FAI - A users guide. Part 1 - What is FAI and what causes it

Part 1: What is FAI and causes FAI?


You're probably reading this article because you've been diagnosed with FAI. It's often a confusing time when the diagnosis is made, and often there are conflicting injuries about what the cause (and solution!) is.


So, some background. FAI is a relatively 'new' condition - the first article ever published on it was in 2003 by Swiss orthopaedic surgeon, Reinhold Ganz.  Of course FAI existed before this, it's just that it was never identified by itself as a cause of hip pain.  We have some old patient notes at the clinic from 1988. The patient had clearly presented with FAI, however back then the diagnosis was 'irritable hip' and the advice was to rest until the symptoms resolved.


FAI, or femoroacetabular impingement, is caused by bony impingement between the ball (head of femur) and socket (acetabulum or pelvis). Some people develop a 'bump' on the ball and some people have a deeper than normal socket that causes this impingement.

The 'bump' (also known as a 'CAM' - the dutch word for 'cog', or 'Ganz lesion' after the surgeon who originally coined the term FAI) is the most common cause of problems, particularly in the athletic population. From here on we will use the term ‘CAM lesion’ to describe the change.

You can also develop 'Pincer' impingement, where the socket is 'too deep' or has too much coverage, and causes bony impingement because of this. Most of the blog today will focus on the CAM lesion.


Photos below show a 'normal' hip joint. The next photo shows the CAM lesion and also a representation of a deep socket and a combined lesion with both CAM and Pincer. Photos from Lavinge 2004. (Click on the images to scroll through the gallery)


So what causes the CAM lesion?

This was unknown until recently some great work by Rintje Agricola, a Dutch PhD student in Rotterdam, demonstrated the development of the bump occurs in adolescence.

Agricola took XRays of a group of 12-19 year olds. One group was pre-professional (junior elite) soccer players, the other was not. 
Both groups had an increase in the prevalence of CAM lesions as age increased from 12-19, however more-so in the soccer group.


Another study by Agricola 2 years later looked at the same group of children, this time the soccer players only. Two years after the initial study, there had been an increase in the number of 'bumps' in the soccer players.

Agricola's 2013 paper is available here and the 2012 paper here


Where the bump occurs is at a region where there is a growth plate (new centre for bone development) in the hip. It's thought an irritation of this growth plate causes the growth of extra bone in the 'wrong' place, causing the CAM lesion.


The gallery below (from Agricola's 2008 paper) show the progression of CAM lesions from non existent, to large.


So what do we know about the cause of FAI:

 - it largely develops during adolescence, particularly from the ages of 12 to 15 or 16.

 - you are more likely to develop it if you are more active as a child, particularly in change of direction and impact sports such as soccer (the study hasn't been repeated in other sports)


What hasn’t so far been established is if there is a genetic risk of developing a CAM lesion.  A 2009 paper by Pollard from Oxford University established that siblings are significantly more likely to develop a CAM lesion than an age matched control, HOWEVER, they didn’t correct the study to take into account sporting activity, which we know from Agricola’s research above has a strong relationship to the development of CAM lesions.

Pollard’s paper is available here if you’re interested: 

There IS an established genetic component to the durability or ‘hardness’ of the articular cartilage, which is the smooth lining of the joint surface. Having a more durable articular cartilage PROBABLY slows the progression of arthritis in a hip joint, however this is a topic for another time.


In our next blog we will look at the problems FAI causes - in particular does it cause hip osteoarthritis and groin pain ('osteitis pubis')