What is Femoral Acetabular Impingement (FAI)?

Femoral acetabular impingement (FAI) is the Cara Delevingne of running injuries. Fret not, this hip injury has nothing to do with a strong eyebrow game, but like Cara, it is very en vogue at the moment. FAI is very much a diagnosis of the modern age. Its birth has dovetailed with the improvement and sensitivity of our imaging techniques like X-rays and MRIs. But what is femoral acetabular impingement, anyway? Let’s look deeper at this diagnosis, and explore FAI symptoms…

What is Femoral Acetabular Impingement?

Simplifying Femoral Acetabular Impingement

In short, it means the hip bones are misshapen. Because of that, they rub against each other and can damage the joint.

For symptoms of femoral acetabular impingement to become problematic you really need a few things happening:

1. A misshapen hip joint
2. Excessive loading of the joint
3. Perceived threat

What is Femoral Acetabular Impingement?

Pincer lesions

A pincer lesion relates to the acetabulum (the socket of the ball and socket joint). If you have a pincer lesion it means that the socket is a bit deeper in some parts so the femoral head (which is the ball, of the ball and socket joint) is deeper into the socket.

The only real consequence of this is that you will run out of range of motion more quickly than if you had a normal-shaped acetabulum.

Visualisation helps us understand what is femoral acetabular impingement; think of it like this: Imagine your acetabulum is like your breakfast bowl and your femur is like the thin end of a baseball bat. If you place the thin end of the baseball bat in the centre of the bowl and, holding the end of the bat in the centre, moved the bat as far as possible in all directions you will eventually get to a place where the shaft of the bat hits the rim of the bowl. That is your end of range.

If, however, you swapped the breakfast bowl for a mug or cup and did the same thing, you would hit the rim of the cup much sooner. This is what a pincer lesion is like.

Cam lesions

A cam lesion relates to the ball (the head of the femur). In a cam lesion, the ball is aspherical, instead of being relatively round, it has a bit of a hump on one side.

The consequences of this are the same as with a pincer lesion, but for slightly different reasons. In this scenario, if you kept your breakfast bowl as the acetabulum but this time turned the baseball bat upside down, so you now have the thick end of the bat in the bowl, you would still run out of range, hitting the rim of the bowl sooner because you have a bigger ball.

Now, this is important. Is having a cam or pincer lesion enough to cause FAI hip pain?

No. 100% No. You can have both cams and pincers — or combinations of both — and have absolutely no issue with your hips. You can continue living your life without any worry.

That is because you need other ingredients in the pot to cause FAI hip pain.

Unfortunately, I feel, we tend to obsess over issues in our body’s structure. The problem here is, if you believe that the structure of the hip is the only issue that could cause FAI symptoms, it means the only solution is to surgically change that structure.

That is grossly oversimplifying the problem and using a sledgehammer treatment when a miniature hammer will do.

So, what causes Symptoms of Femoral Acetabular Impingement?

To get to a place where symptoms of femoral acetabular impingement become problematic you also need excessive loading and/or increased perceived threat.

Excessive loading is where you take the misshapen hip that does not have a huge range of motion, and continuously and repetitively load it at its end of range.

Traditionally the impingement is on the anterior and superior part of the hip joint (front and top). The movements that tend to load this area are hip flexion and internal rotation. This causes excessive antero-superior impingement of the ball on the socket and does not give the joint enough time to recover, get stronger and adapt to the load.

This, in turn, causes the tissues in this area to break down. Your brain will sense this threat to your bodily tissues and protect you by sending you pain signals, your FAI hip pain.

This is why athletes in sports that require repetitive end of range hip flexion and internal rotation FAI more often. Think dancers and cyclists. The hip joint itself is not the problem, it’s the excessive end of range loading of a small part of that hip joint that’s causing the issue.

The third ingredient is not always present, but it’s important to look out for. This is perceived threat. Basically, we are looking for any fear, anxiety, stress, catastrophization (fearing the worst) around the injury or in life in general.

If these thoughts are present, your brain is listening. These thought patterns are telling the brain something is wrong with you or in your life. The brain will perceive this as a threatening state and move to protect you with more FAI hip pain.

So, if we know that all of these three elements can play a part in femoral acetabular impingement, we do not have to look to change the morphology of the hip, we can just look at changing load or threat.

Load & Threat Management in FAI

You could decrease activities that need lots of hip flexion/internal rotation. It could be something as simple as raising your bicycle seat. You can look at movement patterns and change the way you move to offload that part of the hip.

Similarly, if you feel there are wider threats making you stressed or anxious, you can address these by simple education to lower threat levels and provide yourself an environment of perceived safety.

So take a hip with a lesser range of motion, add repetitive end-of-range loading/impingement and sprinkle on some fear or life stress and you have the perfect recipe for hip pain, symptoms of femoral acetabular impingement.

Now we’ve hopefully better answered the question ‘What is Femoral Acetabular Impingement?’, let’s step away from the scalpel and look at the simple things first. Options that do not include cutting through my lovely healthy tissue and shaving away my bone, thank you very much!

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