Summary
The biomechanical properties of the atlanto-axial joint in a young Yorkshire Terrier
dog with spontaneous atlantoaxial instability were compared to those of another young
toy breed dog with a healthy atlantoaxial joint. The range-of-motion was increased
in flexion and lateral bending in the unstable joint. In addition, lateral bending
led to torsion and dorsal dislocation of the axis within the atlas. On gross examination,
the dens ligaments were absent and a longitudinal tear of the tectorial membrane was
observed. These findings suggest that both ventral and lateral flexion may lead to
severe spinal cord compression, and that the tectorial membrane may play a protective
role in some cases of atlantoaxial instability.
Keywords
Atlantoaxial instability - biomechanics - dog