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Research Papers

Biomechanical Analysis of a Long-Segment Fusion in a Lumbar Spine—A Finite Element Model Study

[+] Author and Article Information
Raghu N. Natarajan

Rush University Medical Center,
Suite 204 F,
Orthopedic Ambulatory Building,
1611 West Harrison,
Chicago, IL 60612
e-mail: raghu_natarajan@rush.edu

Kei Watanabe

Department of Orthopaedic Surgery,
Niigata University Medical and
Dental General Hospital,
1-757, Asahimachidori, Chuoku,
Niigata City,
Niigata 951-8510, Japan
e-mail: keiwatanabe_39jp@live.jp

Kazuhiro Hasegawa

Niigata Spine Surgery Center,
2-5-22 Nishi-machi, Konan-ku,
Niigata 950-0165, Japan
e-mail: kazu3795jp@yahoo.co.jp

1Corresponding author.

Manuscript received December 28, 2017; final manuscript received March 27, 2018; published online May 24, 2018. Assoc. Editor: James C Iatridis.

J Biomech Eng 140(9), 091011 (May 24, 2018) (7 pages) Paper No: BIO-17-1604; doi: 10.1115/1.4039989 History: Received December 28, 2017; Revised March 27, 2018

Examine the biomechanical effect of material properties, geometric variables, and anchoring arrangements in a segmental pedicle screw with connecting rods spanning the entire lumbar spine using finite element models (FEMs). The objectives of this study are (1) to understand how different variables associated with posterior instrumentation affect the lumbar spine kinematics and stresses in instrumentation, (2) to compare the multidirectional stability of the spinal instrumentation, and (3) to determine how these variables contribute to the rigidity of the long-segment fusion in a lumbar spine. A lumbar spine FEM was used to analyze the biomechanical effects of different materials used for spinal rods (TNTZ or Ti or CoCr), varying diameters of the screws and rods (5 mm and 6 mm), and different fixation techniques (multilevel or intermittent). The results based on the range of motion and stress distribution in the rods and screws revealed that differences in properties and variations in geometry of the screw-rod moderately affect the biomechanics of the spine. Further, the spinal screw-rod system was least stable under the lateral bending mode. Stress analyzes of the screws and rods revealed that the caudal section of the posterior spinal instrumentation was more susceptible to high stresses and hence possible failure. Although CoCr screws and rods provided the greatest spinal stabilization, these constructs were susceptible to fatigue failure. The findings of the present study suggest that a posterior instrumentation system with a 5-mm screw-rod diameter made of Ti or TNTZ is advantageous over CoCr instrumentation system.

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Figures

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Fig. 1

Finite element mesh of the lumbar spine with posterior instrumentation. Figure shows the FE mesh distribution in the various components of the lumbar spine along with FE mesh distribution in the posterior instrumentation.

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Fig. 2

Comparison of the motion at each of the lumbar spine motion segment obtained from the FE model with cadaver results. The range of motion at all levels calculated using FE models corresponded well and was within one standard deviation under flexion/extension and axial rotation. Results from FE models were within two standard deviations at most levels under lateral bending.

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Fig. 3

Comparison of the range of primary motions obtained from FE models with in vivo results available from the literature [16]. Contributions of the motion from each segment to the total lumbar spine motion from FE model results were compared with corresponding in vivo-measured values. Percent range of primary motion obtained from FE models was comparable with the in vivo results under the torsion and lateral bending mode and at a certain level under flexion/extension.

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Fig. 4

Comparison of the range of primary motions at the L3/L4 motion segment with in vivo results available from the literature [17]. Contributions of the motion from L3/L4 segment to the total lumbar spine motion from FE model results were compared with corresponding in vivo-measured values. Percent range of primary motion obtained from FE models was comparable with the in vivo results under the torsion and lateral bending mode and at a certain level under flexion/extension.

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Fig. 5

Effect of different flexural stiffnesses of spinal rods on the stiffness of the instrumented lumbar spine. The normalized construct stiffness (normalized with respect to Ti rod-screw system) with rods and screws of TNTZ and CoCr obtained in the present study was compared with the measured [7] normalized construct stiffness for superelastic (E = 36 GPa) and variable rods (i.e., E varies from 36 GPa to 83 GPa). For all three loading modes, the instrumented spinal stiffness with the TNTZ screw-rod system (E = 58 GPa) obtained using the FE model compared well with the experimentally determined spinal construct stiffness with a variable rod of similar stiffness (average E = 60 GPa).

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Fig. 6

Effect of different flexural stiffnesses of spinal rods on the stiffness of the instrumented spine. Differences in screw-rod flexural properties moderately affected the stiffness of an instrumented lumbar spine. The increase in spine stiffness due to posterior instrumentation was greatest under twisting and least under lateral bending, which leads us to conclude that the posterior screw-rod system is more stable under torsion and least stable under lateral bending.

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Fig. 7

Effect of Ti screw and rod diameter, and placement of pedicle screws on stiffness of the instrumented spine. Differences in screw-rod geometrical properties moderately affected the stiffness of an instrumented lumbar spine. Screw fixation either at all levels or an intermittent levels did not affect spine stiffness under flexion/extension or twisting.

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