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Research Papers

A Computational Biomechanical Investigation of Posterior Dynamic Instrumentation: Combination of Dynamic Rod and Hinged (Dynamic) Screw

[+] Author and Article Information
Deniz U. Erbulut

Department of Mechanical
Engineering and Neurosurgery,
Koc University, Sariyer,
Istanbul 34450, Turkey
e-mail: erbulutdeniz@gmail.com and
derbulut@ku.edu.tr

Ali Kiapour

Departments of Bioengineering
and Orthopedic Surgery,
E-CORE, University of Toledo,
Toledo, OH 43606
e-mail: Kiapour@asme.org

Tunc Oktenoglu

Vehbi Koc Foundation American Hospital,
Nisantasi, Istanbul 34365, Turkey
e-mail: tuncoktenoglu@gmail.com

Ali F. Ozer

Department of Neurosurgery,
Koc University, Sariyer,
Istanbul 34450, Turkey
e-mail: alifahirozer@gmail.com

Vijay K. Goel

Departments of Bioengineering
and Orthopedic Surgery,
E-CORE, University of Toledo,
Toledo, OH 43606
e-mail: vijay.goel@utoledo.edu

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received October 9, 2013; final manuscript received February 26, 2014; accepted manuscript posted March 6, 2014; published online April 10, 2014. Assoc. Editor: James C. Iatridis.

J Biomech Eng 136(5), 051007 (Apr 10, 2014) (7 pages) Paper No: BIO-13-1486; doi: 10.1115/1.4027060 History: Received October 09, 2013; Revised February 26, 2014; Accepted March 06, 2014

Currently, rigid fixation systems are the gold standard for degenerative disk disease treatment. Dynamic fixation systems have been proposed as alternatives for the treatment of a variety of spinal disorders. These systems address the main drawbacks of traditional rigid fixation systems, such as adjacent segment degeneration and instrumentation failure. Pedicle-screw-based dynamic stabilization (PDS) is one type of these alternative systems. The aim of this study was to simulate the biomechanical effect of a novel posterior dynamic stabilization system, which is comprised of dynamic (hinged) screws interconnected with a coiled, spring-based dynamic rod (DSDR), and compare it to semirigid (DSRR and RSRR) and rigid stabilization (RSRR) systems. A validated finite element (FE) model of L1-S1 was used to quantify the biomechanical parameters of the spine, such as range of motion, intradiskal pressure, stresses and facet loads after single-level instrumentation with different posterior stabilization systems. The results obtained from in vitro experimental intact and instrumented spines were used to validate the FE model, and the validated model was then used to compare the biomechanical effects of different fixation and stabilization constructs with intact under a hybrid loading protocol. The segmental motion at L4–L5 increased by 9.5% and 16.3% in flexion and left rotation, respectively, in DSDR with respect to the intact spine, whereas it was reduced by 6.4% and 10.9% in extension and left-bending loads, respectively. After instrumentation-induced intradiskal pressure at adjacent segments, L3-L4 and L5-S1 became less than the intact in dynamic rod constructs (DSDR and RSDR) except in the RSDR model in extension where the motion was higher than intact by 9.7% at L3-L4 and 11.3% at L5-S1. The facet loads were insignificant, not exceeding 12N in any of the instrumented cases in flexion. In extension, the facet load in DSDR case was similar to that in intact spine. The dynamic rod constructions (DSDR and RSDR) led to a lesser peak stress at screws compared with rigid rod constructions (DSRR and RSRR) in all loading cases. A dynamic construct consisting of a dynamic rod and a dynamic screw did protect the adjacent level from excessive motion.

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Figures

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Fig. 1

Three-dimensional, nonlinear finite element model (L1-S1) with instrumentation

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Fig. 2

(a) dynamic rod and (b) hinged (dynamic) screw

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Fig. 3

Hybrid bending moment for surgery cases compared to intact (10Nm)

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Fig. 6

Maximum von Mises stresses in pedicle screws (in MPa)

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Fig. 5

Motion values in degrees at (a) the superior adjacent segment (L3-L4) and (b) the inferior adjacent segment (L5-S1) in intact, destabilized, and implanted models

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Fig. 4

Average ROM (in degrees) for intact, destabilized and destabilized + instrumentation models at L4-L5

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