Imaging description

Femoroacetabular impingement (FAI) is a common cause of hip pain and eventual osteoarthritis of the hip. It is screened for using an AP radiograph of the pelvis. The radiograph must be aligned as perfectly as possible with the coccyx centered at the symphysis pubis so that no rotation of the pelvis is present. Femoroacetabular impingement is broken down into three major categories: cam-type, pincer-type, and mixed type. Patients with the mixed type of FAI have coexistence of both the cam- and pincer-type. The cam-type can be thought of as abnormal formation/development of the femoral head/neck junction. This can be seen as an osseous bump or as a pistol grip deformity (loss of normal concavity) at the femoral head/neck junction (Figure 45.1). The pincer-type of FAI is abnormal coverage or positioning of the acetabulum. The pincer-type can be diagnosed on radiographs by the presence of coxa profunda, acetabular protrusion, or acetabular retroversion. Coxa profunda and acetabular protrusion are diagnosed radiographically by abnormal positioning of the femoral head and acetabulum relative to the ilioischial line (Figure 45.2). Acetabular retroversion can be diagnosed by presence of the cross-over sign, posterior acetabular wall sign, or the prominent ischial spine sign (Figure 45.3).

Importance

Femoroacetabular impingement is important to recognize in patients with hip pain as treatment of this disease has the best outcome if it is recognized early so that osteoarthritis has not yet developed. Development of osteoarthritis is felt to be the eventual outcome in symptomatic FAI patients if the disease goes untreated. The basic pathology is abnormal abutment of the femoral head/neck junction on the acetabular rim during flexion. This abutment causes labral and cartilage damage at the hip leading to incongruity of the hip joint. Continued usage of an incongruous joint leads to the development of osteoarthritis.