Masterclass: Femoro-acetabular impingement (FAI) – Part I

In the first part of this 2-part article, Chris Mallac looks at the surgery and rehab options for Femoroacetabular impingement (FAI), an increasingly common musculoskeletal disorder that can affect young to middle aged athletes. In part two, Chris will describe in detail the post-operative rehabilitation period required to take an athlete back to full competition

2018 Rotherham United’s Ryan Williams in action with Scunthorpe United’s Rory McArdle Action Images/Ed Sykes


The idea that abnormal hip morphology was a possible cause of hip degeneration and arthritis was initially suggested by Stulberg et al (1975), when they identified the ‘pistol grip deformity’ in the xray findings of abnormal head of femur (HOF) clearance from the acetabulum in patients with hip arthritis1. This led to the idea that mechanical impingement may be one of the underlying causes of hip degeneration and hip joint pain.

Over the next few decades, with improvements in radiology and hip joint surgery, and more sophisticated clinical assessment skills, it was recognised that a number of atypical hip joint morphologies commonly existed that may give rise to hip joint pain in athletes.

In the late 1990’s, Mayer et al (1999)2 were the first to use the term ‘Femoroacetabular impingement’ or ‘FAI’ to describe the morphology of the hip as being a causative factor in hip joint arthritis, and the concept of FAI was further pioneered by Reinhold Ganz3 and colleagues during the early 2000s.

An FAI is a genuine complication for an athlete, particularly for young male athletes, as it can not only lead to hip joint pain and labral and cartilage damage, but it can also give rise to other athletic injuries such as osteitis pubis and inguinal hernias due to the detrimental effect that a lack of hip joint rotation plays on pelvis stress injuries4.

The purpose of this Rehabilitation Masterclass is to explain the relevant anatomy and biomechanics of the hip joint, explain the pathogenesis of FAI and offer management ideas and an in depth overview of the rehabilitation of the hip joint post-FAI surgery.

Figure 1: Hip anatomy

Figure 1: Hip anatomy

Relevant anatomy and biomechanics

The hip joint is a ball and socket joint that enjoys a wide range of movement into all planes of motion. It can also accommodate a large weight bearing force during locomotion (see figure 1). The ‘normal’ hip morphology is identified as5;

  1. The femoral head is slightly more than half a sphere.
  2. The femoral neck is cylindrical, narrowest in the midpoint and widest laterally, and attaches the head to the shaft.
  3. The acetabulum is cup shaped.
  4. The labrum is triangular in cross section with its base attached to the acetabular rim.

Usually a space is maintained between the femoral head neck junction and the acetabular rim, and this provides unhindered hip movement. This is achieved with an offset between the anterior and superior surface of the femoral neck, and the corresponding surface of the femoral head. The normal head neck offset is 9 mm6.


Abnormal morphology of the hip joint can lead to impingement type symptoms. A deviation away from the normal alignment of the hip can lead to a mechanical blockage between the acetabular rim and/ or head-neck of femur. This deviation away from the normal may be due to inherent geometry faults in the patient’s hip joint complex. Inherent factors that may predispose to impingement are7:

  • Pistol grip deformity of the femoral neck (see figure 2).
  • Reduced femoral neck offset in anterolateral aspect of head neck junction (see figure 3).
  • Reduced femoral anteversion (see figure 4). Other factors include:
  • Loss of spherecity in the femoral head.
  • Excessive coverage of acetabulum
  • Retroversion of acetabulum (causes over-coverage and restricts flexion of the femoral neck).
  • Coxa profunda deformity floor of the acetabulum touches the ilio-ischial line) increases the depth of the acetabulum.

Pistol grip deformity of the femoral neck

Figure 2: Pistol grip deformity of the femoral neck

Reduced femoral neck offset in antero-lateral aspect of head neck junction

Figure 3: Reduced femoral neck offset in antero-lateral aspect of head neck junction

Reduced femoral anteversion

Figure 4: Reduced femoral anteversion

FAI as an injury

Although some examples of hip joint impingement are caused by inherent hip joint abnormalities, FAI as an injury is an anatomical mal-alignment between the head and neck of femur and the acetabulum. This causes compression of the labrum and articular cartilage during hip flexion manifesting as anterior hip pain and progression to early osteoarthritis of the hip. It is a condition that seems to affect younger more active people, particularly male. For example, in a radiological study of 155 young active patients 18-50 years of age with complaints of hip pain, it was found that a staggering 87% of subjects had evidence of an FAI8.

The natural progression of an FAI to full blown hip joint disease has been postulated by McCarthy et al (2001)9. They propose that initially an FAI causes excessive loads on the acetabular labrum at extreme ranges of movement. As the labrum frays along the articular margins, this progresses to a more consequential tear of the labrum. The articular cartilage then delaminates away from the articular margin adjacent to the labral lesion. This then leads to abnormal shear forces across the joint, which over a long term, causes more global labral and articular degeneration.

Furthermore, more recent research suggests a strong link between FAI and other athletic groin pain syndromes, in  particular athletic pubalgia . Economopoulos et al (2014) found that in 43 patients who underwent surgery for athletic pubalgia, 86% of them had evidence of a FAI in at least one hip, with Cam lesions being the most prevalent (see below for Cam lesion)10. This highlights the possible interaction between forces acting across the pelvis and hip joint. Hammoud et al (2014) provide an in-depth discussion on how a FAI can possibly lead mechanical compensation patterns, which can in turn lead to a host of sports injuries such as osteitis pubis, sacroiliac joint problems, posterior hip impingement and proximal hamstring syndrome just to name a few11. In the context of FAI, two main manifestations of FAI have been described. These are ‘Cam’ and ‘Pincer’ lesions (see figure 5).

Cam lesions

The Cam lesion is attributed to an asymmetric or nonspherical portion of the femoral head or neck abutting against an acetabular rim. This abutment is most evident in flexion and internal rotation where a shear force occurs along the anterolateral edge of the acetabulum. This deformity decreases the head neck offset, increases the femoral head radius of curvature and results in relative retroversion of the femoral head.

The smooth motion between the femoral head and acetabulum is lost, which causes an outside-in abrasion of the acetabular cartilage with avulsion from the labrum and subchondral bone (delamination). This can lead to separation of the cartilage from the antero superior aspect of the labrum12. Labral injuries are more likely to affect the anterior region possibly due to its poor blood supply. Also, it is a mechanically weaker area of the labrum and the anterior labrum experiences more loads and shear than other areas of the labrum13.

Cam lesions are common in young active men with a ratio of around 25% in asymptomatic hips14, and up to 78% in symptomatic hips15. It is more prevalent in men than in women at a ratio of 3:116. It is possible that a major factor in the development of a Cam lesion is a slipped capital femoral epiphysis where posterior displacement of the capitis leaves a prominence of the anterior neck, resulting in severely limited internal rotation of the hip17. Furthermore pistol grip deformities can lead to more subtle forms of aspherical head of femurs, and these have been associated with early onset osteoarthritis in adults.

Causes of Cam lesions
1. Idiopathic
2. Developmental (Non spherical femoral head, Coxa vara)
3. Traumatic (malunited femoral neck fracture; post traumatic retro version of the femoral head
4. Childhood orthopaedic conditions (Perthes disease; slipped capital femoral epiphysis)
5. Iatrogenic (femoral osteotomy)

Pincer lesions

Pincer lesions create repeated contact between a normal femoral head neck junction and an over covered acetabular rim. This results in multiple cleavage planes in the labrum and subsequent labral degeneration, intrasubstance ganglion formation, ossification of the acetabular rim, and ultimately a deepening of the acetabulum further compounding the pincer lesion.

Unlike in cam lesions, the chondral damage associated with pincer lesions is more circumferential at the anterior and supero-lateral acetabular rim, where force transmission is larger and as a ‘contrecoup’ lesion in the posterior capsularlabral junction due the ‘levering’ out the femoral head along the fulcrum created by the anterior pincer lesion whilst the hip is flexing18 19.

Retroversion of the pelvis can also cause the acetabular rim to roll backward. It is to be noted that spinal deformities like scoliosis or kyphosis can cause pelvic rotation and functional retroversion of the acetabulum which may act as a pincer lesion. There can also be an extra bit of bone along the anterior rim of the acetabulum known as an ‘os acetabulum’20. Pincer lesions are more common in active females21.

Causes of Pincer lesions
1. Idiopathic
2. Developmental (retroverted acetabulum; Coxa profunda; os acetabuli; protrusio acetabuli; chronic residual dysplasia of the acetabulum)
3. Traumatic (post traumatic deformity of the acetabulum)
4. Latrogenic (over correction of retroversion in dysplastic hips)

Mixed lesions

Rarely do cam and pincer lesions occur in isolation. Most type of FAI occurs from mixed cam and pincer pathology at the anterior femoral neck and anterior superior acetabular rim. However, Cobb et al. (2010) argued against this after analyzing the morphology of 60 acetabula with CT scans22. They reported that acetabula with cam hips were shallower than normal hips, which in turn were shallower than pincer hips. Hence, they concluded, that cam and pincer lesions were different patho-anatomic entities with cam hips being shallow while the pincer hips are deeper than normal hips.

Figure 2: Cam and pincer lesions

Figure 5: Cam and pincer lesions

Signs and symptoms of FAI

FAI in the athlete may present as insidious onset groin pain worsened by hip flexion and hip flexion with internal rotation type movements. It is more common in male athletes due to the male predisposition towards Cam impingements. Sports with a higher incidence of FAI are sports that involve a lot of forced flexion with internal rotation such as treading water in water polo, breaststroke kicking, football sports such as soccer and AFL, rugby, field hockey, combat sports such as wrestling and Brazilian Jujitsu and ice hockey.

The athlete themselves may aid the diagnosis by offering information regarding childhood diseases that can lead to abnormal head neck junctions. Conditions such as Perthes disease (3-12 year olds) or slipped capital femoral epiphysis (SCFE) (12-14 year olds) may alert the sports medicine practitioner that an underlying FAI may be present.

  1. The typical collection of signs and symptoms are; Insidious onset of symptoms in active young and middle aged adults.
  2. Groin pain associated with activity and no prior history of trauma.
  3. Inability to perform activities such as high hip flexion or prolong sitting.
  4. May hurt moving from sit or squat to standing.
  5. Sharp stabbing pain with movements such as aggressive turning or pivoting.
  6. Painful clicking, locking or instability from a labral tear secondary to undiagnosed FAI.
  7. Reduced range of motion especially flexion, adduction and internal rotation. Often athletes notice that they are not as flexible as their team-mates.

Objective signs

Due to inherent hip muscle weakness associated with the hip joint pathology, the patient may often show poor proximal pelvic control during single stance tests and a Trendelenburg sign may be present. Passive range of motion will be limited compared with the non-affected side, particularly hip flexion (usually less than 110°) and hip internal rotation (less than 15°). A number of tests have been proposed that may assist the sports medicine practitioner in clinically identifying an FAI in an athlete.

Impingement tests

FADDIR sign23 – This test involves passive hip joint flexion, adduction and internal rotation (Flexion-ADDuction- Internal Rotation). Pain is felt deep in the anterior hip/groin area and the assessor may feel a mechanical block to movement (see figure 6).

Figure 3: FADDIR test

Figure 6: FADDIR test

Posterior inferior impingement test24 – With the hip in hyper extension, passively by hanging the leg over the end of the bed, the affected hip is passively externally rotated. The test is positive if it elicits similar pain as complained by the patient.

FABER test25 – With the hip in flexion, abduction and external rotation (Flexion- ABDuction, External Rotation), abutment of the labrum and cartilage also can occur (see figure 7). The test is positive if it elicits similar pain as complained of by the patient,or if the distance between the lateral knee and the examtable differs between the symptomatic and contra lateral hip.

Figure 4: FABER test

Figure 7: FABER test


Anterior/posterior (AP) views

Well centred AP views are necessary to view Cam lesions (due to flattened headneck junction, pistol grip deformity), any degenerative hip joint changes (sclerosis, osteophytes, reduced joint space) and a profunda socket. AP views will also define the acetabular version such as the crossover sign. This may also be seen indicative of a prominent anterior acetabular wall (pincer impingement). AP views will show acetabular retroversion, acetabular depth, os acetabulum and/or herniation pit in the neck of femur and/or impingement cysts may be present.

Standard Lateral View

This will identify subchondral bone sclerosis and cyst formation of the anterior acetabular rim.

Cross table lateral views/frog lateral views

This can be done in 10-15° internal hip rotation to compensate for femoral anteversion. This is best to view the Cam impingement lesion and reactive fibro cystic changes of the anterior head-neck junction. Other radiological measurements that can be made include head-neck offset26, alpha angles27, and anterior offset ratio28.

Magnetic resonance imaging (MRI) and arthrography (MRA)

MRI is useful if hip labral pathology is suspected in conjunction with FAI, and may show articular cartilage thinning. Increased signal on T2-weighted images in the anterior acetabulum may be indicative of subchondral edema, which may be incorrectly interpreted as a stress fracture of the anterior inferior iliac spine. It is more likely indicative of a subchondral stress reaction in the anterior acetabulum due to failure of the subjacent articular surface. Thus, anterior acetabular subchondral edema is suggestive of significant articular pathology29.

MRA is much more sensitive for viewing intra-articular pathology including labral lesions. This can also be useful, as the gadolinium injection associated with the MRA also has local anaesthetic that can be used as a diagnostic block for intra-articular joint pathology.

Computerised tomography (CT)

CT 3D reconstructions are useful to assess the shape of the femoral head neck junction and this is considered useful for surgical resection. A new CT view has been developed by Pritchard and O’Donnell to demonstrate FAI lesions30. CT scanning in the position of discomfort (POD view) allows symptomatic Cam deformity to be accurately mapped in a 3-dimensional reconstruction (see figure 8). This procedure helps in diagnosis of the problem by visualisation and subsequent planning for surgical resection.

Figure 7: A 3-dimensional reconstruction of Cam lesion

Figure 8: A 3-dimensional reconstruction of Cam lesion


Due to the mechanical nature of FAI, conservative management along with physiotherapy usually fails as ultimately, the impingement of bone on bone will prove to be restrictive to the athlete. Often conservative programs are only effective as acting as identifiers of FAI and then to prevent further mechanical symptoms by avoiding the offending movements. Gym based movements such as squats past 60° hip flexion, Bulgarian squats and leg press are avoided, or modified to be performed in external rotation. However, ultimately if pain persists then surgery is the only way to manage these problems.

Prior to the development of more advanced hip joint surgery techniques, athletes with ‘FAI’ would have lived and performed within the confines of their symptoms. They simply would have avoided movements that created hip joint pain and if this proved too difficult to avoid, they simply would have ceased to compete.

In this current day, advancements in not only diagnosis but also both open and arthroscopic hip joint surgery has allowed simple management of these conditions. Therefore surgery is often recommended for the young athlete with FAI. The purpose of surgery is to remove the mechanical block of the proximal femur against the acetabulum and to also address any associated labral pathology and articular damage. Open and closed arthroscopic techniques have been described.

Prior to 2001, direct access into the hip joint was very difficult due to the risk of avascular necrosis, therefore most hip joint impingements were managed with labral debridement and/or labral reattachment. In 2001, Ganz et al described a surgical technique that involved surgical dislocation with a ‘trochanteric flip osteotomy’ that preserved the blood flow to the femoral head31. This avoided the serious side effect of avascular necrosis of the femoral head associated with hip joint dislocation procedures. This technique led to better identification of FAI as a genuine hip joint pathology and correcting this surgically was now possible.

Figure 5: AP xray showing a prominent femoral neck

Figure 9: AP xray showing a prominent femoral neck

Figure 6: Alpha angle of the head neck of femur

Figure 10: Alpha angle of the head neck of femur

Open surgery

This involves dislocation of the hip anteriorly using a trochanteric flap osteotomy. This allows a full view of the head neck junction. Any osseous impingement visualised can then be excised to restore the normal contour of the femoral neck junction. Retroversion of the acetabulum can be managed with resection of the prominent anterior acetabular rim. Any labral tear should be treated with excision or repair.

Ongoing groin pain following an open procedure can be caused by insufficient restoration of the femoral or acetabular morphology, advanced joint degeneration, joint space narrowing, ongoing cartilage damage, or scar adhesions between the hip joint capsule and femoral neck resection, and between the labrum and hip joint capsule32(31). If this scarring is thick then they may interfere with motion between the femur and acetabulum. These can impinge during flexion and internal rotation movements.

Arthroscopic repair

Arthroscopic intervention is now becoming increasingly more popular. Two areas can examined under arthroscopy33;

  1. Central compartment – the structures medial to the labrum.
  2. Peripheral compartment – the structures outside the labrum but inside the hip capsule.

The arthroscopic procedure is performed with the patient lying supine or lateral decubitus. A traction device is used to separate the femur from the acetabulum to visualise the central compartment. Traction is not needed for peripheral compartment procedures. The joint is filled with saline and two to three portals are used (anterior, anterolateral and/or posterolateral).

The procedures achieved by hip arthroscopy at present include;

  • Debridement of labral tear
  • Repair of torn labrum
  • Chondroplasty of acetabular lesions
  • Drilling or micro fracture of acetabular lesions
  • Acetabular rim excision (pincer lesions)
  • Debridement of acetabular chondral delamination
  • Capsular excision
  • Femoral osteochondroplasty

Philippon et al (2007) found that at 93% of athletes returned to competitive sport following arthroscopically managed FAI34. The small number who did not had advanced OA changes at the time of surgery. Conclusion FAI is an increasingly more common musculoskeletal disorder affecting the young to middle aged athlete that can lead to insidious onset groin pain and damage the hip joint labrum and lead to early OA changes. It is common in sports that require hip flexion and hip rotation positions as part of the skill execution. Cam and pincer lesions are the two varieties of FAI lesions and these can combine to form double lesions. Conservative management usually fails in the young athlete to completely manage their pain, therefore surgery, either open or arthroscopic is usually the recommended treatment. Post operative rehabilitation is addressed in part II of this series.

  1. The Proceedings of the Third Open Scientific Meeting of the Hip Society. St Louis, MO: CV Mosby;1975:212–2.
  2. Ann Surg. 2008;4:656-665
  3. Clin Orthop Relat Res. 2003;417:112–20
  4. Clin Ortop. 1999; 363:93–9
  5. Curr Rev Musculoskelet Med. 4:23–32
  6. Curr Orthop. 2008;22:300–10
  7. Curr Rev Musculoskelet Med. 4:23–32
  8. Clin Orthop Relat Res. 2010 October; 468(10): 2710–2714
  9. Clin Orthop Relat Res. 2001 Dec;(393):25-37
  10. Sports Health. 2014. 6(2); pp 171-177
  11. Sports Health. 2014. 6(2); pp 108-118
  12. J Bone Joint [Br]. 2001;83-B:171–6
  13. Curr Rev Musculoskelet Med. 2009; 2: 105– 117
  14. Arthritis Care & Res. 2010;62 (9): 1319-1327
  15. J Bone Joint Surg Am. 2010 Oct 20;92 (14):2436-44
  16. Clin Orthop Relat Res. 2009;467:739-746
  17. J Surg Orthop Adv. 2004;13(1):20-23
  18. Radiology 2006;240(3):778-85
  19. J Bone Joint Surg Br. 2005;87-B:1012–8
  20. Hip Int. 2006;16(4):281-286
  21. Arthritis Rheum. 2011;63 Suppl 10 :1077
  22. Clin Orthop Relat Res. 2010; 468:2143–51
  23. Knee Surg Sports Traumatol Arthrosc. 2007;15:1041–7
  24. J Bone Joint Surg Br. 2005;87- B:1459–62
  25. Clin Sports Med. 2006;25(2):299–308
  26. Clin Orthop Relat Res. 2007;462:115–21
  27. Clin Orthop. 2004;429:170–7
  28. Hip Int. 2001;11:37–41
  29. Sports Health. 2010. 2(4). pp 321-333
  30. Singh PJ, PritchardM, O’Donnell J. The POD CTscan—a betterway to demonstrate FAI and early hip degeneration. Annual Scientific Meeting. International Society for Hip Arthroscopy; Oct 2009
  31. The J of Bone and J Surgery. 83-B (8), pp1119-1124
  32. Tech Orthop. 2005;20:156–62
  33. Knee Surg Sports Traumatol Arthrosc (2007) 15:908–914.
Share this
Follow us