Basilar Invagination: From Diagnosis and Classification to Surgical Decision-Making

Abstract Basilar invagination (BI) is a complex and heterogeneous developmental anomaly of the craniovertebral junction (CVJ) characterized by the upward migration of the odontoid process into the foramen magnum. Although reduction and stabilization are standard procedures, they may not be indicated for all patient subgroups. to propose a pragmatic decision-making framework to identify patients who may be safely managed with limited posterior strategies versus more complex CVJ reconstruction. The historical evolution of BI classification was reviewed, with a focus on diagnostic craniometry and modern surgical paradigms. A decision-making framework was established based on existing concepts to help surgeons choose the best surgical approach for these patients. Patients are categorized into two primary groups: Type A (clearly unstable): Defined by structural instability and atlantoaxial dislocation (AAD). These patients require C1–2 or C0–2 stabilization regardless of the degree of ventral compression. Type B (stable or potentially unstable): Defined by preserved mechanical alignment, often secondary to congenital clivus hypoplasia. Surgical management in this group is selective once it encompasses a heterogeneous population: patients with severe ventral compression or bone anomalies (e.g., atlas assimilation, C2–3 fusion) may have benefits from C1-2 or C0-2 fixation, while those with isolated Chiari malformation and no instability can be treated with posterior fossa decompression (PFD) and duroplasty. Not all cases of BI may require aggressive reduction and fixation. A selective approach based on the presence of AAD and risk factors for PFD failure optimizes clinical outcomes while preserving CVJ mobility and reducing hardware-related morbidity.