gms | German Medical Science

Objectives: Instability and deformity of the occipital bone and upper cervical spine are serious pathological conditions. The most common intervention for instability at the craniocervical junction is posterior fixation of the occipital bone to the cervical vertebrae. Insertion of occipital bone screws requires evaluation of the occipital bone thickness for effective internal fixation in addition to the presence of the intracranial venous sinus for safe screw insertion without vascular injuries.

We created topographical maps of the occipital bone thickness and the presence of the venous sinus (VS) to assess the risk of screw insertion by comparing multiple commercially available occipital plates.

Methods: We analyzed 100 consecutive patients who had a computed tomography angiography (CTA) of the head and cervical spine at a single academic institution between 2008-2018. Our outcome measure were 1) topographical maps of the occipital bone thickness and the likelihood of venous sinus presence under the bone, and 2) risk of screw insertion comparing the topographic maps and screw hole configurations of four occipital plates.

First, we created a matrix of 231 measurements per patient (21 measurements per level) and analyzed the thickness of the occipital bone. For each measurement, we evaluated the presence the venous sinus. Additionally, we evaluated four occipital plates and their screw hole positions in relation to the occipital bone thickness and the presence of the venous sinus.

Results and Conclusion: The thickest point of the occipital bone measured 13.9 ±3.3mm and was at the middle of the occipital bone and 45mm from the foramen magnum around the external occipital protuberance (EOP). The regions of the occipital bone with thicknesses >8mm was 2cm lateral to the EOP at the level of the superior nuchal line and 2.5cm inferior to the EOP. The location with the highest presence of the VS was around the EOP and the superior nuchal line. The right transverse VS was more prominent in both genders. In our analysis, the percentage the VS was present varied for each occipital plate screw hole. Previous studies have shown 8mm as the minimum screw length to reduce the risk of implant failure. In our analysis, only "T"-shaped plates had screw holes with thicknesses >8mm for each screw hole, which could be shown in previous studies as the minimum screw length to reduce the risk of implant failure.

Our topographic maps show that the regions of the occipital bone with thicknesses with enough screw purchase were within a limited area. For every screw hole in our four analyzed occipital plates, there was a possibility of venous sinus presence ranging from 8-33% with the highest presence around the EOP. Our biometric map of the occipital bone thickness and the analysis of the presence of the VS can assist surgeons in preventing complications like VS injuries and to achieve a stable fixation.