Acetabulum-head index measured on arthrograms in children with Legg-Calvé-Perthes disease.

The acetabulum-head index (AHI), which can be used to assess the lateral displacement of the femoral head, was measured on both plain radiographs and arthrograms in 37 children with unilateral Legg-Calvé-Perthes disease. For the identification of the outermost part of the acetabulum on the arthrograms, two measuring points were used: A. the lateral border of the bony acetabulum (arthrogram I) and B. the lateral border of the labrum (arthrogram II). The reproducibility of the measurements, evaluated by duplicate calculations of the AHI on arthrogram II, which was obtained from the unaffected hips, was high; the root mean square error of the AHI was 3.3. On the plain radiographs, an AHI of 80 or less could be used as a 'guideline' to reveal early subluxation of the femoral head. The corresponding figures on arthrograms I and 11 were 70 and 85, respectively.