Compensatory increase in motion at the adjacent level to the fusion is known to be related to adjacent disc diseases. Recently, during the multi-level fusion with pedicle screws, interspinous spacers are sometimes substituted for the mostsuperior level of the fusion in an attempt to reduce the numbe...
Compensatory increase in motion at the adjacent level to the fusion is known to be related to adjacent disc diseases. Recently, during the multi-level fusion with pedicle screws, interspinous spacers are sometimes substituted for the mostsuperior level of the fusion in an attempt to reduce the number of fusion level and likelihood of degeneration process at the adjacent levels. However, its biomechanical efficacies still remain unknown and complications associated with failure of the vertebral pedicles or the interspinous processes need to be elucidated. In this study, a finite element (FE) study was performed to assessbiomechanical efficacies of the interspinous spacer combined with posterior lumbar fusion. Range of motion (ROM) at the operated and adjacent levels, and stress distribution at the vertebral pedicles and spinous processes around interspinous spacer with respect to changes of bone quality were investigated. A previously-validated 3-dimensional FE model of the intact lumbar spine was used (L1-S1). The post-operative models were made by modifying the intact model to simulate the implantation of interspinous spacer and pedicle screws at the L3-4 and L4-5. Four different configurations of the post-op model were considered : (1) a normal spinal model; (2) Type 1, one-level fusion using posterior pedicle screws (Ti6A14V, E=114GPa, n=0.3, Φ=5.5mm) at the L4-5; (3) Type 2, two-level (L3-5) fusion (4) Type 3, Type 1 plus CoflexTM (Titanium, E=115GPa, n=0.3, Paradigm spine, Wurmlingen, Germany) at the L3-4. Here, to place the CoflexTM, excision of supraspinous ligament and partial removal of interspinous ligament were simulated. The bone-pedicle screw interface behavior was accomplished via ‘tie’contact condition to assume complete postoperative bony union. Friction coefficient of 0.3 was also applied to the contact surface between the wings of CoflexTM and the processes to simulate compressive fixationby upper and lower wings of CoflexTM. Stress distribution at the pedicles and spinous processes after implantation of CoflexTM was assessed with respect to the changes of bone quality. To construct an osteoporotic lumbar spine, the elastic moduli of relevant bony structures were reduced from those of the normal spine (by 66% for the cancellous bone and 33% for the cortical shell, the endplates, and the posterior elements) with others left unchanged. These variations resulted in a reduction of the overall stiffness of the lumbar spine by approximately 15%, as compared with the normal spine model. Load was applied at the superior surface of L1 using flexion/extension of 10Nm, axial rotation of 10Nm, and lateral bending of 10Nm with a compressive follower load of 400N. The inferior surface of the S1 vertebral body is not allowed move in any direction. Hybrid protocol was used to study the ROM at the operated and adjacent levels. Likelihood of failure at the pedicles and the spinous processes was evaluated by ratio between the peak von Mises stress (PVMS) and yield strength of each bone. ABAQUS/Standard V6.10 (Simulia Corp., Providence, RI, USA) was used for FE analysis. As compared to the intact model, Type 2 showed the greatest increase (16~68%) in ROM at the adjacent level (L2-3), followed Type 3 (3~59%), and Type 1 (1~19%) depending on the loading type. Here, ROM of Type 3 was closer to those of Type 1 than Type 2 except in extension. At L3-4, ROM of Type 2 was reduced by 34~56% regardless of loading mode, as compared to decrease of 55% in Type 3 only in extension and increase in ROM by 34% in flexion due to excision of supraspinous and interspinous ligaments and decrease in ROM by pedicle screws at the L4-5. ROM of all post-op models at the L4-5 were decreased significantly in flexion (by 58~76%) and extension (by 60~84%). Influence of Type 3 at L3-4 in axial rotation, lateral bending was negligible. In case of normal bone strength model (Type 3_Normal in Figure 3), PVMS at the process and the pedicle remained less than 20% of their yield strengths regardless of loading, except in extension (about 35%). However, for the osteoporotic model (Type 3_Osteoprotic), it reached up to 56% in extension indicating increased susceptibility to fracture. The results showed that interspinous spacer combined with lumbar spinal fusion was able to reduce the adjacent level motion while effectively providing stability at the operated level as compared to thebi-level posterior lumbar fusion. Stress levels around the spacer appeared to be far lower than the surrounding bone strengths. However, concerns for high stress level at the interspinous process still remain especially in extension with the case of the osteoporotic spine. In conclusion, our study demonstrated that interspinous spacer combined with lumbar spinal fusion may be able to contribute to reducing degeneration process at the adjacent levels while effectively providing stability to the operated level. However, potential failure of stressed pedicles and processes need to be addressed.
Compensatory increase in motion at the adjacent level to the fusion is known to be related to adjacent disc diseases. Recently, during the multi-level fusion with pedicle screws, interspinous spacers are sometimes substituted for the mostsuperior level of the fusion in an attempt to reduce the number of fusion level and likelihood of degeneration process at the adjacent levels. However, its biomechanical efficacies still remain unknown and complications associated with failure of the vertebral pedicles or the interspinous processes need to be elucidated. In this study, a finite element (FE) study was performed to assessbiomechanical efficacies of the interspinous spacer combined with posterior lumbar fusion. Range of motion (ROM) at the operated and adjacent levels, and stress distribution at the vertebral pedicles and spinous processes around interspinous spacer with respect to changes of bone quality were investigated. A previously-validated 3-dimensional FE model of the intact lumbar spine was used (L1-S1). The post-operative models were made by modifying the intact model to simulate the implantation of interspinous spacer and pedicle screws at the L3-4 and L4-5. Four different configurations of the post-op model were considered : (1) a normal spinal model; (2) Type 1, one-level fusion using posterior pedicle screws (Ti6A14V, E=114GPa, n=0.3, Φ=5.5mm) at the L4-5; (3) Type 2, two-level (L3-5) fusion (4) Type 3, Type 1 plus CoflexTM (Titanium, E=115GPa, n=0.3, Paradigm spine, Wurmlingen, Germany) at the L3-4. Here, to place the CoflexTM, excision of supraspinous ligament and partial removal of interspinous ligament were simulated. The bone-pedicle screw interface behavior was accomplished via ‘tie’contact condition to assume complete postoperative bony union. Friction coefficient of 0.3 was also applied to the contact surface between the wings of CoflexTM and the processes to simulate compressive fixationby upper and lower wings of CoflexTM. Stress distribution at the pedicles and spinous processes after implantation of CoflexTM was assessed with respect to the changes of bone quality. To construct an osteoporotic lumbar spine, the elastic moduli of relevant bony structures were reduced from those of the normal spine (by 66% for the cancellous bone and 33% for the cortical shell, the endplates, and the posterior elements) with others left unchanged. These variations resulted in a reduction of the overall stiffness of the lumbar spine by approximately 15%, as compared with the normal spine model. Load was applied at the superior surface of L1 using flexion/extension of 10Nm, axial rotation of 10Nm, and lateral bending of 10Nm with a compressive follower load of 400N. The inferior surface of the S1 vertebral body is not allowed move in any direction. Hybrid protocol was used to study the ROM at the operated and adjacent levels. Likelihood of failure at the pedicles and the spinous processes was evaluated by ratio between the peak von Mises stress (PVMS) and yield strength of each bone. ABAQUS/Standard V6.10 (Simulia Corp., Providence, RI, USA) was used for FE analysis. As compared to the intact model, Type 2 showed the greatest increase (16~68%) in ROM at the adjacent level (L2-3), followed Type 3 (3~59%), and Type 1 (1~19%) depending on the loading type. Here, ROM of Type 3 was closer to those of Type 1 than Type 2 except in extension. At L3-4, ROM of Type 2 was reduced by 34~56% regardless of loading mode, as compared to decrease of 55% in Type 3 only in extension and increase in ROM by 34% in flexion due to excision of supraspinous and interspinous ligaments and decrease in ROM by pedicle screws at the L4-5. ROM of all post-op models at the L4-5 were decreased significantly in flexion (by 58~76%) and extension (by 60~84%). Influence of Type 3 at L3-4 in axial rotation, lateral bending was negligible. In case of normal bone strength model (Type 3_Normal in Figure 3), PVMS at the process and the pedicle remained less than 20% of their yield strengths regardless of loading, except in extension (about 35%). However, for the osteoporotic model (Type 3_Osteoprotic), it reached up to 56% in extension indicating increased susceptibility to fracture. The results showed that interspinous spacer combined with lumbar spinal fusion was able to reduce the adjacent level motion while effectively providing stability at the operated level as compared to thebi-level posterior lumbar fusion. Stress levels around the spacer appeared to be far lower than the surrounding bone strengths. However, concerns for high stress level at the interspinous process still remain especially in extension with the case of the osteoporotic spine. In conclusion, our study demonstrated that interspinous spacer combined with lumbar spinal fusion may be able to contribute to reducing degeneration process at the adjacent levels while effectively providing stability to the operated level. However, potential failure of stressed pedicles and processes need to be addressed.
주제어
#극돌기간 삽입기구 척추경 나사못 유합케이지 유한요소해석
※ AI-Helper는 부적절한 답변을 할 수 있습니다.