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Occupational driving as a risk factor for low back pain in active-duty military service members
The Spine Journal 14 (2014) 592–597
Clinical Study
Occupational driving as a risk factor for low back pain in active-duty military service members Jeffrey B. Knox, MDa,*, Joseph R. Orchowski, MDa, Danielle L. Scher, MDb, Brett D. Owens, MDc, Robert Burks, PhDd, Philip J. Belmont, Jr., MDb a Orthopaedic Surgery Service, Department of Surgery, Tripler Army Medical Center, 1 Jarrett White Rd, Honolulu, HI 96859, USA Orthopaedic Surgery Service, Department of Surgery, William Beaumont Army Medical Center, 5005 N. Piedras St, El Paso, TX 79920, USA c Department of Orthopaedic Surgery, Keller Army Hospital, 900 Washington Rd, West Point, NY 10996, USA d Graduate School of Operational and Information Sciences, Naval Postgraduate School, 1411 Cunningham Rd, Monterey, CA 93940, USA
b
Received 15 March 2011; revised 10 August 2012; accepted 1 June 2013
Abstract
BACKGROUND CONTEXT: Although occupational driving has been associated with low back pain, little has been reported on the incidence rates for this disorder. PURPOSE: To determine the incidence rate and demographic risk factors of low back pain in an ethnically diverse and physically active population of US military vehicle operators. STUDY DESIGN/SETTING: Retrospective database analysis. PATIENT SAMPLE: All active-duty military service members between 1998 and 2006. OUTCOME MEASURES: Low back pain requiring visit to a health-care provider. METHODS: A query was performed using the US Defense Medical Epidemiology Database for the International Classification of Diseases, Ninth Revision, Clinical Modification code for low back pain (724.20). Multivariate Poisson regression analysis was used to estimate the rate of low back pain among military vehicle operators and control subjects per 1,000 person-years, while controlling for sex, race, rank, service, age, and marital status. RESULTS: A total of 8,447,167 person-years of data were investigated. The overall unadjusted low back pain incidence rate for military members whose occupation is vehicle operator was 54.2 per 1,000 person-years. Compared with service members with other occupations, motor vehicle operators had a significantly increased adjusted incidence rate ratio (IRR) for low back pain of 1.15 (95% confidence interval [CI] 1.13–1.17). Female motor vehicle operators, compared with males, had a significantly increased adjusted IRR for low back pain of 1.45 (95% CI 1.39–1.52). With senior enlisted as the referent category, the junior enlisted rank group of motor vehicle operators had a significantly increased adjusted IRR for low back pain: 1.60 (95% CI 1.52–1.70). Compared with Marine service members, those motor vehicle operators in both the Army, 2.74 (95% CI 2.60–2.89), and the Air Force, 1.98 (95% CI 1.84–2.14), had a significantly increased adjusted IRR for low back pain. The adjusted IRRs for the less than 20-year and more than 40-year age groups, compared with the 30to 39-year age group, were 1.24 (1.15–1.36) and 1.23 (1.10–1.38), respectively. CONCLUSIONS: Motor vehicle operators have a small but statistically significantly increased rate of low back pain compared with matched control population. Published by Elsevier Inc.
Keywords:
Low back pain; Epidemiology; Motor vehicle operator
Introduction FDA device/drug status: Not applicable. Author disclosures: JBK: Nothing to disclose. JRO: Nothing to disclose. DLS: Nothing to disclose. BDO: Nothing to disclose. RB: Nothing to disclose. PJB: Nothing to disclose. * Corresponding author. Department of Orthopaedic Surgery, Tripler Army Medical Center, 1 Jarrett White Rd, Honolulu, HI 96859, USA. Tel.: (808) 433-3557; fax: (808) 433-1554. E-mail address: [email protected] (J.B. Knox) 1529-9430/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.spinee.2013.06.029
Low back pain is among the most common musculoskeletal conditions worldwide and is estimated to affect up to 85% of the US population over their lifetimes [1]. It is a significant cause of disability in the working population and carries a substantial economic impact, with an estimated annual cost of $28 billion in the United States alone [2,3]. Because of the significant impact on the industry,
J.B. Knox et al. / The Spine Journal 14 (2014) 592–597
much research has been performed on occupational risk factors associated with this condition. One such risk factor is the involvement in occupational driving [4]. Occupational driving has been proposed to be a risk factor for low back pain because of multiple factors, including whole-body vibration exposure [5], poor ergonomics [6,7], and psychosocial factors [8]. Occupational drivers have been shown to have high rates of low back pain with various vehicle types to include taxis [9,10], buses [11], tractor-trailers [12], and other industrial and construction vehicles [12]. Most of these studies consist of cross-sectional evaluations of small populations without control groups. To better understand the population-based epidemiological characteristics of low back pain in occupational drivers, we sought to determine the risk factors. This study is the first of its kind, examining the incidence rates and epidemiologic variables in a closed high-demand population presumed to be at significant risk for developing this condition.
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Context Occupational driving has long been considered a risk for low back pain. The authors report their findings in activeduty military members. Contribution The investigators found a small but statistically significant increased rate of low back pain in occupational drivers. Female drivers, junior enlisted drivers, and those in the Army (versus Marines) had particularly increased rates. Implications The information is valuable for this population. Interventions to prevent back pain might be considered. Whether the findings are generalizable is unclear. —The Editors
Materials and methods
Statistical methods
After receiving institutional review board approval, a retrospective database search was performed using the Defense Medical Epidemiology Database (DMED). This is a comprehensive database compiling International Classification of Diseases, Ninth Revision (ICD-9), coding information for every patient encounter by a US active-duty military service member. The outpatient data in the DMED are gathered from all military treatment facilities and from nonmilitary health-care facilities providing care to activeduty service members, and ICD-9 codes in the DMED are based on the diagnoses from physicians or physician extenders. In addition, the DMED contains patient demographics, military-specific data, and the total number of service members on active duty each year. The ICD-9 code 724.20 (low back pain) was used to find ambulatory first encounters during 1998 to 2006. The DMED was queried only for ‘‘first-occurrence’’ codes, to exclude repeat coding of the same initial diagnosis for multiple visits for low back pain provided to a single service member during the study period. The DMED was also queried for the following demographic data: military vehicle operators, nonvehicle operators, sex, race, rank, military service, martial status, and age. Race data, based on service members’ self-report, are recorded in the DMED as white, black, and other. Marital status, also determined by self-report, is categorized as single, married, and other. The other demographic categories used were age (!20, 20–29, 30–39, and O40 years), rank (junior enlisted [E1–E4] and senior enlisted [E5–E9]), and military service (Army, Air Force, and Marines). To estimate incidence, one exposure year was defined as 1 year that the service member was exposed to the risk factors for low back pain while enlisted in the US Armed Forces.
The incidence rate represents the new onset of low back pain within the study population per 1,000 personyears. Unadjusted incidence rates of low back pain among military vehicle operators and nonvehicle operators were initially calculated with regard to the following demographic variables: sex, race, age, rank, service, and marital status. Multivariate Poisson regression analysis was used to adjust the incidence rate ratios (IRRs) for other variables in the model. Adjusted IRRs were calculated within each group against a referent, the subgroup with the lowest unadjusted incidence of low back pain among military vehicle operators. SAS software version 9.1 (SAS Institute, Inc., Cary, NC, USA) was used to perform all statistical analyses.
Results A total of 11,542 cases of low back pain were documented in our military vehicle operator population at risk of 213,024 person-years. The overall incidence rate of low back pain in our military vehicle operator population was 54.2 per 1,000 person-years. Service members with other occupations had an overall incidence rate of low back pain of 48.3 per 1,000 person-years. Using this control group as the referent category, military vehicle operators had a significantly increased adjusted IRR of 1.15 (95% confidence interval [CI] 1.13–1.17). When examining the age subcategories, we found that the IRR for motor vehicle operators, compared with service members with other occupations, was statistically significant at all the age subcategories. The incidence rates for drivers versus controls among all age groups are presented in Table 1.
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Table 1 Incidence rate ratio (IRR) of low back pain among vehicle operators and controls in the US military, 1998 to 2006, by age group Age group No. of No. of cases Person-years Person-years IR IR Adjusted IRR Adjusted IRR Adjusted IRR (y) cases (MVO) (control) (MVO) (controls) (MVO)* (controls)* (95% CI), MVOy,z (95% CI), controlz,x (95% CI), MVO/control* !20 20–29 30–39 O40
1,259 7,427 2,504 392
46,424 222,913 97,231 31,132
17,963 141,111 47,970 5,980
821,920 4,797,370 2,055,127 558,726
70.1 52.6 52.2 65.6
56.5 46.5 47.3 55.7
1.24 (1.15–1.36) 0.96 (0.91–1.01) N/A 1.23 (1.0–1.38)
1.23 (1.21–1.24) N/A 1.18 (1.16–1.19) 1.48 (1.45–1.50)
1.18 1.20 1.06 1.04
(1.11–1.25) (1.17–1.22) (1.01–1.10) (0.94–1.16)
N/A, not applicable because this category was used as referent; MVO, motor vehicle operator; CI, confidence interval. * IR is per 1,000 person-years. y 30- to 39-year-old group used as referent category. z Adjusted for sex, service, rank group, marital status, and race. x 20- to 39-year-old group used as referent category.
The unadjusted incidence rates of low back pain in military vehicle operators were 91.4 per 1,000 person-years among women and 48.8 per 1,000 person-years among men (Table 2). Female motor vehicle operators, compared with males, had a significantly increased adjusted IRR for low back pain of 1.45 (95% CI 1.39–1.52), controlling for the other demographic variables. The unadjusted incidence rates for low back pain in motor vehicle operators were 45.4 among whites, 59.0 among blacks, and 53.5 among others per 1,000 person-years (Table 2). There was no significant difference in the incidence of low back pain in motor vehicle operators comparing the different races after controlling for the other demographic variables. Table 2 Unadjusted IRs and adjusted IRRs of low back pain among vehicle operators in the US military, 1998 to 2006, by sex, race, rank group, service, and marital status Category
Number of cases
Person-years
Unadjusted IR*
Adjusted IRR (95% CI)
Male Female Black White Other E1 to E4 E5 to E9 Army Air Force Marine Single Married Other
9,079 2,463 3,693 6,703 1,146 7,901 3,641 8,580 1,095 1,867 5,138 6,013 391
186,084 26,940 62,563 125,227 25,234 132,612 80,412 119,307 22,024 71,693 94,337 112,135 6,552
48.8 91.4 59.0 53.5 45.4 59.6 45.3 71.9 49.7 26.0 54.5 53.6 59.7
N/A 1.45 1.07 1.06 N/A 1.60 N/A 2.74 1.98 N/A 0.87 N/A 1.01
(1.39–1.52)y (1.00–1.14)z (1.00–1.13)z (1.5–1.70)x (2.60–2.89)k (1.84–2.14)k (0.84–0.91){ (0.91–1.12){
IRR, incidence rate ratio; CI, confidence interval; N/A, not applicable because this variable was used as the referent category for the category. * IR is per 1,000 person-years. y Adjusted for age, race, rank group, service, and marital status. Male sex was the referent category. z Adjusted for age, sex, rank group, marital status, and service. White race was the referent category. x Adjusted for age, sex, race, marital status, and service. E5 to E9 rank group was the referent category. k Adjusted for age, sex, race, marital status, and rank group. Marine was the referent category. { Adjusted for age, sex, race, rank group, and service. Married was the referent group.
The unadjusted incidence rate for low back pain in motor vehicle operators among the enlisted rank groups was 59.6 for the junior enlisted and 45.3 for the senior enlisted per 1,000 person-years (Table 2). Compared with the senior enlisted as the referent category, the junior enlisted rank group had significantly increased adjusted IRR for low back pain, 1.60 (95% CI 1.52–1.70). The unadjusted incidence rates for low back pain in motor vehicle operators among the three services were 71.9 for the Army, 49.7 for the Air Force, and 26.0 for the Marines per 1,000 personyears (Table 2). Each service, compared with the Marine as the referent category, had a significantly increased adjusted IRR for low back pain, the Army 2.74 (95% CI 2.60–2.89) and the Air Force 1.98 (95% CI 1.84–2.14), after controlling for the other demographic variables. When investigating marital status, the unadjusted incidence rates for low back pain were 54.5 among single, 53.6 among married, and 59.7 among other service members per 1,000 person-years (Table 2). Single compared with married service members had a significantly decreased adjusted IRR for low back pain of 0.87 (95% CI 0.84–0.91). The incidence rates for marital status are presented in Table 2. Service members in the less than 20-year and more than 40-year age group had the highest incidence rates of low back pain among motor vehicle operators, 70.1 and 65.6 per 1,000 person-years, respectively (Table 1). Compared with the 30- to 39-year-old age group, there were a significantly increased adjusted IRRs for the less than 20-year age group, 1.24 (1.15–1.36), and the more than 40-year age group, 1.23 (1.10–1.38).
Discussion In this large-scale database study, we found that occupational drivers had a higher incidence of low back pain compared with control subjects. Additionally, female sex, lower rank, service in the Army and the Air Force, and age less than 20 years and more than 40 years were all risk factors for the development of low back pain in active-duty military occupational drivers. Single marital status was a negative risk factor for developing low back pain in this population.
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This study represents the largest study on low back pain in occupational drivers identified in the literature representing 9 years of data in an ethnically diverse population. In this population of active-duty military motor vehicle operators, we identified an incidence of acute low back pain of 54.2 per 1,000 person-years. This is similar to the previous incidence studies in nonmilitary populations, which demonstrated rates between 24.2 and 44.7 per 1,000 personyears [13,14]. There was a higher incidence compared with matched controls of active-duty military service members, with an odds ratio of 1.15. This difference is on the lower end of the previous studies, which reported odds ratios between 1.15 [15] and 3.0 [12]. Many mechanisms have been proposed by which occupational driving causes an increased risk of low back pain. One such mechanism is related to the ergonomics of the vehicle environment. Occupational drivers are subjected to sitting for prolonged periods while driving, often with poor body posture. Although prolonged sitting has not been demonstrated to be an independent risk factor for low back pain, there is an association between prolonged sitting with awkward posture and low back pain [16,17]. Vehicle seats are typically poorly designed resulting in poor posture and increased force distribution to the lumbar spine [7]. The use of such seats has been associated with the development of acute low back pain [10]. In addition to the effect of the vehicle seats, operating various vehicles requires frequent twisting, turning, and reaching with the upper and lower extremities. Occupations requiring such activities have been associated with increased rates of low back pain, and drivers who report difficulty in such tasks have been shown to have increased rates of low back pain [6]. Whole-body vibration exposure has also been demonstrated to be a risk factor for low back pain in motor vehicle operators [10,15,18–20]. Whole-body vibration has been shown to increase the latency and the magnitude of response of the erector spinae while also increasing fatigue of the back musculature. This effect is greatest with frequencies approaching the resonance frequency of the human spinal system [21,22], which has been shown to be primarily at 4.5 to 5.5 Hz with a lesser region between 9.4 and 13.1 Hz [23]. Vibration within these frequency ranges has been documented in many vehicle types to include semitruck tractors, taxis [24], and multiple industrial vehicles [25]. In addition to the risks attributable to driving itself, there may be an impact related to the other tasks required of occupational drivers. Occupational drivers are often expected to load and unload cargo and to perform other activities requiring frequent lifting and carrying. These activities place significant stress on the lumbar spine and have been correlated with increased rates of low back pain [26]. Psychosocial factors are increasingly being recognized as important risk factors for the development of low back pain, with increased rates found in individuals with low job satisfaction [27], anxiety, and depression [28]. These factors have been shown to be common in bus drivers
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who exhibit high levels of stress, low job satisfaction, and poor mental health scores [29,30]. This may play a role in the high rates of low back pain observed in these populations [4,10,18]. The reason behind the diminished effect of occupational driving on low back pain rates seen in our population is unclear but may indicate a potential protective effect from the active-duty lifestyle. Our active-duty population is required to maintain minimum standards of physical fitness and to adhere to height and weight standards regardless of their occupation. Decreased weight and increased core fitness may provide a protective effect against the negative effects of driving on the lumbar spine. The current literature provides conflicting evidence regarding the rates of low back pain and disc degeneration in athletes with studies demonstrating higher [31] and lower [32] rates in athletes compared with nonathletic controls. These rates, however, demonstrate significant differences among types of sports or activities. The effect of sex on the incidence of low back pain has been documented in previous studies with many demonstrating an increased incidence in women [33–39] and many other studies finding an equal incidence [14,39–43]. In our study, we identified a 50% increased incidence of low back pain in female motor vehicle operators. This is consistent with the previous studies demonstrating increased rates of low back pain in female service members [44] and in female bus drivers [4]. Racial differences in low back pain rates have also been previously described with variable results and little consensus on racial groups that carry elevated risk of low back pain [38,44–46]. Our study failed to identify a significant difference in low back pain rates among different racial groups. Rank was found to be a significant risk factor for low back pain with junior enlisted motor vehicle operators demonstrating higher rates compared with senior enlisted. Individuals with higher rank are though to have more control over their work environment, which may contribute to their decreased incidence of injury. Branch of service was also found to be significantly correlated with low back pain rates with the Army service members having a threefold increased risk compared with Marines. These differences, however, were similar to those demonstrated previously in active-duty military service members regardless of occupation [44]. The reason behind this difference is unclear and likely multifactorial. Differences in training regimens among services may result in differing rates of back pain or back injury. Additionally, because of the nature of the database, difference in careseeking behavior among the services also likely contributed to this apparent difference. Marital status has been shown to be a significant factor in low back pain in multiple previous studies with low back pain rates found to be lowest in single individuals and highest in those divorced or widowed [35,38,44,45,47,48]. Similarly, our study found single status to be a negative risk
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factor for low back pain. The reasons behind these differences are likely related to the psychosocial aspects of low back pain, the role of which is increasingly being recognized in the pathogenesis of low back pain. Our study identified a significant correlation between age and the incidence of low back pain, with the highest rates in the less than 20-year and more than 40-year age groups. Whereas many studies demonstrate an increased prevalence of low back pain in older individuals [34,35,38,46], the effect of age on incidence rates is less well described with variable rates in the literature ranging from the third to the seventh decades of life [14,39,49]. As our analysis identified only the first occurrence of low back pain, the older service members are more likely to have had previous episodes of back pain and therefore were not captured in our data. The strength of this study rests in the large number of active-duty service members who were captured in the closed military health-care system and annotated in the DMED database. However, there are limitations inherent to any large database study. First, we could not account for the frequency and duration of driving in our group of motor vehicle operators. Second, our subjects were all active-duty service members who maintain characteristics that may limit the ability to generalize these results to the civilian population. Additionally, because of the nature of the database, incidents of low back pain were not captured if the service member did not seek medical attention. Although these are presumably less clinically significant, it is likely that our data underrepresent the true incidence in our population to some degree. In conclusion, the study identified a small but statistically significantly increased rate of acute low back pain in active-duty motor vehicle operators compared with matched controls. Additional demographic risk factors were identified for the development of low back pain in this population to include female sex, lower rank, service in the Army and the Air Force, age less than 20 years or more than 40 years, and single marital status. References [1] Pai S, Sundaram LJ. Low back pain: an economic assessment in the United States. Orthop Clin N Am 2004;35:1–5. [2] Maetzel A, Li L. The Economic Burden of Low Back Pain: a review of studies published between 1996 and 2001. Best Pract Res Clin Rheumatol 2002;16:23–30. [3] Luo X, Pietrobon R, Sun SX, et al. Estimates and patterns of direct health care expenditures among individuals with back pain in the United States. Spine 2004;29:79–86. [4] Magnusson ML, Pope MH, Wilder DG, et al. Are occupational drivers at an increased risk for developing musculoskeletal disorders? Spine 1996;21:710–7. [5] Tiemessen IH, Hulshof CJ, Frings-Dresen MW. Low back pain in drivers exposed to whole body vibration: analysis of a doseresponse pattern. Occup Environ Med 2008;65:667–75. [6] Krause N, Ragland DR, Greiner BA, et al. Physical workload and ergonomic factors associated with prevalence of back and neck pain in urban transit operators. Spine 1997;22:2117–26; discussion 2127.
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Clinical Study
Occupational driving as a risk factor for low back pain in active-duty military service members Jeffrey B. Knox, MDa,*, Joseph R. Orchowski, MDa, Danielle L. Scher, MDb, Brett D. Owens, MDc, Robert Burks, PhDd, Philip J. Belmont, Jr., MDb a Orthopaedic Surgery Service, Department of Surgery, Tripler Army Medical Center, 1 Jarrett White Rd, Honolulu, HI 96859, USA Orthopaedic Surgery Service, Department of Surgery, William Beaumont Army Medical Center, 5005 N. Piedras St, El Paso, TX 79920, USA c Department of Orthopaedic Surgery, Keller Army Hospital, 900 Washington Rd, West Point, NY 10996, USA d Graduate School of Operational and Information Sciences, Naval Postgraduate School, 1411 Cunningham Rd, Monterey, CA 93940, USA
b
Received 15 March 2011; revised 10 August 2012; accepted 1 June 2013
Abstract
BACKGROUND CONTEXT: Although occupational driving has been associated with low back pain, little has been reported on the incidence rates for this disorder. PURPOSE: To determine the incidence rate and demographic risk factors of low back pain in an ethnically diverse and physically active population of US military vehicle operators. STUDY DESIGN/SETTING: Retrospective database analysis. PATIENT SAMPLE: All active-duty military service members between 1998 and 2006. OUTCOME MEASURES: Low back pain requiring visit to a health-care provider. METHODS: A query was performed using the US Defense Medical Epidemiology Database for the International Classification of Diseases, Ninth Revision, Clinical Modification code for low back pain (724.20). Multivariate Poisson regression analysis was used to estimate the rate of low back pain among military vehicle operators and control subjects per 1,000 person-years, while controlling for sex, race, rank, service, age, and marital status. RESULTS: A total of 8,447,167 person-years of data were investigated. The overall unadjusted low back pain incidence rate for military members whose occupation is vehicle operator was 54.2 per 1,000 person-years. Compared with service members with other occupations, motor vehicle operators had a significantly increased adjusted incidence rate ratio (IRR) for low back pain of 1.15 (95% confidence interval [CI] 1.13–1.17). Female motor vehicle operators, compared with males, had a significantly increased adjusted IRR for low back pain of 1.45 (95% CI 1.39–1.52). With senior enlisted as the referent category, the junior enlisted rank group of motor vehicle operators had a significantly increased adjusted IRR for low back pain: 1.60 (95% CI 1.52–1.70). Compared with Marine service members, those motor vehicle operators in both the Army, 2.74 (95% CI 2.60–2.89), and the Air Force, 1.98 (95% CI 1.84–2.14), had a significantly increased adjusted IRR for low back pain. The adjusted IRRs for the less than 20-year and more than 40-year age groups, compared with the 30to 39-year age group, were 1.24 (1.15–1.36) and 1.23 (1.10–1.38), respectively. CONCLUSIONS: Motor vehicle operators have a small but statistically significantly increased rate of low back pain compared with matched control population. Published by Elsevier Inc.
Keywords:
Low back pain; Epidemiology; Motor vehicle operator
Introduction FDA device/drug status: Not applicable. Author disclosures: JBK: Nothing to disclose. JRO: Nothing to disclose. DLS: Nothing to disclose. BDO: Nothing to disclose. RB: Nothing to disclose. PJB: Nothing to disclose. * Corresponding author. Department of Orthopaedic Surgery, Tripler Army Medical Center, 1 Jarrett White Rd, Honolulu, HI 96859, USA. Tel.: (808) 433-3557; fax: (808) 433-1554. E-mail address: [email protected] (J.B. Knox) 1529-9430/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.spinee.2013.06.029
Low back pain is among the most common musculoskeletal conditions worldwide and is estimated to affect up to 85% of the US population over their lifetimes [1]. It is a significant cause of disability in the working population and carries a substantial economic impact, with an estimated annual cost of $28 billion in the United States alone [2,3]. Because of the significant impact on the industry,
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much research has been performed on occupational risk factors associated with this condition. One such risk factor is the involvement in occupational driving [4]. Occupational driving has been proposed to be a risk factor for low back pain because of multiple factors, including whole-body vibration exposure [5], poor ergonomics [6,7], and psychosocial factors [8]. Occupational drivers have been shown to have high rates of low back pain with various vehicle types to include taxis [9,10], buses [11], tractor-trailers [12], and other industrial and construction vehicles [12]. Most of these studies consist of cross-sectional evaluations of small populations without control groups. To better understand the population-based epidemiological characteristics of low back pain in occupational drivers, we sought to determine the risk factors. This study is the first of its kind, examining the incidence rates and epidemiologic variables in a closed high-demand population presumed to be at significant risk for developing this condition.
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Context Occupational driving has long been considered a risk for low back pain. The authors report their findings in activeduty military members. Contribution The investigators found a small but statistically significant increased rate of low back pain in occupational drivers. Female drivers, junior enlisted drivers, and those in the Army (versus Marines) had particularly increased rates. Implications The information is valuable for this population. Interventions to prevent back pain might be considered. Whether the findings are generalizable is unclear. —The Editors
Materials and methods
Statistical methods
After receiving institutional review board approval, a retrospective database search was performed using the Defense Medical Epidemiology Database (DMED). This is a comprehensive database compiling International Classification of Diseases, Ninth Revision (ICD-9), coding information for every patient encounter by a US active-duty military service member. The outpatient data in the DMED are gathered from all military treatment facilities and from nonmilitary health-care facilities providing care to activeduty service members, and ICD-9 codes in the DMED are based on the diagnoses from physicians or physician extenders. In addition, the DMED contains patient demographics, military-specific data, and the total number of service members on active duty each year. The ICD-9 code 724.20 (low back pain) was used to find ambulatory first encounters during 1998 to 2006. The DMED was queried only for ‘‘first-occurrence’’ codes, to exclude repeat coding of the same initial diagnosis for multiple visits for low back pain provided to a single service member during the study period. The DMED was also queried for the following demographic data: military vehicle operators, nonvehicle operators, sex, race, rank, military service, martial status, and age. Race data, based on service members’ self-report, are recorded in the DMED as white, black, and other. Marital status, also determined by self-report, is categorized as single, married, and other. The other demographic categories used were age (!20, 20–29, 30–39, and O40 years), rank (junior enlisted [E1–E4] and senior enlisted [E5–E9]), and military service (Army, Air Force, and Marines). To estimate incidence, one exposure year was defined as 1 year that the service member was exposed to the risk factors for low back pain while enlisted in the US Armed Forces.
The incidence rate represents the new onset of low back pain within the study population per 1,000 personyears. Unadjusted incidence rates of low back pain among military vehicle operators and nonvehicle operators were initially calculated with regard to the following demographic variables: sex, race, age, rank, service, and marital status. Multivariate Poisson regression analysis was used to adjust the incidence rate ratios (IRRs) for other variables in the model. Adjusted IRRs were calculated within each group against a referent, the subgroup with the lowest unadjusted incidence of low back pain among military vehicle operators. SAS software version 9.1 (SAS Institute, Inc., Cary, NC, USA) was used to perform all statistical analyses.
Results A total of 11,542 cases of low back pain were documented in our military vehicle operator population at risk of 213,024 person-years. The overall incidence rate of low back pain in our military vehicle operator population was 54.2 per 1,000 person-years. Service members with other occupations had an overall incidence rate of low back pain of 48.3 per 1,000 person-years. Using this control group as the referent category, military vehicle operators had a significantly increased adjusted IRR of 1.15 (95% confidence interval [CI] 1.13–1.17). When examining the age subcategories, we found that the IRR for motor vehicle operators, compared with service members with other occupations, was statistically significant at all the age subcategories. The incidence rates for drivers versus controls among all age groups are presented in Table 1.
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Table 1 Incidence rate ratio (IRR) of low back pain among vehicle operators and controls in the US military, 1998 to 2006, by age group Age group No. of No. of cases Person-years Person-years IR IR Adjusted IRR Adjusted IRR Adjusted IRR (y) cases (MVO) (control) (MVO) (controls) (MVO)* (controls)* (95% CI), MVOy,z (95% CI), controlz,x (95% CI), MVO/control* !20 20–29 30–39 O40
1,259 7,427 2,504 392
46,424 222,913 97,231 31,132
17,963 141,111 47,970 5,980
821,920 4,797,370 2,055,127 558,726
70.1 52.6 52.2 65.6
56.5 46.5 47.3 55.7
1.24 (1.15–1.36) 0.96 (0.91–1.01) N/A 1.23 (1.0–1.38)
1.23 (1.21–1.24) N/A 1.18 (1.16–1.19) 1.48 (1.45–1.50)
1.18 1.20 1.06 1.04
(1.11–1.25) (1.17–1.22) (1.01–1.10) (0.94–1.16)
N/A, not applicable because this category was used as referent; MVO, motor vehicle operator; CI, confidence interval. * IR is per 1,000 person-years. y 30- to 39-year-old group used as referent category. z Adjusted for sex, service, rank group, marital status, and race. x 20- to 39-year-old group used as referent category.
The unadjusted incidence rates of low back pain in military vehicle operators were 91.4 per 1,000 person-years among women and 48.8 per 1,000 person-years among men (Table 2). Female motor vehicle operators, compared with males, had a significantly increased adjusted IRR for low back pain of 1.45 (95% CI 1.39–1.52), controlling for the other demographic variables. The unadjusted incidence rates for low back pain in motor vehicle operators were 45.4 among whites, 59.0 among blacks, and 53.5 among others per 1,000 person-years (Table 2). There was no significant difference in the incidence of low back pain in motor vehicle operators comparing the different races after controlling for the other demographic variables. Table 2 Unadjusted IRs and adjusted IRRs of low back pain among vehicle operators in the US military, 1998 to 2006, by sex, race, rank group, service, and marital status Category
Number of cases
Person-years
Unadjusted IR*
Adjusted IRR (95% CI)
Male Female Black White Other E1 to E4 E5 to E9 Army Air Force Marine Single Married Other
9,079 2,463 3,693 6,703 1,146 7,901 3,641 8,580 1,095 1,867 5,138 6,013 391
186,084 26,940 62,563 125,227 25,234 132,612 80,412 119,307 22,024 71,693 94,337 112,135 6,552
48.8 91.4 59.0 53.5 45.4 59.6 45.3 71.9 49.7 26.0 54.5 53.6 59.7
N/A 1.45 1.07 1.06 N/A 1.60 N/A 2.74 1.98 N/A 0.87 N/A 1.01
(1.39–1.52)y (1.00–1.14)z (1.00–1.13)z (1.5–1.70)x (2.60–2.89)k (1.84–2.14)k (0.84–0.91){ (0.91–1.12){
IRR, incidence rate ratio; CI, confidence interval; N/A, not applicable because this variable was used as the referent category for the category. * IR is per 1,000 person-years. y Adjusted for age, race, rank group, service, and marital status. Male sex was the referent category. z Adjusted for age, sex, rank group, marital status, and service. White race was the referent category. x Adjusted for age, sex, race, marital status, and service. E5 to E9 rank group was the referent category. k Adjusted for age, sex, race, marital status, and rank group. Marine was the referent category. { Adjusted for age, sex, race, rank group, and service. Married was the referent group.
The unadjusted incidence rate for low back pain in motor vehicle operators among the enlisted rank groups was 59.6 for the junior enlisted and 45.3 for the senior enlisted per 1,000 person-years (Table 2). Compared with the senior enlisted as the referent category, the junior enlisted rank group had significantly increased adjusted IRR for low back pain, 1.60 (95% CI 1.52–1.70). The unadjusted incidence rates for low back pain in motor vehicle operators among the three services were 71.9 for the Army, 49.7 for the Air Force, and 26.0 for the Marines per 1,000 personyears (Table 2). Each service, compared with the Marine as the referent category, had a significantly increased adjusted IRR for low back pain, the Army 2.74 (95% CI 2.60–2.89) and the Air Force 1.98 (95% CI 1.84–2.14), after controlling for the other demographic variables. When investigating marital status, the unadjusted incidence rates for low back pain were 54.5 among single, 53.6 among married, and 59.7 among other service members per 1,000 person-years (Table 2). Single compared with married service members had a significantly decreased adjusted IRR for low back pain of 0.87 (95% CI 0.84–0.91). The incidence rates for marital status are presented in Table 2. Service members in the less than 20-year and more than 40-year age group had the highest incidence rates of low back pain among motor vehicle operators, 70.1 and 65.6 per 1,000 person-years, respectively (Table 1). Compared with the 30- to 39-year-old age group, there were a significantly increased adjusted IRRs for the less than 20-year age group, 1.24 (1.15–1.36), and the more than 40-year age group, 1.23 (1.10–1.38).
Discussion In this large-scale database study, we found that occupational drivers had a higher incidence of low back pain compared with control subjects. Additionally, female sex, lower rank, service in the Army and the Air Force, and age less than 20 years and more than 40 years were all risk factors for the development of low back pain in active-duty military occupational drivers. Single marital status was a negative risk factor for developing low back pain in this population.
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This study represents the largest study on low back pain in occupational drivers identified in the literature representing 9 years of data in an ethnically diverse population. In this population of active-duty military motor vehicle operators, we identified an incidence of acute low back pain of 54.2 per 1,000 person-years. This is similar to the previous incidence studies in nonmilitary populations, which demonstrated rates between 24.2 and 44.7 per 1,000 personyears [13,14]. There was a higher incidence compared with matched controls of active-duty military service members, with an odds ratio of 1.15. This difference is on the lower end of the previous studies, which reported odds ratios between 1.15 [15] and 3.0 [12]. Many mechanisms have been proposed by which occupational driving causes an increased risk of low back pain. One such mechanism is related to the ergonomics of the vehicle environment. Occupational drivers are subjected to sitting for prolonged periods while driving, often with poor body posture. Although prolonged sitting has not been demonstrated to be an independent risk factor for low back pain, there is an association between prolonged sitting with awkward posture and low back pain [16,17]. Vehicle seats are typically poorly designed resulting in poor posture and increased force distribution to the lumbar spine [7]. The use of such seats has been associated with the development of acute low back pain [10]. In addition to the effect of the vehicle seats, operating various vehicles requires frequent twisting, turning, and reaching with the upper and lower extremities. Occupations requiring such activities have been associated with increased rates of low back pain, and drivers who report difficulty in such tasks have been shown to have increased rates of low back pain [6]. Whole-body vibration exposure has also been demonstrated to be a risk factor for low back pain in motor vehicle operators [10,15,18–20]. Whole-body vibration has been shown to increase the latency and the magnitude of response of the erector spinae while also increasing fatigue of the back musculature. This effect is greatest with frequencies approaching the resonance frequency of the human spinal system [21,22], which has been shown to be primarily at 4.5 to 5.5 Hz with a lesser region between 9.4 and 13.1 Hz [23]. Vibration within these frequency ranges has been documented in many vehicle types to include semitruck tractors, taxis [24], and multiple industrial vehicles [25]. In addition to the risks attributable to driving itself, there may be an impact related to the other tasks required of occupational drivers. Occupational drivers are often expected to load and unload cargo and to perform other activities requiring frequent lifting and carrying. These activities place significant stress on the lumbar spine and have been correlated with increased rates of low back pain [26]. Psychosocial factors are increasingly being recognized as important risk factors for the development of low back pain, with increased rates found in individuals with low job satisfaction [27], anxiety, and depression [28]. These factors have been shown to be common in bus drivers
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who exhibit high levels of stress, low job satisfaction, and poor mental health scores [29,30]. This may play a role in the high rates of low back pain observed in these populations [4,10,18]. The reason behind the diminished effect of occupational driving on low back pain rates seen in our population is unclear but may indicate a potential protective effect from the active-duty lifestyle. Our active-duty population is required to maintain minimum standards of physical fitness and to adhere to height and weight standards regardless of their occupation. Decreased weight and increased core fitness may provide a protective effect against the negative effects of driving on the lumbar spine. The current literature provides conflicting evidence regarding the rates of low back pain and disc degeneration in athletes with studies demonstrating higher [31] and lower [32] rates in athletes compared with nonathletic controls. These rates, however, demonstrate significant differences among types of sports or activities. The effect of sex on the incidence of low back pain has been documented in previous studies with many demonstrating an increased incidence in women [33–39] and many other studies finding an equal incidence [14,39–43]. In our study, we identified a 50% increased incidence of low back pain in female motor vehicle operators. This is consistent with the previous studies demonstrating increased rates of low back pain in female service members [44] and in female bus drivers [4]. Racial differences in low back pain rates have also been previously described with variable results and little consensus on racial groups that carry elevated risk of low back pain [38,44–46]. Our study failed to identify a significant difference in low back pain rates among different racial groups. Rank was found to be a significant risk factor for low back pain with junior enlisted motor vehicle operators demonstrating higher rates compared with senior enlisted. Individuals with higher rank are though to have more control over their work environment, which may contribute to their decreased incidence of injury. Branch of service was also found to be significantly correlated with low back pain rates with the Army service members having a threefold increased risk compared with Marines. These differences, however, were similar to those demonstrated previously in active-duty military service members regardless of occupation [44]. The reason behind this difference is unclear and likely multifactorial. Differences in training regimens among services may result in differing rates of back pain or back injury. Additionally, because of the nature of the database, difference in careseeking behavior among the services also likely contributed to this apparent difference. Marital status has been shown to be a significant factor in low back pain in multiple previous studies with low back pain rates found to be lowest in single individuals and highest in those divorced or widowed [35,38,44,45,47,48]. Similarly, our study found single status to be a negative risk
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factor for low back pain. The reasons behind these differences are likely related to the psychosocial aspects of low back pain, the role of which is increasingly being recognized in the pathogenesis of low back pain. Our study identified a significant correlation between age and the incidence of low back pain, with the highest rates in the less than 20-year and more than 40-year age groups. Whereas many studies demonstrate an increased prevalence of low back pain in older individuals [34,35,38,46], the effect of age on incidence rates is less well described with variable rates in the literature ranging from the third to the seventh decades of life [14,39,49]. As our analysis identified only the first occurrence of low back pain, the older service members are more likely to have had previous episodes of back pain and therefore were not captured in our data. The strength of this study rests in the large number of active-duty service members who were captured in the closed military health-care system and annotated in the DMED database. However, there are limitations inherent to any large database study. First, we could not account for the frequency and duration of driving in our group of motor vehicle operators. Second, our subjects were all active-duty service members who maintain characteristics that may limit the ability to generalize these results to the civilian population. Additionally, because of the nature of the database, incidents of low back pain were not captured if the service member did not seek medical attention. Although these are presumably less clinically significant, it is likely that our data underrepresent the true incidence in our population to some degree. In conclusion, the study identified a small but statistically significantly increased rate of acute low back pain in active-duty motor vehicle operators compared with matched controls. Additional demographic risk factors were identified for the development of low back pain in this population to include female sex, lower rank, service in the Army and the Air Force, age less than 20 years or more than 40 years, and single marital status. References [1] Pai S, Sundaram LJ. Low back pain: an economic assessment in the United States. Orthop Clin N Am 2004;35:1–5. [2] Maetzel A, Li L. The Economic Burden of Low Back Pain: a review of studies published between 1996 and 2001. Best Pract Res Clin Rheumatol 2002;16:23–30. [3] Luo X, Pietrobon R, Sun SX, et al. Estimates and patterns of direct health care expenditures among individuals with back pain in the United States. Spine 2004;29:79–86. [4] Magnusson ML, Pope MH, Wilder DG, et al. Are occupational drivers at an increased risk for developing musculoskeletal disorders? Spine 1996;21:710–7. [5] Tiemessen IH, Hulshof CJ, Frings-Dresen MW. Low back pain in drivers exposed to whole body vibration: analysis of a doseresponse pattern. Occup Environ Med 2008;65:667–75. [6] Krause N, Ragland DR, Greiner BA, et al. Physical workload and ergonomic factors associated with prevalence of back and neck pain in urban transit operators. Spine 1997;22:2117–26; discussion 2127.
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