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

The acetabulum-head index (AHI), which can be used to assess the lateral di splacement of the femoral head, was measured on both plain radiographs and arthrograms in 37 children with unilateral Legg-Calve-Perthes disease. For the identification of the outermost part of the acetabulum on the arthrogra ms, two measuring points were used: A. the lateral border of the bony aceta bulum (arthrogram I) and B. the lateral border of the labrum (arthrogram II ). The reproducibility of the measurements, evaluated by duplicate calculat ions of the AHI on arthrogram II, which was obtained from the unaffected hi ps, was high; the root mean square error of the AHI was 3.3, On the plain r adiographs, an AHI of 80 or less could be used as a 'guideline' to reveal e arly subluxation of the femoral head. The corresponding figures on arthrogr ams I and 11 were 70 and 85, respectively.