Measuring the Acetabular Index: An Accurate and Reliable Alternative Method of Measurement

OBJECTIVE. In children (4 months to 8 years old), radiographic measurements of the acetabular index are the preferred method to assess developmental hip dysplasia. However, the acetabular index has been criticized as having variable reliability owing to difficulty identifying the correct anatomic landmarks. An alternative method of measuring the acetabular index using the ischium is being proposed to avoid the variability of the triradiate cartilage line as a reference point. With the alternative method, the acetabular index is derived by measuring the angle between a line connecting the ischial tuberosities and a line connecting the inferomedial and superolateral edges of the acetabulum. The purpose of this study was to evaluate the accuracy and reliability of this alternative method of measuring the acetabular index compared with the traditional method. MATERIALS AND METHODS. Children 4 months to 8 years old who presented for evaluation of developmental dysplasia of the hip were included. Two physicians, each using both the traditional and the alternative method, measured acetabular indexes on all radiographs. Accuracy was defined as mean absolute error less than 6 degrees. Reliability was calculated by means of intraclass correlation coefficient (ICC). RESULTS. Pelvic radiographs of 40 children (324 hips) were included. The mean age was 23.7 months (range, 4-96 months) and mean acetabular index was 24.2 degrees (range, 8-50 degrees). The alternative method was associated with mean absolute error of 2.50 degrees, which is significantly below the threshold of 6 degrees (t < 0.001). Intrarater reliability for the traditional method was high (ICC, 0.81) and for the alternative method was very high (ICC, 0.92). Interrater reliability for the traditional method was high (ICC, 0.89) and for the alternative method was very high (ICC, 0.91). CONCLUSION. Measuring the acetabular index using the alternative method has very high accuracy and intrarater and interrater reliability.