Effects of pelvis rotation on the projected radiographic position of the femoral head in relationship to the acetabulum

The aim of this study was to describe the influence of pelvis rotation on the projected radiographic position of the femoral head in relationship to the acetabulum in the standard ventrodorsal hip extended view, i.e. Norberg angle, pelvic thickness at the level of the cranial effective acetabular rim, femoral head subluxation index, femoral head subluxation category and femoral head-acetabular distance. Twelve adult dogs of large and giant breeds were radiographed in standard hip extended view and with 2°, 4° and 6° of rotation on the right and left sides. Norberg angle, pelvic thickness, subluxation index, subluxation category and femoral head-acetabular distance were measured on radiographs and analysed comparing a normal position with positions of different degrees of rotation for the dependent and non-dependent side. All the studied parameters showed adequate repeatability. Norberg angle and femoral head-acetabular distance did not show significant mean differences in all degrees of rotated pelvis (P > 0.05). The pelvic thickness showed an increase with the degree of rotation on the dependent side (P < 0.05) and a slight decrease on the non-dependent side (P > 0.05). The subluxation index and subluxation category showed an increase with the degree of rotation on the dependent side and a decrease on the non-dependent side (P < 0.05). As the subluxation index and subluxation category are parameters used in the classification of the main international hip dysplasia scoring systems, pelvic rotation can impair the final hip score, especially in scoring systems that are based on the worst joint.