RESUMO
Objective To investigate the pattern of load transfer in cervical spine treated with Hybrid surgery using total disc replacement (TDR) and spinal fusion, so as to deepen the understanding of Hybrid surgery from the biomechanical view. Methods A finite element model of cervical spine C3-7 (INTACT model) was built to simulate three types of fusion surgeries at C4-6 degenerative segments: upper TDR combined with lower bone graft fusion (TDR45 model), upper fusion combined with lower TDR (TDR56 model), two-level fusion (Fusion456 model). Results In all surgical models, mobility of the fused levels was almost lost, while mobility of the TDR levels increased. Under the axial load of 160 N, the entire cervical motion was less than 4° in the INTACT model, while the motion in the TDR45 model and TDR56 model increased to 8.2° and 8.9°, respectively. In the TDR56 model, the force transferred through the C5 vertebra decreased by 20%, while the force transferred through the facet joint force was 3.8 times larger than that of the INTACT model. The facet contact force in the TDR45 model increased by 50%. The maximal stress in the INTACT model was 0.8 MPa, while the facet contact force in the TDR45 model and TDR56 model were almost 2 times as that in the INTACT model. Conclusions Due to the increased mobility at the TDR levels, the cervical curvature after Hybrid surgery changes greatly under the axial load. The alteration of spinal alignment will result in a decrease in anterior vertebral section force at the operative level, as well as an increase in facet joint force and facet cartilage stress.
RESUMO
Objective To analyze biomechanical properties of cervical spine after anterior cervical discectomy and fusion (ACDF) and total disc replacement (TDR) surgery. Methods Twelve cadaveric cervical spines (C2-T1) were adopted, and the motion and load distributions of the cervical segments under intact state and after ACDF and TDR surgery were tested using a three-dimensional (3D) optoelectronics measurement system. All the tests were carried out with displacement control in directions of flexion (Flex), extension (Ext), left bending (LB), right bending (RB), left rotation (LR) and right rotation (RR). Motion characteristics of the normal cervical spine and the implant were also discussed. Results In TDR-treated specimens, range of motion (ROM) was well preserved and could restore to the normal ROM distributions, especially in Flex/Ext and LR/RR direction. While in ACDF-treated specimens, ROM presented a large decrease as much as to 73.41% under the same condition compared with TDR, and ROM distributions were also changed obviously in other motions for the segments. Significant changes of ROM in LB/RB direction occurred in both TDR and ACDF group, which were up to 45.92% and 108.06%, respectively. The experimental data indicated that the normal motion of cervical spines was a 3D coupled motion, especially in LB/RB direction, where a 35% rotation around X-axis existed. The cervical spine could recover close to normal coupled motion after TDR surgery. Conclusions TDR surgery can restore the physiological motion of cervical spines more close to the normal state, especially in Flex/Ext and LR/RR direction. The study provides a theoretical basis and quantitative reference for TDR and ACDF surgery in clinic.