Center-edge angle (CEA) is a useful measure for defining both acetabular dysplasia (undercoverage) and global acetabular overcoverage causing pincer impingement (Fig. 45.2) (3). The CEA is defined as an angle formed by a perpendicular to the interteardrop line and the line passing from the center of the femoral head to the lateral edge of the acetabulum (4). An acetabulum with a CEA <20 degrees is considered to be dysplastic, with insufficient coverage and a risk of subluxation (3). An overcovered acetabulum has a CEA >35 degrees, which can cause pincer impingement between a normal femoral neck and the extended acetabulum with hip motion (3,5). Coxa profunda and protrusio acetabuli are two other terms that describe acetabular depth. These are defined based on the depth of the acetabular fossa on an AP pelvis x-ray relative to the pelvic ring. On the x-ray of a normal hip, the acetabular fossa lies lateral to the ilioischial line (Fig. 45.3) (6). In coxa profunda, the acetabular
fossa is deeper and touches or crosses the ilioischial line (Fig. 45.3) (6,8). However, several articles have shown that the depth of the acetabulum is not specific for the amount of acetabular overcoverage (5,9,10). As a result, CEA is now considered to be the more accurate determinant of global overcoverage. The deepest acetabular fossa occurs in protrusio acetabuli (Fig. 45.3) and occurs much more rarely than coxa profunda or global overcoverage. In a protrusio hip, the femoral head lies at or medial to the ilioischial line and the acetabular roof has a negative tilt. In addition, the center of the femoral head is medial to both the anterior and posterior acetabular walls (Fig. 45.3) (6,8).

Focally increased acetabular overcoverage occurs as a result of acetabular retroversion. In a normal hip, the acetabular opening is anteverted (Fig. 45.4). When the acetabulum is retroverted, the anterior edge of the acetabulum is more lateral and the posterior edge is more medial (Fig. 45.4) (11). As a result, the relatively more lateral edge of the anterior acetabulum can cause pincer impingement when the hip is flexed (12). Acetabular retroversion can be either focal (also described as cranial) and due to relative overcoverage of the anterior–superior acetabular rim or global and due to rotation of the entire hemipelvis (13,14,15,16). This is a relevant question because it influences the recommended surgical procedure—for example, is the posterior coverage sufficient enough for arthroscopic anterior rim trimming or should the patient undergo an anteverting periacetabular osteotomy to improve posterior coverage and stability? Retroversion can also coexist with acetabular dysplasia (17,18) and in the dysplastic hip, retroversion was associated with the earlier onset of symptoms as compared to patients with dysplasia and a normally anteverted hip (17).

There are several radiographic findings that are indicative of probable retroversion and pincer impingement (Fig. 45.5).

Isolated pincer impingement occurs predominantly in females (1). In Ganz’s original cohort of FAI patients, pincer impingement was more common in middle-aged women whereas young adult males were more likely to have cam impingement (1). Pincer impingement may, however, become symptomatic in adolescent patients who participate in sports or activities requiring an extreme range of motion (ROM), for example, dance or gymnastics (27). Combined cam and pincer impingement seems to occur with similar frequency in both males and females, generally making up slightly more than 40% of most cohorts (28,29). There may be an evolutionary reason for these demographic patterns. Examination of other mammal pelves reveals that cam morphology—a nonspherical femoral head on a short femoral neck—is common to mammals classified as “runners” and “jumpers” that require a sturdy hip but do not require much ROM (30). Conversely, mammals classified as “swimmers” or “climbers” and that require greater lower extremity ROM have rounder femoral heads. Because humans evolved from “climbers,” one hypothesis is that the “default femoral head” is rounder and that cam morphology in the human hip may be an adaptation in the evolution of humans as “running apes.” It has also been hypothesized that the increasing size of the human brain selected for pelvic width in females. A wider pelvis places the acetabuli farther apart and increases the abductor force necessary to maintain a level pelvis. If the acetabuli move relatively medial, the lever arm of the body weight shortens and reduces the required abductor force (30). This more medial position of the acetabulum results in coxa profunda (30).

The incidence of cam or pincer morphology appears to be quite high; in one asymptomatic cohort, nearly one-third of females and half of males had at least one predisposing factor for FAI (31). In addition, the majority of the findings were bilateral: 66% to 100%, depending on the specific morphology (31). The incidence of bilateral symptomatic FAI may be lower, on the order of 20% in some series (32).

Because of the biomechanics of FAI and hip motion, the pattern of labral and/or cartilage damage seen arthroscopically is usually reflective of the underlying bony morphology. Cam and pincer impingement both have characteristic patterns of intra-articular damage and the arthroscopic findings should confirm the planned bony procedure to address the FAI. Cam lesions cause acetabular cartilage delamination in about two-thirds of patients, (Fig. 45.7) due to the shear force of the bony bump against the cartilage. The cartilage lesion is typically at the anterior or anterosuperior rim of the acetabulum, and is located posteriorly less often (32,33,34,35). Patients with isolated pincer lesions tend to have more circumferential labral damage consistent with a crush-type injury (Fig. 45.7) (32). They are less likely to have anterosuperior acetabular cartilage delamination (33,36). Acetabular overcoverage appears to be slightly protective for patients with combined cam and pincer impingement because the overcoverage may limit the entrance of the cam deformity into the acetabulum (33,37). If the cam lesion is not identified or addressed at the time of surgery, isolated acetabular rim trimming has the potential to accelerate degenerative changes by improving hip motion. Labral calcification is thought to be common in pincer impingement; however, it appears that the labral crush injury results in callus formation and reparative tissue at the acetabular rim. Histologic examination of tissue taken during open acetabular rim trimming showed bony apposition pushing the labrum away from the acetabular rim (38,34). This is seen
most often in coxa profunda and results in a “double line” or recess that can be seen on the plain radiographs. Evidence of new bone formation at the acetabular rim can also be seen on an MRI of the hip (34,38). Patients with pincer impingement may also have secondary posterior impingement as they lever against the anterior rim to gain ROM, sustaining a contrecoup injury to the cartilage (1). This may occur more frequently in patients with protrusio acetabuli given the bony constraints to hip motion (19).