Computer Analysis Of Atlantoaxial Stability And Its Clinical Application

The atlantoaxial segment is the most mobile and fragile of the whole human spine. Loss of its stability will not only result in local functional disturbance but paraplegia or even death. Our study begins with a practical problem we encountered which is how we select the optimal neural arch interval in posterior atlantoaxial fusion, and tends to focus on the differences in relative stability under three-dimensional motion (rotational translation). The optimal position is then determined by means of the coupling (or contact) area calculation and the static mechanical analysis of each part of the joint under different position, and a theoretical basis is provided in how to obtain better instantaneous stabilizing effect on the fusion. The author proposed a new point of view that the coupling area can be used to evaluate relative degree in stability under dynamic motion. We first calculated the dynamic changes in the coupling area between anterior atlantal tubercle and dentoid process via computer model and approved consequently that i) the coupling area of every component joint change synchronically with the same direction; ii) larger coupling area indicate better stability. And then we concluded that a) different moving position corresponds to different degree in stability and the most stable position in atlantoaxial joint is the neutral one; b) the coupling area of the joint may reflect its degree in stability, i.e. larger the area, better the stability and vice versa; c) while the stabilizing mechanism of the atlantodentoid joint is damaged, the relative position between atlas and axis become so important that anterior displacement will occur as the neural arch interval is excessively wide, which is in flexion ; and conversely posterior displacement will occur as excessively narrow, which is in extension; d) the neutral position is the optimal one and the correspondent neural arch interval is the optimal interval on doing the surgery. The study belongs to exploring research without the existed experience by which we can learn in respect to the viewpoint, subject and methodology. Therefore shortcomings or even mistakes cannot be avoided. We hope that more guide and constructive suestion will be offered to facilitate our successive study in this subject.