Femoral Acetabular Impingement: Testing Methods, Diagnostic Procedures, and Imaging

How Do You Test for Femoral Impingement?

Testing for femoral acetabular impingement (FAI) involves a comprehensive clinical assessment, including a detailed patient history, specific physical examination maneuvers designed to provoke symptoms, and often, diagnostic imaging to confirm the diagnosis and assess associated structural damage.

Understanding Femoral Acetabular Impingement (FAI)

Femoral acetabular impingement (FAI) is a condition characterized by abnormal contact between the femoral head/neck junction and the rim of the acetabulum (hip socket), leading to pain and restricted range of motion. This impingement can cause damage to the articular cartilage and the labrum, a ring of cartilage that deepens the hip socket and provides stability.

There are three main types of FAI:

• Cam Impingement: Occurs when the femoral head is not perfectly round, creating an abnormal bony bump (pistol grip deformity) that grinds against the acetabulum, especially during hip flexion and internal rotation.
• Pincer Impingement: Results from an overgrowth of the acetabular rim, causing the femoral neck to pinch against it. This is often due to excessive coverage of the femoral head.
• Mixed Impingement: A combination of both Cam and Pincer deformities, which is the most common presentation.

Patients typically report deep groin pain, especially with activities involving hip flexion, internal rotation, or prolonged sitting.

The Role of Clinical Assessment

Diagnosing FAI is a multi-faceted process primarily initiated by a healthcare professional, such as a physician, orthopedist, or physical therapist. The initial steps involve:

• Detailed Patient History: Eliciting information about the onset of pain, its location (typically deep groin, sometimes lateral hip or buttock), aggravating activities (e.g., squatting, sitting for long periods, athletic maneuvers involving deep hip flexion), relieving factors, and any prior injuries or treatments.
• Symptom Reproduction: The clinician aims to reproduce the patient's characteristic pain through specific movements, which is a hallmark of a positive impingement test.

Key Physical Examination Tests for FAI

Several specialized tests are performed to assess for FAI, each targeting the anatomical structures involved in impingement. It's crucial to perform these tests bilaterally for comparison.

• FADIR Test (Flexion, Adduction, Internal Rotation):

  • Procedure: The patient lies supine. The examiner passively flexes the patient's hip to 90 degrees, then adducts and internally rotates the hip.
  • Positive Finding: Reproduction of the patient's typical groin pain, sometimes accompanied by a "clunk" sensation. This is considered the most sensitive test for FAI and labral tears.

• FABER Test (Flexion, Abduction, External Rotation):

  • Procedure: The patient lies supine. The examiner places the patient's ankle of the affected leg on the opposite knee, creating a "figure-4" position. The examiner then gently applies pressure to the knee of the affected leg while stabilizing the opposite anterior superior iliac spine (ASIS).
  • Positive Finding: Reproduction of groin pain, or pain in the sacroiliac joint or posterior hip, and/or limited range of motion compared to the unaffected side. While not specific for FAI, it helps differentiate FAI from other conditions like sacroiliac joint dysfunction or iliopsoas issues.

• Squat Test / Dynamic Impingement Test:

  • Procedure: The patient performs a deep squat, observing their movement patterns and pain provocation. Alternatively, the clinician may guide the patient into positions that mimic impingement during functional movements.
  • Positive Finding: Reproduction of groin pain at the end range of hip flexion and internal rotation during the squat. This test assesses functional impingement.

• Range of Motion (ROM) Assessment:

  • Procedure: Passive and active assessment of hip flexion, extension, abduction, adduction, internal rotation, and external rotation.
  • Positive Finding: Often, limited hip internal rotation, especially in a flexed position (e.g., <20 degrees at 90 degrees of hip flexion), is a common finding in FAI. Overall hip flexion may also be restricted.

• Gait Analysis:

  • Procedure: Observation of the patient's walking pattern.
  • Positive Finding: Patients with FAI may exhibit a compensatory external rotation of the affected leg during gait to avoid impingement, or a shortened stride length.

• Palpation:

  • Procedure: Gentle palpation around the hip joint, including the groin area, greater trochanter, and gluteal region.
  • Positive Finding: Tenderness in the anterior groin or along the joint line can indicate underlying pathology, though it's not specific to FAI.

Differentiating FAI from Other Hip Conditions

It is critical to differentiate FAI from other conditions that can cause similar hip and groin pain. These include:

• Labral tears (often co-exist with FAI)
• Hip flexor strains or tendinopathy
• Adductor strains
• Sports hernia (athletic pubalgia)
• Piriformis syndrome
• Lumbar spine pathology (referred pain)
• Sacroiliac joint dysfunction
• Early hip osteoarthritis

A thorough clinical assessment helps narrow down the possibilities, and imaging often plays a confirmatory role.

The Role of Diagnostic Imaging

While physical tests are crucial for identifying symptomatic impingement, imaging studies are essential for confirming the presence of bony deformities and assessing associated soft tissue damage.

• X-rays:

  • Purpose: The initial imaging modality to visualize bony morphology.
  • Views: Anteroposterior (AP) pelvis, frog-leg lateral, and cross-table lateral views are standard.
  • Findings: Can reveal Cam deformities (e.g., "pistol grip" deformity, loss of head-neck offset, alpha angle measurement), Pincer deformities (e.g., coxa profunda, protrusio acetabuli, acetabular retroversion), and signs of early osteoarthritis.

• Magnetic Resonance Imaging (MRI) / Magnetic Resonance Arthrography (MRA):

  • Purpose: Provides detailed images of soft tissues, including the labrum and articular cartilage. MRA involves injecting contrast dye into the joint for enhanced visualization.
  • Findings: Essential for detecting labral tears, cartilage damage, subchondral bone edema, and other intra-articular pathologies that often accompany FAI.

• Computed Tomography (CT) Scan:

  • Purpose: Offers excellent bony detail and 3D reconstruction, particularly useful for complex deformities or surgical planning.
  • Findings: Can precisely measure bony morphology and assess the extent of Cam and Pincer lesions.

Interpreting Test Results and Next Steps

A diagnosis of FAI is typically made when a patient presents with classic symptoms, has positive physical examination findings (especially the FADIR test), and imaging confirms the presence of characteristic bony deformities.

Once diagnosed, management strategies range from conservative approaches (rest, activity modification, physical therapy to improve hip mechanics and strength, anti-inflammatory medication) to surgical intervention (arthroscopic hip surgery to reshape the bone and repair damaged soft tissues). The decision depends on the severity of symptoms, extent of structural damage, and the patient's activity level and goals.

Conclusion

Testing for femoral acetabular impingement is a multi-faceted process that integrates a thorough understanding of the patient's symptoms with specific physical examination maneuvers and confirmatory diagnostic imaging. The combination of a positive FADIR test, limited hip internal rotation, characteristic pain reproduction, and radiological evidence of bony impingement provides a comprehensive basis for diagnosis, guiding appropriate management strategies to alleviate pain and restore hip function.