Biomechanical Alterations in Intact Osteoporotic Spine Due to Synthetic Augmentation: Finite Element Investigation

[+] Author and Article Information
Kathryn B. Higgins

Department of Mechanical and Aerospace Engineering, Rutgers,  The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854kbhiggin@caip.rutgers.edu

David R. Sindall

Department of Mechanical and Aerospace Engineering, Rutgers,  The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854sindall@eden.rutgers.edu

Alberto M. Cuitino

Department of Mechanical and Aerospace Engineering, Rutgers,  The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854cuitino@jove.rutgers.edu

Noshir A. Langrana

Department of Mechanical and Aerospace Engineering, Rutgers,  The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854langrana@caip.rutgers.edu

J Biomech Eng 129(4), 575-585 (Dec 04, 2006) (11 pages) doi:10.1115/1.2746379 History: Received July 22, 2005; Revised December 04, 2006

A three-dimensional nonlinear finite element model (FEM) was developed for a parametric study that examined the effect of synthetic augmentation on nonfractured vertebrae. The objective was to isolate those parameters primarily responsible for the effectiveness of the procedure; bone cement volume and bone density were expected to be highly important. Injection of bone cement was simulated in the FEM of a vertebral body that included a cellular model for the trabecular core. The addition of 10% and 20% cement by volume resulted in an increase in failure load, and the larger volume resulted in an increase in stiffness for the vertebral body. Placement of cement within the vertebral body was not as critical a parameter as cement amount. Simulated models of very poor bone quality saw the best therapeutic benefits.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Unit cell of trabecular micromodel. The open hexagonal cell used in the trabecular core is comprised of individual trabeculae and represents low bone density.

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Figure 2

Overall model geometry. Vertebral body height (BH), width (W), and depth (D) as well as shell thickness (T,TL), and wall curvature (S) are mapped to fall within L1 morphology (27,48,52).

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Figure 3

Stress distribution of the vertical normal component for the 60year old model with no cement. (a) Contour plot of the entire shell and end plate region where the half-height plane at the vertebral cross section is the smallest. (b) Contour plot at the half-height cross section. Notice that the stress is nearly uniform along the shell. (c) Distribution of the stress along the shell from the anterior region (A) to the posterior region (P). The value of the plateau stress is about 55MPa.

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Figure 4

Effect of the cement content on the distribution of the normal vertical stress for 80‐year-old model for no cement; 10% central and 20% central cement by volume.

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Figure 5

Effect of the location of the cement on the shell stresses for 60‐year-old model. The location of cement does not significantly change the plateau stress along the shell, which remains at nearly 55MPa (note that the scale limits are different from the ones in Fig. 3). Higher shell stresses are observed in the vicinity of the cement location.

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Figure 6

Shell deformation pattern for the shell for the 60‐year-old model with no cement. The shell is subjected to bending in addition to compression. The increase of curvature in the central region induces higher compressive stresses in the external wall. The constraining effects of the end plates limit the rotation of the shell introducing tensile stresses in the external region of the shell.

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Figure 7

Core stresses for 60- and 80‐year-old models at the half-height cross section. The wire-frame plot corresponds to the mesh at the half-height cross section. The elevation of the surfaces is proportional to vertical normal stress in that section (compression) and the color is mapped to the scale. (a) In both cases the stress in the core remains nearly uniform, where the core with the weaker 80‐year-old model carries lower stresses. (b) Mean value of the stress for the 60- and 80‐year-old model are approximately 4.5MPa and 3.0MPa, respectively, which are indicated with planes of constant height. Note that the stresses in the core are more than ten times lower than in the shell.

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Figure 8

Load versus displacement for the 80‐year-old model, cement placed anteriorly. The inflection at point 1 in the diagram, displacement of 0.312mm, indicated the onset of plasticity effects in one or more trabeculae. Vertebral body failure is taken at 3mm displacement (point 2) as seen experimentally.

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Figure 9

Load at failure versus cement location and amount, 60‐year-old model. Horizontal line indicates failure load for the unfilled 60‐year-old model (5523.8N). Adding cement volumes over 10% fill results in a larger relative increase in failure load.



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