Biomechanical modelling of a direct vertebral translation instrumentation system: preliminary results

Many new spine instrumentation concepts were introduced in recent years, like the incremental direct vertebral translation. The objective was to develop a biomechanical model in order to analyze the biomechanics of this instrumentation system. The patient-specific spine model was built using the 3D reconstruction based on bi-planar radiographs of a scoliotic patient (thoraco-lumbar Cobb: 49 degrees ). The mechanical properties were derived from literature, experiments on cadaver spines and patient's side bending radiographs. Each screw construct was modelled by four rigid bodies connected each other by kinematic joints. The screw-vertebra flexible joint was represented by 3 experimentally derived non-linear springs, and the rods by non-linear flexible elements. The correction manoeuvres were simulated by lowering the rod, tightening the crimps (incremental segmental translation) and applying secondary correction manoeuvres (direct vertebra derotation, compression, distraction and construct tightening). The simulations showed that the system allows a good force distribution among implants. The long post pushing and pulling contributed, to a great extent, to a global correction in the coronal plane, while the crimp tightening had more important effect in the sagittal plane. The preliminary results illustrated the effectiveness of local correction by a direct vertebra translation technique. Our next step is to validate the model and compare the performance of this strategy with other spinal instrumentation systems.