New Radiographic Index for Evaluating Acetabular Version

Background Several qualitative radiographic signs have been described to assess acetabular retroversion. However, quantitative assessment of acetabular version would be useful for more rigorous research purposes and perhaps to diagnose and treat hip disorders.

Questions/purposes We developed a new quantitative index for acetabular version (p/a ratio). We determined the average p/a, compared it with previous radiographic signs for acetabular retroversion, and evaluated its relationship with anatomic acetabular version.

Methods We calculated the p/a ratio by measuring p (distance from acetabular articular surface to posterior wall) and a (distance from acetabular articular surface to anterior wall) on plain hip AP radiographs and dividing p by a. P and a were assessed on the perpendicular bisector of the line between the teardrop and the lateral edge of the acetabulum. Using 185 hip radiographs from patients with suspected idiopathic osteonecrosis, we measured p/a and compared it with previous qualitative signs for acetabular retroversion. Using 62 hip CT images from patients with no osteoarthritis, we measured the anatomic anteversion at the height of the central femoral head and investigated its relationship with p/a.

Results The average p/a was 2.05 in 185 hips, and most patients with a p/a greater than 2.05 had a negative qualitative retroversion sign. A correlation was observed between central anteversion and p/a (r = 0.84).

Conclusions We believe this ratio can be considered a simple quantitative parameter to assess acetabular version using plain AP radiographs.

Level of Evidence Level III, diagnostic study. See Guidelines for Authors for a complete description of levels of evidence.