Browsing by Author "Shih, Shih-Liang"

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Biomechanical comparison of instrumented posterior lumbar interbody fusion with one or two cages by finite element analysis
(2006-09-01)
Chiang, Ming-Fu
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Zhong, Zheng-Cheng
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Chen, Chen-Sheng
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Cheng, Cheng-Kung
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Shih, Shih-Liang
Study Design. Using finite element models to study the biomechanics of lumbar instrumented posterior lumbar interbody fusion (PLIF) with one or two cages. Objective. Analyzing the biomechanics of instrumented PLIF with one or two cages as to evaluate whether a single cage is adequate for instrumented PLIF. Summary of Background Data. Implantation of a single cage in instrumented PLIF of lumbar spine is still controversial. Methods. Three validated finite element models of L3-L5 lumbar segment were established [intact model (INT), one cage model (LS-1), and two cages model (LS-2)]. The available finite element program ANSYS 6.0 (Swanson Analysis System Inc., Houston, TX) was applied. To analyze the biomechanics of these models, 10 Nm flexion, extension, rotation, and lateral bending moment with 150 N of preload were respectively imposed on the superior surfaces of the L3. Results. Compared with the INT model, the decrease of ROM in the LS-1 and LS-2 models were exaggerated from 0.67 degrees to 3.73 degrees and ranged from 37.2% to 86.1% in all motions. The mean subsidence was found to be slightly higher in the LS-1 model. Most of the cage dislodgement in both models was less than 0.03 mm. The mean dislodgement was slightly higher in the LS-1 model. The stress of cage was found to be high in the LS-2 model. The mean stress of screw was raised to 4.5% to 9.7% in the LS-1, which was higher than that in the LS-2 model. In general, stress of adjacent disc was more pronounced in the LS-2 model. The most stress distributed at the anterior portion of the adjacent disc, which could be used to interpret the clinical findings of the early adjacent disc degeneration. Conclusions. A single cage inserted in an instrumented PLIF gains approximate biomechanical stability, slight greater subsidence, and a slight increase in screw stress but less early degeneration in adjacent disc. Adjusting these factors, instrumented PLIF with one cage could be encouraged in clinical practice.
Effect of the cord pretension of the Dynesys dynamic stabilisation system on the biomechanics of the lumbar spine: a finite element analysis
(2011-11-01)
Liu, Chien-Lin
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Zhong, Zheng-Cheng
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Hsu, Hung-Wei
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Shih, Shih-Liang
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Wang, Shih-Tien
The Dynesys dynamics stabilisation system was developed to maintain the mobility of motion segment of the lumbar spine in order to reduce the incidence of negative effects at the adjacent segments. However, the magnitude of cord pretension may change the stiffness of the Dynesys system and result in a diverse clinical outcome, and the effects of Dynesys cord pretension remain unclear. Displacement-controlled finite element analysis was used to evaluate the biomechanical behaviour of the lumbar spine after insertion of Dynesys with three different cord pretensions. For the implanted level, increasing the cord pretension from 100 to 300 N resulted in an increase in flexion stiffness from 19.0 to 64.5 Nm/deg, a marked increase in facet contact force (FCF) of 35% in extension and 32% in torsion, a 40% increase of the annulus stress in torsion, and an increase in the high-stress region of the pedicle screw in flexion and lateral bending. For the adjacent levels, varying the cord pretension from 100 to 300 N only had a minor influence on range of motion (ROM), FCF, and annulus stress, with changes of 6, 12, and 9%, respectively. This study found that alteration of cord pretension affects the ROM and FCF, and annulus stress within the construct but not the adjacent segment. In addition, use of a 300 N cord pretension causes a much higher stiffness at the implanted level when compared with the intact lumbar spine.
Influence of Dynesys System Screw Profile on Adjacent Segment and Screw
(2010-08-01)
Liu, Chien-Lin
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Zhong, Zheng-Cheng
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Shih, Shih-Liang
;
Hung, Chinghua
;
Lee, Yong-Eng
Study Design: Displacement-controlled finite element analysis was used to evaluate the mechanical behavior of the lumbar spine after insertion of the Dynesys dynamic stabilization system. Objective: This study aimed to investigate whether different depths of screw placement of Dynesys would affect load sharing of screw, range of motion (ROM), annulus stress, and facet contact force. Summary of Background Data: In clinical follow-up, a high rate of screw complications and adjacent segment disease were found after using Dynesys. The pedicle screw in the Dynesys system is not so easy to implant into the standard position and causes the screw to protrude more prominently from the pedicle. Little is known about how the biomechanical effects are influenced by the Dynesys screw profile. Methods: The Dynesys was implanted in a 3-dimensional, nonlinear, finite element model of the L1 to L5 lumbar spine. Different depths of screw position were modified in this model by 5 and 10mm out of the pedicle. The model was loaded to 150 N preload and controlled the same ROMs by 20, 15, 8, and 20 degrees in flexion, extension, torsion, and lateral bending, respectively. Resultant ROM, annulus stress, and facet contact force were analyzed at the surgical and adjacent level. Results: Under flexion, extension, and lateral bending, the Dynesys provided sufficient stability at the surgical level, but increased the ROM at the adjacent level. Under flexion and lateral bending, the Dynesys alleviated annulus stress at the surgical level, but increased annulus stress at the adjacent level. Under extension, the Dynesys decreased facet loading at the surgical level but increased facet loading at the adjacent level. Conclusions: This study found that the Dynesys system was able to restore spinal stability and alleviate loading on disc and facet at the surgical level, but greater ROM, annulus stress, and facet loading were found at the adjacent level. In addition, profile of the screw placement caused only a minor influence on the ROM, annulus stress, and facet loading, but the screw stress was noticeably increased.