| Background:The optimal surgical technique for multilevel cervical degenerative disc diseases(DDD) has been controversial. Multilevel anterior cervical discectomy and fusion(ACDF) inevitably leads to greater loss of mobility in operative segments, impact on the biomechanics in the adjacent segments severely than single segment ACDF by biomechanical study. Multilevel artificial cervical disc replacement(ACDR) is theoretically better choice. Because the indication of ACDR is strict, certain segments in multilevel DDD may not be suitable for replacement.In adidition, multilevel ACDR increase the difficulty of operation, prolong operating time, and its possible complications are unknown. In view of this, cervical artificial disc replacement combined with segmental fusion(Hybrid surgery) is both a compromise solution, but also a promising surgery in treatment of multilevel DDD. There are few clinical studies reported that effect of Hybrid surgery in short-term at least is not worse than the traditional multilevel ACDF. At present, detailed comparison of two-level ACDR, two-level ACDF, two-level Hybrid surgery is still relatively scarce,and those conclusions are inconsistent; Furthermore, the biomechanical difference of Hybrid surgery with the different position combinations has not been reported. Therefore, we designed this study.Objectives:1.The biomechanical properties of two-level ACDR, two-level ACDF, and two-level Hybrid surgery were analyzed through the finite element method(FEM). The comparsion of biomechanical differences of three surgeries in operation segments and adjacent segments, can provide foundation for Clinic about biomechanics.2.Three pairs of two swapped-positions Hybrid surgery was constructed by ACDF and ACDR at two continuous cervical spine levels. Biomechanics in two-level Hybrid surgeries corresponding to each other were compared, and then to explore the origin of these biomechanical differences, to provide mechanical reference for the clinical implementation of Hybrid surgery about position combination of ACDF and ACDR.Methods:1.Finite element model of C2-T1 to be established and validated.The CT scan image from healthy volunteers as a data source, finite element model of C2-T1 was established using Mimics 10.1 software, Rapidform 2006 software and Abaqus 6.11 software. And its results were compared with previous literature. Finite element model was used for subsequent study when it is validated to be effective.2.The finite-element-based biomechanical analysis of two-level ACDR, two-level ACDF, two-level Hybrid surgery in C4-C6Arthroplasty was simulated through Prestige-LP prosthesis, and arthrodesis was simulated with anterior plate/screw/allograft. Two-level ACDR, two-level ACDF, two-level Hybrid surgery(C4-C5 ACDR combined with C5-C6 ACDF, Hybrid-M1 group) were constructed in C4-C6 according to the experimental design. Under the displacement controlled, we calculated the overall stiffness, range of motion(ROM), intradiscal pressures(IDP), facet joint Von Mises stresses of each groups, And Comparatively analyzed the mechanical parameters between the three groups.3. The finite-element-based biomechanical analysis of hybrid surgeries at two continuous cervical spine levels with swapped positionsArthroplasty was simulated with Prestige-LP prosthesis, and arthrodesis was simulated with anterior plate/screw/allograft. According to the experimental design, Hybrid-S1 group(C3-C4 TDR combined with C4-C5 fusion),Hybrid-S2 group(C4-C5 TDR combined with C3-C4 fusion), Hybrid-M2 group(C5-C6 TDR combined with C4-C5 fusion), Hybrid-U1 group(C5-C6 TDR combined with C6-C7 fusion)and Hybrid-U2 group(C6-C7 TDR combined with C5-C6 fusion)were constructed. Under the displacement controlled, we calculated the overall stiffness, ROM, IDP, facet joint Von Mises stresses of each groups, And Comparatively analyzed the mechanical parameters about Hybrid-S1 vs Hybrid-S2, Hybrid-M1 vs Hybrid-M2, and Hybrid-U1 vs Hybrid-U2.Results:1.Finite element model of C2-T1 to be established and validated.Geometric model of C2-T1 was established successfully, including vertebral bone structure, cartilage of facet joint, intervertebral disc including the nucleus pulposus and annulus fibrosus(matrix and fiber), and ligaments(including the anterior longitudinal ligament, posterior longitudinal ligament, flaval ligament, interspinous ligament, capsule ligaments of facet joint). Finite element model of C2-T1 include 78631 nodes and 190608 elements. Intact model was applied axial pre-compress load of 75 N, and moment of 1.0 Nm to simulate flexion, extension, left axis rotation, and right lateral bending, produced 35.24°, 17.22°, 17.14°, 14.29° respectively. Comparing to in vitro data in other scholars, the finite element model was validated to be effective.2.The finite-element-based biomechanical analysis of two-level ACDR, two-level ACDF, two-level Hybrid surgery in C4-C6The stiffness of two-level ACDR was lower than the intact. The stiffness of Hybrid surgery was lower than the intact excluding flexion. The stiffness of two-level ACDF was higher than the intact.ROM of two-level ACDR in replacement segments increased, but in the adjacent segments was lower than the intact. ROM of two-level ACDF extremely reduced in fusion segments, but in the adjacent segments was higher than the intact. ROM of Hybrid-M1 increased in the replacement segment and reduced in the fusion segment, but in the adjacent segments was lower than the intact excluding flexion.IDP of adjacent segment was lower than intact in two-level ACDR. IDP of adjacent segment was higher than intact in two-level ACDF. IDP of adjacent segment was lower than intact in Hybrid-M1 excluding flexion.Von Mises stress of facet joint in the replacement segments of two-level ACDR increased when it is in extenxion and left axial rotation, but its adjacent segments lower than intact. Von Mises stress of facet joint in the fusion segments of two-level ACDF decreased extremely,but but its adjacent segments higher than intact. Von Mises stress of facet joint in the replacement segment of Hybrid-M1 increased drastically, but its adjacent segments lower than intact excluding flexion.3. The finite-element-based biomechanical analysis of hybrid surgeries at two continuous cervical spine levels with swapped positionsThe overall stiffness of the C2-T1 is higher in all hybrid surgeries than in intact reference model for the flexion, whereas lower than in intact reference model in extension, left axial rotation and right lateral bending.(1)The C3-C5 hybrid surgury: in flexion/extension, left axial rotation and right lateral bending, the ROM of the replacement segment increased, by 146.5%、207.9% and 172.7% respectively for C3-C4 replacement, and by 193.0% 、 251.4% and 163.6% respectively for C4-C5 replacement. In extension, left axial rotation and right lateral bending, Von Mises stress of facet joint was increased, by 256.5%、186.9% and 185.1% respectively for C3-C4 replacement, and by 50.1%、54.1% and124.4% respectively for C4-C5 replacement. In flexion, IDP of C2-C3 was increased by 45.2% as a result of replacement in C3-C4, and by 38.7% as a result of replacement in C4-C5.(2)The C4-C6 hybrid surgury: in flexion/extension, left axial rotation and right lateral bending,the ROM of the replacement segment increased, by 146.4%,237.6% and145.0% respectively for C4-C5 replacement, and 93.6%,146.9% and110.8% respectively for C5-C6 replacement. In extension, left axial rotation and right lateral bending, Von Mises stress of facet joint from C4-C5 replacement increased by 20.2%,52.7% and111.1% respectively. Von Mises stress of facet joint from C5-C6 replacement reduced by 8.0% in extension, and increased by 279.2%、60.3% in left axial rotation and right lateral bending respectively. In flexion, IDP of C3-C4 was increased by 2.7% for replacement in C4-C5, and 2.1% for replacement in C5-C6.(3)The C5-C7 hybrid surgury: in flexion/extension, left axial rotation and right lateral bending, the ROM of replacement segment increased, by 116.7%、179.2%、122.3% respectively for C5-C6 replacement, and by 154.7%、323.6%、229.8% respectively for C6-C7 replacement. In axial rotation, lateral bending, Von Mises stress of facet joint from C5-C6 replacement was increased by 267.1% and62.7% respectively; Von Mises stress of facet joint from C6-C7 replacement increased by 201.6%,154.9%,203.8% and144.7% respectively in flexion, extension, left axial rotation and right lateral bending. In flexion, IDP of C4-C5 was increased by 13.9% for replacement in C5-C6, and 20.1% for replacement in C6-C7.Conclusions:1.Finite element model of C2-T1 was successfully established, including refined vertebrae, intervertebral discs, ligaments and facet joints, and validated to be effective.2.Compared with the intact, two-level ACDF leads to increase stiffness, ROM, IDP, and Von Mises stress of facet joint in adjacent segments increased. From the view of biomechanics, it may lead to adjacent segment diseases.3.Compared with the intact, two-level ACDR leads to increase ROM, and Von Mises stress of facet joint in replacement segments, but no significant adverse impact on adjacent segments. From the view of biomechanics, it may protect its adjacent segments.4.The ROM and facet joint Mises stress at the replacement segment in the hybrid surgeries both increased. The impact of the hybrid surgeries on the ROM and facet joint Mises Stress in adjacent levels was quite small, but its impact on the IDP of adjacent segments was significant. From the view of biomechanics, it may protect its adjacent segments to some extent.5.The hybrid use of arthroplasty for segments with high physiological mobility and arthrodesis for those with smaller mobility has less impact on the biomechanics of the cervical spine. |
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