In the multivariate model for factors associated with repeated back pain at follow-up, after adjusting for covariates, combat deployers were at an increased odds compared with noncombat deployers (OR = 1.27, 95% CI: 1.08–1.50). However, unlike the main back pain model, a positive association was found between nondeployers and repeated back pain (OR = 1.18, 95% CI: 1.03–1.35) versus noncombat deployers. Compared with functional support/administration occupations, no other occupation was significantly associated with repeated back pain.
In a separate model investigating length of deployment instead of combat experiences, after adjusting for covariates, participants with an increased length of deployments had higher odds of back pain than nondeployers, with adjusted odds of 1.08 (95% CI: 1.02–1.15) for 1 to 270 days and 1.30 (95% CI: 1.23–1.40) for >270 days. No association was observed between repeated back pain and cumulative days deployed 1 to 270 days (OR = 0.94, 95% CI: 0.84–1.05) or >270 days (OR = 1.05, 95% CI: 0.91–1.20) compared with nondeployers.
Combat deployers had a 38% higher odds of reporting back pain at follow-up and 27% higher odds of repeated back pain, than noncombat deployers. Those with longer deployments had higher odds of back pain than nondeployers. Similar to previous studies that assessed a variety of physical health outcomes,24–27 combat experience appears to be the primary risk factor rather than deployment itself, possibly attributed to higher physical demands and psychological load from life-threatening combat situations. Deployment length may impact back pain risk, possibly due to sustained operations and prolonged wearing of protective armor. Nondeployers had 18% higher odds of repeated back pain than noncombat deployers. Chronic back pain may increase the likelihood of being disqualified for deployment or can be a contributing factor to other disqualifying comorbidities. Medical exemptions from deployment are determined at the individual level and are fluid, with multiple reasons that may fluctuate at any given time. These data are challenging to capture at the larger population-based level and further substudies may be warranted.
In military personnel, back pain is among the most frequent reasons for medical visits and lost duty time and has been associated with pain-related disability.28,29 Further, back pain potentially reduces mental and physical health5,30,31 and may result in high medical costs and dependence on pain medication(s).32,33 The importance of addressing back pain to potentially mitigate high costs associated with opioid use cannot be overstated.32,33 The overall rate of lower back pain related medical encounters in active-duty U.S. Armed Forces was 74.1 visits per 1000 person-years, with recurrence within 1 year at 23%.29 In our study, 16% of service members self-reported back pain, while 58% had repeated back pain, highlighting the substantial burden and need for further studies.
In this study, service support/supply handlers had an increased odds of back pain compared with functional support/administrative occupations. Previous research also found higher odds of low back pain among U.S. Marines service/supply versus administrative/other occupations.13 We identified reduced odds of back pain in electrical/mechanical versus functional support/administrative occupations, inconsistent with previous research.13
Potential risk factors for back pain are numerous and may include genetics, age, sex, enlisted rank, obesity, smoking, psychosocial factors (attitude toward employer/pay), high workload and psychological load (high demand/low control), heavy or static work with lifting or vibration, history of back pain, job dissatisfaction, and wearing of body armor.3,9,34,35 Similar to previous studies, we found that back pain is associated with age (born before 1960 vs. 1970–1979), being female, being enlisted, being overweight or obese, being former or current smokers, and engaging in heavy versus light work. We were unable to assess genetics, job dissatisfaction, wearing of body armor, and history of back pain though previous back pain was incorporated in the repeated back pain model. Active-duty members and Army personnel had increased odds of back pain; these subgroups may be more likely to perform physically demanding activities, such as repetitive motions or carrying heavy loads during deployments, warranting targeted prevention efforts.
We also observed a weak association (OR = 1.17, 95% CI: 1.02–1.34) between back pain and positive screen for panic/anxiety disorder consistent with previous studies, although earlier studies differ by definitions of risk or prognostic factors, populations, and assessed outcomes (e.g., incident low back pain, return to work status, persistent disabling low back pain).36–39 A prospective study examining disabling pain after 12 months found anxiety to be an independent predictor (relative risk = 1.84, 95% CI: 1.05–3.25) of back pain in general practice patients,6 while depression, initially identified as a prognostic indicator, was not significant after adjusting for other covariates. Pre-pain psychopathology has been hypothesized to heighten vulnerability to developing chronic pain after acute injury based on a “diathesis-stress” model in which predisposing psychological characteristics are activated by stress.40 In our study, both back pain and repeated back pain models confirmed positive association between back pain and PTSD symptoms, as well as lower mental component scores. Other studies have shown psychosocial and psychological factors, including depression, psychological distress, passive coping strategies, fear-avoidance beliefs, and catastrophizing, to be associated with persistent back pain.7,41–46 A multifactorial relationship between mental health conditions and back pain is supported by these findings.
We observed an increase in odds of back pain in former and current smokers that is consistent with previous reports linking smoking and musculoskeletal disorders. Toxins contained in tobacco smoke may promote damage to vascular structures of discs and joints, as well as tissue damage and changes in neurological pain response.47
Limited physical functioning and daily activities have been reported among those with back pain.3,28 We found lower physical component summary scores in participants with back pain and repeated back pain. Those who sat or engaged in heavy work had increased odds of back pain and repeated back pain than those doing light work. However, occupations requiring long periods of standing had increased odds of repeated back pain compared with light work. It is possible that individuals with back pain choose more sedentary work following injury. Nonsedentary work, without prolonged standing or heavy lifting, may provide activity needed to reduce repeated back pain.
Because this cohort study oversampled females, previously deployed, and U.S. Reserve/National Guard personnel at baseline, it may not be representative of all deployers or the general military population. However, previous reviews of Millennium Cohort baseline data suggested that this cohort is reasonably representative of military personnel in terms of demographic and health characteristics and has reliable self-reported health and exposure information.48–51 The back pain sample is representative of the baseline enrolled sample as well as the invited sample with 74.5% male (compared with 73.2% in the baseline and 76.0% male in the invited).8 Because of the intentional oversampling to ensure adequate power for statistical inferences, it is slightly lower than the U.S. military at 84.7% male.8 Likewise, the top three occupations are combat specialists, functional support/admin, and electrical/mechanical in the back pain sample (20.6%, 19.7%, and 14.1%, respectively), representative of the baseline sample (20.0%, 20.0%, and 14.8% respectively), the invited sample (20.9%, 17.9%, and 16.2%, respectively), and the U.S. military population (21.9%, 17.6%, and 15.1%, respectively).8 Misclassification of back pain may exist despite assessment of individuals at multiple time points, as comprehensive information on duration, frequency, and location of symptoms were not available. Pain estimates are often dependent upon self-report, as there is no definitive test or validated standard for these symptoms.3,52 Although response bias may exist, investigation of the initial cohort responders found little cause for concern. The Cohort had greater than 70% follow-up, and potential loss-to-follow-up nonresponse bias was previously found to have limited to no effect on findings.53
Our study is the first to prospectively investigate back pain over multiple time points and focuses on deployment (differentiated by combat experience) and occupation as primary predictor variables. Other unique strengths include large sample size and inclusion of all Services and components of the military. Approximately 30% of the Cohort deployed in support of the operations in Iraq and Afghanistan from 2001 through 2006, resulting in robust numbers for investigating deployment-related concerns. Finally, self-reported health symptoms, such as back pain, may better frame health issues when there are no validated objective standards for outcome measures or potentially underreported medical visits.
Deployment with combat experiences was found to increase the odds of back pain and repeated back pain in a relatively young U.S. military and veteran population. This study frames the burden of back pain in the military, which may be associated with both reduced physical and mental functioning. Occupational associations identified may aid in targeted efforts to improve overall health and functioning long after leaving military service.
* To our knowledge, this is the first study to prospectively assess deployment and self-reported recent back pain in a population-based U.S. military cohort.
* Deployers with combat experiences had higher odds of recent back pain than noncombat deployers.
* There was no association between recent back pain and nondeployers compared with noncombat deployers.
The authors thank the entire Millennium Cohort Study Team and participants; the professionals from the U.S. Army Medical Research and Materiel Command, especially those from the Military Operational Medicine Research Program; Scott L. Seggerman from the Management Information Division, DMDC; and Michelle LeWark from the Naval Health Research Center.
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Keywords:Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.
back pain; chronic pain; cohort studies; combat disorder; mental health; military personnel; occupational health; outcome assessment; statistics; survey methodology