Femoroacetabular impingement (FAI) is a condition affecting the hip joint where the bones of the hip are abnormally shaped. This abnormal shape causes them to rub against each other during movement, leading to friction and potential damage within the joint. Understanding the commonness of this condition involves looking at both the presence of these bone shapes and how often they actually cause problems. This distinction is important because many people have the bone shapes without experiencing any symptoms.
Understanding Femoroacetabular Impingement
Femoroacetabular impingement occurs when there is unusual contact between the femoral head (the “ball” at the top of the thigh bone) and the acetabulum (the “socket” in the pelvis). This abnormal contact happens because of extra bone growth on either one or both of these structures.
The two primary types of FAI morphology are cam and pincer. Cam impingement involves an extra bump of bone on the femoral head or neck, which prevents it from smoothly rotating within the hip socket. Pincer impingement occurs when the acetabulum, or hip socket, has an excessive bony rim that extends too far, leading to the labrum (a rim of cartilage) being “pinched” during hip movements. Many individuals often have a mixed type, exhibiting characteristics of both cam and pincer morphologies. This bone-on-bone impingement can stress the joint’s soft tissues, including the labrum and cartilage.
Prevalence of FAI Morphology
The anatomical variations associated with FAI are common in the general population, even among individuals who do not experience hip pain. For instance, cam deformity has been observed in approximately 37% of asymptomatic hips, and in about 23.1% of the general asymptomatic population. Pincer deformity is also prevalent in asymptomatic individuals, found in around 67% of asymptomatic hips. Some research suggests that at least one FAI-related morphological feature can be detected in up to 66% of asymptomatic hip joints.
Prevalence of Symptomatic FAI
While the anatomical features of FAI are common, a much smaller proportion of individuals with these morphologies actually develop symptomatic FAI. The prevalence of clinically diagnosed FAI is estimated to be between 10% and 15% in the general adult population.
One study focusing on white adults aged 20 to 49 found symptomatic FAI in about 3% of the population. The overall population-based incidence of FAI diagnosis has been reported as approximately 54.4 per 100,000 person-years. This incidence has shown an increasing trend over time, which may be partly attributed to better recognition and diagnostic capabilities. In specific athletic populations, such as collegiate athletes, the incidence of symptomatic FAI was found to be 3.0% of the total athlete population.
Factors Affecting FAI Occurrence
Several factors influence the likelihood of developing FAI morphology and symptomatic FAI. Age plays a role, with FAI morphology often developing during adolescence and early adulthood, influenced by growth spurts and physical activity. The incidence of diagnosed FAI tends to be higher in younger adults.
Sex differences are also observed in FAI types; cam impingement is more common in males, while pincer impingement is more frequently seen in females. Overall, females may have a higher incidence of FAI diagnosis compared to males, at 73.2 versus 36.1 per 100,000 person-years.
Activity level, particularly involvement in high-impact sports or activities with repetitive hip movements, increases the risk of both developing FAI morphology and experiencing symptoms. Athletes, especially those in sports like ice hockey, soccer, and basketball, have a higher prevalence of cam morphology. This suggests that physical activity during skeletal maturation can contribute to the development of these bone shapes. Genetic predispositions may also contribute to FAI development, with research suggesting an increased risk among siblings of affected individuals.