Femoroacetabular Impingement (FAI)/Impingement Syndrome (FAIS)

What is FAI or FAIS?

Femoroacetabular impingement (FAI) refers to early contact (impingement) between the bones of the hip during movement.  FAI is due to a variation in shape of the bones of the hip joint (thigh bone (femur) and/or hip socket (acetabulum)). This may result in reduced range of motion (stiffness) and, in a relatively small percentage of people, hip pain associated with extra forces being placed across the joint structures such as in the hip socket (acetabulum) including the labrum (a triangular fibrocartilaginous fringe that joins the edge of the hip socket important for hip joint health) and cartilage (tissue that covers and protects the ends of long bones at the joints) (Figure 1).

Painful FAI is referred to as Femoroacetabular Impingement Syndrome (FAIS). A published consensus of world experts¹ agreed that the diagnosis of FAIS requires 3 key features:

1. FAI bony variations observable on XRay or MRI Scans

2. Hip or groin pain related to particular motions or positions

3. Pain and restricted range of motion on clinical tests performed by a health professional


Types of FAI:

3 different bony shape configurations, or morphology (meaning shape) types, may be associated with FAI – Cam, Pincer and Mixed


1: Cam Morphology (shape) – Cam FAI

Cam is an engineering term for relating to the irregularly shaped sliding or rotating piece. In hips, cam morphology or cam-type morphology (Figure 2) is where the femoral head is non-spherical and the junction between the head and neck of the femur is thicker than normal. This thickening means that as the hip moves into flexion (knee to chest) or internal rotation (knee turns in) there is less bony clearance between the head/neck junction and the acetabulum. When the cam area comes into contact with the acetabulum, it is referred to as Cam Impingement or Cam FAI. This may result in restricted range of motion and higher loads than normal being placed on the labrum and the cartilage on the outer edge of the acetabulum.

Pincer type Femoroacetabular Impingement

Figure 3: Pincer morphology (shape) of the acetabulum (socket). The additional bone is indicated by the red region.

3.Cam & Pincer Morphology – Mixed FAI

A mix of cam and pincer shapes (Figure 4) can also occur, referred to as Mixed FAI.

Figure 1: Basic anatomy of the hip joint, as viewed from the front

Cam type Femoroacetabular Impingement

Figure 2: Cam morphology (shape) of the head-neck junction of the femur. The additional bone is indicated by the red region.

2. Pincer Morphology (shape) – Pincer FAI

Pincer morphology or pincer-type morphology (Figure 3) refers to a variation in shape or orientation of the hip socket (acetabulum). Overcoverage of the top of the thigh bone (femoral head) by the acetabulum may occur when the socket is deeper than average (global overcoverage). Alternatively, the edge of the socket may sit lower over the front of the femoral head due to an orientation where the socket faces a little more backwards than normal (acetabular retroversion – focal overcoverage). Both situations may result in restricted movement as the femur and the acetabulum come together earlier in range, referred to as Pincer Impingement or Pincer FAI. The labrum can again be put under higher stress.

Mixed type Femoroacetabular Impingement

Figure 4: Mixed cam and pincer morphology (shape) of the head-neck junction of the femur and the acetabulum respectively. The additional bone is indicated by the red region.

What are the usual symptoms?

  • FAIS commonly presents as a deep pain in the front of the hip or groin, although it can also occur in the side and back of the hip and thigh, and sometimes in the knee.
  • Pain can start suddenly or build up more slowly over time.
  • Movement can become stiff and restricted, often with pain at the end of range
  • Pain can be felt playing sport, but also with sitting, squatting and lunging. If badly flared up, the hip may ache at night and pain may persist for several days after playing sport.
  • Stiffness, clicking, catching, locking or giving way may also be experienced by some people.

Key Point: Variations in bone shape on imaging do not necessarily correspond with pain or FAI symptoms, meaning you can have the bone shape variations and no pain². Up to 67% of people have been reported to have FAI bone variations on imaging but do not have any symptoms³.

What causes FAI?

FAI is a condition related to alterations in the pattern of bony growth4. Bones continue to grow and change shape during childhood and adolescence. Growing bones have cartilaginous growth-plates or epiphyses from which the bone grows and extends, before fusing into hard bone as early adulthood is reached. The growth plates (Figure 5) are influenced by the amount and type of physical loads they are exposed to. The most common type of FAI, Cam FAI, is thought to develop in response to physical activity that places load across the growth plate in the neck of the femur. High load across the growth plate may stimulate excess bone to be laid down in this area.

Hip growth plate

Figure 5: The growth plate of the hip where Cam changes are thought to occur

Researchers have identified a number of possible factors that may contribute to this bony response and risk of developing a ‘cam-type’ bony shape:

  • High exposure to twisting and bending forces across the femoral neck
    Involvement in sports that involve these types of forces – Soccer/football codes, hockey, basketball4
  • Training 4 or more times/week
  • Being male – a cam shape is much more common in males
  • Change in the shape of the head and/or neck of the femur can also be related to childhood conditions such as Legg-Calve Perthes Disease or Slipped Capital Femoral Epiphysis.

Change in the shape of the head and/or neck of the femur can also be related to childhood conditions such as Legg-Calve Perthes Disease or Slipped Capital Femoral Epiphysis.

Getting Help. What is the treatment for FAIS?

Importantly no treatment is required for asymptomatic FAI – if you have a FAI bony shape but no pain, then there is no need to seek active treatment. There was a period where surgeons were offering preventative surgery to ‘correct’ bony variations but there is absolutely no evidence for this approach and problems may be created in hips that may never have become painful. The world’s leading surgeons and other health professionals in this field all agree that PREVENTATIVE SURGERY is NOT RECOMMENDED¹.

Hip Pain professional assessing pelvic movement

Scientific evidence for management of FAIS has only recently started to emerge, with the first high-quality surgical studies (randomised clinical trials) published in 20185 6. Early pilot trials assessing outcomes of high-quality physiotherapy programs have also been published.

The evidence so far suggests:

  • Both surgery and physiotherapist-led rehabilitation provide significant reductions in pain and improvements in function
  • Surgery may provide outcomes that are mildly better than rehabilitation alone, if you only have access to limited, publicly funded care
  • Surgery comes with increased health risks, although complications are uncommon
  • Neither intervention at this point restores full painfree function. 1-2 years later participants in these trials still report ongoing problems and have often not returned to full sporting participation7.
  • Surgery is not a ‘fix’ and optimal outcomes will require substantial commitment to high-quality rehabilitation that targets individual deficits in hip and trunk muscles strength, balance and functional ability8.

Considering the available evidence, a trial of non-surgical care for 3-6 months, with excellent adherence to the specific advice and prescribed exercise program is recommended before considering a surgical intervention. In some circumstances, earlier progression to surgery may be recommended but there are currently no clear guidelines as to who should consider an early surgical option.

Always seek out a health professional with extensive experience and up-to-date knowledge in this area.



1. Griffin, D., Dickenson, E., O’Donnell, J et al. (2016). The Warwick Agreement on femoroacetabular impingement syndrome (FAI syndrome): an international consensus statement. British Journal of Sports Medicine, 50(19), pp.1169-1176.

2. Reiman, M. and Thorborg, K. (2015). Femoroacetabular impingement surgery: are we moving too fast and too far beyond the evidence? British Journal of Sports Medicine, 49(12), pp.782-784.

3. Frank, J., Harris, J., Erickson, B., Slikker, W., Bush-Joseph, C., Salata, M. and Nho, S. (2015). Prevalence of Femoroacetabular Impingement Imaging Findings in Asymptomatic Volunteers: A Systematic Review. Arthroscopy: The Journal of Arthroscopic & Related Surgery, 31(6), pp.1199-1204.

4. Zadpoor, A. (2015). Etiology of Femoroacetabular Impingement in Athletes: A Review of Recent Findings. Sports Medicine, 45(8), pp.1097-1106.

5. Griffin, D., Dickensen, E., Hobson, R., et al. (2018). Hip arthroscopy compared to best conservative care for the treatment of femoroacetabular impingement syndrome: a randomised controlled trial (UK FASHIoN). Osteoarthritis and Cartilage, 26, pp.S24-S25.

6. Mansell, NS., Rhon, DI., Meyer, J., Slevin, JM., Marchant, BG. (2018). Arthroscopic surgery or physical therapy for patients with femoroacetabular impingement syndrome: A randomized controlled trial with 2-year follow-up. American Journal of Sports Medicine, 46(6), pp.1306-1314.

7. Thorborg, K., Kraemer, O., Madsen, AD., Holmich, P. (2018). Patient-reported outcomes within the first year after hip arthroscopy and rehabilitation for femoroacetabular impingement and/or labral injury: The difference between getting better and getting back to normal. American Journal of Sports Medicine, 46(11), pp.2607-2614.

8. Freke, MD., Kemp, J., Svege, I., Risberg, MA., Semciw, A., Crossley, KM. (2016). Physical impairments in symptomatic femoroacetabular impingement: a systematic review of the evidence. British Journal of Sports Medicine 2016, 50(19), pp.1180.

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