Effects of flexion/extension on the axial compression tolerance of the human lumbar spine during dynamic compression

Introduction:: Spine injuries have become increasingly common in modern warfare, particularly in recent American military conflicts where they accounted for 5.5% of all casualties³. This rise in spinal injuries can be largely attributed to the increased use of improvised explosive devices (IEDs), resulting in underbody blast (UBB) injuries^{2, 5, 10}. Despite the implementation of wave deflectors and V-shaped hulls to protect occupants from primary blast injuries, U.S. military ground vehicles were not initially designed to protect soldiers from the vertical accelerations caused by the energy transfer between the IED and vehicle¹¹. Among soldiers wounded in action, the incidence of lumbar spine fractures ranked fourth, and second among those killed in action⁶. These injuries can severely impact a soldier's ability to carry out field operations and are frequently associated with lifetime disabilities that significantly impact their quality of life^{4, 7}.

Vertical loading plays a significant role in the overall loading of the spine during UBB^{11, 12, 14}. Previous axial force injury criterion can be used to develop biofidelic surrogates, establish finite element models, and ultimately contribute to injury mitigation strategies¹³. However, due to the complex geometry and loading path during UBB, changes in spine positioning can redistribute loading and affect the risk of injury. Previous studies have also indicated that introducing flexion in the loading regime can impact the spine's tolerance to injury⁹. Therefore, understanding how changes in spine positioning and flexion or extension can influence the risk of injury during UBB will provide valuable insights into developing effective injury mitigation strategies.


Materials and Methods:: Dynamic compression tests were conducted on 32 unembalmed human cadaveric thoracolumbar spines (T12-S1) at injurious loading conditions across three different specimen initial postures, which represented the nominal seated soldier posture ± a standard deviation⁸. The three tested postures were denoted as nominal (n=3), pre-flexed (n=16), and pre-extended (n=13); and were combined with the previous nominal posture dynamic compression axial loading criterion proposed by Yoganandan et al. (n=43),¹³ to investigate the effects of flexion and extension on the spine injury tolerance. MicroCT images were used to examine the integrity of the lumbar spine specimens. Only specimens that showed no sign of joint degradation or osteophytes were selected for the study to ensure their mechanical response was not compromised. To facilitate sensor placement, the spines were prepared by removing musculature, fat, periosteum, and ribs, while keeping the osteoligamentous structure intact. The specimens were then secured to a custom servohydraulic Materials Testing Machine (MTS®, Eden Prairie, MN, USA) allowing motion only in the sagittal plane and they were aligned to match one of the three investigated postures. A six-axis load cell (model MC5-6-5000, AMTI, Watertown, MA, USA) recorded forces and moments at the proximal (T12) end of the thoracolumbar spine at 100 kHz. Signals were filtered with a phaseless 4-pole low-pass Butterworth filter with a cutoff frequency of 3000 Hz. Three parametric distributions were assessed to determine the injury risk for the tested postures. The distribution with the lowest Anderson-Darling (AD) was selected as the best representation of the injury risk¹.


Results, Conclusions, and Discussions:: The investigated dataset consisted of 75 male cadaveric thoracolumbar spines. Out of these, 64 specimens suffered vertebral body fractures, 2 had no injuries, and the remaining 9 experienced other injury types. The male specimens had a mean age, weight, stature, and body mass index of 66±11 years, 81±12kg, 1.77±0.05m, and 26±4kg/m², respectively. Among the tested postures, the loglogistic distribution had the lowest AD coefficient and was therefore chosen to represent the injury risk. The nominal posture presented a higher injury tolerance to the axial force, with 5569N representing the 50% injury risk. The pre-flexed and pre-extended postures had similar injury tolerance, with the 50% risk being at 4618N and 4493N, respectively (Figure 1).

These results are in agreement with previous studies, indicating that the introduction of flexion reduces the injury tolerance of the lumbar spine⁹. Further, extension was also found to have a similar effect of increasing the injury risk to axial force during dynamic compression. From the perspective of beam theory, differences in the injury tolerance across postures could be explained by an increased moment contribution to failure in the flexed and extended postures. Since the moment is not included in the axial compression injury assessment, the effect of bending would be to reduce the apparent axial tolerance in off-nominal conditions. Contributing to base beam theory effects, the off-nominal postures may change the center of moment action of the complex system which can further introduce moments that are making a higher contribution to the injury event. An additional explanation may be the reduction of the spine range of motion incurred by the loss of lordosis of the pre-flexed and pre-extended postures. This could result in different anatomical interactions that happen at higher displacements and forces in the compressive event for the nominal posture. This is similar in concept to the beam theory approach.

The results in this study suggest that an axial force metric alone does not capture the full potential of the injury risk during UBB, making the case for the development of injury criteria that consider the contributions of the bending moment in the loading regime.


Acknowledgements (Optional): :

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