C1-2 Fusion in Atlantoaxial Dislocation and Basilar Invagination with and without Chiari Malformation: Clinical/Radiological and Craniometric Results.

OBJECTIVE: Patients with Chiari malformation (CM) associated with atlantoaxial dislocation (AAD) and basilar invagination (BI) may present with a small posterior cranial fossa, but data on the volumetric analysis are lacking. Additionally, whether additional foramen magnum decompression (FMD) is needed together with atlantoaxial fusion remains controversial. This study evaluated the volumetric alterations of the posterior cranial fossa in these patients and analyzed the radiological and clinical outcomes after posterior C1-C2 reduction and fixation plus C1 posterior arch resection. METHODS: Thirty-two adult CM patients with AAD and BI (CM-AAD/BI group) and 21 AAD and BI patients without CM (AAD/BI-only group) who received posterior atlantoaxial fusion plus C1 posterior arch resection were retrospectively studied. The clinical and radiological outcomes and volumetric measurements of the posterior cranial fossa were evaluated. RESULTS: The majority of CM-AAD/BI patients (94%) improved clinically and radiologically at 12 mo postoperatively, and none required additional FMD. Morphological analysis revealed a significant reduction in the bony posterior cranial fossa volumes of the CM-AAD/BI group (P < 0.01) and the AAD/BI-only group (P < 0.01) relative to those of the CM group. No significant differences were observed between the CM-AAD/BI and AAD/BI groups. CONCLUSIONS: Compared with patients with simple CM, patients with AAD/BI with or without CM demonstrated a considerably and equally reduced bony posterior cranial fossa volume. No additional FMD is needed in the treatment of CM-AAD/BI patients after posterior reduction and fusion plus C1 posterior arch resection.

Indirect Ventral Brainstem Decompression by Posterior C1-C2 Distraction and Fixation for Basilar Invagination.

OBJECTIVE: Basilar invagination usually shows a decrease of clivus axis angle (CAA), which could give rise to progressive neural compression. Exploring a safe and effective fixation technique to achieve atlantoaxial stability and neural decompression remains necessary. In this study, we introduce a modified posterior C1-C2 distraction and fixation technique by which we obtained indirect ventral neural decompression and atlantoaxial stability in a series of patients with decreased CAA. METHODS: Thirty patients of basilar invagination were enrolled in our series. All patients underwent thin-slice computed tomography (CT) scan, magnetic resonance imaging, and dynamic plain radiography examinations before surgery, at discharge and during the follow-ups. Posterior C1-C2 facet joint release and intraoperative reduction by fastening rods were performed in all patients. The CAA was measured on midsagittal CT scans. Patients' neurologic status was evaluated by the Japanese Orthopaedic Association score. RESULTS: No neurovascular injury and serious postoperative complication occurred in all patients. Complete ventral brainstem decompression was achieved in 20 patients and partial in 10 patients. The mean postoperative CAA significantly improved to 132.6 degrees compared with the preoperative 123.6 degrees (P < 0.01). The bone fusion was confirmed in all patients on the basis of the last follow-up spine CT scans. CONCLUSIONS: Indirect ventral brainstem decompression by posterior C1-C2 distraction and fixation is a safe and effective technique for treatment of basilar invagination.

Chiari Malformations Type I without Basilar Invagination in Adults: Morphometric and Volumetric Analysis.

BACKGROUND: Chiari malformation type I (CMI) cases are frequently associated with basilar invagination (BI), which complicates the understanding of the pathology of CMI. We specifically evaluated the morphometric and volumetric alterations in the bony structures of CMI patients without BI. METHODS: Fifty adult CMI patients without BI treated at our institution from January 2015 to December 2019 were retrospectively studied. The morphometric and volumetric characteristics of the posterior cranial fossa (PCF) were analyzed using thin-slice computed tomography images. RESULTS: Compared with the controls, the clivus length (P < 0.001), supraoccipital length (P < 0.001), Klaus height index (P < 0.001), axial length (P < 0.001), clivo-axial angle (P < 0.001), tentorial angle (P < 0.05), and bony PCF volume (P < 0.001) of the CMI-only group were significantly smaller, and the distance between the Chamberlain line and the dens axis (P < 0.001), clivus angle (P < 0.001), and basal angle (P < 0.001) of the CMI-only group were significantly larger, while the distance between the McRae line and the dens axis, McRae line, anteroposterior diameter of the PCF, occipital angle, occipital canal angle, and tentorial Twining line angle showed no significant difference between the 2 groups. CONCLUSIONS: Hypoplasia of the clivus and occipital bone were confirmed in CMI patients without BI, thus providing further evidence for the notion that CMI is secondary to the underdevelopment of the PCF.