Morphometric study of the atlantooccipital and the lateral atlantoaxial joints in small breed dogs with and without atlantoaxial instability.

Introduction: The combination of atlantoaxial instability (AAI) with several abnormalities and malformations of the craniovertebral junction in small breed dogs has been described, however there is limited knowledge regarding morphometric alterations of the synovial joints in this region. This study aimed to evaluate the morphometric differences of the lateral atlantoaxial and the atlantooccipital joints between dogs with and without AAI using computed tomography (CT) scans. Materials and methods: This retrospective observational case-control study analyzed multiplanar reconstructed CT scans of 33 small and toy breed dogs (11 with AAI and 22 controls without AAI). The depth-to-length ratios of the articular surfaces of the atlantooccipital and lateral atlantoaxial joints were measured in dorsal and sagittal reconstructions and tested for significant differences between AAI and control groups. Results: The depth-to-length ratio of the occipital condyles in sagittal ( Discussion and conclusion: The atlantooccipital joint surfaces were more shallow in small breed dogs with AAI compared to controls. These results suggest that a certain degree of atlantooccipital joint dysplasia may be more commonly present in dogs with AAI, whereas morphometric alterations of the lateral atlantoaxial joints appear more complex and less consistently differentiated between groups.