The effect of the parameters of posterior occipital condylar screws on the safety of screw placement

Abstract

Objective To investigate the safety of the occipital condylar screw with vertical position and evaluate the selection strategy of the posterior approach of the posterior occipital condylar screw in Chinese people. Methods The clinical imaging data of 60 outpatients from September 2013 to September 2015 were retrospectively analyzed, 36 male and 24 female, the average age was 41.6±9.2 (range from 25-58), Excluded occipitocervical injury, tumor and deformity patients. We built a three-dimensional digital model and simulated placing screw by utilizing CT data on Mimics software, after that we took the occipital condyle posterior medial and lateral midpoint as the entry point, then made 2 points equidistantly to the midpoint in vertical direction. We put 3.5 mm diameter virtual screws in 4 different conditions: largest cranial angle, smallest cranial angle, longest screw path and shortest screw path. Then we assessed the anatomical relationship between the screw and the hypoglossal canal or the atlanto-occipital joint by a three-dimensional window and measured the cranial angle, medial angle and length of screw path, then calculated the safety angle of the cranial angle, the successful rate of setting screw, and compared the safety of different screw points by 3-Matic software. Results 120 occipital condyles were obtained from the CT data of 60 patients by Mimics software. There was no significant difference in the data of the cranial angle, medial angle, safety range and length between both left and right sides. The obtained safe cranial angle of each point respectively was 20.9°±6.0° (lowest point), 17.0°±6.2° (middle point), and 11.6°±7.1°(top point), obviously the largest angle was in the lower point and the smallest was in the top point. The difference was statistically significant. We then acquired the successful rates of different cranial angle of each point, the highest successful rate was 99.17%, 96.67%, 74.17% in lowest, middle and top point when cranial angle were 3°or 4°, 3°and 0°respectively. The successful rates of lower point and middle point were significantly higher than the top point, and the difference was statistically significant. The medial angle parameters obtained were 34.41°±2.59° on left and 34.06°±2.44° on right, and there was no significant difference. The length parameters of the longest screw path acquired were 23.09±1.47 mm, 22.84±1.40 mm and 23.15±1.45 mm at top, middle and lowest entry point. The average value of shortest screw path of each point was 21 mm, and there was no significant difference among every entry point. Conclusion Among the occipital condyle posterior screw entering points, selecting the lower point can improve the success rate and safety; the change of nail enter point in the vertical direction has little effect on the length of the nail. We can increase the safety and reduce the risk of occipital condylar screw placement as far as possible through the three-dimensional digital technology. Key words: Internal fixators; Cervical vertebrae; Image processing, computer-assisted; Treatment outcome