The C2 isthmus screw provided sufficient biomechanical stability in the setting of atlantoaxial dislocation

Abstract

Abstract Background. The emerging of the C2 isthmus screw fixation technique is gaining popularity in the setting of atlantoaxial dislocation or other conditions requiring fixation of C2. However, the biomechanical stability of this fixation is poorly understood. Purpose. To compare and elucidate the biomechanical stability of C2 pedicle screw (C2PS), C2 isthmus screw (C2IS) and C2 short isthmus screw (C2SIS) fixation techniques in atlantoaxial dislocation (AAD). Method. A three-dimensional finite element model (FEM) from occiput to C3 was established and validated from a healthy male volunteer. Three FEMs, C1 pedicle screw (PS)-C2PS, C1PS-C2IS, C1PS-C2SIS were also constructed. The range of motion (ROM) and the maximum von Mises under flexion, extension, lateral bending and axial rotation loading were analysed and compared. The pullout strength of the three fixations for C2 were also evaluated. Result. C1PS-C2IS showed the greatest decrease in ROM with flexion, extension, lateral bending and axial rotation. C1PS-C2PS showed the least ROM reduction under all loading condition than both C2IS and C2SIS. The C1PS-C2PS model had the largest von Mises under all directions followed by C1PS-C2SIS, and lastly the C1PS-C2IS. Under axial rotation and lateral bending loading, the three models showed the largest and least von Mises stress respectively. The stress of the three models was mainly located in the connection of the screw and rod. Overall, the maximum pullout strength for C2PS, C2IS and C2SIS were 729.41N, 816.62N, 640.54N respectively. Conclusion. In patients with atlantoaxial dislocations, the C2IS fixation provided comparable stability, with no significant stress concentrations. Furthermore, the C2IS had sufficient pullout strength when compared with C2PS and C2SIS. C2IS is an effective and safe fixation modality in the treatment of atlantoaxial dislocations.