Relevance of C1/2 Facet Configurations and Clivus-Canal-Angles for Adult Patients with Chiari I Malformation with and without Basilar Invagination

OBJECTIVE: C1/2 facet configurations and clivus-canal-angles (CXAs) have been proposed as criteria for posterior fusion in Chiari I malformation (CMI). METHODS: Three-hundred and forty adults with CMI without basilar invagination (BI), 111 with CMI with BI, and 100 age- and sex-matched controls were studied using sagittal T2-weighted magnetic resonance imaging scans analyzing preoperative and postoperative values with their impact on progression-free survival rates. RESULTS: For CMI without BI, C1/2 facet configurations and CXA were similar to controls (142 +/- 11 degrees and 144 +/- 10 degrees, respectively) with low rates for posterior C1 displacements (7.1% and 10%, respectively). In CMI with BI, C1 facet displacements were common (54.9%) with lower CXA (120 +/- 15 degrees). After foramen magnum decompression (FMD) in CMI without BI (n = 169), 1.8% developed posterior C1 facet displacements without CXA changes and a 97% progression-free survival rate for 10 years. In CMI with BI, patients without ventral compression or instability underwent FMD without fusion (n = 19). Among them, 5.3% developed a posterior C1 facet displacement without CXA changes and a 94% progression-free survival rate for 10 years. The remainder of CMI with BI underwent FMD with C1/2 fusion (n = 48). Among these, CXA values increased with 10-year progression-free survival rates of 74% and 93% with and without ventral compression, respectively. CONCLUSIONS: For adult CMI without BI, C1/2 facet configurations and CXA are irrelevant. FMD alone provides excellent long-term outcomes. In CMI with BI, anterior C1 facet displacements indicate C1/2 instability. Posterior fusions can be reserved for patients with ventral compression or C1/2 instability.