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Research Papers

Stress Changes in Intervertebral Discs of the Cervical Spine Due to Partial Discectomies and Fusion

[+] Author and Article Information
Abraham Tchako

Department of Mechanical Engineering, Union College, Schenectady, NY 12308

Ali M. Sadegh

Department of Mechanical Engineering, The City College of The City University of New York, New York, NY 10031

J Biomech Eng 131(5), 051013 (Apr 15, 2009) (11 pages) doi:10.1115/1.3118763 History: Received August 02, 2007; Revised March 04, 2009; Published April 15, 2009

Spine discectomy and fusion is a widely used surgical procedure to correct irreversible degenerative diseases and injuries to the intervertebral disk. The surgical procedure involves the removal of the damage disk material, the decortication of the fusion site, and the placement of the bone graft. Fusion is believed to generate additional stresses in the neighboring disks, which can subsequently lead to new disk degeneration and re-operation. The autologous bone has proven to be the best material for the fusion. However, the autologous bone has three major disadvantages: the high rate of donor site morbidity, the limited and sometimes poor quality of the amounts of bone available, and the extra operative time needed for harvest. For these reasons this study is undertaken to estimate the optimum amount of bone graft needed for a discectomy and correlate it to the change in stress in adjacent levels. A detailed and validated 3D finite element model of the complete human cervical spine (C1-T1) was altered to simulate segmental full and partial discectomies. One full fusion (bone graft occupies about 90% of the vertebral body) and seven partial fusions (bone graft occupies about 10%, 20%, 30%, 40%, 50%, 65%, and 75% of the vertebral body) were simulated at each of the four mid- and lower single levels of the cervical spine and the relationship between the change in stresses in the adjacent levels and the bone graft size (area) was studied. The changes in stress were compared with the previously obtained results of the unfused models. The fused and unfused models were preloaded with a 73.6 N compressive force representing the weight of the head and with a 1.5 Nm physiological moment in flexion, extension, lateral bending, and axial rotation. More than 132 cases were analyzed. The results showed that the necessary amount of bone graft needed for discectomy depends on the cervical disk level to be fused and varies between 30% and 75% of the disk area. The results also suggested that there is a threshold size of the bone graft area, before and/or after which, the long-term effects of the change in stresses in adjacent disks are biomechanically consequential.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 2

Typical vertebra of the FE model

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Figure 3

Typical disk of the FE model

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Figure 4

Simulation of partial discectomies; darker color shows bone graft area

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Figure 5

Maximum von Mises stresses (MPa) in intervertebral disk in the intact model

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Figure 6

Percentage of stress changes P in neighboring disks as a function of the percentage of discectomy/bone graft and fusion at disk34—with the models loaded in (a) flexion, (b) extension, (c) lateral bending, and (d) axial rotation

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Figure 7

Percentage of change in stresses P in neighboring disks as a function of the percentage of discectomy/bone graft and fusion at disk45—with the models loaded in (a) flexion, (b) extension, (c) lateral bending, and (d) axial rotation

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Figure 8

Percentage of change in stresses P in neighboring disks as a function of the percentage of discectomy/bone graft and fusion at disk56—with the models loaded in (a) flexion, (b) extension, (c) lateral bending, and (d) axial rotation

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Figure 9

Percentage of change in stresses P in neighboring disks as a function of the percentage of discectomy/bone graft and fusion at disk67—with the models loaded in (a) flexion, (b) extension, (c) lateral bending, and (d) axial rotation

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Figure 10

The stress fields of the cervical disks for four types of loadings; flexion, extension, lateral bending, and axial rotation, when only disk45 is 30% partially fused in the cervical spine model. The model is fixed at C7-T1 vertebrae. Note that the posterior is generally lower left side and the anterior is upper right side.

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Figure 1

The global FE model

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