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Research Papers

Tension and Combined Tension-Extension Structural Response and Tolerance Properties of the Human Male Ligamentous Cervical Spine

[+] Author and Article Information
Alan T. Dibb, Jason F. Luck, V. Carol Chancey, Lucy E. Fronheiser, Barry S. Myers

Department of Biomedical Engineering, Division of Orthopaedic Surgery, Duke University, Durham, NC 27708-0281

Roger W. Nightingale1

Department of Biomedical Engineering, Division of Orthopaedic Surgery, Duke University, Durham, NC 27708-0281rwn@duke.edu

1

Corresponding author.

J Biomech Eng 131(8), 081008 (Jul 06, 2009) (11 pages) doi:10.1115/1.3127257 History: Received September 12, 2008; Revised March 25, 2009; Published July 06, 2009

Tensile loading of the human cervical spine results from noncontact inertial loading of the head as well as mandibular and craniofacial impacts. Current vehicle safety standards include a neck injury criterion based on beam theory that uses a linear combination of the normalized upper cervical axial force and sagittal plane moment. This study examines this criterion by imposing combined axial tension and bending to postmortem human subject (PMHS) ligamentous cervical spines. Tests were conducted on 20 unembalmed PMHSs. Nondestructive whole cervical spine tensile tests with varying cranial end condition and anteroposterior loading location were used to generate response corridors for computational model development and validation. The cervical spines were sectioned into three functional spinal segments (Occiput-C2, C4-C5, and C6-C7) for measurement of tensile structural response and failure testing. The upper cervical spine (Occiput-C2) was found to be significantly less stiff, absorb less strain energy, and fail at higher loads than the lower cervical spine (C4-C5 and C6-C7). Increasing the moment arm of the applied tensile load resulted in larger head rotations, larger moments, and significantly higher tensile ultimate strengths in the upper cervical spine. The strength of the upper cervical spine when loaded through the head center of gravity (2417±215N) was greater than when loaded over the occipital condyles (2032±250N), which is not predicted by beam theory. Beam theory predicts that increased tensile loading eccentricity results in decreased axial failure loads. Analyses of the force-deflection histories suggest that ligament loading in the upper cervical spine depends on the amount of head rotation orientation, which may explain why the neck is stronger in combined tension and extension.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the testing apparatus used during tension testing. The specimen head was mounted in a cradle that allowed translation and rotation. The cranial end condition could be varied by fixing these degrees of freedom. The cranial loading location could be varied by positioning the head anterior or posterior relative the rotational bearing. The specimen is loaded by a downward translation of the hydraulic actuator, which is coupled to T1 by a six-axis load cell.

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Figure 2

Schematic of the four cranial end conditions tested during whole spine tension testing: (a) free, (b) rotational constrained, (c) translational constrained, and (d) fixed

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Figure 3

Schematic of the four loading locations tested during whole spine tension testing: (a) 3 cm posterior of the condyles (tension-flexion loading), (b) aligned over the OC (pure tension loading), (c) through the CG (head inertial loading), and (d) 3 cm anterior of the CG (tension-extension loading)

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Figure 4

Schematic of the testing apparatus used during tension testing of the (a) upper cervical (O-C2) and (b) lower cervical (C4-C5 and C6-C7) spinal segments. The lower cervical spinal segments were mounted with the addition of an eccentricity bracket to maintain their lordotic orientation.

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Figure 5

Average kinematic response corridors during whole cervical spine tensile loading with varying cranial end conditions. The plots are presented with the applied load on the abscissa versus (a) axial displacement, (b) anterior-posterior (A-P) head translation, and (c) sagittal head rotation. Cranial end condition had a significant effect between the fixed end condition and rotational constrained compared with the free end condition and the translational constrained (Table 2).

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Figure 6

Representative tensile response plots of the whole cervical spine from specimen T31 for two cranial end conditions: fixed and free. Stiffness was determined from the slope of the linearly regressed applied tensile load versus displacement response between 150 N and 300 N.

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Figure 7

Average kinematic response corridors during whole cervical spine tensile loading with varying loading locations. The plots are presented with the applied load on the abscissa versus (a) axial displacement, (b) anterior-posterior (A-P) head translation, and (c) sagittal head rotation. Loading location had a significant effect between all loading locations except between loading through the CG and loading 3 cm posterior the OC (Table 3).

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Figure 8

Average kinematic response corridors during fixed-fixed spinal segment tensile stiffness testing with (a) loading phase only and (b) loading and unloading phases. The plots are presented with the applied load on the abscissa versus axial displacement. The O-C2 spinal segment was found to be less stiff and dissipate more strain energy than the C4-C5 and C6-C7 segments (Table 4).

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Figure 9

Average stiffness response of the upper cervical spine at four levels of head extension rotation. The head was rotated and then fixed at the level of extension for the duration of the test. Head rotation had a significant effect on axial displacement (Table 5).

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Figure 10

Representative failure plots of the O-C2 spinal segment for the two loading locations. Specimens T22 and T33 were failed with the tensile load aligned over the OC and through the CG, respectively. Major failure was defined as either a 10% decrease in load or a 20% decrease in stiffness during continued loading. Ultimate failure was defined as the maximum load each spinal segment was able to withstand.

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