Incidence of Abnormalities of the Second and Third Cervical Vertebral Junction in Dogs with Atlantoaxial Instability: A Multicentre Study.

To evaluate the occurrence and frequency of abnormalities at the second and third cervical vertebral junction (C2/3) in dogs with and without atlantoaxial instability (AAI).  Retrospective multi-institutional case-controlled case series.  One hundred and seventeen dogs with AAI and 117 dogs without AAI.  Radiographs, together with computer tomographic images or magnetic resonance images or both, of the cranial cervical spine of dogs were reviewed for the presence or absence of intervertebral disc-related anomalies, osseous fusion of the vertebrae, spondylosis, or any other anomaly of the C2/3.  The incidence of anomalies affecting the C2/3 in dogs with AAI was 38.46% ( = 45) and in the control group it was 11.97% ( = 14). The majority of the observed anomalies involved the intervertebral disc. In conjunction with AAI, intervertebral disc-related anomalies were noted in 33.34%, spondylosis in 2.56%, osseous fusion in 1.71% and a hypoplasia of the spinous process in 0.85% of the cases. Summarized under the term intervertebral disc-related anomalies, a morphological alteration of the intervertebral disc was noted in 10 cases with AAI, characterized by a spherical outer shape and a minimally reduced size and a dorsal positioning in the intervertebral space.  There is a significantly higher incidence of anomalies affecting the C2/3 in association with AAI. In conjunction with AAI, intervertebral disc-related anomalies are the most frequent pathological finding affecting the C2/3.