Evaluation of the measurement methods for protrusio acetabuli in normal children

The normal values of the radiological lines most frequently used as references in the diagnosis of adult protrusio acetabuli were prospectively determined in 150 children (300 hips) without femoral pathology and aged between 2 and 15 years, mean age 8 years. The teardrop shape was modified according to the child's age, the "crossed" type predominating (48.7%). The mean centre edge (CE) angle fluctuated, with a median value of 31.2 degrees +/- 6.2 degrees, increasing with age and with slightly greater values in females; a large spread of CE angle values was observed within age groups. Acetabular ilioischial line values ranged from +7 to -5 mm, with a mean of +1.8 +/- 2 mm. This last method was the most suitable because it was not modified by changes in incidence of X-rays in radiological studies. With this method protrusio is diagnosed in children when the acetabular line is projected medially, provided that the ilioischial line is 1 or 3 mm or more for boys and girls, respectively. We conclude that the most trustworthy method available to evaluate PA in children should be "line crossing".