Center of the femoral head: a magnetic resonance imaging study.

Determination of the center of the femoral head (CFH) may be difficult because of incomplete epiphyseal ossification of the femoral head in children younger than 8 years. The purpose of this study is to find a practical method for determining the center of the femoral head in normal hips of children younger than 8 years. Twenty-seven children who had magnetic resonance imaging (MRI) examinations of their hips (N = 54) were reviewed retrospectively. The average age of the subjects was 4.7 years (range, 1.2-8 years). Measurements were performed on the closest section to the midcoronal plane of the femoral head (the widest spherical femoral head seen on the MRI section). The CFH on the MRI section was found using a constructed circular frame by using a computer program. The medial and lateral edges of the proximal femoral ossific nucleus were marked, and the midpoint of the ossific nucleus adjacent to the growth plate (MPON) was determined. The distance between the MPON and the CFH was calculated. The average distance was 1.5 mm (range, 0-5 mm). The distance between these 2 points was less than or equal to 2 mm in 40 hips (74%). In conclusion, this study shows that the MPON can be used as a landmark to determine the center of the femoral head in normal hips of children younger than 8 years and can be used to measure the lower extremity mechanical axis.