Femoroacetabular Impingement (FAI) is a hip condition where abnormally shaped bones of the hip joint cause friction and restricted movement. This occurs when the femur (thigh bone) collides prematurely with the rim of the acetabulum (hip socket). FAI is a common cause of hip pain, particularly among younger, active adults.
FAI is a structural problem that develops over time, often during childhood growth, rather than an injury. When the hip bones do not fit together perfectly, the abnormal contact damages the soft tissues inside the joint. This chronic rubbing leads to pain, stiffness, and long-term joint degeneration.
The Anatomy of Impingement
FAI is a mismatch between the shape of the femoral head (the ball) and the acetabulum (the socket). This structural issue causes the femoral neck to abut the acetabular rim during hip motion, especially with flexion. The continuous impact damages the labrum, a ring of cartilage that deepens the socket, and the articular cartilage lining the joint.
FAI is categorized into three patterns based on the abnormal bone growth. Cam impingement involves extra bone forming on the femoral head, making the ball less round. This bump grinds against the socket cartilage when the hip is flexed. Pincer impingement occurs when the acetabulum rim has an excessive bony overhang, causing the socket to cover too much of the femoral head. Mixed impingement is the most common form, presenting characteristics of both Cam and Pincer deformities.
Recognizing the Signs of FAI
The primary complaint for individuals with FAI is activity-related pain in the hip or groin area. This pain often begins as a dull ache but can become sharp and catching, especially during movements that bring the knee toward the chest. Activities involving deep hip flexion, such as prolonged sitting, squatting, or getting in and out of a low car, frequently trigger discomfort.
Patients sometimes describe the pain location using the “C-sign,” cupping their hand around the front and side of the hip. While the groin is the most common location, pain can also radiate to the outer hip, buttock, or lower back. Stiffness or a reduction in the hip’s range of motion, particularly internal rotation, is also reported.
Confirming the Diagnosis
Diagnosing FAI requires a combination of symptoms, physical signs, and imaging findings. The process begins with a physical examination focused on provocative maneuvers. A primary diagnostic test is the Flexion, Adduction, and Internal Rotation (FADIR) test. If the doctor moving the patient’s knee toward the chest and rotating it inward reproduces the characteristic hip pain, it suggests mechanical impingement.
Imaging visualizes underlying bony abnormalities and soft tissue damage. Standard X-rays are the initial tool, providing clear views of the bone structure to identify Cam and Pincer deformities. X-rays allow doctors to measure specific angles, such as the alpha angle for Cam lesions, and assess acetabular over-coverage.
Magnetic Resonance Imaging (MRI) evaluates soft tissues, detecting tears in the labrum and damage to the articular cartilage. A Computed Tomography (CT) scan may also be ordered, offering a detailed, three-dimensional view of the bony anatomy helpful for surgical planning. To confirm the hip joint is the source of pain, a diagnostic injection of a numbing agent into the joint space can be performed; temporary pain relief strongly suggests FAI.
Treatment Pathways for FAI
Management of FAI ranges from conservative non-operative methods to surgical intervention, depending on symptom severity and joint damage. The initial step is a conservative approach, starting with activity modification to avoid painful movements. This is often combined with nonsteroidal anti-inflammatory drugs (NSAIDs) to manage pain and inflammation.
Physical therapy is a fundamental component of non-operative care, focusing on improving hip strength, stability, and neuromuscular control. A personalized physical therapy program may successfully improve pain and function for many patients. If conservative treatment fails to provide adequate relief, surgical options are considered.
The standard surgical procedure for FAI is hip arthroscopy, a minimally invasive technique. The goal is to restore normal hip mechanics by reshaping the bone and repairing damaged soft tissues. For Cam lesions, the surgeon performs an osteoplasty, trimming excess bone on the femoral head and neck. For Pincer lesions, the overhanging acetabular rim is trimmed, and any labral tears are repaired or reconstructed. Surgery may offer superior short-term results for young, active patients whose symptoms persist after physical therapy.