Orthopaedic Surgery - Femoroacetabular Impingement

Basics

Femoroacetabular impingement (FAI) results from abnormal contact between the femoral head-neck junction and the acetabular rim during hip motion.

Repeated impingement can produce hip pain and functional limitation, particularly in young and active patients.

Over time, abnormal contact may damage the acetabular labrum and adjacent articular cartilage and may contribute to the development of hip osteoarthritis.

Classification

FAI is classified according to whether the predominant structural abnormality arises from the femur, acetabulum, or both.

The three principal patterns are cam, pincer, and mixed impingement.

Cam Impingement

Cam impingement results from an abnormal contour of the femoral head-neck junction.

Instead of maintaining a smooth spherical transition between the femoral head and neck, the proximal femur develops a prominence that contacts the acetabular rim during hip flexion and rotation.

Femoral Abnormalities

Associated findings may include a decreased femoral head-neck ratio, reduced head-neck offset, femoral neck retroversion, and an aspherical femoral head.

These abnormalities reduce clearance between the femur and acetabulum.

Mechanism of Damage

During flexion and internal rotation, the abnormal femoral prominence enters the acetabulum and may shear the adjacent labrum and cartilage.

Repeated loading can lead to progressive chondrolabral injury.

Pincer Impingement

Pincer impingement results from excessive acetabular coverage of the femoral head.

The acetabular rim contacts the femoral neck earlier than normal during hip motion.

Acetabular Abnormalities

Potential causes include acetabular retroversion, acetabular protrusio, coxa profunda, and excessive focal or global acetabular rim coverage.

Mechanism of Damage

Repeated contact between the acetabular rim and femoral neck may damage the labrum.

A countercoup cartilage lesion may also develop posteriorly in some patients because the femoral head is levered within the socket.

Mixed Impingement

Many patients demonstrate both femoral and acetabular abnormalities.

This is referred to as mixed cam-pincer impingement.

Epidemiology

Radiographic morphology associated with FAI is common even among people who have no hip symptoms.

Reported prevalence of cam-type morphology in asymptomatic individuals has ranged from approximately 5–57%, while pincer-type features have been reported in approximately 29–67%.

Therefore, imaging abnormalities alone do not establish symptomatic FAI.

Symptomatic Patients

Among patients undergoing surgery for symptomatic FAI, one series found approximately 47% with predominantly cam impingement, 8% with pincer impingement, and 45% with combined cam and pincer abnormalities.

Etiology

Athletic Participation

High-impact athletic activity during skeletal development may contribute to the development of cam morphology.

Sports commonly associated with repetitive hip loading include soccer, basketball, and hockey.

Pediatric Hip Disease

Residual deformity after childhood hip disorders can predispose to FAI.

A particularly important example is slipped capital femoral epiphysis (SCFE).

Other developmental abnormalities can also alter the relationship between the femoral head, neck, and acetabulum.

Trauma

Previous injury to the proximal femur or acetabulum may alter hip anatomy and result in secondary impingement.

Genetics

Genetic and developmental factors may influence proximal femoral and acetabular morphology and contribute to susceptibility.

Diagnosis

Signs and Symptoms

The typical patient reports deep groin pain, although pain may also be felt in the buttock or lateral hip.

Symptoms are often associated with activities requiring hip flexion, rotation, or prolonged sitting.

Groin Pain

Groin pain is the classic presentation.

Patients may describe a deep ache or sharp pain during squatting, running, pivoting, or sitting in a low chair.

Gluteal Pain

Some patients localize discomfort posteriorly around the gluteal region rather than directly in the groin.

Lateral Hip Pain

Pain over the greater trochanter may develop secondarily because altered hip mechanics can overload the abductor tendons or trochanteric bursa.

Pain With Hip Flexion

Patients commonly report difficulty with deep hip flexion.

Activities such as moving from standing to sitting, getting into a car, squatting, or bringing the knee toward the chest may aggravate symptoms.

Mechanical Symptoms

Clicking, catching, popping, or locking may occur.

These symptoms often suggest associated labral or chondral pathology.

Physical Examination

Hip Flexion

Hip flexion may be reduced because of pain or mechanical impingement.

Internal Rotation

Loss of internal rotation, particularly with the hip flexed, is common.

The restriction may be caused by bony morphology, pain, or both.

Resting Position

The lower extremity may rest in slight external rotation in some patients.

Provocative Tests

Anterior Impingement Test

The anterior impingement maneuver is performed by placing the hip in flexion, adduction, and internal rotation.

Reproduction of the patient’s groin or hip pain represents a positive test.

This maneuver is commonly referred to as the FADIR test.

Posterior Impingement Test

The posterior impingement test places the hip in extension and external rotation.

Reproduction of hip or groin pain suggests posterior impingement or related intra-articular pathology.

Imaging

Plain Radiographs

Plain radiographs are essential for evaluating hip morphology.

A standard assessment commonly includes an AP pelvis view together with lateral views of the proximal femur.

AP Pelvis

The AP pelvis view is used to assess acetabular coverage, joint-space preservation, femoral head shape, acetabular version, and degenerative changes.

Cross-Table Lateral View

An axial cross-table lateral radiograph can demonstrate abnormalities of the anterior femoral head-neck junction.

Frog-Leg Lateral View

A frog-leg lateral view may help identify loss of the normal femoral head-neck offset or a cam-type prominence.

False-Profile View

For a false-profile view, the patient stands with the pelvis rotated approximately 65° relative to the imaging plate.

This view is useful for assessing anterior acetabular coverage of the femoral head.

MRI

MRI can evaluate both osseous morphology and associated soft-tissue abnormalities.

It is particularly useful for identifying labral tears and articular cartilage damage.

Femoral Head-Neck Anatomy

MRI can also characterize the contour of the femoral head-neck junction and help define cam morphology.

Excluding Other Disease

MRI is useful for excluding alternative causes of hip pain, including avascular necrosis of the femoral head.

Pathological and Radiographic Findings

Cam Morphology

A cam lesion may produce a characteristic pistol-grip deformity, with loss of the normal concavity of the femoral head-neck junction and an aspherical femoral head.

Pincer Morphology

Findings suggesting pincer-type impingement include acetabular protrusio, coxa profunda, and acetabular retroversion.

Crossover Sign

On an appropriately positioned AP pelvis radiograph, acetabular retroversion may produce a crossover sign, in which the projected anterior acetabular wall crosses over the posterior wall.

Alpha Angle

The alpha angle is one method used to quantify asphericity at the femoral head-neck junction.

It is formed by a line through the center of the femoral head and neck and a second line extending from the center of the femoral head to the point where the anterior or anterolateral head-neck contour departs from the expected spherical outline.

Historically, lower thresholds such as approximately 42° have been described, although interpretation should be made in conjunction with symptoms, examination, and the imaging technique used.

Differential Diagnosis

Hip Osteoarthritis

Degenerative arthritis may produce groin pain, restricted internal rotation, and stiffness similar to FAI.

Radiographs help determine the degree of joint-space loss.

Trochanteric Bursitis

Greater trochanteric pain syndrome produces predominantly lateral hip pain and tenderness rather than classic deep groin pain.

Gluteal or Adductor Tendinopathy

Tendon disorders around the hip may reproduce pain during resisted muscle testing and should be distinguished from intra-articular impingement.

Snapping Hip Syndrome

Snapping hip may cause audible or palpable clicking during movement and can mimic the mechanical symptoms of FAI.

Acetabular Labral Tear

Labral tears frequently coexist with FAI but may also occur independently.

They commonly cause groin pain, catching, or clicking.

Juvenile Arthritis

Inflammatory arthritis should be considered in younger patients with persistent pain, stiffness, or systemic features.

Femoral Head Osteonecrosis

Avascular necrosis can cause progressive hip and groin pain and should be excluded, particularly when risk factors are present.

Athletic Pubalgia

Sports hernia or athletic pubalgia may present with groin pain in active patients and can resemble FAI clinically.

Treatment

General Principles

Treatment depends on the severity of symptoms, degree of functional impairment, associated labral or cartilage injury, and presence of established osteoarthritis.

Asymptomatic radiographic morphology alone generally does not require treatment.

Nonoperative Management

Patients with mild or moderate symptoms should usually begin with nonoperative treatment.

Conservative care can improve symptoms but does not change the underlying bony morphology.

Activity Modification

Activities that repeatedly place the hip in deep flexion, adduction, or internal rotation may need to be reduced or modified.

Examples include deep squatting, low sitting positions, or repeated pivoting activities.

Physical Therapy

Physical therapy focuses on improving hip mobility within a nonprovocative range, pelvic control, core stability, and strength of the hip musculature.

Movement patterns that repeatedly reproduce impingement should be minimized.

Anti-Inflammatory Medication

Oral NSAIDs may be used for short-term control of pain and inflammation when appropriate.

Surgery

Surgery may be considered when persistent symptoms and functional limitation continue despite an adequate trial of conservative treatment and imaging demonstrates correctable structural pathology.

Hip Arthroscopy

Hip arthroscopy is widely used to treat symptomatic FAI.

It allows treatment of both the bony abnormality and associated labral or cartilage lesions through minimally invasive portals.

Advantages

Compared with traditional open surgery, arthroscopy may be associated with less soft-tissue morbidity, faster rehabilitation, reduced postoperative pain, and fewer wound-related complications in appropriately selected patients.

Osteochondroplasty

For cam impingement, an osteochondroplasty or femoroplasty is performed.

The prominent portion of the femoral head-neck junction is resected to recreate a more normal head-neck offset.

Acetabular Rim Trimming

For pincer impingement caused by focal excessive acetabular coverage, the acetabular rim may be carefully trimmed.

The amount of resection must be controlled to avoid producing iatrogenic acetabular undercoverage or instability.

Labral Treatment

Associated acetabular labral pathology may be treated with repair or selective debridement.

When technically possible, preservation and repair of viable labral tissue are generally favored because the labrum contributes to hip stability and the fluid seal of the joint.

Open Surgical Hip Dislocation

Open surgical dislocation may be considered when deformity is too extensive or complex for arthroscopic management.

Examples include selected patients with major acetabular deformity, severe protrusio, substantial cartilage defects, or marked acetabular retroversion.

Femoral Osteotomy

A femoral osteotomy may be required when abnormal femoral version or other major structural deformity contributes to impingement and cannot be adequately corrected by simple osteochondroplasty.

Periacetabular Osteotomy

Severe acetabular malorientation may require a periacetabular osteotomy to reorient the acetabulum rather than simply trimming the rim.

This is particularly important when rim removal would worsen underlying instability or dysplasia.

Follow-Up

Prognosis

Both arthroscopic and open surgical treatment can produce substantial improvement in appropriately selected patients.

Many patients report improved function and marked reduction in pain after surgery.

Historical series have reported pain reduction approaching 95% at approximately 1 year in selected populations.

Factors Affecting Outcome

The best results generally occur in patients with limited pre-existing cartilage degeneration and preserved joint space.

Advanced osteoarthritis decreases the likelihood that impingement surgery will provide durable benefit.

Complications

Heterotopic Ossification

Ectopic bone formation may occur around the hip after surgery and can cause pain or restricted motion.

Femoral Neck Fracture

Excessive resection during femoral osteochondroplasty can weaken the femoral neck and create a risk of fracture.

Lateral Femoral Cutaneous Nerve Injury

Temporary or persistent numbness related to lateral femoral cutaneous nerve irritation may occur, particularly after procedures involving anterior hip exposure or traction.

Labral Injury

The acetabular labrum may be injured during the original disease process or during surgery.

Cartilage Injury

Articular cartilage damage may be present before treatment or can occur as a surgical complication.

Progressive chondral loss may eventually contribute to osteoarthritis.

Patient Monitoring

Follow-up should assess pain, hip range of motion, internal rotation, mechanical symptoms, gait, strength, and return to desired activities.

After surgery, monitoring should also evaluate wound healing, rehabilitation progress, heterotopic ossification, residual impingement, recurrent labral symptoms, and progression of degenerative joint disease.