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

Abstract

The infrapatellar fat pad (IFP) is an intracapsular, extra-synovial adipose tissue located beneath the patellar tendon. It serves as a protective cushion for the knee, mitigating the mechanical impact of various movements. Furthermore, the IFP possesses a robust vascular network and strong secretory capacity, enabling it to contribute to the resolution of various inflammatory processes associated with knee pathology. Magnetic resonance imaging (MRI) is widely used to assess the IFP, but it is limited by high cost, long examination times, and an inability to provide real-time dynamic evaluation. Ultrasound offers a cost-effective, portable, and clinically accessible alternative for IFP imaging. Beyond visualizing the dynamic morphology of the IFP, advances in Doppler ultrasound and elastography now enable the assessment of blood supply and mechanical properties. This review summarizes the ultrasound features of the IFP under physiological and pathological conditions, explores its potential applications in the diagnosis and evaluation of IFP-related disorders, and highlights future directions, with particular emphasis on the role of dynamic ultrasound in evaluating anterior knee disorders in sports medicine.

Introduction

The infrapatellar fat pad (IFP) is an intracapsular, extra-synovial adipose tissue located beneath the patellar tendon. It cushions the knee against mechanical impact during movement and has been shown to possess a rich vascular network, abundant nerve distribution, and strong secretory capacity [1, 2]. Under inflammatory conditions, such as knee osteoarthritis (OA) and the postoperative knee, the IFP undergoes a series of changes, including immune cell infiltration, fibrosis, and vascularization [3, 4]. These pathological changes are closely linked to the loss of the IFP’s shock-absorbing function and an abnormal secretory phenotype, both of which contribute to the development of clinical symptoms.

Magnetic resonance imaging (MRI) is commonly used in clinical and research settings to evaluate inflammation, fibrosis, and scarring in the IFP [5, 6]. In addition, MRI also serves as a valuable diagnostic tool for IFP-related diseases, including synovial pathologies, such as synovitis, pigmented villonodular synovitis, and symptomatic infrapatellar plica, as well as some relatively rare mass-like abnormalities of the IFP [5]. However, MRI is limited by its inability to provide dynamic evaluation, high cost, and lengthy examination times.

Ultrasound is a portable, cost-effective, and clinically accessible musculoskeletal imaging modality that offers a promising alternative to MRI for IFP assessment [5, 6]. Although ultrasound is widely used for soft tissue assessment, only a limited number of studies have described its potential role in assessing pathological conditions of the IFP. Ultrasound allows direct visualization of the dynamic morphological characteristics of the IFP [7]. Moreover, recent studies have further validated the clinical utility of B-mode ultrasound and elastography in detecting IFP stiffness and structural alterations, reinforcing its translational value in assessing early degenerative changes [7, 8]. Ultrasound has also been employed to evaluate the knees of asymptomatic high-performance athletes, revealing alterations and endogenous predisposing factors for overuse injuries [9]. Despite this growing body of research, the literature remains fragmented: individual studies have reported isolated sonographic features in specific conditions, but a clinically oriented synthesis that explicitly links ultrasound findings to diagnostic and management decisions is lacking. Moreover, the level of evidence supporting many applications has not been critically appraised, making it difficult for clinicians to distinguish established uses from experimental ones. This review, therefore, aims to provide a focused synthesis of the current evidence on ultrasound assessment of the IFP, with an emphasis on its clinical utility in sports medicine, particularly for conditions in which the IFP may be involved. Specifically, we examine how B-mode, Doppler, and elastography findings can characterize IFP abnormalities and contribute to the diagnosis and assessment of IFP-related disorders. Rather than offering an exhaustive technical catalog, this review critically evaluates what ultrasound can and cannot currently reveal about IFP pathology, and identifies key gaps that warrant further investigation.

To synthesize the available evidence, we performed a literature search of the PubMed and Web of Science databases for articles published from inception to April 2026. The search strategy employed the following Boolean logic: ("infrapatellar fat pad" OR "Hoffa's fat pad") AND ("ultrasound" OR "sonography" OR "ultrasonography"). Given the narrative design of this review and the rapidly evolving nature of musculoskeletal ultrasound research, both original investigations and instructive case reports were considered eligible for inclusion. The reference lists of retrieved articles were also manually screened to identify any additional relevant sources.

Ultrasound Characteristics of the Healthy Infrapatellar Fat Pad

General Ultrasound Characteristics of the Healthy Infrapatellar Fat Pad

In healthy subjects, the IFP exhibits a distinctive two-layered echo structure (Fig. 1). The echogenicity differs between the superficial and deeper layers: the deeper layer appears brighter, indicating differences in acoustic impedance between the layers. The superficial layer is heterogeneously hypoechoic, resembling the subcutaneous fat immediately overlying the patellar tendon, while the deeper layer is homogeneously echogenic and lacks prominent hyperechoic connective tissue septa [8]. This two-layered sonoanatomical appearance has been consistently confirmed in pilot studies of asymptomatic young subjects, with the superficial layer containing hyperechoic septa and the deeper layer appearing more homogeneous [8]. However, another study reported heterogeneous echogenicity of the deeper layer, a view corroborated by histological findings that the IFP possesses larger lobules in the superficial layer and smaller lobules with thicker septa in the deeper layer [10].

Fig. 1.

Sagittal ultrasound image of the right knee in flexion from a 24-year-old healthy volunteer. IFP-S superficial layer of the infrapatellar fat pad, IFP-D deeper layer of the infrapatellar fat pad

In addition to echogenicity, the morphology of the IFP has been the subject of considerable research. The IFP is located in the anterior compartment of the knee, and its shape and position are influenced by the motion of the knee joint and muscle contraction. During knee extension, the movement velocity of the deep portion of the IFP was significantly faster than that of the superficial portion [11]. With regard to thickness changes during flexion, the superficial layer undergoes a marked reduction (20.6%), accompanied by significant changes in the areas and perimeters of the superficial lobules, whereas the deep layer shows only a 1.3% reduction, and the deep lobules do not change significantly [10]. The thickness of the IFP during walking is significantly greater than that measured during supine and standing conditions. Moreover, the IFP thickness waveform during gait demonstrates a gradual increase during the stance phase and a decrease during the swing phase of the gait cycle. The morphological change of the IFP during walking was found to be 1.35 ± 0.42 mm and showed a significant correlation with the knee flexion moment [12]. Recent research has further elucidated the relationship between muscle activity and IFP dynamics, demonstrating that increased co-contraction of the lateral quadriceps and hamstring muscles during the stance phase of gait is associated with restricted morphological changes (ΔIFP) in patients with knee OA [13]. In addition to knee extension and walking, quadriceps contraction also alters the shape of the IFP. It causes the patellar tendon to straighten, resulting in an anterior bulge of the IFP on both sides of the patellar tendon and a distal bulge into the deep infrapatellar bursa [14]. A study comparing IFP characteristics in different athletic populations demonstrated a significant reduction in IFP thickness in outdoor soccer players compared with futsal players. However, this difference was only statistically significant in the left leg [15]. The flexibility of the IFP is closely related to the function and mobility of the knee joint. The thickness change ratio of the IFP between flexion and extension is often used to assess its flexibility. In one physical therapy study, the thickness change ratio of the IFP between 90° and 0° flexion was calculated before and after intervention, and the results indicated that neither manual therapy nor hot pack intervention resulted in a significant change in IFP flexibility [16]. In summary, the majority of notable alterations in IFP morphology resulting from movement occur instantaneously. However, the evidence remains inconclusive as to whether the mechanical load generated by exercise is sufficient to induce adaptive structural changes in the IFP.

Blood and Oxygen Supply of the Healthy Infrapatellar Fat Pad Assessed by Ultrasound

The detection rate of IFP blood flow has been evaluated in several studies using color Doppler imaging. In the study of Vera-Pérez et al., blood flow was detected in 55.5% of young, healthy, asymptomatic IFPs [8]. Nakanishi et al. proposed that the isometric contraction of the quadriceps (ICQ) might affect the hardness and blood flow in the IFP [17]. Tissue hemoglobin (Hb), oxygenated Hb, deoxygenated Hb, and total Hb exhibited a notable decline during ICQ and a subsequent increase following ICQ [18]. In addition, after 15 min of treadmill exercise, the diameter of the largest vessel showed a marked expansion, accompanied by a tendency toward dilation in asymptomatic knees [19]. Nevertheless, further investigation is required to elucidate the long-term consequences of exercise-induced alterations in IFP blood flow and oxygen supply for the knee joint.

Ultrasound Elastography of the Healthy Infrapatellar Fat Pad

The elasticity of the IFP has been evaluated using two elastography techniques: strain elastography (STE) and shear wave elastography (SWE). In healthy asymptomatic subjects, STE showed greater elasticity in the superficial layer of the IFP than in the deeper layer [8]. Consistent with the aforementioned histological differences between the two layers, finite element modeling has shown that adipose lobules in the micro-chambers undergo less deformation than those in the macro-chambers, partially explaining the difference in mechanical properties between the two layers [10]. SWE is an elastography technique that employs acoustic radiation force impulses to vibrate the target tissue and measures the generated shear wave propagation speed [20]. In contrast to STE, SWE provides quantitative assessment of the IFP and is less operator-dependent. Previous research has indicated that IFP stiffness (as reflected by SWE) increases during quadriceps contraction, which may simultaneously compress local blood vessels and induce a transient ischemic state [18]. In community-dwelling older adults, a higher level of light physical activity has been associated with reduced IFP stiffness, suggesting that physical activity may play a protective role in maintaining IFP mechanical health [21].

Ultrasound Features of Inflammatory Pathological Changes in the Infrapatellar Fat Pad

As the IFP actively responds to inflammation, corresponding changes can be observed on ultrasound, including alterations in IFP thickness, fat lobule morphologic features, and echo intensity. Some of these changes are closely associated with clinical symptoms, particularly anterior knee pain (AKP), which has been demonstrated to diminish the capacity to engage in activities of daily living and sports participation [9, 22]. Among the IFP inflammatory conditions, the most commonly reported are IFP impingement, IFP involvement in patellar tendinopathy, postoperative IFP changes, and OA-related IFP changes. The key ultrasound features of these conditions are summarized in Table 1.

Table 1.

Ultrasound characteristics of IFP-related conditions

IFP-related conditions	Ultrasound characteristics	References
Impingement	• IFP thickening and edema • Diminished overall compressibility of the IFP lobule • Smaller change in the largest vessel diameter following exercise • Hypoechoic regions within the IFP adjacent to the patellar tendon	[19, 23, 24]
Patellar tendinopathy	• Hypoechoic changes at the patellar tendon–IFP interface • Altered microcirculation on high‑sensitivity color/power Doppler • Thickening or abnormal echotexture of the anterosuperior IFP	[25]
Postoperative knee	• Smaller thickness of the superficial part of the IFP post-ACLR • Smaller IFP thickness change ratio between different knee flexion angles post-ACLR • Increased blood flow post-ACLR • Higher IFP echo-intensity post-ACLR • Restricted morphological change during walking post-ACLR • Increased neovascularity within IFP in arthrofibrosis after TKA	[26–33]
Knee OA	• Smaller changes in IFP hardness and Hb concentration during ICQ • Increased SWE stiffness • Synovial thickening and vascularity • Higher echo intensity of the superficial medial IFP, associated with knee symptoms • Restricted morphological change during walking, correlated with KOOS pain	[17, 34–40]

IFP infrapatellar fat pad, ACLR anterior cruciate ligament reconstruction, TKA total knee arthroplasty, ICQ isometric contraction of the quadriceps, OA osteoarthritis, Hb hemoglobin, SWE shear wave elastography

Ultrasound Features of IFP Impingement

Acute trauma or repetitive microtrauma can lead to patellar tendinopathy and IFP impingement, resulting in inflammation and fibrotic changes within the IFP and associated symptoms [41]. In patients with IFP impingement, ultrasound examination may reveal thickening and edema of the IFP [23]. An underlying degree of chronic fibrosis has been observed in symptomatic knees with IFP impingement, which is manifested by diminished compressibility of the IFP lobule. Following exercise, the compressibility of the symptomatic knees increases toward a more normal percentage, which is expected given that exercise typically augments blood flow and edema [19]. In addition, dilated vessels have been observed in the IFP of patients with IFP impingement [42]. However, after exercise, the vessels appeared constricted or remained unchanged in these abnormal IFPs, in contrast to the vasodilation observed in healthy controls following exercise [19]. This aberrant response may be attributed to microscarring and hypertrophy of the IFP and indicates impaired circulatory function.

IFP Involvement in Patellar Tendinopathy

Patellar tendinopathy is 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 IFP means that the fat pad is frequently involved in the disease process. A recent high-resolution ultrasound study proposed a standardized protocol for assessing the patellar tendon–IFP interface, demonstrating that modern ultrasound equipment can visualize the anterosuperior portion of the IFP and the loose connective tissue of the deep paratenon with its microvascular plexus as potential pain generators in patients with proximal patellar tendinopathy [25]. High-sensitivity color and power Doppler can further evaluate the microcirculation of the soft tissues beneath the patellar tendon at this interface. These findings suggest that IFP involvement may contribute to the pain and dysfunction experienced by athletes with patellar tendinopathy. However, the current evidence is limited to a small number of methodological and proof-of-concept studies. The prognostic value of IFP imaging features in patellar tendinopathy has not been established; one recent cohort study found no significant association between baseline imaging abnormalities (including IFP edema on MRI) and 24-week clinical outcomes after exercise therapy [43]. Further research is needed to define the specific sonographic characteristics of IFP involvement in patellar tendinopathy and their clinical relevance.

Ultrasound Features of the IFP After Knee Surgery

In the context of knee surgery, the IFP is frequently subjected to partial resection because it may obstruct the surgical field. Furthermore, knee surgery itself can stimulate the IFP, resulting in an inflammatory response. Anterior cruciate ligament reconstruction (ACLR) is one of the most common knee surgeries performed in clinical practice. The ultrasound manifestations of inflammatory changes in the IFP following ACLR have been documented in numerous studies, and the altered ultrasound characteristics at different postoperative timepoints have been reported (Table 1).

The flexibility of the IFP is directly related to the flexibility of the knee joint. The thickness of the superficial layer of the IFP is typically measured at 0°, 10°, 30°, and 90° of knee flexion using ultrasound, and the change ratio, defined as (thickness at 0° flexion–thickness at 90° flexion)/thickness at 0° × 100%, is calculated to reflect the flexibility of the IFP. Three months following ACLR, a significant reduction in the thickness of the superficial IFP at 90° knee flexion and a notable decline in the thickness change ratio of the IFP were observed in the reconstructed knees, compared with the contralateral knees [26]. Similar postoperative alterations have been documented even after less extensive arthroscopic procedures. In a cross-sectional study of patients who underwent arthroscopy via a central patellar portal for isolated meniscectomy, ultrasonography at a mean follow-up of 28 months revealed significant thickening of the patellar tendon and contour irregularities with focal hypoechoic areas within the IFP, despite the absence of clinical symptoms [44]. More recently, a dynamic ultrasound study demonstrated that patients post-ACLR exhibited insufficient morphological changes in the IFP during walking, and unlike in healthy controls, the normal correlation between ΔIFP and knee flexion moment was absent, indicating impaired dynamic responsiveness to mechanical loading during gait [32]. In patients who had undergone ACLR 3 months previously, the change ratio of IFP thickness was significantly greater in the group able to sit in the seiza style compared with the group unable to do so [27]. Moreover, the flexibility of the IFP has also been linked to AKP. Postoperative AKP has been observed in up to 18.5–50.0% of knees following a bone–patella tendon–bone autograft ACLR and 9.0–30.0% following a hamstring tendon graft ACLR [45, 46]. In patients 3-month post-ACLR, a smaller IFP thickness change ratio has been identified as a contributing factor to AKP during squatting [28]. In patients who had undergone ACLR with a hamstring tendon autograft 6 months previously, increased blood flow was also associated with AKP, in addition to the previously mentioned correlation between the thickness change ratio and AKP [29, 30]. Univariate analysis revealed that age, quadriceps strength, and increased blood flow in the IFP were significantly associated with the presence of AKP. Multivariate logistic regression analysis further indicated that increased blood flow in the IFP was an independent factor associated with AKP [30]. It has been reported that over one-third of individuals will develop radiographic OA within 10-year post-ACLR [31]. In patients 1–5 years after ACLR, persistent inflammatory changes in the IFP can be detected by ultrasound; specifically, higher limb symmetry index values of IFP echo intensity have been found in patients with MRI‑confirmed IFP synovitis compared with those without [47]. Early detection of changes in IFP echo intensity could serve as an easily identifiable marker for impending post-ACLR OA joint alterations, thereby enabling timely interventions.

Furthermore, ultrasonography has demonstrated utility in the evaluation of the fibrotic knee following total knee arthroplasty (TKA). In patients with clinically diagnosed arthrofibrosis after TKA, gray-scale and power Doppler ultrasound revealed significantly increased synovial membrane thickening and, notably, enhanced neovascularity within the IFP compared with asymptomatic controls with well-functioning prostheses [33]. The presence of hyperemia within the IFP in this context suggests that persistent local inflammation or fibrotic remodeling of the fat pad may contribute to the global joint stiffness characteristic of post-TKA arthrofibrosis. More broadly, ultrasound can identify scarring of the IFP as a contributor to periarticular knee pain after arthroplasty [48]. These findings expand the postoperative relevance of IFP sonographic assessment beyond ligament reconstruction to include joint replacement sequelae.

Ultrasound Features of OA-Related IFP Changes

While ultrasound has traditionally been used in knee OA to assess cartilage morphology, synovitis, and osteophytes [34, 35, 49], growing recognition of the IFP's active role in OA pathogenesis has shifted attention toward its sonographic evaluation [50–52]. In healthy individuals, ICQ increased IFP hardness and was followed by an increase in oxygenated Hb concentration, consistent with reactive hyperemia. However, these changes in IFP hardness and Hb concentration were not significant in the knee OA group during ICQ, possibly suggesting impaired local microcirculation in the OA–IFP [17]. Fibrosis of the IFP may not only alter its echographic appearance and affect its blood supply, but also impact its elasticity. In a study of patients with knee OA, IFP stiffness was significantly associated with AKP in univariate analysis, and this association remained significant in multivariate analysis after adjusting for age, sex, femorotibial Kellgren–Lawrence grade, and MRI findings, including IFP synovitis and bone marrow lesions [36]. This finding is supported by evidence showing that higher IFP shear modulus is associated with increased severity of knee symptoms, and that IFP stiffness increases notably during deep knee flexion in OA patients [37, 38]. In addition, ultrasound assessment of echo intensity in the superficial medial IFP, along with medial meniscal extrusion, has been identified as a factor independently associated with knee symptoms in community-dwelling older adults [39]. Moreover, restricted morphological change in the IFP during walking has been revealed as a dynamic feature specific to symptomatic knee OA, showing a significant positive correlation with KOOS-pain scores [40].

Ultrasound Findings in Under-reported IFP Lesions

Ultrasound is widely recognized as a valuable tool in the assessment of superficial masses around the knee joint. Given that the IFP is an intracapsular, extra‑synovial structure situated beneath the patellar tendon, space‑occupying lesions involving this tissue can be readily detected. However, most of the reported lesions are rare overall, and the evidence is limited to case reports. To provide a broader differential diagnosis without disproportionately expanding the text, we summarize the key ultrasound features of the more commonly encountered lesion types in Table 2. Representative sonographic images of true space‑occupying lesions are shown in Fig. 2.

Table 2.

Ultrasound characteristics of common IFP space‑occupying lesions

Lesion	Key ultrasound characteristics	Clinical notes	References
Ganglion cyst	Cystic, hypoechoic, lobulated, well‑defined lesion; may appear anechoic with sparse internal echoes and no internal vascularity; may contain septations; can extend through a defect of the lateral patellar retinaculum	Non‑positional, non‑reducible mass; presents as soft or firm, non‑tender or tender; can be aspirated or injected under ultrasound guidance; post‑injection reduction in size documented	[53–56]
Synovial hemangioma	Circumscribed hypoechoic lesion; color Doppler demonstrates abnormal vascular regions; dynamic compression reveals fluid‑filled and vascular areas; may appear as a thick‑walled, hypoechoic lesion with tortuous vascular channels; can be multiple; weak arterial flow may indicate cavernous type	Rare benign vascular tumor; symptoms: pain, swelling, spontaneous hemarthrosis, restricted mobility; occurs predominantly in children and young adults; ultrasound useful for excluding arteriovenous malformations and as first‑line imaging in pediatric patients; familial occurrence reported as a cause of anterior knee pain	[57–61]
Lipoma	Well‑delineated, homogeneously hyperechoic fatty mass; unchanged during knee flexion/extension; non‑reducible under transducer pressure	Non‑tender, long‑standing, slow‑growing mass; typically located in the anterolateral aspect of the knee	[53]

Fig. 2.

a Ganglion cyst: sagittal sonogram showing an anechoic ganglion cyst that has posterior acoustic enhancement over IFP as well as septations within the cyst [56]. b Lipoma: axial sonogram showing a fatty mass in the anterolateral aspect of the knee, which developed on both sides of the lateral patellar retinaculum (arrows), through a focal defect (asterisk), with a superficial component (solid arrowheads) and a deep component (contoured arrowheads) [53]. c, d Synovial hemangioma: a focal interruption of the medial patellar ligament of about 4 × 3 mm with subcutaneous herniation of a small portion of swelling described (c), with large liquid and vascular areas detected via Doppler interrogation (d) [60]

Unlike true mass lesions, IFP herniation occurs when intra-articular tissue protrudes through a focal defect, most commonly in the lateral patellar retinaculum [62]. Clinically, it presents as a painless mass that becomes apparent during knee flexion [63, 64]. Idiopathic herniation of the IFP appears to be more prevalent in children; however, it may also occur in adults, particularly in the postoperative setting [64]. It should be noted that a herniated mass in this region may also signal other underlying conditions, including synovial chondromatosis, focal pigmented villonodular synovitis, osteochondroma, and chondrosarcoma [53]. Ultrasound imaging reveals focal herniation of IFP tissue within the infrapatellar region through a defect in the lateral retinaculum. Importantly, this sonographic finding is frequently only demonstrable during dynamic scanning with the knee in flexion, and thus may be missed on MRI performed with the knee in extension (Fig. 3) [63, 65]. In addition to spontaneous herniation, iatrogenic entrapment of fibroadipose tissue may occur following arthroscopic procedures. A case of fat pad entrapment within the suprapatellar pouch has been described as a rare postoperative complication subsequent to resection of the IFP and medial plica, highlighting the utility of dynamic ultrasound in identifying mobile loose bodies tethered to portal sites [66]. Judicious use of dynamic ultrasound is, therefore, essential for the accurate diagnosis of IFP herniation, thereby obviating the need for unnecessary MRI examinations and potential sedation.

Fig. 3.

IFP herniation: a sagittal ultrasound in extension demonstrated a focal defect (arrow) in the lateral retinaculum (arrowheads). b Sagittal ultrasound in flexion demonstrated an ill-defined mixed hypoechoic/hyperechoic mass (similar to the appearance of the IFP) extending into the subcutaneous soft tissues (arrows) [63]

Discussion

This review synthesizes the evolving role of ultrasound in characterizing the IFP across a spectrum of physiological and pathological states. While MRI remains the conventional reference standard for comprehensive soft tissue evaluation, ultrasound offers distinct advantages that extend beyond mere morphological depiction. Its capacity for real-time dynamic imaging reveals functional deficits, such as restricted morphological change during gait or reduced flexibility during flexion, that are not discernible on static MRI examinations [32, 40]. Furthermore, the integration of SWE and color Doppler imaging provides quantitative biomarkers of tissue stiffness and vascularity, which have been increasingly correlated with reported symptoms in knee OA and postoperative cohorts [37, 39]. In a prospective diagnostic accuracy study of AKP involving 143 patients, ultrasonography demonstrated an overall sensitivity of 85.3% and specificity of 100% for detecting various periarticular abnormalities when compared with MRI as the reference standard [67]. This level of diagnostic performance substantiates the role of ultrasound as a reliable first-line imaging modality for the initial screening and assessment of painful conditions involving the anterior knee compartment, including those arising from the IFP.

Despite these advancements, several limitations temper the immediate clinical translation of these findings. First, the evidence for many ultrasound applications in IFP pathology is limited and heterogeneous. Case reports, small observational studies, and larger cohort studies are often given similar weight in the literature. In this review, we have attempted to clearly distinguish established clinical uses (e.g., dynamic assessment of IFP mobility after ACLR) from emerging or speculative ones (e.g., elastography for early tendinopathy). Second, although ultrasound can quantify echogenicity, stiffness, and vascularity, the precise biological and structural meaning of these measurements remains incompletely understood. For instance, the correlation between SWE values and histological fibrosis grade has not been rigorously validated, and changes in echo intensity may reflect fibrosis, edema, or fatty infiltration. This uncertainty must be explicitly acknowledged when interpreting sonographic findings. Third, the diagnosis of IFP space-occupying lesions via ultrasound is currently confined to case reports. While dynamic scanning is invaluable for detecting intermittent herniation or vascular malformations, its sensitivity relative to MRI for deep intra-articular masses has not been rigorously established. Fourth, the reproducibility of ultrasound measurements, particularly echo intensity and stiffness, can be confounded by probe orientation and operator dependency, necessitating standardized acquisition protocols before widespread clinical adoption.

Conclusion

Ultrasound is a clinically valuable imaging tool providing useful information on IFP morphology, flexibility, vascularity, and stiffness. Dynamic B-mode ultrasound reliably assesses IFP deformation, with the thickness change ratio serving as a functional marker after ACLR. Doppler imaging detects IFP hypervascularity associated with AKP in post-surgical and overuse conditions, while SWE quantifies stiffness changes linked to tendinopathy and early degeneration. In mass lesions, ultrasound aids differential diagnosis when combined with dynamic maneuvers, though MRI remains the reference standard. Current evidence is heterogeneous, and the biological interpretation of parameters such as echo intensity and stiffness remains a limitation. Standardized protocols and larger longitudinal studies are needed to integrate these findings into routine clinical and return-to-sport decision-making.

Abbreviations

ACLR

Anterior cruciate ligament reconstruction

AKP

Anterior knee pain

Hb

Hemoglobin

ICQ

Isometric contraction of the quadriceps

IFP

Infrapatellar fat pad

KOOS

Knee Injury and Osteoarthritis Outcome Score

OA

Osteoarthritis

SWE

Shear wave elastography

TKA

Total knee arthroplasty

Author Contributions

C.Z. and XX.X. contributed equally to this work. C.Z. performed the literature search and wrote the initial manuscript. XX.X., HN.W., M.Y., HJ.H., LG.C., JQ.W. and J.C. reviewed, edited and wrote sections of the final manuscript. All authors revised the draft critically and agreed on the final version.

Funding

This work was supported by the National Natural Science Foundation of China (82472499 and 82372418), and Beijing Nova Program (20250484832).

Data Availability

Data are available upon reasonable request from the corresponding author.

Declarations

Ethics Approval and Consent to Participate

Not applicable.

Consent for Publication

Written informed consent for publication of the ultrasound image included in this review was obtained from the healthy volunteer prior to its use. The image has been anonymized to remove personally identifiable information.

Competing Interests

All authors declare no competing interests.