Two alternative methods for the measurement of MZ distance on a hip radiograph

Background. The aim of this study was to assess the use of two alternative measurement methods for the MZ distance, the distance between the acetabular centre (M) and the femoral head centre (Z), on an anteroposterior hip radiograph.

Methods. The MZ distance was measured in 900 normal and 34 dysplastic adult hips by using the classic method (MZ-c) and two alternative methods (MZ-m and MZ-x). MZ-m utilized the vectorial distance between points M and Z, like MZ-c, but was expressed as a positive or negative value according to the localization of the points with respect to each other. MZ-x was the projected distance between points M and Z on the x-axis, which was the line perpendicular to the two parallel lines passing through points M and Z, and was expressed as a positive or negative value according to the localization of the points with respect to each other.

Results. There was a significant correlation between the centre-edge angle and the three measurement methods in both normal and dysplastic hips. The lowest normal limits for MZ-m and MZ-x in adult hips were considered to be –3 mm and –2 mm, respectively (point Z is lateral to point M). Intraobserver and interobserver measurement variations of both methods were less than 2 mm.

Conclusion. Both alternative methods were considered more useful than the classic one in the exact determination of the distance between the centres of the femoral head and the acetabulum.