Angular stable anterior plating following thoracolumbar corpectomy reveals superior segmental stability compared to conventional polyaxial plate fixation

Study Design. Biomechanical in vitro testing of primary and secondary stability in 12 human thoracolumbar spinal specimens using a spine simulator. Objective. In a corpectomy model anterior plate systems were investigated for their ability to restore spinal stability particularly focusing on the influence of angular stability, bone mineral density (BMD) and failure mode. Summary of Background Data. The concept of isolated anterior column reconstruction following thoracolumbar fractures using newly developed minimally invasive spine surgical techniques has attracted major clinical interest. In analogy to angular stable plate systems in long bone fixation the application of locking plates to the spine is aimed to limit loss of reduction and to improve stability. Methods. Twelve human spinal specimens (Th11-L3) were tested in a 6-degree-of-freedom spine simulator under pure moments of 7.5 Nm to investigate primary and secondary stiffness of 2 different anterior reconstruction options: (1) Synex II cage and MACS TL polyaxial anterior plating system, (2) Synex II cage and ArcoFix angular stable anterior plating system. An increasing 4-step cyclic loading model was included. Results. The angular stable plate system showed superior stability compared to the nonangular system in axial rotation and lateral bending. Flexion/ extension loading demonstrated no difference between the systems in range of motion. A positive correlation between BMD and the number of load cycles until failure for the nonangular stable system (R-2 = 0.90) was found. Different failure modes were investigated for the plating systems. The MACS system showed loosening at the connection between screw and plate inducing tilting under flexural load and final failure. The ArcoFix system revealed increased stability under cyclic loading and failed by parallel sintering to the endplate. Conclusion. Anterior angular stable fixation showed higher primary and secondary stability following thoracolumbar corpectomy. In specimens with lower BMD the use of angular stable systems substantially increased stability. Angular stable systems, however, differ in the way of construct failure.