| Objective: To describe the anatomical and morphologic characteristics of the congenital craniovertebral junction (CVJ) malformation, including assessment of the discrepancies of bony anomalies, variations of the route of vertebral artery (VA) in the subgroup with occipitalization of the atlas, biomechanical diversities of atlantoaxial facet joints morphology of occipitalization of the atlas, and anatomical suitability of screw fixation for the subgroup with C2-3 fusion.Methods: 41 patients with congenital CVJ malformation were analyzed. After spiral CT scan, the image files in D1C0M format were transferred to the three-dimensional visualization workstation based on a set of Amira software. Three dimensional (3D) models of occipito-atlanto-axial complex were constructed. Anatomical and morphological evaluations were examined by selective display and cutting, arbitrary rotation of the 3D models in the virtual environment. Also the related measurements were made with respect to the selected landmarks.Results: The congenital CVJ malformation compromises anomalies form occiput, atlas and axis. In this series, often the malformed osseous architectures were different combinations of platybasia, basilar invagination, occipitalization of atlas, and C2-3 fusion. Several uncommon anomalies were also found in association, such as unilateral atlantal lateral mass hypertrophy, spina bifida of axis, and occipital paracondylar process. Detailed analysis of the bony anomalies suggested the concomitant dysplasia was common in anomalies of clivus, occipital condylar, atlantal lateral mass, and atlantal posterior arch. Occipitalization of atlas in some cases manifested rotated fusion, representing the rotated misalignment of atlas to occiput. Comparative observations of bonyanomalies and variations of the route of VA in 23 cases of occipitalization of atlas revealed that the VAs may take several discrete routes to enter the crania in respect of the varied extent of fusion and dysplasia between atlas and foramen magnum. In 35 cases of occipitalization of atlas, three dimensional morphometrics of atlantal inferior facet were carried out, showing a great scope of variation of facet angles both on the sagittal and coronal planes. Excessive anterior tilting and/or asymmetry of atlantal inferior facet may be in relation to the various types of atlantoaxial dislocation. Anatomical measurement of C2-3 fusion for evaluation of screw placement, which were performed in 17 cases of C2-3 fusion, demonstrated that narrowing of C2 pars interaticularis or C3 pedicle, which caused unsuitability for a 3.5mm screw placement, on both side was seen in one or 3 cases, on single side was seen in 4 or 7 cases, respectively.Conclusion: 1. The essential configuration of congenital CVJ malformation is the diversiform segmentation failure on the basis of the dysplasia to various extents in occiput, atlas and axis; hence the prevalence of individual discrepancy regarding the detail anatomy. In those patients where existed the occipitalization of atlas, several regular patterns of intracranial route of VA were found, which may or may not resemble the normal anatomy. These patterns were accompanied by some detailed variation of the bony malformation, and also further suggested some embryologic discrepancy in the formation process of the unsegmented occipito-axial structure. The surgeons should familiar to the bony and vascular variations in order to avoid the unexpected injuries such as the compromise of VA during a surgery. 2. Prevalence of anterior tilting and asymmetric articulations can be seen in the load-bearing joints of atlantoaxial complex in patients with occipitalization of atlas. The resulting biomechanical changes may be a more proximate cause for atlantoaxial dislocation in these patients. 3. Although quite a few morphological variations was seen in congenital C2-3 fusion, application of screw fixation techniques in this structure was still feasible in majority of the patients.
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