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Research Papers

Adjacent-Level Hypermobility and Instrumented-Level Fatigue Loosening With Titanium and PEEK Rods for a Pedicle Screw System: An In Vitro Study

[+] Author and Article Information
Aakas Agarwal

Engineering Center for Orthopaedic
Research Excellence (E-CORE),
Department of Bioengineering and
Orthoapedic Surgery,
University of Toledo,
2801 West Bancroft Street,
Toledo, OH 43606
e-mail: aakash.agarwal@rockets.utoledo.edu

Marcel Ingels

Engineering Center for Orthopaedic
Research Excellence (E-CORE),
Department of Bioengineering and
Orthoapedic Surgery,
University of Toledo,
2801 West Bancroft Street,
Toledo, OH 43606
e-mail: marcel.ingels@rockets.utoledo.edu

Manoj Kodigudla

Engineering Center for Orthopaedic
Research Excellence (E-CORE),
Department of Bioengineering and
Orthoapedic Surgery,
University of Toledo,
2801 West Bancroft Street,
Toledo, OH 43606
e-mail: Manoj.kodigudla@utoledo.edu

Narjes Momeni

Engineering Center for Orthopaedic
Research Excellence (E-CORE),
Department of Bioengineering and
Orthoapedic Surgery,
University of Toledo,
2801 West Bancroft Street,
Toledo, OH 43606
e-mail: Narjessmomeni@gmail.com

Vijay Goel

Engineering Center for Orthopaedic
Research Excellence (E-CORE),
Department of Bioengineering and
Orthoapedic Surgery,
University of Toledo,
2801 West Bancroft Street,
Toledo, OH 43606
e-mail: vijay.goel@utoledo.edu

Anand K. Agarwal

Engineering Center for Orthopaedic
Research Excellence (E-CORE),
Department of Bioengineering and
Orthoapedic Surgery,
University of Toledo,
2801 W. Bancroft Street,
Toledo, OH 43606
e-mail: anand.agarwal@utoledo.edu

1Corresponding author.

Manuscript received June 30, 2015; final manuscript received February 23, 2016; published online March 25, 2016. Assoc. Editor: Brian D. Stemper.

J Biomech Eng 138(5), 051004 (Mar 25, 2016) (8 pages) Paper No: BIO-15-1322; doi: 10.1115/1.4032965 History: Received June 30, 2015; Revised February 23, 2016

Adjacent-level disease is a common iatrogenic complication seen among patients undergoing spinal fusion for low back pain. This is attributed to the postsurgical differences in stiffness between the spinal levels, which result in abnormal forces, stress shielding, and hypermobility at the adjacent levels. In addition, as most patients undergoing these surgeries are osteoporotic, screw loosening at the index level is a complication that commonly accompanies adjacent-level disease. Recent studies indicate that a rod with lower rigidity than that of titanium may help to overcome these detrimental effects at the adjacent level. The present study was conducted in vitro using 12 L1-S1 specimens divided into groups of six, with each group instrumented with either titanium rods or PEEK (polyetheretherketone) rods. The test protocol included subjecting intact specimens to pure moments of 10 Nm in extension and flexion using an FS20 Biomechanical Spine Test System (Applied Test Systems) followed by hybrid moments on the instrumented specimens to achieve the same L1-S1 motion as that of the intact specimens. During the protocol's later phase, the L4-L5 units from each specimen were segmented for cyclic loading followed by postfatigue kinematic analysis to highlight the differences in motion pre- and postfatigue. The objectives included the in vitro comparison of (1) the adjacent-level motion before and after instrumentation with PEEK and titanium rods and (2) the pre- and postfatigue motion at the instrumented level with PEEK and titanium rods. The results showed that the adjacent levels above the instrumentation caused increased flexion and extension with both PEEK and titanium rods. The postfatigue kinematic data showed that the motion at the instrumented level (L4-L5) increased significantly in both flexion and extension compared to prefatigue motion in titanium groups. However, there was no significant difference in motion between the pre- and postfatigue data in the PEEK group.

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Figures

Grahic Jump Location
Fig. 8

Adjacent level segmental motion after instrumentation at L4-L5 and L2-L3 (and its comparison with intact) with PEEK rods. P value < 0.05 indicates statistical significance with a 95% confidence interval.

Grahic Jump Location
Fig. 7

Adjacent level segmental motion after instrumentation at L4-L5 (and its comparison with intact) with PEEK rods. P value < 0.05 indicates statistical significance with a 95% confidence interval.

Grahic Jump Location
Fig. 6

Adjacent level segmental motion after instrumentation at L4-L5 and L2-L3 (and its comparison with intact) with titanium rods. P value < 0.05 indicates statistical significance with a 95% confidence interval.

Grahic Jump Location
Fig. 5

Adjacent level segmental motion after instrumentation at L4-L5 (and its comparison with intact) with titanium rods. P value < 0.05 indicates statistical significance with a 95% confidence interval.

Grahic Jump Location
Fig. 4

L4-L5 instrumented specimen under cyclic loading

Grahic Jump Location
Fig. 3

Pedicle screw system with PEEK rods at L4-L5 (left) in conjunction with cage at L4-L5. Pedicle screw system with PEEK rods at L2-L3 + Pedicle screw system with PEEK rods in conjunction with cage at L4-L5 (right)

Grahic Jump Location
Fig. 2

Pedicle screw system with titanium rods at L4-L5 (left) in conjunction with cage at L4-L5. Pedicle screw system with titanium rods at L2-L3 + Pedicle screw system with titanium rods in conjunction with cage at L4-L5 (right)

Grahic Jump Location
Fig. 1

L1-S1 specimen on the kinematic testing machine

Grahic Jump Location
Fig. 9

Instrumented level motion at L4-L5 pre and post fatigue with titanium rods and PEEK rods at 10 Nm of moment. P value < 0.05 indicates statistical significance with a 95% confidence interval.

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