A neck compression injury criterion incorporating lateral eccentricity

Abstract

There is currently no established injury criterion for the spine in compression with lateral load components despite this load combination commonly contributing to spinal injuries in rollover vehicle crashes, falls and sports. This study aimed to determine an injury criterion and accompanying tolerance values for cervical spine segments in axial compression applied with varying coronal plane eccentricity. Thirty-three human cadaveric functional spinal units were subjected to axial compression at three magnitudes of lateral eccentricity of the applied force. Injury was identified by high-speed video and graded by spine surgeons. Linear regression was used to define neck injury tolerance values based on a criterion incorporating coronal plane loads accounting for specimen sex, age, size and bone density. Larger coronal plane eccentricity at injury was associated with smaller resultant coronal plane force. The level of coronal plane eccentricity at failure appears to distinguish between the types of injuries sustained, with hard tissue structure injuries more common at low levels of eccentricity and soft tissue structure injuries more common at high levels of eccentricity. There was no relationship between axial force and lateral bending moment at injury which has been previously proposed as an injury criterion. These results provide the foundation for designing and evaluating strategies and devices for preventing severe spinal injuries.

Figure 1

Dynamic “impact” test setup looking toward the anterior side of the specimen for the end constraint allowing lateral translation and rotation of the superior end of the specimen (left, Translation Free) and the end constraint allowing only lateral rotation and not translation (right, Translation Fixed) showing the load applied at a predefined initial eccentricity (e).

Figure 2

Compression force (Fz), contralateral lateral shear force (Fy) and ipsilateral lateral bending moment (Mx) coronal plane loads on the inferior disc for a specimen subjected to a left eccentric compression force (left) and calculated coronal plane resultant force, Fyz, and eccentricity, Eyz (right).

Figure 3

Axial force-time response (top) and lateral bending moment-time response (bottom) for the low (left), medium (centre) and high (right) initial eccentricity specimen groups. Solid lines indicate the unconstrained lateral translation end condition and dashed lines indicate the constrained lateral translation end condition. Positive force is tension and positive lateral bending moment is to the ipsilateral side.

Figure 4

Lateral bending moment (ipsilateral is positive) and axial force (tension is positive) at failure for each specimen in the following initial eccentricity and end constraint groups: low-fixed (blue square), low-free (blue circle), medium-fixed (black rhombus), medium-free (black triangle), high-fixed (red star), high-free (red hexagon). Previously published data^,, with a revised time of injury, are the low-fixed and high-fixed specimens. The linear regression line (solid line), equation, 95% confidence interval (dotted lines) and coefficient of determination are shown for the combined data.

Figure 5

Coronal plane eccentricity at injury calculated from the lateral bending moment and coronal plane resultant force at injury plotted with the magnitude of coronal plane resultant force at injury for each specimen in the following initial eccentricity-end constraint groups: low-fixed (blue square), low-free (blue circle), medium-fixed (black rhombus), medium-free (black triangle), high-fixed (red star), high-free (red hexagon). The linear regression line (solid line), equation, 95% confidence interval (dotted line) and coefficient of determination are shown for the combined data of this study. Black crosses show data from compression tests with lateral eccentricity described by Toomey et al. (2012). The shaded green area indicates a region of coronal plane eccentricity and force in which no specimens initiated injury in the current data.

Figure 6

Coronal plane eccentricity and resultant coronal plane force for male specimens (left) and female specimens (right). Initial eccentricity-end constraint groups are indicated as follows: low-fixed (blue square), low-free (blue circle), medium-fixed (black rhombus), medium-free (black triangle), high-fixed (red star), high-free (red hexagon). Linear regression lines (solid lines), equations, 95% confidence intervals (dotted lines) and coefficients of determination are shown.