Can Posterior Reduction Replace Odontoidectomy as Treatment for Patients With Congenital Posterior Atlantoaxial Dislocation and Basilar Invagination?

BACKGROUND: For patients with odontoid process protrusion and basilar invagination, posterior screw-rod fixation can usually achieve satisfactory horizontal reduction, but in some cases satisfactory reduction in the vertical direction cannot be achieved at the same time. OBJECTIVE: To propose a method for calculation of the theoretical maximum vertical reduction possible in individual patients. METHODS: The computed tomography imaging data of patients with occipitalization and basilar invagination who were treated at our institute between January 2013 and June 2016 were retrospectively analyzed. The direction of odontoid reduction was decided by the inclination of the lateral joint. The atlanto-dental distance was assumed to be the maximum possible reduction in the horizontal direction. The maximum vertical reduction possible was calculated based on these values. RESULTS: A total of 82 patients (34 males and 48 females) were included. The theoretical vertical reduction value was 4.2 +/- 3.0 mm, which was significantly smaller than that of the dental protrusion (14.5 +/- 3.8 mm, P = .000). Analysis of follow-up data (29 cases) showed that, the difference between the theoretical vertical reduction value H (4.7 +/- 3.5 mm) and the actual vertical reduction value Ha (5.6 +/- 3.5 mm) was not significant (P = .139). CONCLUSION: The theoretical calculation method we proposed can well predict the actual degree of vertical reduction. The theoretical vertical reduction value is significantly lower than the odontoid protrusion value, indicating that satisfactory reduction in the vertical direction is difficult with a posterior approach alone.