Low back pain becomes the universal disability that experienced by roughly 80% of adults. The prevalent etiology of chronic low back pain is degenerative disc disease (DDD). The DDD commonly occurs in the lumbar area which caused by loading history, genetic inheritance, and aging of the disc. Posterior lumbar interbody fusion (PLIF) surgery had been widely applied to treat the DDD problem. However, the effects of the PLIF surgery on the fixation stability and adjacent segment degeneration are still not well understood. Computer simulation is a possible technique to explore this problem. Thus, the aim of this study was to discover a more suitable rod material to improve both the fixation stability and adjacent segment degeneration problem using T10-S1 multilevel spine models. The intact spine model was scanned from a healthy volunteer. Nine vertebrae from T10 to S1 were developed, and the intervertebral discs were created to connect each vertebra. Seven types of spinal ligaments were considered and simulated using tension-only spring elements. The spine model with DDD at L4-L5 was created, and it was treated with pedicle screw and rod fixation and a titanium cage. The motions in flexion, extension, lateral bending, and axial rotation were applied in the spine models. Different rod materials, which ranged from 1 GPa to 1 MPa, were considered to investigate the effects on the fixation stability and adjacent segment degeneration. All the numerical models were analyzed using ANSYS Workbench. In post-processing, the intersegmental rotation at the index level (fixation stability) and the rotation at the adjacent level (adjacent segment degeneration) were calculated. Additionally, the implant stress and disc stress were also calculated. The results showed that the rod material of 1 GPa has the lowest intersegmental rotation at the index level compared to the other rod materials. It indicated that the highest fixation stability could be obtained. However, the high-stiffness rods had higher risk of the implant failure and adjacent segment degeneration problem. After reducing the stiffness of the rods, the intersegmental rotation at the index level of the treatment models was close to that of the intact model for the use of 19.78 MPa rod materials in the lateral bending and 1 MPa rod material in the flexion and extension. However, changing the rod materials did not affect the intersegmental rotation in the axial rotation. The effects of the rod materials on the fixation stability and adjacent segment degeneration could be investigated using finite element method. The optimum rod materials, which ranged from 1 MPa to 19.78 MPa, retained both the fixation stability and adjacent segment degeneration. This study could provide useful information to surgeons and help them to understand the effects of rod materials on spine biomechanics.

Low back pain becomes the universal disability that experienced by roughly 80% of adults. The prevalent etiology of chronic low back pain is degenerative disc disease (DDD). The DDD commonly occurs in the lumbar area which caused by loading history, genetic inheritance, and aging of the disc. Posterior lumbar interbody fusion (PLIF) surgery had been widely applied to treat the DDD problem. However, the effects of the PLIF surgery on the fixation stability and adjacent segment degeneration are still not well understood. Computer simulation is a possible technique to explore this problem. Thus, the aim of this study was to discover a more suitable rod material to improve both the fixation stability and adjacent segment degeneration problem using T10-S1 multilevel spine models. The intact spine model was scanned from a healthy volunteer. Nine vertebrae from T10 to S1 were developed, and the intervertebral discs were created to connect each vertebra. Seven types of spinal ligaments were considered and simulated using tension-only spring elements. The spine model with DDD at L4-L5 was created, and it was treated with pedicle screw and rod fixation and a titanium cage. The motions in flexion, extension, lateral bending, and axial rotation were applied in the spine models. Different rod materials, which ranged from 1 GPa to 1 MPa, were considered to investigate the effects on the fixation stability and adjacent segment degeneration. All the numerical models were analyzed using ANSYS Workbench. In post-processing, the intersegmental rotation at the index level (fixation stability) and the rotation at the adjacent level (adjacent segment degeneration) were calculated. Additionally, the implant stress and disc stress were also calculated. The results showed that the rod material of 1 GPa has the lowest intersegmental rotation at the index level compared to the other rod materials. It indicated that the highest fixation stability could be obtained. However, the high-stiffness rods had higher risk of the implant failure and adjacent segment degeneration problem. After reducing the stiffness of the rods, the intersegmental rotation at the index level of the treatment models was close to that of the intact model for the use of 19.78 MPa rod materials in the lateral bending and 1 MPa rod material in the flexion and extension. However, changing the rod materials did not affect the intersegmental rotation in the axial rotation. The effects of the rod materials on the fixation stability and adjacent segment degeneration could be investigated using finite element method. The optimum rod materials, which ranged from 1 MPa to 19.78 MPa, retained both the fixation stability and adjacent segment degeneration. This study could provide useful information to surgeons and help them to understand the effects of rod materials on spine biomechanics.

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