Ultrasound guided knee fat pad impingement injection

What is knee fat pad impingement?

What is the knee fat pad?

The knee fat pad is a cushion of fatty tissue located below the patella (knee cap) and behind the patellar tendon (which runs from the knee cap to the shin bone). This is sometimes referred to as the Hoffa’s fat pad or the infrapatellar fat pad (because of its location below the patella). It is a soft structure that can be easily felt by pressing just underneath your kneecap. In addition to fatty tissue, the knee fat pad contains blood vessels and many nerve endings, making it sensitive to pain. Therefore, problems here can lead to significant pain in the front of the knee. We do not fully understand the function of the knee fat pad, but there is evidence that it acts as a shock absorbent important for normal knee mechanics. Knee fat pad impingement occurs if the fat pad is pinched between the patella (knee cap) and the lateral femoral condyle (the lower end of the thigh bone). This will lead to inflammation, swelling, pain, and sometimes scar tissue formation.

What are the causes of knee fat pad impingement?

The causes of infrapatellar (Hoffa’s) fat pad impingement includes:

Overuse, particularly during sports activities (like running) and activities that cause increase load upon the quadriceps tendon (like football).
Direct trauma to the knee.
Altered lower limb mechanics (for example, due to hip muscle weakness) cause impingement of the fat pad.
Due to the close relationship between the knee fat pad and the other knee structures, fat pad inflammation can be seen in association with other knee conditions like:
- Patellar tendinosis.
- Patellofemoral joint (knee cap) pain

And less likely

How is knee fat pad impingement diagnosed?

The clinical findings are not very specific. Sometimes the condition is suspected clinically and confirmed with imaging. In other cases, the diagnosis is only picked up with knee imaging. The best imaging modality to assess the knee fat pad is an MRI examination. This will demonstrate the inflammation and fluid within the fat pad and confirm the diagnosis. Ultrasound can be useful in the assessment also as it can show increase blood flow to the area of inflammation within the fat pad, but MRI is more sensitive. Ultrasound is particularly useful in performing ultrasound-guided injections to the knee fat pad.

What are the treatment options for knee fat pad impingement?

Treatment of knee fat pad impingement starts by managing the underlying cause. As we have mentioned, fat pad inflammation can be associated with knee conditions. In addition, a dedicated physiotherapy program is usually trailed. This usually consists of a rehabilitation program to strengthen specific muscle groups within the hip and thigh and specific lower limb stretching exercises. Treatment also includes education about abnormal movement/posture and how to correct this, and a taping technique might be used.

Can a steroid injection help with knee fat pad impingement?

If the pain persists despite the above measures, then an ultrasound-guided steroid injection can be considered.

What is a steroid/cortisone injection?

Corticosteroid (cortisone) is a well-established anti-inflammatory medicine. It acts by reducing the inflammation in the targeted area, thus providing a strong and local anti-inflammatory effect to help control the symptoms and allow the patient to manage the condition, usually by undergoing a physiotherapy rehabilitation program. The injection can be both therapeutic (help with the pain) and diagnostic (confirm the diagnosis if you notice a good response from the injection). Please read our FAQs for more information about steroid injections.

What is the benefit of having an ultrasound-guided injection for knee fat pad impingement?

There is significant evidence that when injections are done under ultrasound guidance, they result in superior outcomes, with better pain relief, longer effects and more improvement in function compared to injections done without ultrasound. This is because ultrasound guidance allows for accurate needle placement and, thus, delivery of the medicine to the exact site of inflammation. Before the injection, the doctor will perform an ultrasound examination and correlate the findings with any MRI images to determine the exact sire of inflammation and impingement within the knee fat pad. Then, the exact site will be targeted for the injection. Injecting under ultrasound guidance also allows for visualisation of the adjacent structures so they can be assessed before performing any injection and reduces the risk of tissue injury at the injection site.

Summary

Knee (Hoffa’s) fat pad inflammation/impingement is not an uncommon cause of pain at the front of the knee. It can be associated with other knee conditions. Treatment starts with physiotherapy. MRI is useful in confirming the diagnosis and ultrasound is essential to guide treatment, if a steroid (cortisone) injection is indicated.