Causes of Femoroacetabular  Impingement (FAI) or Femoroacetabular Impingement Syndrome (FAIS): What is it?

causes of Femoroacetabular Impingement, cam impingement, pincer impingement
causes of Femoroacetabular Impingement, cam impingement, pincer impingement

What is FAI or FAIS?

What are the causes of femoroacetabular impingement and what is it? You may have been diagnosed with femoroacetabular impingement, femoroacetabular impingement syndrome or hip impingement, or if you’ve seen more than one health professional, you may well have been given two different diagnoses.   Or maybe you’ve just been searching the internet to make your own diagnosis and are confused about the different terminology. Let’s explore the different diagnoses and causes of FAIS. 

Figure 1: Basic Anatomy of the Hip as Viewed From the Front

Femoroacetabular impingement (FAI) refers to early contact (impingement) between the bones of the hip during movement, due to a variation in shape of the   bones of the hip joint (femur (thigh bone) and/or acetabulum (hip socket)).  This may result in reduced range of motion (stiffness) and, in a relatively small percentage of peoplehip pain associated with extra forces being placed across joint structures such as the acetabular labrum and cartilage (Figure 1) 

What is Femoroacetabular Impingement Syndrome (FAIS)?

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: Bone Changes that are Attributed to The Causes of FAI

3 different bony shape configurations may be associated with and result in the cuases of FAI

1. Cam Morphology (shape) – Cam FAI

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

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.   



2. Pincer Morphology (shape) – Pincer FAI 

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

Pincer morphology or pincer-type morphology (Figure 3) refers to a variation in shape or orientation of the acetabulum.  of the whole femoral head by the acetabulum may occur when the socket is deeper than average (global overcoverage) or the edge of the socket may sit lower at 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. 

3. Cam & Pincer Morphology – Mixed FAI 

Cam & Pincer Morphology - Mixed FAI

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

What are the Causes of 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. 

Figure 5. The growth plates across which stresses occur that are thought linked to the development of FAI related bone growth

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.

How Can A Hip Pain Professional Help?

An interview and physical assessment with a Hip Pain  Professional  can help clarify which factors may have contributed to the development of your hip pain. A specific, personalised plan can then be actioned to help you overcome your problem. 


  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 normalAmerican Journal of Sports Medicine46(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 201650(19), pp.1180. 

This blog was written by Dr. Alison Grimaldi, with contribution from one of our Hip Pain Professionals, Kirsty McNab, experienced physiotherapist.

Dr. Alison Grimaldi is a globally recognised expert physiotherapist, researcher, and educator, who has over 30 years professional clinical experience helping patients recover from a wide range of hip and pelvic conditions.

Dr Alison Grimaldi - Hip Pain Professional

Dr. Alison Grimaldi  BPhty, MPhty(Sports), PhD, FACP 

Dr Grimaldi has completed Bachelor of Physiotherapy, Master of Physiotherapy and  Doctor of Philosophy (Physiotherapy) degrees. She is a fellow of the Australian College of Physiotherapists, Practice Principal of PhysioTec Physiotherapy, an Australian Sports Physiotherapist , an Adjunct Senior Research Fellow at the University of Queensland, as well as an author and global educator. Her passion is helping people with hip pain, and educating other health professionals around how to help more people with hip pain.

Check Out More You Can Read on this Area at HipPainHelp:

Anterior Hip Pain: scroll down to the “Joint Related” section and read more about the front (anterior) of the hip joint and how it can be invovled in FAI, FAIS and hip pain.

Groin Pain: scroll down to the “Joint Related” section and read more about the groin region and the hip joint and how they can be invovled in FAI, FAIS and hip pain.

Make sure you check out our next blog which examines how you know if you have FAI syndrome and how a diagnosis is reached.


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