MRI fundamentally cannot: real-time, dynamic imaging of living tissue as the knee bends, extends, and bears weight. In a prospective diagnostic accuracy study of anterior knee pain involving 143 patients, ultrasonography achieved a sensitivity of 85.3 percent and a specificity of 100 percent for detecting periarticular abnormalities compared with MRI, a performance level that substantiates its role as a reliable first-line screening tool.

What does a healthy fat pad look like on an ultrasound screen? The review describes a distinctive two-layered echo structure. The superficial layer appears heterogeneously hypoechoic, resembling the subcutaneous fat immediately overlying the patellar tendon and containing hyperechoic connective tissue septa, while the deeper layer is generally more homogeneously echogenic. This layered architecture is not merely cosmetic; it reflects genuine histological differences, with larger fat lobules in the superficial layer and smaller lobules with thicker septa in the deeper layer. Finite element modeling has shown that adipose lobules confined within these micro-chambers undergo less deformation than those in larger chambers, which helps explain why the two layers behave so differently under load. Strain elastography studies in healthy, asymptomatic subjects have confirmed that the superficial layer is more elastic than the deeper one, linking the sonographic appearance directly to measurable mechanical properties.

The dynamic behavior of the fat pad is where ultrasound truly separates itself from static imaging. During knee extension, the deep portion of the fat pad moves significantly faster than the superficial portion. When the knee flexes, the superficial layer undergoes a marked thickness reduction of about 20.6 percent, while the deep layer shrinks by only 1.3 percent, and the deep lobules barely change at all. During walking, fat pad thickness is significantly greater than when measured lying down or standing, and the thickness waveform rises during the stance phase and falls during the swing phase, with a mean morphological change of 1.35 plus or minus 0.42 millimeters that correlates significantly with the knee flexion moment. Quadriceps contraction adds another layer of complexity: it straightens the patellar tendon, producing an anterior bulge of the fat pad on both sides of the tendon and a distal bulge into the deep infrapatellar bursa. These instantaneous, movement-driven alterations are invisible on MRI performed with the knee at rest.

Blood flow within the fat pad tells its own story. Color Doppler studies have detected blood flow in 55.5 percent of young, healthy, asymptomatic fat pads, and the tissue’s hemodynamics respond briskly to activity. Tissue hemoglobin, oxygenated hemoglobin, deoxygenated hemoglobin, and total hemoglobin all decline during isometric quadriceps contraction and rebound afterward, a pattern consistent with transient compression of local vessels followed by reactive hyperemia. After fifteen minutes of treadmill exercise, the largest vessel within the fat pad of asymptomatic knees shows marked expansion, a tendency toward dilation that appears to reflect healthy circulatory function. Shear wave elastography, which uses acoustic radiation force impulses to vibrate the tissue and measure the speed of resulting shear waves, adds a quantitative dimension: fat pad stiffness increases during quadriceps contraction, potentially compressing vessels and inducing a temporary ischemic state. Intriguingly, in community-dwelling older adults, higher levels of light physical activity are associated with reduced fat pad stiffness, hinting that everyday movement may protect the mechanical health of this tissue.

When pathology strikes, the ultrasound signature changes in characteristic ways. In fat pad impingement, caused by acute trauma or repetitive microtrauma, examination may reveal thickening and edema, along with diminished compressibility of the fat lobules that signals underlying chronic fibrosis. Perhaps most telling is the vascular response: while healthy fat pads dilate their vessels after exercise, impinged fat pads show vessels that constrict or remain unchanged, an aberrant reaction the authors attribute to microscarring and hypertrophy, and one that indicates impaired circulatory function. In patellar tendinopathy, one of the most common overuse injuries in jumping and running athletes, the close anatomical relationship between the proximal patellar tendon and the anterosuperior portion of the fat pad means the tissue is frequently drawn into the disease process. High-resolution ultrasound can now visualize the tendon-fat pad interface and the microvascular plexus of the deep paratenon as potential pain generators, although the authors caution that the prognostic value of these imaging features remains unestablished, with one recent cohort study finding no association between baseline imaging abnormalities and clinical outcomes after exercise therapy.

The postoperative knee provides some of the most compelling evidence for ultrasound’s clinical utility. Anterior cruciate ligament reconstruction, one of the most common knee surgeries in sports medicine, frequently involves partial resection or irritation of the fat pad, and the resulting inflammatory changes are readily tracked sonographically. Three months after reconstruction, patients show significant reduction in superficial fat pad thickness at ninety degrees of flexion and a notable decline in the thickness change ratio compared with the uninjured contralateral knee. A dynamic ultrasound study found that reconstructed knees exhibit insufficient morphological changes in the fat pad during walking, and that the normal correlation between fat pad deformation and knee flexion moment seen in healthy controls is absent, indicating impaired responsiveness to mechanical loading. These deficits matter clinically: anterior knee pain affects between 18.5 and 50 percent of knees after bone-patellar tendon-bone graft reconstruction and 9 to 30 percent after hamstring graft procedures, and a smaller thickness change ratio has been identified as a contributing factor to pain during squatting. In patients six months after hamstring graft reconstruction, increased fat pad blood flow on Doppler imaging emerged as an independent factor associated with anterior knee pain on multivariate analysis.

The implications extend beyond ligament surgery. More than one-third of individuals develop radiographic osteoarthritis within ten years of anterior cruciate ligament reconstruction, and ultrasound can detect persistent inflammatory changes in the fat pad one to five years postoperatively, with higher echo intensity symmetry values found in patients with MRI-confirmed fat pad synovitis. After total knee arthroplasty, gray-scale and power Doppler ultrasound of patients with arthrofibrosis reveals significantly increased synovial thickening and enhanced neovascularity within the fat pad compared with asymptomatic controls, suggesting that persistent inflammation or fibrotic remodeling of the pad may contribute to global joint stiffness. In knee osteoarthritis itself, fat pad stiffness measured by shear wave elastography is significantly associated with anterior knee pain even after adjusting for age, sex, disease grade, and MRI findings, while restricted morphological change during walking appears to be a dynamic feature specific to symptomatic disease, correlating with pain scores. In osteoarthritic knees, the normal hemoglobin responses to quadriceps contraction are blunted, pointing to impaired local microcirculation.

Ultrasound also shines in the diagnosis of rare space-occupying lesions and, above all, in catching what static imaging misses. Ganglion cysts appear as anechoic structures with posterior acoustic enhancement, lipomas as fatty masses that may herniate through focal defects in the lateral patellar retinaculum, and synovial hemangiomas as lesions with large vascular areas on Doppler interrogation. Fat pad herniation, most common in children but also seen in adults after surgery, presents as a painless mass apparent during knee flexion, and it is frequently demonstrable only on dynamic scanning, meaning it can be missed entirely on MRI performed with the knee extended. The review’s authors are candid about the limitations: the evidence base is heterogeneous, the biological meaning of echo intensity and stiffness measurements remains incompletely understood, and reproducibility can be confounded by probe orientation and operator dependency. Standardized acquisition protocols and larger longitudinal studies are needed before these findings can be integrated into routine clinical and return-to-sport decision-making. Still, the message is clear: a cheap, portable scan of a once-ignored fat pad may soon help clinicians diagnose knee pain faster, monitor recovery after surgery, and spot the earliest signs of joint degeneration, all in real time.

Subject of Research: Ultrasound imaging of the infrapatellar fat pad in healthy and diseased knees

Article Title: Ultrasound Characteristics of the Infrapatellar Fat Pad Under Physiological and Pathological Conditions

Article References: Zhang, C., Xie, X., Wang, H., Yang, M., Huang, H., Cui, L., Wang, J., & Cheng, J. (2026). Ultrasound Characteristics of the Infrapatellar Fat Pad Under Physiological and Pathological Conditions. Sports Medicine – Open, 12(1), Article 142. https://doi.org/10.1186/s40798-026-01113-x

Image Credits: AI Generated

DOI: 10.1186/s40798-026-01113-x

Keywords: infrapatellar fat pad, Hoffa's fat pad, ultrasound, elastography, Doppler imaging, knee osteoarthritis, anterior knee pain, ACL reconstruction, patellar tendinopathy, sports medicine, musculoskeletal imaging, arthrofibrosis