Hip Impingement (FAI)
Femoroacetabular impingement (FAI) is abnormal contact between the ball and socket of the hip caused by extra bone. Repeated impingement during normal activity tears the labrum and damages the cartilage, eventually causing osteoarthritis in younger patients. Mr Shakir Hussain, Consultant Orthopaedic Surgeon at the Royal Orthopaedic Hospital Birmingham, treats end-stage FAI with hip replacement and hip resurfacing, and refers younger patients with preserved cartilage for joint-preserving hip arthroscopy.
What is femoroacetabular impingement (FAI)?
Femoroacetabular impingement (FAI) is abnormal contact between the femoral head-neck junction and the rim of the acetabulum during hip movement, caused by extra bone on one or both surfaces. There are three types: cam (extra bone on the femoral side), pincer (extra bone on the socket side), and mixed (both). Repeated impingement during normal activity tears the labrum and damages cartilage, eventually progressing to hip arthritis.
The hip is a ball-and-socket joint between the femoral head and the acetabulum. In a normal hip, the round femoral head rotates smoothly inside a deep socket through a wide range of motion. In FAI, the joint surfaces are shaped abnormally, so deep flexion or rotation produces premature bone-on-bone contact at the rim.
The three types of FAI describe where the extra bone sits:
- Cam impingement. The femoral head has lost its perfect roundness at the head-neck junction. Instead of a smooth concave waist, there is a convex bulge (sometimes called a "pistol grip deformity"). This aspherical head jams into the acetabular rim during flexion. Cam morphology is most common in young male athletes.
- Pincer impingement. The acetabular socket is over-deep or rotated backwards (retroversion), so its rim covers too much of the femoral head. The rim catches on the femoral neck during flexion. Pincer morphology has a slight female predominance.
- Mixed (combined) impingement. Most symptomatic patients have features of both types. The 2016 Warwick Agreement requires symptoms, signs, and imaging together before the diagnosis of FAI syndrome is made.
FAI is a leading cause of hip pain in young active adults. It is also the underlying reason a significant proportion of hip replacements done in patients under 50 are needed, because long-standing impingement progresses to secondary osteoarthritis.
What does hip impingement feel like?
Hip impingement typically causes groin pain on hip flexion and rotation, pain after sitting for long periods, clicking or catching during movement, and reduced internal rotation. Many patients are young active adults whose symptoms started gradually and worsened with sport. The classic clinical examination finding is a positive FADIR test (pain on flexion, adduction, and internal rotation).
FAI typically presents in active adults aged 20 to 50. Pain is mechanical and position-related rather than the constant ache of established osteoarthritis. Many patients first present with a labral tear caused by the underlying FAI.
Groin pain on flexion and rotation
Deep groin pain provoked by bending the hip up and rotating the leg inward, mirrored by the FADIR test in clinic. Patients often point to the groin with a "C" shape grip around the hip.
Pain after sitting
Stiffness and groin pain after sitting for long periods (driving, flights, desk work). Patients often need to stretch out and "uncrook" the hip before walking off.
Clicking, catching, or locking
Mechanical symptoms suggest a labral tear caused by the impingement. A torn labrum produces clicks during specific movements (sitting cross-legged, rotating the leg out).
Reduced internal rotation in flexion
The hallmark physical sign. With the hip and knee at 90 degrees, the affected hip cannot rotate inward as far as the other side. Mr Hussain checks this on examination.
Pain after sport
Cyclists, footballers, runners, dancers, and ice hockey players are commonly affected. Pain typically builds during sport and is worst the next day rather than at the time.
Long diagnostic delay
Many patients see several clinicians before FAI is diagnosed. Symptoms often misattributed to groin strain, sports hernia, or "muscular" hip pain before a properly oriented X-ray makes the diagnosis.
What causes hip impingement?
Cam impingement develops during late adolescent growth in response to high-impact loading: the cartilage at the top of the femur remodels into a thicker, aspherical shape. This is why cam morphology is heavily over-represented in young male athletes who trained intensively before age 12. Pincer impingement is more idiopathic, slightly more common in women, and often associated with acetabular retroversion or coxa profunda.
Cam and pincer FAI have different origins, even though their clinical effects converge into the same impingement syndrome.
The most important risk factors are:
- Adolescent high-impact sport (for cam). Elite male soccer players have cam morphology prevalence around 70 per cent. Ice hockey players 69 to 85 per cent. The peak vulnerability window is age 11 to 14 years, when the femoral head growth plate is still active and responds to load by thickening.
- Male sex (for cam). Mean alpha angle (the measurement that defines cam morphology) is around 60 degrees in male athletes versus 48 degrees in female athletes.
- Acetabular retroversion (for pincer). The socket rotates backwards instead of facing forward, so the front rim sticks out and catches the femoral neck.
- Coxa profunda (for pincer). A deeper than normal acetabular socket that over-covers the femoral head globally.
- Family history. Family clustering is documented and FAI is more common in first-degree relatives of affected patients.
- Slight female predominance (for pincer). Pincer morphology is more idiopathic and women are over-represented.
Asymptomatic cam morphology is common in athletes and does not always cause problems. The 2016 Warwick Agreement explicitly distinguishes between cam or pincer "morphology" (an imaging finding) and "FAI syndrome" (the clinical condition). Only the latter requires treatment.
How is hip impingement diagnosed?
The 2016 Warwick Agreement defines FAI syndrome by three criteria together: appropriate symptoms (groin pain related to movement and position), positive clinical signs (FADIR test, restricted internal rotation in flexion), and imaging findings of cam and/or pincer morphology. The most useful imaging measurement is the alpha angle on a 45-degree Dunn lateral view: values above 55 to 60 degrees support cam morphology.
Diagnosis depends on combining the clinical picture with the right radiographic views. Cam morphology is missed on AP-only views and only confirmed on a properly performed lateral hip view.
Investigations used in hip impingement:
- Clinical examination. Range-of-motion testing with the hip and knee at 90 degrees, looking for reduced internal rotation. FADIR test (flexion, adduction, internal rotation) provokes anterior impingement pain. FABER and Stinchfield tests help distinguish hip from spine pain.
- AP pelvic X-ray. Assesses for pincer features (cross-over sign indicating retroversion, posterior wall sign, ischial spine sign), joint space loss, and the Tönnis grade of any secondary osteoarthritis.
- Lateral hip view. A 45-degree Dunn view is preferred for measuring the alpha angle. Sensitivity 91 per cent and specificity 88 per cent for cam detection. Frog-leg lateral and cross-table lateral views are alternatives.
- Alpha angle measurement. The standard quantitative measure of cam morphology. Values over 55 to 60 degrees support cam-type FAI when combined with symptoms.
- MRI or MR arthrogram. Essential when joint preservation is being considered. Shows labral tears, cartilage damage, and bone marrow oedema. Gadolinium arthrogram improves sensitivity for subtle labral lesions.
- CT scan. Used for 3D bony morphology and surgical planning, particularly for measuring alpha angle in multiple planes.
Can hip impingement be treated without hip replacement?
Most patients start with activity modification, physiotherapy, and NSAIDs. For symptoms that do not settle, hip arthroscopy is the gold-standard joint-preserving operation in younger patients with preserved cartilage. The UK FASHIoN randomised trial (Lancet 2018) showed hip arthroscopy outperforms personalised physiotherapy at 12 months and continues to do so at 3-year follow-up. Hip replacement is reserved for end-stage cases with established secondary osteoarthritis.
Treatment for FAI sits on a spectrum from conservative care to hip arthroscopy to hip replacement. The right point on that spectrum depends on age, the state of the cartilage, and how the symptoms are affecting daily life and sport.
- Activity modification. Avoiding deep hip flexion sports during flare-ups (cycling, deep squats, full splits) can reduce symptoms substantially. Many patients can find a sustainable level of activity that minimises impingement.
- Targeted physiotherapy. Hip neuromuscular control programmes focusing on the deep stabilisers, gluteal strengthening, core stability, and posterior chain. International prehabilitation consensus 2025 supports structured protocols.
- NSAIDs. Topical and oral, used during symptom flares.
- Image-guided intra-articular injection. Diagnostic (confirms the hip as the source of pain) and therapeutic (provides several months of symptom control). A positive response to injection helps predict response to surgery.
- Hip arthroscopy with osteoplasty and labral repair. The gold-standard joint-preserving operation for symptomatic FAI in younger patients with preserved cartilage. Performed under traction through two or three small portals: the cam deformity is removed (femoral osteoplasty), the acetabular rim is trimmed where appropriate (acetabuloplasty), and any labral tear is repaired or reconstructed. The UK FASHIoN trial showed hip arthroscopy produces clinically significant improvement compared with conservative care.
Hip arthroscopy is a hip preservation subspecialty. Mr Hussain does not personally perform hip arthroscopy and refers appropriate younger patients to a dedicated hip arthroscopist with the volume and case-mix that produces the best results. The UK has a small number of high-volume hip arthroscopy centres.
If you take prescribed medication, particularly blood-thinners or anti-inflammatories, please review Mr Hussain's patient guide on medications to pause before hip or knee surgery as you approach a surgical decision.
When should I consider hip replacement for FAI?
Hip replacement is the right next step when FAI has progressed to symptomatic secondary osteoarthritis (Tönnis grade 2 or 3) with established cartilage loss. At that point, joint preservation through hip arthroscopy is no longer effective. Long-standing FAI is one of the main reasons patients in their 40s and 50s need hip replacement.
The treatment decision is age-and-cartilage driven. For younger patients with preserved cartilage, hip arthroscopy is the right operation and Mr Hussain refers to a specialist hip arthroscopist. For patients with established secondary arthritis, the decision shifts to hip replacement or hip resurfacing.
Markers Mr Hussain looks for when deciding that hip replacement (or resurfacing) is appropriate:
- Imaging showing Tönnis grade 2 or 3 secondary osteoarthritis (joint space loss, sclerosis, cysts)
- Failed prior hip arthroscopy without lasting symptom improvement
- Pain that wakes you at night, more than twice a week
- Walking distance reduced and pain affecting work, sport, or daily life
- Age generally over 45, where the cartilage damage typically exceeds what hip arthroscopy can address
- Imaging-confirmed cam morphology with established cartilage damage (a strong predictor of THR within five years)
Earlier intervention with joint preservation (before cartilage damage develops) gives the best long-term hip preservation, which is why timely diagnosis matters. The decision tree between arthroscopy and replacement is driven primarily by the state of the cartilage on imaging.
Private consultations with Mr Hussain are available at the Royal Orthopaedic Hospital, Priory Hospital Edgbaston, and Harborne Hospital. See the private consultation and surgery fees page for pricing and recognised insurers, or book a consultation directly.
Hip replacement or hip resurfacing for end-stage FAI: which is right for me?
For patients with end-stage FAI-derived secondary osteoarthritis, Mr Hussain offers both total hip replacement and hip resurfacing. Total hip replacement suits most patients, particularly those over 60, women, or with reduced bone quality. Hip resurfacing is the bone-preserving option best suited to active men under 60 with end-stage FAI-related arthritis, good bone quality, and an adequate femoral head size.
Patients with FAI-derived arthritis are often younger and more active than the typical osteoarthritis patient, which makes the resurfacing versus replacement discussion particularly relevant. Mr Hussain trained in both techniques and tailors the choice to the individual.
Total Hip Replacement
The femoral head (including any cam deformity) is removed and replaced with a prosthetic head and stem. The acetabular surface is replaced with a prosthetic cup. Identical technique to primary OA THR, with similar outcomes in this patient group.
- Suitable for nearly all patients with end-stage FAI arthritis
- Modern implants designed for 25 to 30 year lifespan
- Removes any underlying cam deformity completely
- Cemented or uncemented fixation tailored to bone quality
- Preferred for older patients, women, or those with osteoporosis
Hip Resurfacing
Only the surface of the femoral head is capped; the socket is lined. Any cam deformity is reshaped as part of femoral preparation or replaced by the resurfacing component. The patient profile (active male, end-stage FAI arthritis, preserved femoral bone stock) overlaps closely with the classic resurfacing candidate.
- Best for active men under 60 with good bone quality
- Requires a femoral head measuring 48 millimetres or larger
- Lower dislocation risk than total replacement
- Particularly suited to patients who want to return to demanding sport
- Easier conversion to a standard replacement if ever needed
For a more detailed patient comparison covering recovery, return to sport, and implant choice, read Mr Hussain's patient guide on hip resurfacing versus total hip replacement. Active patients may also find returning to sport after hip resurfacing useful for setting realistic expectations.
How successful are the treatments for hip impingement?
Hip arthroscopy gives clinically significant pain and function improvements that are durable to at least 10 years in patients with preserved cartilage. The UK FASHIoN randomised trial established hip arthroscopy as the gold-standard joint-preserving operation. For end-stage FAI-related arthritis, total hip replacement and hip resurfacing produce outcomes comparable to primary osteoarthritis surgery.
Treatment outcomes differ markedly by stage. Joint preservation in younger patients with intact cartilage produces excellent long-term results in the majority. Hip replacement in end-stage cases gives similar pain relief and function gains to those seen in primary osteoarthritis patients.
Expertise in end-stage hip impingement treatment in Birmingham
Consultant at the Royal Orthopaedic Hospital
Mr Hussain practises at the Royal Orthopaedic Hospital Birmingham, one of the largest specialist orthopaedic hospitals in Europe, alongside Priory Hospital Edgbaston and Harborne Hospital.
3,000+ arthroplasty cases
From a total of more than 5,000 procedures performed, giving the operative volume and case complexity required for consistently excellent outcomes. Read more about Mr Hussain's training and background.
British Hip Society Travelling Fellowship
Trained at ENDO-Klinik Hamburg under Professor Thorsten Gehrke and Professor Mustafa Citak, the international reference centre for complex hip surgery.
Both hip replacement and hip resurfacing
Many surgeons offer only one technique. Mr Hussain trained in both, allowing the choice to be tailored to the individual patient rather than to a single technique.
Doctify Outstanding Patient Experience 2024, 2025, and 2026
Awarded in three consecutive years, recognising consistently high patient-reported outcomes and communication.
4.98 out of 5 from verified reviews on Doctify. Outstanding Patient Experience Award 2024, 2025, and 2026.
Frequently asked questions about hip impingement
For more questions about surgery, recovery, fees, and what to expect, see the full frequently asked questions page or read recent patient testimonials.