Tae-Ahn Jahng1, Young Eun Kim, Kyung Yun Moon. 1. Department of Neurosurgery, Seoul National University, 300 Gumi-dong, Bundang-gu, Seongnam-si, Gyeonggi-do 464-707, Korea.
Abstract
BACKGROUND CONTEXT: Recently, nonfusion pedicle-based dynamic stabilization systems (PBDSs) have been developed and used in the management of degenerative lumbar spinal diseases. Still effects on spinal kinematics and clinical effects are controversial. Little biomechanical information exists for providing biomechanical characteristics of pedicle-based dynamic stabilization according to the PBDS design before clinical implementation. PURPOSE: To investigate the effects of implanting PBDSs into the spinal functional unit and elucidate the differences in biomechanical characteristics according to different materials and design. STUDY DESIGN: The biomechanical effects of implantation of PBDS were investigated using the nonlinear three-dimensional finite element model of L4-L5. METHODS: An already validated three-dimensional, intact osteoligamentous L4-L5 finite element model was modified to incorporate the insertion of pedicle screws. The implanted models were constructed after modifying the intact model to simulate postoperative changes using four different fixation systems. Four models instrumented with PBDS (Dynesys, NFlex, and polyetheretherketone [PEEK]) and rigid fixation systems (conventional titanium rod) were developed for comparison. The instrumented models were compared with those of the intact and rigid fixation model. Range of motion (ROM) in three motion planes, center of rotation (COR), force on the facet joint, and von Mises stress distribution on the vertebral body and implants with flexion-extension were compared among the models. RESULTS: Simulated results demonstrated that implanted segments with PBDSs have limited ROM when compared with the intact spine. Flexion motion was the most limited, and axial rotation was the least limited, after device implantation. Among the PBDS selected in this analysis, the NFlex system had the closest instantaneous COR compared with the intact model and a higher ROM compared with other PBDS. Contact force on the facet joint in extension increased with an increase of moment in Dynesys and NFlex; however, the rigid or PEEK rod fixation revealed no facet contact force. CONCLUSIONS: Implanted segments with PBDSs have limited ROM when compared with the intact spine. Center of rotation and stress distribution differed according to the design and materials used. These biomechanical effects produced a nonphysiological stress on the functional spinal unit when they were implanted. The biomechanical effects of current PBDSs should be carefully considered before clinical implementation.
BACKGROUND CONTEXT: Recently, nonfusion pedicle-based dynamic stabilization systems (PBDSs) have been developed and used in the management of degenerative lumbar spinal diseases. Still effects on spinal kinematics and clinical effects are controversial. Little biomechanical information exists for providing biomechanical characteristics of pedicle-based dynamic stabilization according to the PBDS design before clinical implementation. PURPOSE: To investigate the effects of implanting PBDSs into the spinal functional unit and elucidate the differences in biomechanical characteristics according to different materials and design. STUDY DESIGN: The biomechanical effects of implantation of PBDS were investigated using the nonlinear three-dimensional finite element model of L4-L5. METHODS: An already validated three-dimensional, intact osteoligamentous L4-L5 finite element model was modified to incorporate the insertion of pedicle screws. The implanted models were constructed after modifying the intact model to simulate postoperative changes using four different fixation systems. Four models instrumented with PBDS (Dynesys, NFlex, and polyetheretherketone [PEEK]) and rigid fixation systems (conventional titanium rod) were developed for comparison. The instrumented models were compared with those of the intact and rigid fixation model. Range of motion (ROM) in three motion planes, center of rotation (COR), force on the facet joint, and von Mises stress distribution on the vertebral body and implants with flexion-extension were compared among the models. RESULTS: Simulated results demonstrated that implanted segments with PBDSs have limited ROM when compared with the intact spine. Flexion motion was the most limited, and axial rotation was the least limited, after device implantation. Among the PBDS selected in this analysis, the NFlex system had the closest instantaneous COR compared with the intact model and a higher ROM compared with other PBDS. Contact force on the facet joint in extension increased with an increase of moment in Dynesys and NFlex; however, the rigid or PEEK rod fixation revealed no facet contact force. CONCLUSIONS: Implanted segments with PBDSs have limited ROM when compared with the intact spine. Center of rotation and stress distribution differed according to the design and materials used. These biomechanical effects produced a nonphysiological stress on the functional spinal unit when they were implanted. The biomechanical effects of current PBDSs should be carefully considered before clinical implementation.