Choosing the Route for Odontoidectomy for Type B Basilar Invagination: The Role of Boogard’s and Welcher’s Base Angles

Introduction: Basilar Invagination (BI) presentation varies from asymptomatic to symptoms related to instability at craniovertebral junction (CVJ) (e.g., cervical pain) or to direct compression of the brainstem by CVJ structures (i.e., neurologic deficit). When surgical treatment is indicated, the transoral approach has been considered the standard approach for odontoidectomy. However, Type B BI produces a more cranial disposition of the odontoid dens, making the transoral approach less suitable. We consider that Skull base angles (i.e., Welcher and Bogard angles) measured in preoperative Computerized Tomography (CT) imaging can have a role in the surgical approach decision-making process.

Methods: We performed a literature review regarding BI classification, Type B BI approaches, and skull base radiographic angles that can be measured through CT head and cervical spine scans. Finally, we expose a case review, focusing on the workflow to choose the most appropriate odontoidectomy route and postoperative outcome

Results: In Type B basilar invagination (BI), the atlantodental interval (ADI) is preserved. This invagination primarily results from occipital condyle and/or clivus hypoplasia, leading to a basal skull base angle (Welcher’s angle) inclined toward platybasia. These wide angles cause a cranial displacement of the C1-C2 complex, sometimes reaching a height similar to the sella. The Boogard angle, partially formed by the McRae line, indirectly indicates platybasia and the high position of C1-C2. A 51-year-old patient presented with facial and lower cranial nerve palsy and progressive quadriparesis. Diagnosed with Type B BI and clivus hypoplasia, the C1 and odontoid process were positioned posterior to the sphenoid sinus. An endoscopic transnasal odontoidectomy led to significant postoperative recovery, with gradual improvement in neurological deficits.

Conclusion : Welcher and Boogard angles above 143 not only indicate the presence of platybasia but also suggest that in cases where an odontoidectomy is planned, the transnasal approach is anatomical, straightforward, safe, and reliable in Type B BI.