Femoroacetabular impingement (FAI), a recently described hip condition in adolescents and young adults, results from abnormal physical contact between the proximal femur and the acetabulum.1 FAI is usually characterized by the site of the predominant morphologic abnormality—proximal femur (cam-type FAI), acetabulum (pincer-type FAI), or both (mixed impingement). Cam-type FAI is typified by the aspherical extension of the articular surface at the anterosuperior head–neck junction of the proximal femur with loss of the normal offset. With hip motion, especially in the maximal ranges of flexion and internal rotation, the aspherical proximal femur repeatedly contacts the anterosuperior acetabulum, damaging the chondrolabral junction and ultimately the labrum itself. In pincer-type impingement, femoral head overcoverage caused by acetabular retroversion and/or coxa profunda directly damages the anterior labrum when the acetabular rim contacts the proximal femur during physiologic motion. “Contrecoup” injury of the posterior-inferior acetabular cartilage may also occur. Over time, recurrent microtrauma to the acetabular cartilage and/or labrum may lead to degenerative changes of the hip and ultimately to premature osteoarthritis.1,2
Patients with FAI typically present with groin pain that may be activity-related or that may occur with prolonged sitting with the hip in a flexed position. Physical examination findings suggestive of FAI include decreased passive internal hip rotation and reproducible pain with adduction and internal rotation of the flexed hip—the impingement sign, or the flexion, adduction, and internal rotation (FADIR) test.3 Diagnostic imaging evaluation initially includes radiographs of the pelvis and hips. These radiographs may show a “pistol-grip” deformity and/or decreased head–neck offset (as determined by increased alpha angle) in the setting of cam-type impingement (Figure 1).4 Pincer-type impingement may be associated with a crossover sign, coxa profunda, and an increased center-edge angle (CEA). Advanced imaging studies, such as computed tomography (CT), magnetic resonance imaging (MRI) arthrogram, and delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), are commonly used to better delineate bony deformity and concomitant injuries of the labrum and cartilage (Figure 2).
Treatment for FAI often consists initially of activity modification, use of anti-inflammatory medications, and physical therapy. Intra-articular corticosteroid injections may be used both diagnostically and therapeutically. When nonsurgical measures fail to adequately relieve symptoms, surgery may be warranted. Whether performed open or arthroscopically, surgery is directed first at correcting the underlying osseous abnormality—performing an osteoplasty of the proximal femur to remove the cam lesion, performing an acetabular osteoplasty (“rim-trimming”) to address a focal pincer lesion, and/or performing a periacetabular osteotomy to decrease global acetabular overcoverage (Figure 3).5
Sex-Based Differences in FAI Incidence
Traditionally, it was thought that cam-type impingement occurred predominantly in young, athletic males, whereas pincer-type impingement resulting from acetabular overcoverage occurred primarily in females during their fourth decade. However, our understanding of the sex-based differences in the incidence and presentation of FAI has evolved, and it is now clear that the interplay of sex, radiographic signs of impingement, and development of symptoms requiring treatment is more complex.
In recent large population-based studies, investigators have attempted to better characterize the sex-based differences in the incidence of osseous FAI deformity. Gosvig and colleagues2 examined radiographic and questionnaire outcomes of 3620 patients (age range, 21-90 years) and found that males were more likely than females to have a pistol-grip deformity of the hip (19.6% vs 5.2%); that deep acetabular sockets were common in both sexes (15.2% vs 19.4%); and that the presence of pistol-grip deformity or deep socket was significantly associated with development of osteoarthritis, independent of sex.
In a study of 2081 asymptomatic patients (mean age, 18.6 years), Laborie and colleagues4 reported similar radiographic findings. Males were significantly more likely than females to have a cam-type deformity, as evidenced by pistol-grip deformity, focal prominence of the femoral neck, and/or flattening of the lateral aspect of the femoral head. Males were also more likely than females to have a pincer deformity, though radiographic signs of pincer deformity—a crossover sign, excessive acetabular coverage (defined by increased CEA), and a posterior wall sign—were common in both sexes, occurring in 16.6% of females and 34.3% of males. Bilateral findings of FAI-associated deformity were also more common in males than in females, both for cam-type deformity (24.7% vs 6.3%) and pincer-type deformity (21.7% vs 9.7%).
Sex-Based Differences in FAI Presentation
In males and females, the clinical presentation of FAI is similar—insidious onset of deep groin pain, often exacerbated with activity, and physical examination findings of decreased hip motion (particularly internal rotation) and a positive impingement test.3 Nevertheless, the sexes’ clinical presentation differs in several ways. Specifically, in a study using 3-dimensional CT to assess bony deformity in both symptomatic and asymptomatic patients, Beaulé and colleagues6 reported that alpha angles were significantly higher in symptomatic males than in symptomatic females (73.3° vs 58.7°). Hetsroni and colleagues7 recently reported similar results in a study of 217 symptomatic young adults treated arthroscopically for hip pain. Preoperative CT showed that alpha angles were significantly larger in males than in females (63.6° vs 47.8°). The authors postulated that females may be more likely to be symptomatic in the setting of smaller cam lesions because of the increased peak hip flexion and frontal plane motion commonly demonstrated by females during drop landings in sport. The authors further hypothesized that sex differences in muscle mass (which contributes to dynamic hip stability) and ligamentous laxity (a component of static hip stability) may result in larger physiologic ranges of motion for many females. As a result, bony impingement may occur in the setting of smaller anatomical lesions in females. The authors further noted that, compared with their male counterparts, females being treated for symptomatic FAI had significantly more femoral and acetabular anteversion.