Secondary ossification centre at the acetabular dorsal rim in a dog: radiographic and MRI observations

The aim of this study was to describe development of the secondary ossifica tion centre at the acetabular dorsal rim in a dog (os coxae quartum) using radiographic and magnetic resonance imaging (MRI). A female Beagle dog underwent weekly hip radiographic and MRI examination, from 12 to 22 weeks of age. On radiographic examination, the os coxae quartum was visible from 16 to 22 weeks of age. Using MRI, the cartilage, in which the os coxae quartum appe ared, had an estimated cranio-caudal length of 4.5 mm, a maximal width of 5 .0 mm from 12 to 16 weeks of age; 2.7 mm and 6.0 mm from 17 to 19 weeks of age. The os coxae quartum was observed at 20 and 21 weeks of age, it had an estimated cranio-caudal length of 2.7 mm and a maximum width of 4.5 mm. MRI might have failed to give satisfactory measurements of the craniocaudal length of the cartilage and of the os coxae quartum but width measurements were indicative of real sizes. This study contributed to an estimation of the shape of the os coxae quartum and permitted a representative drawing to be made. MRI enabled the progressive ossification of the acetabular dorsal rim to be followed : it could help to determine whether or not the develop ment of the os coxae quartum is aplastic or hypoplastic in cases of canine hip dysplasia.