In case, instability at the atlanto-axial joint is detected, then the patient should mandatorily undergo a posterior stabilization. The term 'basilar invagination' was coined before the advent of the computed tomographic (CT) scan and magnetic resonance imaging (MRI), after assessing various craniometric lines and angles on plain radiographs or poly-tomograms. All these craniometric lines showed the relationship of the cranium with the axis (C2 body as well as the odontoid process) and the relationship between C1 and C2 was forgotten. The relationship of C1 and C2 could not be studied well in all dimensions on plain radiographs or on polytomography. Therefore, the high position of odontoid due to clival or occipital condylar hypoplasia or due to AAD could not be seen. Nowadays, with the advent of high quality CT scans, we can observe the anatomy of atlas (C1) and axis (C2) and their relationship very clearly in all the dimensions of space. Therefore, it can be clearly seen that the so- called BI is only due to clival hypoplasia or it is due to the abnormal relationship of C1 and C2, i.e., C2 invaginating into the C1 ring (which may be called axial invagination/vertical dislocation/central dislocation). The clival hypoplasia by itself does not require any treatment. The axial invagination is dislocation at the atlanto-axial joints and should, therefore, be reduced, stabilized and fused.
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