Craniovertebral Junction Anomalies in Small Breed Dogs with Atlantoaxial Instability: A Multicentre Case–Control Study

Objective  The main purpose of this study was to define criteria to systemically describe craniovertebral junction (CVJ) anomalies and to report the prevalence of CVJ anomalies in small breed dogs with and without atlantoaxial instability (AAI). Materials and Methods  Retrospective multicentre matched case–control study evaluating magnetic resonance imaging and computed tomographic images of small breed dogs with and without AAI for the presence of CVJ anomalies. Results  One hundred and twenty-two dogs were enrolled (61 with and 61 without AAI). Only dogs with AAI had dens axis anomalies such as separation ( n  = 20) or a short-rounded conformation ( n  = 35). Patients with AAI were more likely to have atlantooccipital overlapping based on transection of McRae's line by the dorsal arch of the atlas (odds ratio [OR] = 5.62, p p p p p Clinical Significance  Small breed dogs with AAI are more likely to have other CVJ anomalies such as atlantooccipital overlapping or dens anomalies. The grade of brachycephaly does not differ between patients with and without AAI. Certain objective criteria from human literature were found useful for the assessment of both AAI and atlantooccipital overlapping such as McRae's line, Wackenheim's clivus line, and clivus canal angle. The classification criteria used can help to evaluate CVJ anomalies in a more systematic way.