Femoroacetabular Impingement (FAI): Understanding Cam, Pincer, and Mixed Types

What are the Three Types of FAI?

Femoroacetabular Impingement (FAI) is a condition characterized by abnormal bone morphology in the hip joint, leading to impingement and potential damage to the articular cartilage and labrum. It is primarily categorized into three distinct types: Cam, Pincer, and Mixed impingement, each defined by the location of the excess bone formation.

Understanding Femoroacetabular Impingement (FAI)

Femoroacetabular Impingement (FAI) is a common cause of hip pain and dysfunction, particularly in active individuals. It occurs when there is abnormal contact between the femoral head/neck junction (the ball of the hip joint) and the acetabular rim (the socket of the hip joint). This irregular contact, especially during hip flexion, adduction, and internal rotation, can lead to repetitive microtrauma, causing damage to the articular cartilage and the acetabular labrum – a fibrocartilaginous ring that deepens the socket and provides joint stability. Understanding the specific type of FAI is crucial for accurate diagnosis and effective management.

Type 1: Cam Impingement

Cam impingement describes an abnormality of the femoral head and neck. In this type, there is an extra bone growth, or a "bump," on the femoral head-neck junction. This bump is typically located anteriorly or anterosuperiorly.

• Mechanism: As the hip flexes and internally rotates, this non-spherical portion of the femoral head jams against the acetabular rim. Instead of smoothly rotating within the socket, the enlarged femoral head essentially acts like a "cam" (an eccentric rotating part), grinding or levering against the cartilage and labrum inside the acetabulum. This can shear the labrum off the bone and delaminate the articular cartilage.
• Commonly Seen In: Cam impingement is more prevalent in young, active males, particularly those involved in sports requiring deep hip flexion and repetitive rotational movements, such as hockey, soccer, and martial arts.

Type 2: Pincer Impingement

Pincer impingement involves an abnormality of the acetabulum (the hip socket). This type is characterized by excessive bone growth on the rim of the acetabulum, leading to an "over-coverage" of the femoral head.

• Mechanism: The excessive bony rim of the acetabulum pinches the labrum between the rim and the femoral neck, particularly during hip flexion. Unlike cam impingement, where the femoral head itself is misshapen, in pincer impingement, the socket is too deep or has an extended rim. This repetitive pinching can lead to labral tears, ossification of the labrum, and cartilage damage.
• Commonly Seen In: Pincer impingement is more frequently observed in middle-aged women and can sometimes be associated with conditions like acetabular retroversion (where the socket faces slightly backward).

Type 3: Mixed Impingement

Mixed impingement is the most common presentation of FAI, occurring in approximately 80-90% of cases. As the name suggests, it is a combination of both Cam and Pincer morphologies.

• Mechanism: Individuals with mixed impingement exhibit both the abnormal femoral head-neck junction (cam lesion) and the excessive acetabular rim coverage (pincer lesion). This combination often leads to more severe and complex patterns of damage to the labrum and articular cartilage due to the compounded mechanical stress.
• Prevalence: Given its high prevalence, many individuals presenting with FAI symptoms will have features of both cam and pincer types on imaging.

Clinical Implications and Management

Regardless of the type, FAI typically presents with groin pain, especially during activities involving deep hip flexion, such as sitting for long periods, squatting, or athletic maneuvers. Early diagnosis, often involving physical examination and imaging (X-rays, MRI), is crucial.

Management strategies for FAI vary depending on the severity of symptoms and the extent of joint damage.

• Conservative Management: This includes activity modification, physical therapy focusing on improving hip strength, stability, and movement patterns, and anti-inflammatory medications. The goal is to reduce impingement forces and alleviate symptoms.
• Surgical Intervention: For cases unresponsive to conservative treatment or those with significant structural damage, hip arthroscopy (minimally invasive surgery) is often performed. During surgery, the orthopedic surgeon can reshape the femoral head-neck junction (femoral osteoplasty) and/or trim the excessive acetabular rim (acetabular rim trimming) to restore normal hip mechanics and address any labral or cartilage damage.

Conclusion

The three types of Femoroacetabular Impingement—Cam, Pincer, and Mixed—describe the specific anatomical abnormalities that lead to hip joint impingement. While each type has distinct characteristics, they all result in abnormal contact within the hip joint, potentially leading to pain and degenerative changes. Understanding these distinctions is fundamental for healthcare professionals to accurately diagnose FAI and tailor effective treatment plans, ultimately aiming to preserve hip joint health and function.