What is Femoroacetabular Impingement (FAI)?
Femoroacetabular impingement (FAI) syndrome is a motion-related hip disorder where abnormal bone contact occurs between the proximal femur and acetabulum, causing groin pain and progressive joint damage that can lead to early osteoarthritis. 1
Core Pathophysiology
FAI results from abnormal bone morphology that creates premature mechanical contact during hip motion, specifically between the femoral head-neck junction and the acetabular rim. 1, 2 This repetitive abnormal contact generates progressive damage to three key structures:
- Acetabular labrum - tears from repetitive trauma 1, 3
- Chondrolabral junction - delamination at the cartilage-labrum interface 1
- Articular cartilage - progressive wear leading to early osteoarthritis 1, 2
Morphological Types
FAI presents in three distinct patterns based on the location of bony abnormality:
- Cam morphology: Abnormal femoral head-neck junction (alpha angle >60°) where the femoral head is not perfectly spherical, creating a "bump" that jams into the acetabulum 2, 4
- Pincer morphology: Acetabular overcoverage (lateral center edge angle >40°) where the socket covers too much of the ball 2, 5
- Mixed pattern: Combination of both cam and pincer deformities 1, 2
Clinical Presentation
The hallmark symptom is groin pain, though pain may radiate to the back, buttock, or thigh. 1, 2, 4 Pain is motion-related and worsened by hip flexion, adduction, and internal rotation. 2 Patients commonly report clicking, catching, buckling, stiffness, giving way, and limited hip range of motion. 3
Critical diagnostic point: The absence of groin pain helps exclude FAI syndrome and labral tears. 1, 4
Diagnostic Triad
FAI syndrome requires all three components for diagnosis:
- Symptoms: Hip-related groin pain 1, 2
- Clinical signs: Positive FADIR test (flexion-adduction-internal rotation), though this has good sensitivity but poor specificity 2, 4
- Imaging findings: Cam, pincer, or mixed morphology on radiographs 1, 2
Major pitfall: Never treat imaging findings alone—cam and pincer morphology are extremely common in asymptomatic individuals and do not warrant intervention without corresponding symptoms and clinical signs. 2, 5
Affected Population
FAI syndrome typically affects young to middle-aged active adults (20-50 years), including athletes and individuals with repetitive hip flexion activities. 2 It is a common cause of hip pain in this demographic with significant impact on hip-related function and quality of life. 6
Long-term Consequences
The repetitive mechanical overload from FAI is implicated in the development of early hip osteoarthritis. 1, 2, 3 Early recognition and treatment may prevent progression to end-stage osteoarthritis. 7, 8