Hoffa’s Fat Pad and Anterior Knee Pain: An MRI Reading Pathway

On knee MRI, high signal in Hoffa’s infrapatellar fat pad (HFP) often attracts attention in patients with anterior knee pain. Edema is a starting point, not a diagnosis: it may accompany impingement, trauma, postoperative change or adjacent disease, and can also be asymptomatic. Using the 2016 pictorial review by Draghi and colleagues, this article follows anatomy, signal distribution and the source of pathology rather than attributing all anterior knee pain to the fat pad.

1｜Start with normal anatomy: fat, recesses and plicae

The HFP is intracapsular and extrasynovial, posterior to the patellar tendon and anterior to the synovium, with attachments to the anterior meniscal horns. Normal fat is bright on non-fat-suppressed T1- and T2-weighted images, while fibrous septa are dark. Vertical suprahoffatic and horizontal infrahoffatic recesses may contain joint fluid; their normal configuration should be recognized before diagnosing a cyst or a tear.

2｜Superolateral edema and impingement: location matters, but so do symptoms

Repetitive microtrauma can inflame the fat pad. The review illustrates focal superolateral edema in a young athlete and an enlarged, edematous fat pad in a soccer player with Hoffa’s disease. With superolateral edema, review patellar height, the patellofemoral joint and adjacent patellar tendon. The review describes an association with patella alta, but edema alone does not establish a symptomatic impingement syndrome.

3｜Acute trauma: read the fat pad in the context of the whole knee

Direct contusion, twisting injury and patellar dislocation may produce HFP edema or hemorrhage. Interpret increased signal on fluid-sensitive fat-suppressed sequences together with the patella, femoral condyles, tibial plateau, ligaments and joint fluid. Do not summarize a knee with substantial adjacent injury merely as fat-pad inflammation.

4｜Postoperative change: distinguish fibrosis from a Cyclops lesion

Establish whether the patient has undergone arthroscopy, anterior cruciate ligament (ACL) reconstruction or another operation. Fibrosis may appear as bands or ill-defined, confluent low-signal tissue. A Cyclops lesion is a nodular fibrous lesion after ACL reconstruction, typically anterior to the graft in the posterior HFP. In a patient with extension loss, its location and potential mechanical impingement deserve particular attention.

5｜Trace the source: meniscus and patellar tendon

The HFP borders the menisci, ligaments and extensor mechanism, making secondary edema common. A low-signal structure within the pad may be a displaced meniscal flap rather than a primary mass. Patellar tendon thickening, increased signal or graft-harvest change can coexist with edema. In children and adolescents, also assess the tibial tubercle apophysis for Osgood–Schlatter disease.

6｜Cystic lesions: meniscal cyst or ganglion?

When a fluid-signal lesion is found in the pad, trace its relationship to the meniscus and normal synovial recesses. Meniscal cysts commonly accompany a meniscal tear. Ganglia may be unilocular or multilocular, round or lobulated. If the adjacent meniscus appears normal, reconsider the origin instead of naming the lesion by location alone.

7｜Synovial disease: low-signal proliferation and loose bodies

When the abnormality extends beyond the fat pad, follow the synovium and joint recesses. The review uses pigmented villonodular synovitis (PVNS) to demonstrate hemosiderin-associated low-signal proliferation, and synovial chondromatosis to illustrate intra-articular loose bodies. Associated HFP edema should not distract from these more specific findings.

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