Hip Impingement: Causes, Types, and Contributing Factors

What causes hip impingement?

Hip impingement, formally known as femoroacetabular impingement (FAI), primarily results from abnormal bone shapes within the hip joint that cause the femoral head or neck to abnormally contact the acetabulum, often exacerbated by repetitive motion and specific activities.

Understanding Hip Impingement (Femoroacetabular Impingement - FAI)

Femoroacetabular impingement (FAI) is a condition where extra bone grows along one or both of the bones that form the hip joint – the femoral head/neck (top of the thigh bone) or the acetabulum (hip socket). This abnormal bone growth causes the bones to rub against each other during hip movement, particularly during deep flexion, internal rotation, or adduction. Over time, this repetitive friction can damage the articular cartilage (the smooth surface covering the bones) and the labrum (a rim of cartilage around the socket that helps deepen it and provide stability), leading to pain, stiffness, and potentially osteoarthritis.

Primary Anatomical Causes of FAI

The fundamental cause of FAI lies in subtle, yet significant, structural abnormalities of the hip joint. These anatomical variations are typically classified into three main types:

• Cam Impingement: This occurs when there is an extra bone bump or prominence on the femoral head or neck, making it less spherical. As the hip flexes and internally rotates, this non-spherical portion "jams" into the front of the hip socket (acetabulum), grinding the cartilage and labrum. This type is more common in young, active males.
• Pincer Impingement: This type involves extra bone growth on the rim of the acetabulum, extending out over the femoral head. This "over-coverage" causes the femoral neck to pinch against the rim of the socket, particularly during hip flexion, leading to labral tears and cartilage damage. Pincer impingement is more frequently observed in middle-aged women.
• Mixed Impingement: The most common form of FAI, mixed impingement, involves a combination of both cam and pincer lesions. This means there are anatomical abnormalities present on both the femoral head/neck and the acetabulum, leading to a complex pattern of impingement.

Developmental and Genetic Factors

The abnormal bone shapes characteristic of FAI are believed to develop during childhood and adolescence. While the exact mechanisms are not fully understood, several factors contribute:

• Growth Plate Activity: During periods of rapid growth, particularly around puberty, the growth plates (epiphyses) of the femur and acetabulum can develop these non-spherical shapes.
• Genetic Predisposition: There appears to be a genetic component, as FAI can run in families, suggesting a hereditary influence on bone development.
• Developmental Conditions: Certain childhood hip conditions, such as slipped capital femoral epiphysis (SCFE), where the growth plate of the femoral head slips, or Legg-Calvé-Perthes disease, which involves avascular necrosis of the femoral head, can lead to the development of cam deformities and increase the risk of FAI later in life.

Biomechanical and Activity-Related Contributing Factors

While anatomical abnormalities are the prerequisite for FAI, symptoms often arise or are exacerbated by specific biomechanical factors and activity levels that repeatedly stress the hip joint in vulnerable positions:

• Repetitive Motion: Sports and activities that involve deep hip flexion, internal rotation, and pivoting movements are significant contributors. Examples include:
  • Squatting and lunging (especially with poor form or excessive depth)
  • Martial arts
  • Ice hockey
  • Soccer
  • Dance
  • Cycling (with aggressive hip flexion)
• High-Impact Activities: Activities involving repeated forceful loading, jumping, and landing can accelerate the wear and tear on compromised hip joints.
• Muscle Imbalances:
  • Weakness in gluteal muscles (especially gluteus medius and maximus) can lead to compensatory movements and altered hip mechanics.
  • Tightness in hip flexors, adductors, or hamstrings can restrict normal hip range of motion and force impingement positions.
  • Poor core stability can also contribute to suboptimal movement patterns.
• Altered Movement Patterns: Individuals may develop compensatory movement strategies to avoid pain, which can inadvertently place more stress on other parts of the joint or adjacent structures.
• Hypermobility/Instability: In some cases, excessive joint laxity or instability can lead to increased joint translation and subsequent impingement-like symptoms, even without significant bony abnormalities.

Other Potential Contributing Factors

While less common as primary causes, other factors can influence the development or progression of FAI symptoms:

• Trauma: A direct blow to the hip or a significant injury can sometimes lead to the development of bony spurs or alter joint mechanics, contributing to impingement.
• Osteoarthritis: While FAI often leads to osteoarthritis, pre-existing degenerative changes in the joint can also alter its mechanics and contribute to impingement symptoms.
• Capsular Laxity: Excessive laxity in the hip joint capsule can affect stability and potentially lead to abnormal contact between the femoral head and acetabulum.

The Role of Load and Activity Level

It's crucial to understand that many individuals may have the underlying anatomical bone shapes associated with FAI but remain asymptomatic. The development of pain and symptoms often depends on the level and type of physical activity. High loads, repetitive movements, and participation in specific sports can trigger symptoms in an anatomically predisposed hip. Conversely, reducing or modifying these activities can often alleviate symptoms, even if the underlying bone morphology remains.

Conclusion: A Multifactorial Condition

In summary, hip impingement is a complex, multifactorial condition. Its primary cause lies in subtle anatomical variations of the hip joint, predominantly cam and pincer deformities, which are largely developmental. These structural abnormalities set the stage for impingement, but the onset of symptoms is frequently triggered or exacerbated by repetitive movements, specific athletic activities, and biomechanical imbalances that repeatedly force the hip into positions where the abnormal bone surfaces collide. Understanding these contributing factors is key to both prevention strategies and effective management of FAI. A thorough assessment by a healthcare professional is essential for accurate diagnosis and personalized treatment.