The study on the feasibility of posterior occipitoatlantoaxial transarticular screw fixation

Objective To explicit the entry point, optimal angle of insertion, and allowable range and length of the trajectory in posterior atlanto occipital transarticular screw fixation; To discuss the indication, complication and procedure of craniovertebral junction fixation with transarticular screws. MethodsPosterior occipitoatlantoaxial transarticular K wire implantation was performed under visual control on 20 dried bony specimens that contained complete atlanto occipital articulations, and cephalocaudal and lateral views were filmed by X ray, on which the optimal angle and length of the screw trajectory were measured. The atlanto occipital articulations of 30 normal adults were three dimensionally reconstructed by CT, on which the adjustable range of allowable angle of the posterior atlanto occipital transarticular screw trajectory was measured. Posterior atlanto occipital transarticular screw fixation was performed on 12 fresh craniovertebral junction specimens. Anteroposterior and lateral views were filmed by X ray and the three dimensions were reconstructed by CT postoperatively. Results Measurement of the dried specimens on analog X ray films showed that the atlanto occipital transarticular screw trajectory was upper tilting in sagittal plane and inside tilting in coronary plane; the ideal angle of the trajectory was: 53.3°±3.4° upper tilting in sagittal plane and 20.0°±2.6° inside tilting in coronary plane. Length of the trajectory was (29.28±2.46) mm. Measurement on three dimensional CT reconstruction showed that there was a range for adjusting the upper tilting angle between the direction of the trajectory and the sagittal plane, usually within 49.6°±2.0°. Likewise, there was also a range for adjusting the inside tilting angle in coronary plane, usually within 40.4°±3.4°. The distance between the dens of axis and the top of the anterior atlas was 9.80～17.7 mm, mean (13.43±1.93) mm. Imaging examination demonstrated that atlanto occipital transarticular screw fixation according to the above results ensured the insertion of the trajectory through the atlanto occipital articulation without running the risk of going into the neural tube. Conclusion Posterior atlano occipital transarticular fixation is clinically feasible; there is a definite direction for implantating the screws and an adjustable range for the allowable length and angle of the trajectory. When used in combination with C 1～2 transarticular fixation, posterior atlanto occipital transarticular fixation can be considered to be an ideal fixation modality for traniovertebral fusion. A good understanding of appropriate indication and a good command of the surgical skills are essential for the success of the procedure.