Morphologic and radiologic anatomy of the occipital bone

Several diseases may cause craniovertebral instability warranting occiput-cervical fusion. As occipital screw and rod constructs are becoming more popular, requiring that screws be placed either medially or laterally in the occipital bone, the need for clearer anatomical and computed tomography (CT)-confirmed data regarding the relative thickness of the occiput in its various localities has become more critical. In 18 cadaveric specimens, the occipital bone was divided into 35 measurable segments. Transversely, the occipital bone was divided into five lines starting at the level of the inion; horizontal lines then proceeded inferiorly in 1-cm segments, 1, 2, 3, and 4 cm below the level of inion. In a comparable fashion, the occipital bone was divided vertically, starting at the midline, and proceeding laterally also in 1-, 2-, and 3-cm segments. Anatomical measurements of thickness were directly performed using a Vernier caliper. Results were directly correlated with axial CT measurements of bony thickness. Anatomical and CT measurements closely correlated within the same specimen, but there was significant interspecimen variability. The marked differences in the occipital bone anatomy noted between specimens indicates that patients undergoing occipital screw placement for cranial-cervical instability would benefit from preoperative occipital CT evaluations.