Vertical atlantoaxial dislocation as a cause of failure of midline fixation.

We report an 11-year-old girl who had previously undergone an operation for basilar invagination involving a foramen magnum decompression and midline wire fixation. After improving initially, her neurological condition worsened again. Repeated investigations showed a firm midline craniovertebral fixation and bone fusion. However, she was found to have a vertical mobile and reducible atlantoaxial dislocation. Treatment of the vertical dislocation by lateral mass fixation resulted in lasting relief from her symptoms. Vertical instability at the atlantoaxial joints needs to be identified and appropriately treated as it may be a cause of failure of midline fixation.