Femoro-acetabular Impingement (FAI)

I’m quite excited about this!….this is our first post from a Physio Guest Blogger on RunningPhysio. Ultramarathon runner, Physio and all round legend Andy “Coomba” Coombs has written us an excellent piece on Femoro-acetabular Impingement! He’s due to run a 52 mile ultramarathon in September for the Rockinghorse Foundation, if you’d like to sponsor him you can do here.

Over to you Andy….

FAI appears to be receiving a great deal of attention at present which is probably, in part, due to the fact it is a relatively new diagnosis in the orthopaedic world and secondly due to advances in scanning and operative procedures. Previously, FAI may have been considered a form of early arthritis, however it is now considered a separate condition. I have therefore decided to write this to provide some information regarding what the condition is and how it can be managed.

First of all I feel it is worth breaking down the jargon as it can become somewhat confusing.

Femoro – Femur or thigh bone

Acetabulum – Socket of the hip joint

So the femoro-acetabular joint is the medical term for the hip joint.

Impingement is a term to describe the catching or squashing of tissues in the body. In this case a pinching between the head/ neck of the femur and the rim of the acetabulum.

In a normal hip joint there is a round spherical ball (the head of the femur) and a matched socket which is a equally spherical cup shape (figure 1).The acetabulum is orientated downwards and forwards, this has been indicated as a remnant feature of our ancestors who walked on 4 legs. To further increase the stability of the hip there is a layer of cartilage called the acetabular labrum which is attached to the rim of the acetabulum. This works in two ways, the first is to provide a physical increase in the size of the acetabulum and the second is it maintains a seal on the joint which allows a vacuum to form within the joint pulling the femoral head inwards and reduces impact forces through the synovial fluid. There is a nice video of hip impingement here. Check here for more details on hip anatomy and musculature.

Figure 1: Normal anatomy of the hip from Lewis (1918) Gray’s Anatomy 20th ed.

FAI has been sub-categorized into two primary boney causes Cam and Pincer.

A CAM in engineering terms refers to an oval shaped cog that converts rotational motions into up and down motions, like the Cam shaft in a car. The CAM shape of the head of femur occurs when there is some extra bone growth on the neck of the femur or a pistol grip deformity – see figure 1A. The bone itself isn’t really an issue the problem arises when you attempt to move your leg into extremes of movement. Movements of Flexion (bringing hip towards your chest) Adduction (moving the leg inwards) and internal rotation and in some cases abduction (moving leg outwards) will lead to a catching of the femoral head on the acetabulum and squash the tissues in between. This type of impingement tends to be most prevalent in young active men.

Pincer impingements are related to an increase in the depth of acetabulum, leading to an over coverage of the femoral head and impingement. There are a number of to abnormalities to consider with this but all essential lead to the same end result. Interestingly these appear more prevalent in middle aged active women.

Figure 1A Cam Impingement, note the additional bone growth on the femur identified by the 3 black arrows. 1B Pincer impingement, note the additional bone growth on the rim of the acetabulum identified by the 3 black arrows. Images reproduced from Epinosa et al. 2006 freely available online here. This show the hip as if looking down from the top through the joint.

Figure 1C: Cross-sectional view of normal hip anatomy from Byrne et al. 2010 freely available here.

Table 1: Causes and development of FAI.

Figure 3 depicting the top of the femur and the angluation of the femoral neck. Smaller angles are termed coxa vara and greater angles coxa valga. Credit for this picture to Behrang Amini MD/PhD.

For the majority of athletes with FAI it is likely that it has an idiopathic development (no known causes). The table above lists other potential causes but I would assume that you may know if you have had any of these.


To have a confirmed diagnosis of FAI either an x ray or MRI is required. MRI will be more sensitive to subtle hip changes. To identify if there is also a labral tear a further investigation called an MRA (Magnetic resonance arthorgram) is required as MRI is only considered about 30% accurate whereas MRA is about 90%. 3 dimensional imaging is now being used in the diagnosis of impingement. This article has details and excellent pictures.

Signs, symptoms and investigations

In a western population there is a higher incidence of anterior FAI meaning that the impingement is occurring at the front of the hip. As a result the most common area of pain is in the groin though symptoms can also be present on the side of the hip or in the deep buttock region. It is commonly described as a deep aching sensation and is often with no history of trauma. It usually occurs after activities that have involved extremes of movement, for instance dancers or athletes who play ball sports with lots of twisting or pivoting. There may also be a clicking, locking or feeling of instability which could be attributed to a labral tear, as around 73% of patients with FAI also had concurrent labral tears (Kang et al. 2009)

Physical testing for FAI is relatively tricky as the best tests we have involve positioning the leg into a provocative position of Flexion, adduction and medial rotation and aiming to elicit pain (e.g.Frederick’s, FADIR, FABER (Patrick’s test). The problem with these tests is that they will be positive with most hip joint pathologies, but they are reasonably good at ruling out potential causes of groin pain such as adductor strain, sportsman hernia (Gilmore’s groin), iliopsoas tendinopathy/ bursitis to name a few. In addition to identifying hip joint pathology, a thorough physical examination will also involve the identification of factors that may contribute to the problem.

Figure 4: impingement test combining hip flexion, adduction and medial rotation from Kaplan et al. 2010 freely available here.

In of particular relevance to runners may be:

Any force that may cause the femoral head to be shifted forwards in its socket may contribute to symptoms of FAI. These include positions of hyper-extension, either during movement or rest, or alternatively tightness in the back of the hip. This will contribute to the symptoms as it places a greater load on the top of the acetabulum.

Hyperextension of the hip during running.

Runners who over extend their hip during the push off phase of gait may be at risk of irritating a sensitive hip. The reason for this is that during hip extension the femoral head translated forwards in is socket (by a small degree). This places an increase in load bearing through the top rim of the acetabulum and the superior labrum. This article has extensive details of hip movements during gait and running.

Tight posterior muscles and ligaments.

The deep muscles in the the back of the hip like piriformis, obturator internus/ externus, gemelus superior/ inferior and quadratus femoris as well as the posterior hip capsule and ligaments can apply a forward force on the head of femur. This will particularly play a role during hip flexion/ medial rotation movements further exacerbating the impingement. All of these muscles play a role in externally rotating the hip and it is impossible to clinically identify which may be at fault. However because the all have similar roles and are positioned in similar positions treatment would be the same.

Imbalances of the hip musculature

Some of the leading research scientists in hip impingement have shown that insufficency (either strength or co-ordination) of iliopsoas during hip flexion or gluteus maximum during hip extension can lead to an increase in the anterior glide of the femoral head (research abstract here).

Reduced shock absorbancy of the hip – If runners also have a Labral tear

The acetabular labrum helps to maintain vacuum seal on the hip joint. This vacuum helps with some of the shock absorbancy of the hip by keeping the synovial fluid in the places it is needed. Therefore if the labrum is torn then there is likely to be a reduction in the vacuum in the hip joint and therefore a reduction in shock absorption.

A stiff hip

Obviously with extra bone either on the femur or the acetabulum there will be a reduction in the amount of movement available, but in addition to this avoidance of stretching or taking the hip through its full range due to pain is likely to lead to a tightening of soft tissues leading to a greater restriction in range of movement. This in itself can be painful. An extreme example would be if you have had a limb in a plaster cast for some time. When the limb is removed it feels very stiff and when taken to its end of range it becomes painful. This is quite often due to soft tissue tightness and the same is true for all joints that are not moved through their full range regularly. So if you have been avoiding particular movement because of pain then it may stiffen and some of that discomfort is related to a stiff hip and some due to the impingement.

Identification of any of these factors has the potential to influence the pain of FAI.


From my experience and the available research on rehabilitation for FAI, physiotherapy does have a place in identifying factors that may be contributing to the problem and treating those. For example if it is indetified that running form is an issue then changing it can have an impact. Mobilizations and stretches for stiff or tight hip can help, or correction of muscle imbalances. However even with the identification of these factors and treatment of them unfortunately some patients will fail to improve and this is because there are significant anatomical changes in the hip which need to be addressed that no amount of rehabilitation will change. If you do want to rehab a suspected FAI it is not something that can be easily self managed and so it is important to ask for assistance from a Physio or hip specialist to guide you. They are likely to work on the following areas;

  • Rest from aggravating factors – One of the key principles of treatment of FAI is reducing aggravating factors such as moving the hip repeatedly into uncomfortable positions. This may occur with running but is perhaps more likely with other sports that involve more rotation or deeper squatting positions e.g. Football or rugby. It might be that you need to rest from these sports during rehab to allow things to settle. It may mean modification, such as change of running form mentioned above.
  • Improving hip range of movement – There is a challenge of balancing restoring range of movement without aggravating symptoms. This can sometimes be achieved by gently working on stiff movements within a comfortable range. Certain Physio techniques (called MWM’s) may help.
  • Increasing strength of gluteal muscles – these help support the hip and pelvis – details here.
  • Stretching tight muscles – this may include deep hip muscles or tight hip flexors (which can be stretched using the sofa stretch – video in this article on ITB)
  • Improving balance and control of movement – in particular preventing excessive hip extension or sideways movements. Details on rehabbing balance here.

If the decision is made to trial rehab then consider giving it a good go for around 8 – 10 weeks and if there has been no change in that period it is unlikely that continued work will suddenly make all the difference. That said this has to be for people who have a confirmed FAI, not a suspected one from positive impingement test because as I mentioned earlier physical testing is not accurate enough to state that a patient has an FAI only good enough to state they have a problem with the hip joint.


Surgical intervention is fairly varied but most now are performed as key hole surgery and will involve correction of bone abnormalities, and repair or resection of the labrum. This is not my area of expertise as I am a physiotherapist but I refer you to a reputable journal article here for more information as well as a proposed rehabilitation programme from Mr Richard Villar, a leading hip surgeon in the UK.



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