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INTRODUCTION: Fighter pilots and military helicopter aircrew experience chronic neck and back pain at rates significantly higher than civilians. Their unique loading conditions, including high-G exposures and whole-body vibration, likely contribute to accelerated degenerative changes that can alter the response of the spine to external loads and contribute to spine-mediated pain. This study aimed to characterize mechanical changes to the annulus fibrosus (AF) associated with repeated loading modeling in the military aviation environment. METHODS: Fresh porcine AF specimens were used in this study which have been previously identified as representative of human tissue. Test specimens were harvested from the anterior superficial region of the AF and exercised in tension using a protocol that quantified pre- and post-damage stiffness and viscoelastic (i.e., time/rate dependent) properties. Tissue damage was induced using repeated subfailure tension loading with one of nine pre-defined groups with a combination of different strain magnitudes (11%,28%,44%) and cycle counts (400,1600,6400). Damage loading time ranged from 1 to 81 minutes. Following damage and post-damage material characterization, specimens were quasi-statically pulled to failure. RESULTS: Stiffness and viscoelasticity of AF tissues decreased significantly after damage loading for all damage groups and changes were correlated with both the magnitude of applied loading and the total number of cycles. Despite significant dose-dependent changes to the viscoelastic and elastic properties, the ultimate properties during quasistatic distraction to failure did not change. This indicates the mechanical response during low-strain magnitudes is significantly affected, but not the high-strain mechanical response. DISCUSSION: This study quantified changes in AF mechanics caused by tensile damage loading and found decreases in viscoelastic and elastic properties post-damage. These changes reduce the ability of the intervertebral disc to absorb energy associated with external loads and can contribute to disc laxity, which can lead to spine segmental instability. These changes can increase the loading of the surrounding apophyseal joints and may contribute to discogenic and radicular pain symptoms. These findings may partially explain a mechanism of acute flight-related pain symptoms in military aircrew, highlighting a need to reduce the magnitude of cyclic loads an AF is exposed to during flight. Learning Objectives 1. The audience will learn how the mechanical response of the annulus fibrosus changes in response to damage caused by cyclic loading. 2. The audience will be able to understand why changes in the mechanical response of the annulus fibrosus may contribute to discogenic and radicular pain symptoms. [ABSTRACT FROM AUTHOR]

ABSTRACT: Quantification of biomechanical tolerance is necessary for injury prediction and protection of vehicular occupants. This study experimentally quantified lumbar spine axial tolerance during accelerative environments simulating a variety of military and civilian scenarios. Intact human lumbar spines (T12‐L5) were dynamically loaded using a custom‐built drop tower. Twenty‐three specimens were tested at sub‐failure and failure levels consisting of peak axial forces between 2.6 and 7.9 kN and corresponding peak accelerations between 7 and 57 g. Military aircraft ejection and helicopter crashes fall within these high axial acceleration ranges. Testing was stopped following injury detection. Both peak force and acceleration were significant (p < 0.0001) injury predictors. Injury probability curves using parametric survival analysis were created for peak acceleration and peak force. Fifty‐percent probability of injury (95%CI) for force and acceleration were 4.5 (3.9–5.2 kN), and 16 (13–19 g). A majority of injuries affected the L1 spinal level. Peak axial forces and accelerations were greater for specimens that sustained multiple injuries or injuries at L2–L5 spinal levels. In general, force‐based tolerance was consistent with previous shorter‐segment lumbar spine testing (3–5 vertebrae), although studies incorporating isolated vertebral bodies reported higher tolerance attributable to a different injury mechanism involving structural failure of the cortical shell. This study identified novel outcomes with regard to injury patterns, wherein more violent exposures produced more injuries in the caudal lumbar spine. This caudal migration was likely attributable to increased injury tolerance at lower lumbar spinal levels and a faster inertial mass recruitment process for high rate load application. Published 2017. This article is a U.S. Government work and is in the public domain in the USA. J Orthop Res 36:1747–1756, 2018. [ABSTRACT FROM AUTHOR]

Background: Lumbar back pain and radiculopathy are common diagnoses. Unfortunately, conventional magnetic resonance imaging (MRI) fi ndings and clinical symptoms do not necessarily correlate in the lumbar spine. With upright imaging, disc pathologies or foraminal stenosis may become more salient, leading to improvements in diagnosis. Materials and Methods: Seventeen adults (10 asymptomatic and 7 symptomatic volunteers) provided their informed consent and participated in the study. A 0.6T upright MRI scan was performed on each adult in the seated position. Parameters were obtained from the L2/3 level to the L5/S1 level including those pertaining to the foramen [cross-sectional area (CSA), height, mid-disc width, width, thickness of ligamentum fl avum], disc (bulge, height, width), vertebral body (height and width), and alignment (lordosis angle, wedge angle, lumbosacral angle). Each parameter was compared based on the spinal level and volunteer group using twofactor analysis of variance (ANOVA). Bonferroni post hoc analysis was used to assess the differences between individual spinal levels. Results: Mid-disc width accounted for 56% of maximum foramen width in symptomatic volunteers and over 63% in asymptomatic volunteers. Disc bulge was 48% greater in symptomatic volunteers compared to asymptomatic volunteers. CSA was generally smaller in symptomatic volunteers compared to asymptomatic volunteers, particularly at the L4-L5 and L5-S1 spinal levels. Thickness of ligamentum fl avum (TLF) generally increased from the cranial to caudal spinal levels where the L4-L5 and L5-S1 spinal levels were significantly thicker than the L1-L2 spinal level. Conclusions: The data implied that upright MRI could be a useful diagnostic option, as it can delineate pertinent differences between symptomatic volunteers and asymptomatic volunteers, especially with respect to foraminal geometry. [ABSTRACT FROM AUTHOR]

The objective of this investigation was to develop probabilistic finite element (FE) models of the anterior longitudinal ligament (ALL) and posterior longitudinal ligament (PLL) of the cervical spine that incorporate the natural variability of biological specimens. In addition to the model development, a rigorous validation methodology was developed to quantify model performance. Experimental data for the geometry and dynamic properties of the ALL and PLL were used to create probabilistic FE models capable of predicting not only the mean dynamic relaxation response but also the observed experimental variation of that response. The probabilistic FE model uses a quasilinear viscoelastic material constitutive model to capture the time-dependent behaviour of the ligaments. The probabilistic analysis approach yields a statistical distribution for the model-predicted response at each time point rather than a single deterministic quantity (e.g. ligament force) and that response can be statistically compared to experimental data for validation. A quantitative metric that compares the cumulative distribution functions of the experimental data and model response is computed for both the ALL and PLL throughout the time histories and is used to quantify model performance. [ABSTRACT FROM AUTHOR]

The objective was to examine significant differences in the bony structure of cervical spine vertebrae based on gender and spinal level that may influence injury risk in women following automotive rear impact. Male and female subjects were recruited for a separate study and data from two subsets were selected for inclusion in this study. Subjects were size-matched based on sitting height (17 males, 11 females) and head circumference (9 males, 18 females). Axial CT scans were obtained of the cervical spine from the C1 through C6. Bony boundaries of cervical vertebrae were defined using image-analysis software and biomechanically-relevant dimensions were derived at spinal levels C2 through C6. Six of seven vertebral dimensions were significantly dependent upon gender and spinal level in both subgroups. Male vertebrae had larger dimensions for each metric. Depth dimensions were greatest at caudal and cranial extents, whereas width dimensions were smallest at C2 and increased caudally. Greater linear and areal dimensions in size-matched male subjects indicates a more stable cervical spinal column that may be more capable of resisting inertial loading of the head-neck complex during automotive rear impacts. Although the explanation for greater injury susceptibility in females is likely multi-factorial, including differences in spinal material properties, soft tissue tolerance thresholds, occupant-seatback orientation, and neck muscle size/orientations, the present study has identified significant differences in cervical spine anatomical dimensions that may contribute to greater rates of whiplash injury in that population.