Application of complex neck loads to human spine at the occipital condyle joint: Implications for nonstandard postures for automated vehicles.

Objective: The automotive industry's shift toward automated vehicles allows the occupants to assume postures different from the standard upright seated position. Injury criteria assessments are needed under these nonstandard postures to advance safety. The objective of this study is to develop a new device that can position the human cadaver head-neck structures in different nonstandard pre-postures using custom devices and apply external loading anticipated in modern and future automotive and military scenarios.

Methods: An isolated head to T1 human cadaver specimen was attached to a load cell at T1. The load cell was fixed to the top of a six-degree-of-freedom custom spinal positioning device to orient the specimen such that the occipital condyle joint was in line with the torque axis of a custom angular displacement test device. The angular device converted the linear motion of a vertically oriented electro-hydraulic piston to a torque about the occipital condyle joint of the specimen. The head was pre-rotated in the axial plane, approximately 20 degrees to the left, while maintaining the coronal alignment of the lower cervical spine. Targets were secured at the head and spine (details in the body of the manuscript), and their three-dimensional positions were measured using a seven-camera optical motion capture system. Right and then left lateral bending tests were conducted. Occipital condyle joint loads were determined from the superior load cell, and the stiffness difference between the left and right lateral bending was determined.

Results: The peak coronal bending moments were 27.1 Nm and 47.6 Nm for the right and left lateral bending tests. At the time of the peak x-moment, the y moments were 1.6 and 9.1 Nm, and the z moments were 3.1 and 4.8 Nm. The head angle with respect to T1 at the time of peak x-moments was 28.1 and 27.7 deg about x, 11.0 and 11.7 deg about y, and 33.9 and 21.8 deg about z axes for the right and lateral bending tests. C1 left lateral mass fractured following the left lateral bending test.

Conclusions: The stiffness of the spine increased by approximately three times due to asymmetries in posture and loading. The present system of custom spinal positioning and angular displacement test devices and loading methodologies can be used in conjunction with a conventional piston testing apparatus to conduct additional experiments to delineate the injury patterns and mechanisms and develop injury criteria applicable to modern and future vehicle environments.