Canine hip dysplasia is a debilitating hereditary orthopaedic disease with a high prevalence in dogs. The aim of this study was to describe the influence of internal or external rotation of the femur on the projected radiographic position of the patella within the trochlear groove, and on the femoral head in relationship to the acetabulum in the standard ventrodorsal hip extended view, i.e., medial or lateral patella displacement, Norberg angle, femoral head subluxation index and femoral head subluxation category. Eleven adult dog cadavers of large and giant breeds were radiographed in standard ventrodorsal hip extended view and with internal and external femoral rotation. The medial and lateral patella displacement, Norberg angle, subluxation index and subluxation category variables were measured on radiographs, and analysed comparing the normal position with positions of different degrees of internal or external rotation of the femur. In the normal ventrodorsal hip extended view, the patella was centred within the distal femoral metaphysis (P > 0.05). A mean ± SD internal femoral rotation of 16.0 ± 5.9° resulted in a medial patella index displacement of 0.23 ± 0.09, and a mean external femoral rotation of 17.9 ± 6.7° resulted in a lateral patella index displacement of 0.24 ± 0.1. The mean Norberg angle was 105.3 ± 4.3°, 107.7 ± 5.5° and 104.2 ± 4.3° (P < 0.05); the subluxation index was 0.15 ± 0.06, 0.12 ± 0.05 and 0.18 ± 0.06 (P < 0.05); the subluxation category was 1.55 ± 0.6, 1.46 ± 0.7 (P > 0.05) and 1.96 ± 0.65 (P < 0.05) in normal, internal and external femoral rotation ventrodorsal hip extended views, respectively. In conclusion, as the Norberg angle, subluxation index and subluxation category are parameters used for classification in the main international hip dysplasia scoring systems, adequate femoral position with the patella centred in the distal metaphysis is of uppermost importance to ensure the technical quality of radiographs.
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