Jasmin Ordobazari, Charlotte Pfeiffer, Adriano Wang-Leandro, A. VolkHolger, Georga T. Karbe

Background
Atlantoaxial instability (AAI) in dogs, particularly in toy breeds, is a congenital or developmental condition characterized by excessive movement between the first (atlas) and second (axis) cervical vertebrae. Diagnosis typically involves radiography or advanced imaging, but optimal diagnostic markers and the role of computed tomography (CT) in assessing AAI severity are not fully established. This study aimed to evaluate CT-based morphometric differences in the craniovertebral junction of dogs with and without AAI and to identify anatomical variations that may predispose to or result from the condition.

Methods
A retrospective study analyzed CT scans from 67 dogs: 38 with confirmed AAI and 29 controls without AAI. Various morphometric parameters were measured on multiplanar reconstructed CT images, including the dens angle, length and height of the dens, atlas canal height, and atlantoaxial space. Measurements were compared between groups, and statistical models assessed associations between morphometric values and AAI diagnosis. Intra- and inter-observer reliability was also evaluated to ensure measurement consistency.

Results
Dogs with AAI exhibited significantly smaller dens dimensions (length and height) and larger atlantoaxial spaces compared to controls. The dens angle was also significantly increased in AAI dogs, indicating greater cranial deviation. Additionally, several parameters of the atlas and axis, such as reduced atlas canal height and altered shape of the atlantal fovea, differed between groups. Logistic regression identified dens angle and dens length as strong predictors of AAI. Measurement reproducibility was high, supporting the clinical utility of the proposed CT-based metrics.

Limitations
This study was retrospective and limited to dogs that had undergone CT scanning, potentially introducing selection bias. Additionally, the findings may not generalize to breeds outside of the toy and small breed groups predominantly represented in the sample. The lack of long-term clinical outcome data also limits correlation between morphometric findings and disease progression or prognosis.

Conclusions
CT-based morphometric analysis reveals distinct anatomical differences in the craniovertebral junction of dogs with AAI, particularly involving the dens and atlantoaxial space. These parameters can aid in the diagnosis and potentially in the surgical planning or prognostication for affected dogs. The high reliability of measurements supports their integration into routine diagnostic workflows.

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