| BACKGROUNDCervical artificial disc replacement emerged due to its biomechanical superiority to anterior cervical discectomy and fusion, that is to preserve the segmental motion at the index level and prevent the degeneration of adjacent segment. Previous biomechanical studies with respect to cervical disc arthroplasty were mostly either radiography or Ex vivo cadaver models. The most common studied parameter was range of motion (ROM). Cervical internal responses were few measured. Finite element analysis have the greatest advantage in this area. Previous finite element model of cervical artificial disc was few and simple. There is no systematic and comparative biomechanical study with respect to several different types of protheses that are common at home and abroad. There is no conclusion with respect to biomechanical information of cervical artificial disc. Despite its encouraging short-term outcomes with artificial cervical disc arthroplasty, the long-term clinical benefit remains unknown. With the development of finite element analysis, we think it is necessary to study the biomechanical characters of low cervical vertebrae after disc arthroplasty using the method. Part 1 Establishment of Cervical Three Dimensional Finite Element ModelObject:To develop a validated three dimensional finite element model of C3-C7 level.Methods:The C3-C7 CT image data of a healthy adult man in sequential DICOM files were loaded into Mimics software and then Geomagic software to form geometrical model, which were meshed to form the finite element model in Hypermesh software. Appropriate material properties, contacts as well as boundary and loading conditions were defined for the model. Moment loads were applied on the model in flexion/extension, lateral bending and axial rotation. These loads correspond to the loads applied in previous studies. The developed finite element model was validated by comparing its predictions with previously published experimental and numerical results.Results:The C3-C7 finite element model has 40905 nodes and 222065 elements. This model comprised the vertebral bodies, the bony posterior elements (transverse processes, pedicles, laminae, spinous processes, and facet joints), the discs, the endplates, and the ligaments at each of the five levels. The results after loading were almost the same as previously published experimental and numerical results.Conclusion:We have established a validated three dimensional finite element model of C3-C7 level which can be used for the biomechanical research of cervical artificial disc replacement. Part 2 Biomechanical Research of Cervical Artificial Disc ReplacementObject:To study the biomechanical behavior of C3-C7 level after the placement of Prodisc-C, PCM, Prestige LP, Mobi-C, Bryan disc prostheses at C5-C6 segment as well as C4-C5 and C5-C6 segment.Methods:The finite element models of anterior cervical plate and above five disc prostheses were developed and positioned at C5-C6 segment as well as C4-C5 and C5-C6 segment of intact model of C3-C7. For intact and each surgical models, the loading was applied to the superior surface of C3 while the inferior surface of C7 was fully fixed in all directions. The following loadings were applied to each of these models:73.6N axial compression pre-load+1Nm flexion, extension, lateral bending, axial rotation moment. The following parameters were determined for each model:instantaneous center of rotation(ICR), range of motion(ROM), intradiscal pressure (IDP), facet loads, tension on ligaments.Results:The ICR and ROM at the index segment, as well as ROM and IDP at the adjacent intact segments following artificial disc replacement(ADR) were not significantly different from those of the normal cervical spine model. For single-level arthroplasty, the facet loads at C5/6 following ADR of Prodisc-C, PCM, Prestige LP, Mobi-C and Bryan increased by 27.9%,34.9%,39.5%,25.6%,-2.3% respectively during extension,19.6%,13.0%,30.4%,41.3%,23.9% respectively during lateral bending,22.2%,24.4%,46.7%,44.4%,44.4% respectively during axial rotation. For double-level arthroplasty, the facet loads following ADR of Prodisc-C, PCM, Prestige LP, Mobi-C and Bryan increased by 6.25%,10.0%,12.5%,15.0%,-7.5% respectively at C4/5 and 25.6%,16.3%,18.6%,25.6%,-2.3% respectively at C5/6 during extension,10.0%,11.7%,15.0%,28.3%,25.0% respectively at C4/5 and 15.2%,13.0%,19.6%,23.9%,30.4% respectively at C5/6 during lateral bending,26.3%,23.7%,39.5%,42.1%,36.8% respectively at C4/5 and 37.8%,26.7%,55.6%,55.6%,51.1% respectively at C5/6 during axial rotation. For single-level arthroplasty, tension at C5/6 following ADR of Prodisc-C, PCM, Prestige LP, Mobi-C and Bryan increased by 15.0%, 25.0%,25,0%,37.5%,100.0% respectively on posterior longitudinal ligament,43.8%,37.5%,50.0%,50.0%,75.0% respectively on capsular ligaments,142.9%,102.9%,114.3%,228.6%,171.4% respectively on interspinous ligaments during flexion. For double-level arthroplasty, tension following ADR of Prodisc-C, PCM, Prestige LP, Mobi-C and Bryan increased by 8.7%,15.2%,30.4%,47.8%,56.5% respectively at C4/5 and 5.0%,2.5%,25.0%,50.0%,37.5% respectively at C5/6 on posterior longitudinal ligament; 0.0%,11.1%,22.2%,24.4%,44.4% respectively at C4/5 and 35.7%,22.9%,42.9%,54.3%,100.0% respectively at C5/6 on interspinous ligaments during flexion. Conclusion:We can conclude that cervical artificial disc replacement may avoid or at least slow down the degeneration of adjacent level due to its preservation of motion. However, it maybe accelerate the degeneration of surgical level due to the wrong position of prosthesis. We should choose different type of disc prosthesis individualizedly. Confirmed clinical result will rely on long-term follow-up. Part 3 Complication-related Biomechanical Research of Cervical Artificial Disc ReplacementObject:To study the biomechanical mechanism of subsidence and wear of device after cervical artificial disc replacement.Methods:For the models of single-level ADR with Prodisc-C, PCM, Prestige LP, Mobi-C and Bryan, the loading was applied to the superior surface of C3 while the inferior surface of C7 was fully fixed in all directions. The following loadings were applied to each of these models: 73.6N axial compression pre-load 1Nm flexion, extension, lateral bending, axial rotation moment. Stress at the interface between upper and lower end plates of prosthesis and upper and lower vertebrae were compared among different types of cervical prostheses. Stress distribution throughout the implant were also compared.Results:Prestige LP and Prodisc-C discs caused higher stress at the interface between upper and lower end plates of prosthesis and upper and lower vertebrae than PCM, Mobi-C, and Bryan discs. Stress throughout the polymer cores of Mobi-C and Bryan discs was lower. But there were some stress concentrations on their polymer cores. Stress at the interface between upper and lower prosthesis end plates of Prestige LP disc was higher. And there were significant stress concentrations at the interface. Although Stress concentrations on polymer core of Prodisc-C disc were not significant, the whole stress was higher. There were high stress concentrations only at the edge of polymer core of PCM disc.Conclusion:Subsidence of device after cervical artificial disc replacement are closely related to anchorage and internal designs of disc prosthesis. Wear of device after cervical artificial disc replacement are closely related to kinematic and geometry designs of disc prosthesis. |
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