Tension and combined tension-extension structural response and tolerance properties of the human male ligamentous cervical spine.

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+/-215 N) was greater than when loaded over the occipital condyles (2032+/-250 N), 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.

Comparative structural neck responses of the THOR-NT, Hybrid III, and human in combined tension-bending and pure bending.

This study evaluated the biofidelity of both the Hybrid III and the THOR-NT anthropomorphic test device (ATD) necks in quasistatic tension-bending and pure-bending by comparing the responses of both the ATDs with results from validated computational models of the living human neck. This model was developed using post-mortem human surrogate (PMHS) osteoligamentous response corridors with effective musculature added (Chancey, 2005). Each ATD was tested using a variety of end-conditions to create the tension-bending loads. The results were compared using absolute difference, RMS difference, and normalized difference metrics. The THOR-NT was tested both with and without muscle cables. The THOR-NT was also tested with and without the central safety cable to test the effect of the cable on the behavior of the ATD. The Hybrid III was stiffer than the model for all tension-bending end conditions. Quantitative measurement of the differences in response showed more close agreement between the THOR-NT and the model than the Hybrid III and the model. By contrast, no systematic differences were observed in the head kinematics. The muscle cables significantly stiffened the THOR-NT by effectively reducing the laxity from the occipital condyle (OC) joint. The cables also shielded the OC upper neck load cell from a significant portion of the applied loads. The center safety significantly stiffened the response and decreased the fidelity, particularly in modes of loading in which tensile forces were large and bending moments small. This study compares ATD responses to computational models in which the models include PMHS response corridors while correcting for problems associated with cadaveric muscle. While controversial and requiring considerable diligence, these kinds of approaches show promise in assessing ATD biofidelity.