Purpose or Case Report: Femoroacetabular impingement (FAI) results from incongruence of the femoral head and acetabulum, and is a clinical diagnosis supported by imaging findings. Despite the traditional categorization of FAI into “pincer” and “cam” types in young and middle-aged adults, the etiology is often unclear with contributing factors from both sides of the hip joint, as well as the surrounding muscles and tendons. Many patients first become symptomatic during adolescence. Comprehensive early treatment, which includes both surgery and intensive physical therapy, both relieves symptoms and prevents the premature onset of osteoarthritis. Pediatric radiologists must provide relevant and actionable reporting on pre-operative imaging in order to maintain value. In addition to a descriptive assessment, the most commonly used quantitative measurements are acetabular version, α angle, and femoral version.

This image-rich exhibit reviews common acetabular and femoral morphologies associated with FAI , outlines our low-dose CT protocol, and simplifies obtaining proper reformations and measurements. At our institution, we utilize a low-dose CT protocol (equivalent to approximately 3-5 AP pelvis radiographs) for pre-operative planning, which allows for easy creation of the 2-D and 3-D reformatted images.

Normally, the acetabulum is anteverted 10-15 degrees to allow for physiologic movement. Decreased anteversion is correlated with pincer-type FAI. Measurement requires correction for pelvic tilt and is explained in Fig. 1. This method has been shown to be equivalent to the more complicated 3-D measurements.

The α angle is obtained from radial reformations. A normal α angle is 55-60 degrees or less, and an increased α angle is associated with cam-type FAI. Cam-type FAI most often results from deficient femoral head-neck offset in the anterosuperior quadrant, and α angles should be reported for each position in that quadrant. Creation of radial reformations and measurement of the α angle are explained in Fig. 2.

Assessing femoral version is important because many pediatric conditions that lead to FAI are associated with abnormal femoral version, including developmental hip dysplasia, Legg-Calve-Perthes disease, slipped capital femoral epiphyses, and septic arthritis/osteomyelitis. The femur is normally 10-20 degrees anteverted. Both decreased and increased femoral version are associated with FAI. The method for calculating femoral version is explained in Fig. 3.
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