Femoroacetabular Impingement (FAI): Causes, Contributing Factors, and Risk Factors

What causes FAI?

Femoroacetabular Impingement (FAI) is fundamentally caused by an abnormal shape of the bones of the hip joint (femoral head/neck and/or acetabulum), leading to premature contact and restricted motion. This underlying structural anomaly is often exacerbated or made symptomatic by various biomechanical and activity-related factors.

Understanding FAI: A Brief Overview

Femoroacetabular Impingement (FAI) is a condition characterized by abnormal contact between the femoral head and/or neck (thigh bone) and the acetabulum (hip socket). This "impingement" or collision occurs during normal hip movements, especially flexion and internal rotation, and can lead to damage of the articular cartilage, labrum, and subchondral bone within the hip joint. While the structural abnormalities are often present from a young age, symptoms typically emerge later due to repetitive stress.

Primary Structural Causes of FAI

The root cause of FAI lies in subtle deformities of the hip joint's bony architecture. These deformities are generally classified into three main types: Cam, Pincer, or a combination of both (Mixed).

• Cam Impingement

  • Description: This type involves an abnormal, non-spherical shape of the femoral head and/or a prominence at the junction of the femoral head and neck. Instead of a smooth, spherical transition, there's an extra bump or "pistol grip" deformity.
  • Mechanism: As the hip flexes and internally rotates, this extra bone on the femur jams into the front-top (anterosuperior) rim of the acetabulum. This repeated grinding action can shear the articular cartilage away from the bone (delamination) and damage the labrum.
  • Prevalence: More common in young, active males.

• Pincer Impingement

  • Description: Pincer impingement results from an over-coverage of the femoral head by the acetabulum. This can be due to an excess bone growth on the rim of the acetabulum, or the socket being angled abnormally (e.g., retroversion – facing too far backward, or coxa profunda – excessively deep socket).
  • Mechanism: The prominent acetabular rim pinches the labrum between itself and the femoral neck, leading to crushing and degeneration of the labrum. Over time, ossification (bone formation) can occur in the labrum, further exacerbating the impingement.
  • Prevalence: More common in middle-aged women.

• Mixed Impingement

  • Description: This is the most common presentation of FAI, where both Cam and Pincer deformities are present in the same hip.
  • Mechanism: The combined structural abnormalities lead to a more complex pattern of impingement and often more extensive joint damage.

Contributing Factors to FAI Development

While the primary structural deformities are the cause, several factors can contribute to the development of symptoms and the progression of joint damage in individuals with FAI.

• Genetics and Bone Development

  • The abnormal bone shapes associated with FAI are often believed to develop during childhood and adolescence, particularly during periods of rapid bone growth. Genetic predisposition likely plays a role, influencing the development of the femoral head and acetabulum. Certain growth plate disorders or conditions like Slipped Capital Femoral Epiphysis (SCFE) can also lead to FAI-like deformities.

• Activity Level and Sport Participation

  • High-impact sports and activities involving repetitive hip flexion, internal rotation, and pivoting motions are significant contributing factors. Sports such as ice hockey, soccer, martial arts, dance, football, and certain track and field events place extreme and repeated stress on the hip joint, accelerating the wear and tear in an already compromised joint. It's not the activity that causes the bone deformity, but rather the activity that causes the symptoms to manifest due to repeated impingement.

• Repetitive Motions and Chronic Stress

  • Even in non-athletes, occupations or daily activities that involve prolonged sitting with hip flexion, deep squatting, or repetitive twisting can contribute to the onset of FAI symptoms by repeatedly forcing the abnormal bone shapes into contact.

• Hip Joint Loading Patterns

  • How forces are distributed across the hip joint during movement and weight-bearing can influence the impact of FAI. Abnormal loading, possibly due to poor movement mechanics or muscle imbalances, can concentrate stress in areas prone to impingement.

• Muscle Imbalances and Biomechanical Dysfunctions

  • Weakness or dysfunction in key hip musculature, such as the gluteal muscles (stabilizers of the pelvis and femur) or core muscles, can alter hip kinematics. For example, gluteal weakness might lead to excessive hip adduction and internal rotation during movement, increasing the likelihood of impingement. Similarly, tightness in hip flexors or adductors can pull the joint into positions that promote impingement.

• Trauma or Injury

  • While not a direct cause of the underlying bone shape, a traumatic event (e.g., a fall, sports injury) can sometimes trigger or exacerbate FAI symptoms by causing acute damage to the already vulnerable labrum or cartilage.

• Other Hip Conditions

  • Pre-existing conditions like hip dysplasia (a shallow hip socket), Legg-Calve-Perthes disease, or previous hip surgeries can alter hip mechanics and contribute to the development or progression of FAI.

The Role of Impingement: How Causes Lead to Symptoms

The abnormal bony contact, regardless of whether it's Cam or Pincer (or both), leads to a cascade of events:

1. Mechanical Stress: The repeated "bumping" or "pinching" of bone on soft tissue.
2. Labral Damage: The labrum (a rim of cartilage that deepens the socket and provides stability) is often the first structure to be damaged, leading to tears.
3. Cartilage Damage: Over time, the articular cartilage lining the bones begins to wear away, fray, or detach, leading to chondral lesions.
4. Inflammation and Pain: The damaged tissues become inflamed, causing pain, stiffness, and reduced range of motion.
5. Osteoarthritis Progression: Chronic FAI, if left untreated, significantly increases the risk of developing early-onset hip osteoarthritis due to the progressive loss of articular cartilage.

Who is at Risk?

Individuals at higher risk for developing symptomatic FAI typically include:

• Athletes involved in sports with repetitive hip flexion and rotation (e.g., soccer, hockey, martial arts).
• Individuals with certain genetic predispositions or a history of pediatric hip conditions.
• Young to middle-aged adults, as symptoms often manifest in this age range.
• Those with occupations or hobbies requiring frequent deep hip flexion or twisting.

Conclusion: A Multifactorial Condition

In summary, FAI is primarily caused by structural abnormalities of the hip joint bones, which are largely developmental. However, the manifestation of symptoms and the progression of joint damage are often influenced by a complex interplay of activity levels, biomechanical factors, and the cumulative stress placed on the joint. Understanding these underlying causes and contributing factors is crucial for accurate diagnosis, effective management, and the development of targeted rehabilitation strategies to mitigate pain, improve function, and potentially slow the progression of hip osteoarthritis. If you suspect you have FAI, consulting with an orthopedic specialist or sports medicine physician is essential for a comprehensive evaluation.