Radiographic Evaluation of Atlas and Axis Anomalies in Toy Breed Dogs With and Without Atlantoaxial Subluxation.

Abstract

Atlantoaxial subluxation (AAS) is generally a congenital condition that mainly affects toy breed dogs. Previous studies in several toy breed dogs revealed that dogs with AAS had a relatively high proportion of incomplete ossification (IO) of the atlas and dens anomalies compared to dogs without AAS. These anatomical characteristics may be important in surgical decision-making. Thus, the present study evaluated morphological differences in the atlas and axis between Maltese dogs with and without AAS. The medical records of Maltese dogs with and without AAS from 2015 to 2020 were analyzed. Abnormalities of the atlas and axis were evaluated using computed tomography (CT). A total of 45 dogs were reviewed. Maltese dogs with AAS revealed a higher ratio of IO of the atlas (56%) than non-affected dogs (19%). Dens anomalies were identified in 78% of the dogs with AAS and in 26% of non-affected dogs. The shape of the atlas has been identified as thin, solid compact bone in Maltese dogs. Dogs that revealed IO of the dorsal arch of the atlas showed significantly lower CT values (in Hounsfield units) than dogs without IO. The CT values of the midline of the dorsal arch were significantly lower than those of the outer surrounding region. Dens hypoplasia was defined by measuring the dens-to-axis length ratio according to a previous study. A significantly lower ratio was identified in dogs with AAS than in non-affected dogs. The incidence ratio of abnormalities of the atlas and axis in Maltese dogs with AAS was similar to that of previous studies. The dorsal arch of the atlas is composed of thin cortical bone with a vulnerable midline region. As dogs with AAS are more likely to be afflicted with abnormalities in the atlas and axis, considering these morphological features is important when planning the surgical stabilization of AAS.