Spectrum of radiographic femoroacetabular impingement morphology in adolescents and young adults: an EOS-based double-cohort study.

BACKGROUND: Symptomatic femoroacetabular impingement is a known prearthritic condition. Impingement morphology is poorly defined in the adolescent population. The purpose of this study was to document the prevalence of radiographic impingement morphology in adolescents with no symptomatic hip problems.

METHODS: Ninety anteroposterior images of the hip in forty-five consecutive adolescents with scoliosis met the inclusion criteria. Sex distribution was equal. The second cohort (ninety hips) was an age-matched group with no scoliosis. None had symptomatic hip problems. Images were analyzed for coxa profunda, protrusio acetabuli, Tönnis angle, anteroposterior alpha angle, center-edge angle, acetabular crossover, ischial spine sign, and neck-shaft angle.

RESULTS: Of the 180 hips, 92.8% demonstrated at least one parameter suggesting impingement morphology, whereas 52.2% showed at least two signs. Evidence of coxa profunda was seen in 81.7% of the hips, while a negative Tönnis angle was seen in 31.1% and a center-edge angle indicative of acetabular overcoverage was seen in 15%. An acetabular crossover sign was detected in 27.2% of the hips, while an abnormal anteroposterior alpha angle was found in 5.6% of the hips in male patients and 6.7% of the hips in female patients. Statistical analysis revealed that abnormal alpha angles (p = 0.029), crossover signs (p = 0.029), and ischial spine signs (p = 0.026) were more common in the cohort without scoliosis, and coxa profunda was more common in females (p = 0.034).

CONCLUSIONS: There was a high prevalence of radiographic impingement morphology beyond the spectrum of normal in this double-cohort study of adolescents. Femoroacetabular impingement remains a dynamic problem, and we caution against relying only on the use of hard-set static radiographic parameters when evaluating femoroacetabular impingement. This study raises the important question of what morphologic characteristics should be defined as abnormal, when at least one finding of impingement morphology is noted in such a large segment of the population. On the basis of the normative data obtained, reference values for radiographic parameters of femoroacetabular impingement morphology should be redefined. Normal values for a Tönnis angle were between -8° and 14°, the upper limit of the center-edge angle was 44°, and the normal values for femoral neck-shaft angle were between 121° and 144°. Surgical indications should be tailored to physical examination findings and not radiographic findings alone.