Quantitative anatomy of the occiput and the biomechanics of occipital screw fixation

Study Design. The surgically relevant osseous anatomy of the human anatomy was carefully studied and described. The stability of cortical and cancellous screws placed in anatomic sites commonly used for internal fixation of the occiput was tested. Objectives. To define the bony anatomy of the occiput in quantitative terms and to measure the ability-of cortical and cancellous screws inserted at sites commonly used for internal fixation. Summary of Background Data. To the authors' knowledge, no previous studies described the gross anatomy of the occiput in specific relation to the internal venous structures in the cranium and to the biomechanical strength of screw fixation in different areas, of the occiput. Methods. Thirty-seven human occiputs were carefully measured using calipers. Thin sections from six such specimens were analyzed with specific attention to cortical thicknesses. Stability of screws placed:in various locations in the occiput were tested in axial pullout. Results. The thickness of the occiput varied from extremely thin to a 0.1-mm thickness in the region of the cerebellar fossa and increased to a maximum of 8.3 mm at the level of the superior nuchal line and at the transverse sulcus. Results of pullout testing showed that the cancellous screws were as strong as the cortical screws in this area. In areas of the occiput thicker : than 7 mm, unicortical fixation was as strong as bicortical fixation. Conclusion. There is a wide variation in thickness of the bone of the occiput. The strength of screw fixation was proportional to the bone's thickness.