Atlantoaxial Fixation for Basilar Invagination without Obvious Atlantoaxial Instability (Group B Basilar Invagination): Outcome Analysis of 63 Surgically Treated Cases.
BACKGROUND
We discuss the rationale of surgical treatment of group B basilar invagination by atlantoaxial facet joint stabilization and segmental arthrodesis.


METHODS
From January 2010 to April 2016, 63 patients with group B basilar invagination were surgically treated. All patients had varying degree of myelopathy-related functional disability. Fifty-two patients had both Chiari malformation and syringomyelia. All patients were treated by atlantoaxial plate and screw fixation with the techniques described by us in 1994 and 2004. Foramen magnum decompression or syrinx manipulation was not carried out in any patient. Occipital bone and subaxial spinal elements were not included in the fixation construct.


RESULTS
Three patients died in the immediate postoperative phase. In the remaining patients, there was clinical improvement and no patient's neurologic function worsened after surgery. In 12 of 38 patients in whom postoperative magnetic resonance imaging was possible, at a follow-up of at least 3 months, there was reduction in the size of the syrinx.


CONCLUSIONS
The pathogenesis of basilar invagination in group B is related to atlantoaxial instability. The clinical outcome suggests that the surgical treatment in these cases should be directed toward atlantoaxial stabilization and aimed at segmental arthrodesis. Inclusion of the occipital bone in the fixation construct is not necessary. Foramen magnum decompression and procedures involving manipulation of Chiari malformation and syringomyelia are not necessary.