Anatomical considerations for the placement of cervical transarticular screws.

OBJECT

the object of this study was to determine the safe screw placement technique for cervical transarticular screw fixation.

METHODS

twenty cadaveric adult cervical spines were studied. All soft tissues surrounding the cervical spinal nerves from C-2 to T-1 were dissected carefully to expose the lateral mass, facet joint, transverse process, vertebral artery (VA), and spinal nerves (ventral and dorsal rami). After the proper entrance and exit points for the transarticular screws were determined, posterior transarticular screw implantation was performed under direct visualization from C2-3 to C5-6. A CT scan was performed to check the screw placement. The angle and length of the transarticular screw trajectory, the distance between the tip of the screw and the VA, and the sagittal safety angle were measured on the CT scan. Statistical analysis was performed using ANOVA (p < 0.05). Sagittal and axial orientations of transarticular screws were carefully analyzed.

RESULTS

there was no nerve or artery impingement or penetration. The average caudal angle of the screws in the sagittal plane was 37.3° ± 5.0° and the lateral angle in the axial plane was 16.6° ± 4.6°. The average distance between the tip of the screw and the VA (the posterior border of the VA foramen) was 5.8 ± 1.5 mm. The average sagittal safety angle was 41.9° ± 5.6°. No difference was observed according to the vertebral level. The average bone purchase was 18.7 ± 1.4 mm. Bone purchase was significantly greater at C2-3 (23.2 ± 1.6 mm) than at C3-4 through C5-6 (17.2 ± 1.3 mm, p < 0.05).

CONCLUSIONS

this study establishes anatomical guidelines to allow for safe cervical transarticular screw insertion. The starting point of transarticular screws should be 1 mm medial to the midpoint of the lateral mass. The "ideal" drilling angle is approximately 37° in the inferior direction and 16° in the lateral direction for the C2-3 through the C5-6 levels. The screw should be directed as laterally as possible in the axial plane without causing the lateral mass to fracture and as caudally as the occipital bone permits in the sagittal plane. The ideal screw size would be 3.5 mm in diameter and 18 mm in length.