Femoroacetabular Impingement (FAI): Understanding Hip Bone Abnormalities

What Is Femoroacetabular Impingement?

Femoroacetabular impingement (FAI) is a condition in which abnormal contact (impingement) occurs between the femoral head (ball) and the acetabular rim (socket edge) during normal hip movement. This results from abnormal bony morphology that creates a mechanical mismatch between the ball and socket.

FAI is now recognized as a major cause of hip labral tears and — over years of repeated impingement damage — may be a significant contributor to early hip osteoarthritis in young adults.

Types of FAI

Cam impingement: An abnormal bump (cam lesion) on the femoral head or head-neck junction reduces the spherical shape of the femoral head. When the hip is flexed, this non-spherical portion jams against the acetabular rim, shearing the adjacent articular cartilage and labrum from inside-out. More common in young, active males.

Pincer impingement: Excessive coverage of the femoral head by the acetabular rim (over-coverage). The rim impinges on the femoral head-neck junction with flexion. More common in middle-aged women.

Mixed impingement: Both cam and pincer deformities present — the most common pattern.

Causes

FAI morphology (the abnormal bone shape) is primarily developmental — the bone grows into this shape during adolescence. Contributing factors include:

Genetics
High-volume athletic training during adolescent bone growth (particularly hip loading sports)
Prior childhood hip conditions (Perthes disease, slipped capital femoral epiphysis)

The presence of FAI morphology does not guarantee symptoms — many people have FAI bony shape without pain. Symptoms develop when repetitive mechanical impingement damages the labrum and cartilage.

Symptoms

Groin pain: The primary symptom — often deep, aching groin or anterior hip pain
C-sign: Patient cups the lateral hip in a C shape to indicate pain location
Pain with hip flexion activities: Sitting for prolonged periods, driving, squatting, bending
Pain with athletic activities: Pivoting, kicking, deep hip flexion
Restricted internal rotation and hip flexion
Snapping or clicking (often from associated labral tear)

Diagnosis

Clinical examination: FADIR test (flexion, adduction, internal rotation) is the primary impingement test
X-ray: Identifies cam deformity (alpha angle measurement), pincer over-coverage, and crossover sign
MR arthrogram: Evaluates labral integrity and cartilage damage extent
CT scan: 3D reconstruction helps surgical planning for cam resection

Treatment

Conservative Treatment

FAI without significant labral or cartilage damage may be managed with:

Activity modification (avoid deep hip flexion, provocative sports)
Physical therapy (hip flexibility, core stability, movement pattern modification)
NSAIDs for acute flares

Conservative care does not correct the underlying bony morphology — if the cause of impingement is not addressed, labral and cartilage damage will progress.

Hip Arthroscopy (FAI Correction)

Arthroscopic hip surgery is the treatment of choice for symptomatic FAI with labral pathology:

Cam resection (femoroplasty): Reshaping the femoral head-neck junction to restore spherical shape
Rim trimming (acetabuloplasty): Removing excess acetabular rim bone
Labral repair or reconstruction
Cartilage treatment (microfracture, cartilage restoration if damage is focal)

Return to sport: 3–6 months (earlier for lower-demand activities, 6+ months for contact and pivot sports).