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KNEE FAT PAD IMPINGEMENT MRI APPEARANCE
MRI OF FAT PAD IMPINGEMENT AROUND THE KNEE
Knee fat pad impingement syndromes are characterized by pain and inflammation of the fat pads surrounding the knee joint and can significantly impact knee function.
This blog post looks at the anatomy of the knee fat pads, the MRI appearance of impingement, and key areas to focus on MRI, to make accurate a diagnosis and guide appropriate treatment decisions.
There are four fat pads around the knee that can become impinged and can be symptomatic.
WHAT DOES NORMAL FAT PAD LOOK LIKE
WHICH FAT PADS
Quadriceps fat pad:
Located above the patella and posterior to the quadriceps tendon.Supratrochlear fat pad:
Situated anterior to the femoral trochlea.Hoffa’s fat pad:
Positioned behind the patella/ patella tendon and tibia.
MRI NORMAL KNEE FAT PADS
Normal fat pads are High Signal on PD or T1 and will be dark on PDFS T2FS or STIR as they will completely fat saturate.
Image Above: Location for quadriceps fat pad (Blue Arrow), Supratrochlear Fat Pad (Pink arrow) & Hoffa’s Fat Pad (yellow arrow).
MRI KNEE FAT PAD IMPINGEMENT: WHAT TO LOOK FOR
MRI FAT PAD IMPINGEMENT: WHAT TO LOOK FOR
On MRI Impingement of any of the fat pads has a similar appearance and the best sequence to diagnose it is on a STIR/T2FS or PDFS sequence.
Look for
Increased signal intensity on fluid-sensitive sequences (PDFS, STIR, T2-weighted) in the affected fat pad.
Low signal intensity on T1-weighted images.
Edema may extend to surrounding soft tissues.
- Expansion of the contours of the fat pad which become convex. This is usually seen in the Quadriceps and Supratrochlear fat pads and not in Hoffa’s fat pad.
- C+: There is no need to give contrast but if you do, impingement in the acute and subacute stages will enhance.
MRI QUADRICEPS FAT PAD IMPINGEMENT
- The quadriceps fat pad lies between the quadriceps tendon and the suprapatellar recess.
- On MRI, quadriceps fat pad impingement typically presents with a convex posterior margin and may show increased signal intensity on fat-saturated sequences, suggesting inflammation or edema.
Image Above: Impingement of quadriceps fat pad (yellow arrow). Due to swelling the posterior margin can become convex (blue arrow).
MRI SUPRATROCHLEAR FAT PAD IMPINGEMENT
- The supratrochlear fat pad, located just superior to the trochlear groove, may demonstrate high T2/STIR signal and subtle contour bulging when impinged.
- MRI knee fat pad impingement involving the supratrochlear pad is often overlooked unless specifically assessed for shape and signal asymmetry.
Image Above: Impingement of supratrochlear fat pad (yellow arrow). Same appearance as impingement elsewhere.
MRI HOFFA’S (INFRAPATELLAR) FAT PAD IMPINGEMENT
- Hoffa’s fat pad impingement is a common cause of anterior knee pain and appears as a region of high signal intensity on MRI, Proton density fat-saturated images.
- Hoffa’s fat pad impingement may be central or more laterally particularly when there is patella mal tracking.
- The fat pad, also known as the infrapatellar fat pad, is highly vascular and innervated, making it sensitive to repetitive trauma or mechanical impingement.
Image Above: Impingement of Hoffa’s fat pad (yellow arrow).
TEST YOURSELF ON SOME COMMON & FREQUENTLY ASKED QUESTIONS
WHAT DOES FAT PAD IMPINGEMENT LOOK LIKE ON MRI?
MRI knee fat pad impingement refers to inflammation or compression of the knee’s fat pads, typically seen in the quadriceps, supratrochlear, or Hoffa’s fat pads on MRI.
WHICH FAT PADS ARE COMMONLY INVOLVED IN KNEE IMPINGEMENT?
The quadriceps fat pad, supratrochlear fat pad, and Hoffa’s fat pad are most commonly affected.
WHAT DOES FAT PAD IMPINGEMENT LOOK LIKE ON MRI?
It appears as increased signal intensity (edema), convex posterior margins, and irregularity in the involved fat pad.
HOW CAN YOU DIFFERENTIATE NORMAL VS ABNORMAL FAT PADS ON MRI?
Normal fat pads show uniform low signal with smooth margins, while abnormal ones show bright signal and margin bulging or irregularity.
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