Acetabulum-head index in children with normal hips: a radiographic study of 154 hips.

The acetabulum-head index (AHI), which is used to assess femoral head coverage on plain radiographs, was measured in 77 children (154 hips) with normal hips aged 2 to 14 years. The mean AHI value was 94 (range, 79-114). Both the intraobserver and the interobserver reproducibility of the measurements was high. The AHI values tended to decrease with increasing age. The mean AHI minus 2 standard deviations, which was used to define the border value for subluxation of the femoral head, was 80. The authors propose that an AHI < or = 80 can be used as a guideline to reveal abnormal lateral displacement of the femoral head in children.