RESUMEN
PURPOSE: To analysis the effects of reduction of stiffness in transverse ligament and wire diameter on atlanto-axial posterior fusion and fixations using finite element model. MATERIALS AND METHODS: The finite element model of occipito-atlanto-axis was made based on cadaver with a normal occiput and cervical spine. In order to investigate the effects of the wire diameter on the posterior fusion and fixations, an unstable atlanto-axial finite element model was made to reduce stiffness of the transverse ligament orderly (50%, 75%, 100%). And the wire was tied according to Brooks and Jenkins'method after modeling the grafted bone to fit in the space between the atlas and axis. The wire diameter was 18G, 20G and 22G. Anterior atlanto-dens interval (AADI) was measured after placing 1.5 Nm pure flexion movement on the occiput. RESULTS: The AADI was increased as the stiffness of transverse ligament was reduced. In the case of 50%, 75% and 100% decrement, the AADI were 3.37 mm, 3.87 mm and 4.94 mm. In the case of 50% and 75% decrement, the sufficient fixations were obtained although we used thin wire. However, in the 100% decrement, the AADI was exceeded the 3 mm when we used thin wire (20G, 22G) and sufficient fixations were obtained with only 18G. CONCLUSION: The sufficient fixations were obtained regardless of wire diameter in the partial tear of transverse ligament. In the complete tear, the minimal 18G wire diameter was useful to acquire sufficient fixations. However, the results were derived from the finite element model analysis. Thus further verification should be necessary to confirm the results using cadaver experiments.