What is a fat pad!? Despite common beliefs about fat, in actuality fat in your body is really functional and necessary – you just don't want it in excessive amounts. Even if you already knew that, we don't usually consider fat as an anatomical structure. Well it is! And it is actually a really sensitive type of structure found throughout the body. There are three fat pads just in the knee.
The fat pad that's most commonly affected in the knee is the infrapatellar fat pad. You can see below that it's the largest of the three. It's also the most vulnerable to injury based on its size and position. You can imagine how a direct blow or a direct fall on to the knee could cause some damage.
What is it exactly?
The fat pad is kind of like a pouch of fat only it's not distinct from the rest of the knee joint. It actually has connection all throughout the knee joint. It connects to both the the tibia and femur leg bones, to the ACL itself, to the meniscus, and the knee cap as well. It's yellow, squishy and mobile, so as the leg changes positon and the knee joint moves the fat pad can take on different shapes. Most importantly it's not just a glob of fat – it's full of blood vessels and nerve endings. That means it's a dynamic structure and unfortunately can be a real source of pain. In fact it's one of the most pain sensitive structures in the body.
Why is it there?
The function of the infrapatellar fat pad is not actually crystal clear. Perhaps the most obvious theory is that it is a shock absorber. Based on its location and texture you can see how it could have a protective effect if you fall on your knees for example. But it's possible that it also helps with how your kneecap tracks and other biomechanical functions. Most interestingly, some scientists believe that it actually acts as a store for regenerative cells to help with recovery after injury.
Why should I care that the fat pad exists?
Knee pain is such a common complaint across ages, activity levels, genders. And maybe because it's so common, people have come up with generic names to describe these injuries (runner's knee, jumper's knee etc.) and even generic treatments (ice, braces, glucosamine etc.) But to effectively treat knee pain the cause has to be investigated thoroughly so that treatment can be specific to the tissue that is injuried. Most often it's a problem with joint mechanics or one of the ligaments or tendons, but sometimes it's the fat pad.
How does the fat pad get injured?
There are three major ways the fat pad gets injured.
1) Direct blow to the knee
2) Repetitive microtrauma. In other words, due to a cumulation of minor injuries
3) Side effect from surgery. This can occur post ACL repairs or knee replacements.
What does a fat pad injury feel like?
Since the fat pad is located below the knee cap and behind the patellar tendon, that's where the pain is felt. Think of that squishy area right under your knee cap, if you slide your fingers to the outside borders of that area – that's your infrapatellar fat pad. If that's the structure that's injured, usually pressure to that area is painful. More generally it's a deep feeling of aching and burning, as opposed to something sharp and acute.
Once the fat pad is injured it can become inflammed and swollen, which translates to a potential pinching of the fat pad under the knee cap. That's why straightening your leg out all the way, either while standing or going up the stairs can also be painful. Often we see that people who have fat pad injury stand with their knees hyperextended. On the other extreme, having your knee bent for an extended period of time can also be uncomforatable.
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What can be done about it?
Fortunately there are a number of ways to treat this injury. The first step is to figure out how it was injured in the first place. Was it a traumatic injury? Was it due to poor alignment (this could be anywhere between the hips and the feet)? Or was there another injury or surgery? Once we've determined the cause we can do something about it.
Taping – This sounds simple but it can be a really effective way to take the load off the fat pad. Tape is placed below the fat pad to give it a lift and de-stress the tissue, while the strip above actually tilts the knee cap up to un-pinch the tissue.
Muscle Training – Often a predisposing factor to fat pad injury is poor hip and thigh muscle strength and control; it is seen particularly in those who hyperextend their knees on a regular basis. The hyperextension can occur during walking or standing – or both. Muscle strengthening can help improve the stability of the knee in that range – where the knee has to support your body weight as the other foot comes off the ground. The best way to do this is in a closed chain fashion, which is a fancy way of saying exercises that keep your feet on the ground. If you're an athlete, these exercises should be progressed to move advanced movements like jumping and running.
Movement Retraining – Sometimes the issue is not the strength of a muscle, it's the commands your brain sends to your muscles. In this case it's important to retrain the body's movement patterns. Patients are shown strategies to improve knee control, especially to prevent hyperextension.
Non-conservative Treatment – If necessary, medical treatment such as injections and surgery are also available to treat a fat pad syndrome. This can involve anything from injecting a local anesthetic to cutting out part or the entire fat pad. This treatment can follow conservative treatment or can be in conjunction with it as well.
The fat pad is not a commonly known part of our body. But having some knowledge that this structure exists can be really helpful in getting down the source of you knee pain. And the more specific your 'knee pain' diagnosis can be, the faster and more effective your treatment will be.
1) Dragoo, J. L., Johnson, C. & McConnell, J. Evaluation and treatment of disorders of the Infrapatellar fat pad. Sports Medicine 42, 51–67, 10.2165/11595680-000000000-00000 (2012)
2) Eymard, F. & Chevalier, X. Inflammation of the infrapatellar fat pad. Joint Bone Spine 83, 389–393, 10.1016/j.jbspin.2016.02.016 (2016)