Femoroacetabular Impingement (FAI): Symptoms, Physical Exam, and Imaging for Diagnosis

How to diagnose FAI?

Diagnosing Femoroacetabular Impingement (FAI) is a multi-faceted process that integrates a thorough clinical history, a comprehensive physical examination, and advanced imaging studies to pinpoint the specific anatomical abnormalities and their contribution to hip pain and dysfunction.

Understanding Femoroacetabular Impingement (FAI)

Femoroacetabular Impingement (FAI) is a condition characterized by abnormal contact between the femoral head/neck and the acetabular rim during hip movement, leading to cartilage damage, labral tears, and pain. This structural mismatch can be categorized into three main types:

• Cam Impingement: An abnormal bump on the femoral head-neck junction that jams into the acetabulum, typically damaging the articular cartilage.
• Pincer Impingement: Over-coverage of the femoral head by the acetabular rim, leading to crushing of the labrum between the rim and the femoral head/neck.
• Mixed Impingement: A combination of both cam and pincer morphologies, which is the most common presentation.

Accurate diagnosis is crucial for effective management, ranging from conservative physical therapy to surgical intervention.

Initial Clinical Assessment: Symptom Analysis and Patient History

The diagnostic journey begins with a detailed assessment of the patient's symptoms and medical history. This initial step helps to establish a strong suspicion of FAI and differentiate it from other hip pathologies.

• Common Symptoms Reported:

  • Groin pain: The most frequent complaint, often deep and aching.
  • Lateral hip pain: Can also be present, sometimes radiating to the buttock or thigh.
  • Mechanical symptoms: Clicking, catching, or locking sensations within the joint.
  • Stiffness: Especially after prolonged sitting or activity.
  • Aggravation with specific movements: Pain often worsens with hip flexion, adduction, and internal rotation (e.g., sitting, squatting, getting in and out of a car, sports activities).
  • "C-sign": Patients often cup their hand around the greater trochanter to indicate the general area of pain.

• Patient History Questions:

  • Onset of symptoms: Gradual or sudden?
  • Activity level: Does pain correlate with specific sports or activities?
  • Previous injuries or surgeries: Relevant to the hip or lower extremity.
  • Impact on daily life: How does the pain affect work, sleep, and recreational activities?
  • Response to previous treatments: Physical therapy, medication, injections.

The Physical Examination: Uncovering Clinical Signs

A comprehensive physical examination is vital to identify objective signs of FAI and rule out other conditions.

• Observation and Palpation:

  • Gait analysis: Checking for limping or compensatory movements.
  • Posture: Assessing pelvic tilt or rotational asymmetries.
  • Palpation: Tenderness around the greater trochanter, iliopsoas, or adductor tendons.

• Range of Motion (ROM) Assessment:

  • Reduced hip flexion: Often limited, sometimes with a "hard" end-feel.
  • Limited internal rotation: Especially in flexion (e.g., at 90 degrees of hip flexion).
  • Pain at end-range movements: Particularly with combined movements that provoke impingement.

• Specific Impingement Tests: These provocative tests aim to reproduce the patient's pain by stressing the impingement area.

  • Flexion, Adduction, Internal Rotation (FADIR) Test: Considered the most sensitive test for FAI.
    • Procedure: Patient supine, hip flexed to 90 degrees, then adducted and maximally internally rotated.
    • Positive Sign: Reproduction of the patient's typical groin pain, often with a feeling of apprehension or mechanical block. This indicates impingement between the femoral neck and the anterior-superior acetabular rim.
  • Flexion, Abduction, External Rotation (FABER) Test (Patrick's Test): Primarily used to assess hip joint pathology and sacroiliac joint dysfunction.
    • Procedure: Patient supine, one ankle placed on the opposite knee, allowing the hip to flex, abduct, and externally rotate. The examiner then gently presses down on the flexed knee.
    • Positive Sign: Reproduction of groin pain (suggests hip joint pathology, including FAI or intra-articular issues) or pain in the sacroiliac joint area.
  • Posterior Impingement Test: Less common, but may be positive in cases of posterior FAI.
    • Procedure: Patient supine or prone, hip extended, adducted, and externally rotated.
    • Positive Sign: Reproduction of posterior hip or buttock pain.

• Differential Diagnosis Considerations: It's crucial to differentiate FAI from other conditions that cause similar symptoms, such as:

  • Labral tears (isolated, not due to FAI)
  • Hip flexor strain or tendinopathy
  • Adductor strain or tendinopathy
  • Sports hernia (athletic pubalgia)
  • Lumbar spine pathology (referred pain)
  • Sacroiliac joint dysfunction
  • Osteoarthritis of the hip
  • Piriformis syndrome

Advanced Imaging: Visualizing the Hip Joint

Imaging studies are essential to confirm the presence of FAI, characterize the specific morphology, and assess for associated intra-articular damage.

• Plain Radiographs (X-rays):

  • Views: Anteroposterior (AP) pelvis, lateral hip, and specific FAI views (e.g., Dunn 45° or 90° lateral, frog-leg lateral).
  • What they show:
    • Cam morphology: Presence of a "pistol grip" deformity, reduced head-neck offset, or asphericity of the femoral head.
    • Pincer morphology: Deep acetabulum, acetabular retroversion (crossover sign), or coxa profunda.
    • Joint space narrowing: Suggestive of concomitant osteoarthritis.

• Magnetic Resonance Imaging (MRI):

  • What it shows: Excellent visualization of soft tissues, including the labrum, articular cartilage, joint capsule, and surrounding muscles.
  • Key findings: Labral tears (often at the anterior-superior aspect), articular cartilage damage (chondral lesions), subchondral bone edema (stress reaction), and effusion.

• MR Arthrography (MRA):

  • Procedure: An MRI performed after injecting a contrast dye directly into the hip joint.
  • What it shows: Considered the gold standard for detecting labral tears and subtle cartilage abnormalities, as the contrast outlines these structures more clearly.

• Computed Tomography (CT) Scan:

  • What it shows: Provides detailed bony anatomy, superior to X-rays for assessing femoral and acetabular morphology in three dimensions.
  • Use: Particularly useful for surgical planning, allowing for precise measurement of impingement lesions and correction angles.

Diagnostic Injections: Confirming Pain Origin

A diagnostic injection can be a critical step in confirming that the hip joint is indeed the source of the pain, especially when other potential sources of pain (e.g., lumbar spine, sacroiliac joint) are considered.

• Intra-articular Anesthetic Injection:
  • Procedure: A local anesthetic (e.g., lidocaine) is injected directly into the hip joint, often under fluoroscopic or ultrasound guidance for accuracy.
  • Positive Sign: Significant, temporary relief of pain (typically 75-100% reduction) immediately following the injection strongly indicates that the hip joint pathology (e.g., FAI, labral tear) is the primary pain generator. This helps guide treatment decisions.

The Diagnostic Process: A Collaborative Effort

The diagnosis of FAI is typically made by an orthopedic surgeon or a sports medicine physician with expertise in hip pathology. They integrate all aspects of the diagnostic process: the patient's history, findings from the physical examination, and the results of imaging studies and diagnostic injections. This holistic approach ensures an accurate diagnosis and appropriate treatment planning.

Conclusion: The Importance of Comprehensive Diagnosis

Diagnosing Femoroacetabular Impingement is a meticulous process that requires a thorough understanding of hip anatomy, biomechanics, and pathology. By combining a detailed patient history, a targeted physical examination, advanced imaging, and sometimes diagnostic injections, healthcare professionals can accurately identify FAI. Early and precise diagnosis is paramount to preventing further joint damage, alleviating symptoms, and guiding patients toward the most effective management strategies, whether conservative or surgical, ultimately preserving hip function and improving quality of life.