Accessory bone clasping the C5 spinous process in a case of craniovertebral junction instability

We report the case of a 13-year-old male with craniovertebral junction anomaly characterized by Group A basilar invagination and atlantoaxial instability, presenting with progressive neurological deficits. Imaging revealed an unusual accessory bone in continuity with the C5 spinous process, extending inferiorly toward the posterior elements of C6–C7, appearing to “clasp” the spinous process. The possible developmental origin and adaptive biomechanical role of this anomalous bone are discussed. Following atlantoaxial stabilization, the patient showed significant clinical improvement.