Occupational Exposure to Whole Body Vibration Among U.S. Military Veterans: Acute and Chronic Contributions to Musculoskeletal Disorders and Spine-Area Pain

Musculoskeletal (MSK) disorders and spine-related pain are major health concerns among veterans. Low back pain is the most common chronic pain condition among Veterans, contributing significantly to healthcare utilization and rising treatment costs within the Veteran's Health Administration (VHA). As VHA community care expenditures more than doubled between 2017 and 2021, the growing demand for chronic pain services underscores the need for improved understanding of the mechanisms that contribute to spine degeneration in this population. There is evidence that occupational exposure to whole body vibration (WBV) is associated with an increased risk for a multitude of negative health outcomes, including MSK pain in the back and neck. However, despite extensive research on low back pain etiology and disc degeneration, major gaps persist around early detection of endplate degeneration and the impact of acute or chronic WBV exposure on IVD health.

The ubiquitous nature of low back pain in the general public but especially in veterans shows that there is a strong medical need for better understanding of the biomechanics of spine health. This study can help clinical practice by identifying early detection biomarkers with non-invasive MRI techniques and to investigate hydration as a protective mechanism.

The problem of low back pain and spine health is especially relevant within veterans because while service members are generally healthier than the typical U.S. adult of similar age at the time they enter service (because of rigorous physical and mental screening requirements to join the military, continued fitness standards, and access to comprehensive healthcare), research has shown that Veterans have poorer overall health-related quality of life than nonveterans and a high prevalence of back pain-related conditions. Veterans have often been exposed to WBV during their military career while in armored or heavy vehicles or in aircraft. Many veterans also continue to be exposed to WBV in their post-military careers with 28.5% working in natural resources, construction and maintenance or production, transportation and material moving. These jobs and many others may expose workers to WBV while operating vehicles (trucks, construction and heavy machinery), or operating tools where the vibrational amplitude is large enough to transmit to the body. These WBV exposures during their military career and post military career carries an increased risk of negative health outcomes including MSK pain in the back and neck.

While we know that many veterans are exposed to these conditions and they are associated with an increased risk of MSK pain, understanding the biomechanics will help understand how to prevent and treat those long term or chronic injuries. These are knowledge gaps that we have identified in relation to intervertebral disc health and exposure to WBV:

1. Segment- and region-specific disc fluid dynamics under WBV are poorly understood.

While diurnal disc fluid migration patterns have been documented, the impact of acute or chronic WBV on nucleus pulposus and annulus fibrosis fluid distribution, especially across lumbar levels, remains unknown. 2. Early endplate degeneration cannot currently be detected noninvasively. Modic bone marrow changes observable with MRI lack sensitivity and reflect late-stage pathology. No clinical imaging method reliably identifies early degenerative alterations of the cartilaginous or bony endplates. 3. The relationship between endplate health and disc fluid transport under WBV exposure is unclear.

Endplate degeneration likely disrupts regional fluid exchange and may drive segment-dependent susceptibility to WBV-induced disc injury. 4. No prior studies have integrated advanced quantitative MRI techniques to evaluate endplate perfusion, disc fluid movement, and WBV response.

Although dynamic contrast-enhanced MRI can measure endplate perfusion, concerns about gadolinium retention limit its use. Non-contrast alternatives such as arterial spin labeling (ASL) have not been applied to endplate assessment.

This study will address those gaps by combining multi-parametric MRI and groups selected for both acute and longitudinal occupational exposure to WBV to evaluate IVD fluid transport and endplate health. By addressing these gaps, this study will advance understanding of the biomechanical and physiological impacts of WBV on spine health.

We seek to understand the connection between WBV exposure and spine health by comparing groups of veterans with history of or ongoing exposure to WBV and using advanced MRI techniques to study intervertebral disc (IVD) and vertebral endplate health. We also seek to study the acute changes in IVD and vertebral endplate biology before and after exposure to WBV and whether individual hydration state influences IVD water dynamics.

The specific aims are:

Aim 1: Determine the impact of acute and longitudinal WBV on lumbar spine endplate health.

Aim 2: Determine how segmental lumbar and regional IVD fluid distribution and transport are acutely affected by WBV and determine if there are correlations with endplate health.

Aim 3: Determine whether individual hydration state influences IVD water dynamics following acute vibrational exposure.

We hypothesize the following

Specific Aim 1 Hypothesis 1: Multi-parametric MRI will reveal Veterans with previous military WBV exposure have more degenerative endplate markers when compared to Veterans with no occupational WBV exposure.

Hypothesis 2: Multi-parametric MRI in Veterans with current occupational exposure to WBV, but with no previous military WBV exposure, will have earlier expression of degenerative endplate markers when compared to Veterans with no WBV exposure.

Hypothesis 3: The rate of change for degenerative endplate markers over the four-year duration of the study will be greater for Veterans with current occupational WBV exposure than for Veterans that had previous military occupational WBV exposure.

Specific Aim 2 Hypothesis 1: Veterans who routinely experience occupational WBV will exhibit less IVD regional fluid distribution before and after experimental WBV, while those without daily WBV will have greater changes in IVD regional fluid distribution following experimental WBV, with water moving from the NP into the AF or out of the disc.

Hypothesis 2: Veterans who routinely experience occupational WBV will exhibit a cranial to caudal decrease in water transport as well as a decrease in baseline water distribution.

Hypothesis 3: Degenerative endplate health will be correlated with decreased lumbar spine water distribution and transport.

Specific Aim 3 Hypothesis 1: Participants in a low-hydration state will exhibit a larger reduction in intervertebral disc (IVD) water content and altered fluid-transport metrics after 30 minutes of controlled seated vibration than participants in a normal/high-hydration state.

This research is relevant to the VA due to approximately 50% of veterans experiencing chronic pain with lower back pain being the most common type of chronic pain. This is one of the most common reasons for seeking care within the VHA and this creates a significant financial burden on the Veterans Health Administration. There is also data that shows despite service members being generally healthier than the typical U.S. adult when entering service, there is a high incidence of lower back pain among service members and that veterans have poorer overall health related quality of life than non-veterans. Occupational exposure to vibration is associated with an increased risk for a multitude of negative health outcomes, including MSK pain in the back and neck.

Study Design:

This study will investigate acute and chronic effects of occupational whole-body vibration on the health of lumbar spine endplates and intervertebral discs by studying Veterans that had occupational exposure to WBV in their military occupation and Veterans with current civilian occupations that involve WBV. The approximate study timeline, as outlined in the grant proposal, is shown below. Lumbar spine endplate characteristics and intervertebral disc perfusion and hydration in both study groups will be compared to age- and sex-matched Veteran controls with no history of occupational WBV. The volunteer population will be chosen to represent the ethnic and sex distribution of the Zablocki Veterans Affairs Medical Center patient population. Current and military occupations will be selected to maximize consistency in WBV characteristics. For example, the ZVAMC patient population includes both truck drivers and heavy equipment operators. However, given that the WBV characteristics (e.g., frequency and magnitude of vibration) between those occupations are different, which will likely have different acute and chronic effects on the spine, preference in the research population be given to maximize consistency in the WBV environment across the research subject population. We will follow a similar procedure for choosing Veterans with military occupational WBV. Occupations unique to the military, such as helicopter aircrew, will be given priority although other occupations such as truck drivers, etc. will be targeted if we are not able to enroll Veterans with previous military helicopter occupation in sufficient numbers. The initial questionnaire will also include the Oswestry Disability Index (ODI), which will provide information on the impact of low back pain for each prospective subject's daily activities and function. The ODI will also be used in the selection of subjects for the MRI arm of the study to ensure a consistent sample. All interested Veterans will be consented and enrolled into the study prior to the pre-screening interview and Veterans that fit into the selected groups for the MRI study will receive a separate invitation to participate in that arm of the study. The five groups of Veterans that will be required to complete this study are described below.

* Current WBV Group (Group 1) will consist of Veterans with no history of active duty occupational WBV exposure, but with civilian occupational WBV exposure as a Veteran. These individuals will be less than 45 years old and have less than 5 years of civilian occupational WBV exposure. This group will demonstrate the effects of civilian occupational WBV exposure on endplate degeneration, IVD fluid dynamics, and spine structural degeneration metrics. Three multi-parametric MRI scans obtained over the course of this four-year protocol (aim 1) will allow for tracking of endplate health and identification of progressive changes in spine structural characteristics such as lumbar Cobb angle, disc/foraminal dimensions, endplate degeneration, and intervertebral disc perfusion and hydration. These changes will be correlated with WBV logs maintained by each enrolled subject that will provide weekly estimates of the type and duration of occupational exposure over the duration of the study. An experimental acute WBV protocol (aim 2) will outline the acute response of the lumbar spine endplates and intervertebral discs to WBV This protocol will consist of a baseline upright MRI scan in the morning, 30 minutes of experimental WBV, and a second post-WBV upright MRI scan in the afternoon. Subjects will be selected for participation in the MRI arm of the study to ensure consistency in occupational WBV characteristics (e.g., heavy equipment operators versus long haul truck drivers) and duration of occupation to reduce the variability in ongoing occupational WBV that may influence the current spine health at the time of enrollment * Military WBV Group (Group 2) will consist of Veterans with at least 6 years of active duty WBV exposure (ex. heavy equipment operators, helicopter pilots, small watercraft operators). These individuals will be less than 45 years old and have no civilian occupational exposure to WBV. This group will undergo two se