In Reply: A Staged Approach for Surgical Management of Basilar Invagination

To the Editor: We thank Dr Bajaj for his instructive comment1 on our study about surgical management of basilar invagination.2 He advocates that all basilar invaginations can be managed with a single-stage surgery aiming at realigning C1 and C2, dispensing for the need of a direct anterior decompression. We do agree that, when basilar invagination is due to C1-C2 hypermobility as evidenced by anterior displacement of C1, pushing C2 in relation with C1 will ensure effective and indirect anterior decompression, but, while atlantoaxial realignment should be the aim whenever feasible, it might not always be the case. Indeed, there are cases of basilar invagination which do not involve C1-C2 displacement. For instance, basilar invagination might not be due to C1-C2 instability but rather to condyle hypoplasia, leading to posterior displacement of C0 relatively to C1 (Figure). In such cases, the “rocker” technique is useless because there is no C1-C2 reduction needed. Reduction of C0-C1 subluxation could be advocated but is technically challenging because the C1 lateral masses are very thin.FIGURE.: A case of basilar invagination due to occipital condyles hypoplasia where indirect anterior decompression could not be achieved with posterior approach: preoperative CT A and MRI B; postoperative CT C and MRI D. CT, computed tomography.This example also illustrate a second limitation to the “single-stage for all cases” approach: instrumentation of C1 or C2 may not always be technically feasible. In many cases of craniovertebral junction malformations, anatomical particularities (such as high riding vertebral arteries in C2, atlas assimilation, condyles hypoplasia, etc.) do not allow for safe placement of screws, hence limiting the possibilities of in situ reduction. Altogether, we agree that, if basilar invagination is due to C1-C2 subluxation and whenever these vertebrae can be safely instrumented, in situ reduction should be the aim to achieve indirect anterior decompression in a single-stage strategy (this is the middle stage of our surgical algorithm), but there will always be cases in which indirect decompression is not anatomically or technically achievable (or could not be attained during the initial posterior approach). In such cases, we argue that there is still room for direct anterior decompression.