Is C1/2 fusion required for all patients with Chiari I Malformation or basilar invagination?


C1/2 fusion has been proposed as standard treatment for Chiari I Malformation (CMI) and basilar invagination (BI). The alignment of C1/2 facettes and the clivus canal angle (CCA) before and after surgery were analyzed as parameters for C1/2 stability and craniocervical sagittal balance.


All spinal malformations have been entered into a spinal cord register since 1991. The CCA and C1/2 facettes were analyzed in 391 patients with CMI and/or BI. Postoperative results were determined after 3 months and progression-free survival rates calculated with Kaplan-Meier statistics.


In 291 patients with CMI without BI, the CCA was in the normal range (143+11ᵒ) in contrast to 12 patients with BI without CMI (122+12ᵒ) and 88 patients with CMI+BI (120+15ᵒ; p<0.0001). The C1/2 facettes were in normal alignment in 94% in CMI without BI, 58% in BI without CMI and 36% in CMI+BI (p<0.0001). Anterior subluxations of the C1 facette occured in BI with ventral medullary compression only, posterior subluxations in all groups. In CMI without BI (n=132) and CMI+BI without ventral compression (n=29) posterior decompressions without fusion left the CCA unchanged (-0.4+3ᵒ and -0.1+5ᵒ, respectively) irrespective of facette orientation. In BI with ventral compression (n=36) decompressions with realignment and fusion increased the CCA from 113+16ᵒ by 6+7ᵒ on average. Clinical results after 3 months (improved status for 77% in CMI without BI, 86% with CMI+BI after decompression, 79% with BI after decompression plus fusion) and progression-free survival rates for 10 years (91% for CMI without BI, 77% for CMI+BI after decompression and 84% for BI after decompression plus fusion) showed no significant differences.


Patients with CMI without BI and BI without ventral compression do not require C1/2 fusion and can be managed by posterior decompression alone. Patients with BI and ventral compression should undergo posterior decompression, realignment and fusion.