Anatomical feasibility for safe occipital condyle screw fixation.

Purpose: The occipital condyle (OC) screw can be a viable alternative option for the occipito-cervical fixation. However, the risk of vertebral artery (VA) injury during the direct OC screw fixation has not been adequately assessed. The purpose of this study was to establish the course of the VA (V3) relative to the nearby osseous structures to estimate the feasibility of OC screw fixation and describe its anatomical relationship depending on patient's age and sex.

Methods: A total of 387 three-dimensional computed tomographic angiograms (3D-CTA) were used and compared between two age groups. The vertebral artery diameter and two kinds of bony space were measured. The occipito-C1 arch space (O-C1S) and VA-occipital bone distance (VOD, six entry points) were measured on both sides. The feasibility of direct OC screw fixation can be represented by the VOD value; the minimum feasible value was determined to be 4 mm. Angular measurements (O-C1A and O-C2A) were also taken to assess their relationship to the bony space.

Results: The mean value of the O-C1S ranged from 9.0 to 9.9 mm. The mean value of the VOD ranged from 3.2 to 3.5 mm, and the proportion of individuals for which direct OC screw fixation was considered feasible ranged from 32 to 42 % in both age groups and there was no significant difference between two age groups. The VOD value was not affected by laterality or by gender (P > 0.05). The mean kyphosis of the O-C1A (-5° ± 5.2°, range -22° to 8°) was significantly smaller in the young age group compared to the older age group (-7.6° ± 5.3°, range -26° to 2°). The mean lordosis of the O-C2A (12.4° ± 6.4°, range 3°-33°) was significantly higher in the young age group compared to the older age group (10.4° ± 6.4°, range 0°-36°).

Conclusions: The direct OC screw fixation was not possible in a considerable number of cases due to the limited space and the position of the VA regardless of age group. Only about one quarter (21-24 %) of all patients was affordable to place the direct OC screw bilaterally. There was more space available to place the direct OC screw when the angle of the upper cervical spine is more kyphotic.