Job strain, iso-strain, and the incidence of low back and neck injuries. A 7.5-year prospective study of San Francisco transit operators

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Abstract

Work-related musculoskeletal disorders account for the largest single category of lost-time occupational injury or disease episodes in industrialized countries. In this study we analyzed the impact of the psychosocial work environment, conceptualized by the demand–control–support model, on the incidence of low back and neck injury in a cohort of 1221 public transit operators followed for 7 years and 6 months. The two main exposure variables were “job strain” (mismatch of high psychological demands and low decision latitude) and “iso-strain” (job strain plus exposure to low social support at work). Analyses controlled for demographic factors, physical workload, and pain at baseline. For low back injuries, increased hazard rates were found for job strain and iso-strain based on tertiles, with hazard ratios (HR) of 1.30 (95% CI=0.96–1.75) and 1.41 (95% CI=0.98–2.01), respectively. Job strain and iso-strain based on median split or analyzed as continuous variables were not associated with low back injuries. For neck injuries, job strain and iso-strain based on median split showed HRs of 1.27 (95% CI=0.99–1.63) and 1.33 (95% CI=1.01–1.77), respectively. Job strain and iso-strain based on tertiles had HRs of 1.52 (95% CI=1.13–2.05) and 1.73 (95% CI=1.21–2.45), respectively. When analyzed as continuous variables, a one-point increase on the job strain and iso-strain scales led to an 8% (95% CI=0.98–1.19) and 14% (95% CI=1.02–1.27) increased hazard of neck injuries, respectively. This study shows the importance of the psychosocial work environment in the etiology of musculoskeletal injuries among transit operators. Since reviews have shown that psychosocial workplace conditions in this occupational group can be changed, these findings indicate a potential for prevention at the workplace.

Introduction

Work-related injuries and illnesses are a major public health and economic problem in highly developed industrialized countries (Frank, Pulcins, Kerr, Shannon, & Stansfeld, 1995; Skov, Borg, & Orhede, 1996). Work-related musculoskeletal disorders account for more than one-third of lost-time occupational injuries and illnesses in the United States (Bureau of Labor Statistics, 2003) and form the single largest category of such episodes that are recordable by US Occupational Safety and Health Administration (OSHA) criteria and subject to workers’ compensation insurance claims (Bernard, 1997; Bureau of Labor Statistics, 2003). The costs of work-related occupational injury and illness totaled $171 billion in the US in 1992, with $49 billion for low back injuries alone (Leigh, Markowitz, Fahs, Shin, & Landrigan, 1997). Work-related back injuries represent about 20% of all workers’ compensation claims and are responsible for 33% of all workers’ compensation costs in the United States (Andersson, Pope, Frymoyer, & Snook, 1991). Neck pain occurs both alone and in combination with shoulder or low back pain, adding substantially to the burden of musculoskeletal diseases. A population-based study in Saskatchewan found a lifetime prevalence of neck pain of 66% and a point prevalence of 22% (Côté, Cassidy, & Carroll, 1998). The prevention of spinal injuries and associated disability has become a major challenge for employers, insurance carriers, and occupational health specialists. However, the development of effective intervention strategies has been hampered by limited information on occupational risk factors that are amenable to change (Krause, Frank, Dasinger, Sullivan, & Sinclair, 2001).

Findings from numerous studies suggest that low back and neck injuries are caused by multifactorial aspects of the work environment, including exposure to physical and ergonomic hazards like awkward body postures, vibration, and repeated and forceful motions, as well as psychosocial factors such as low job satisfaction and low job control (Ariëns, van Mechelen, Bongers, Bouter, & van der Wal, 2001; Bigos et al., 1991; Davis & Heaney, 2000; Kerr et al., 2001; Krause, Ragland, Fisher, & Syme, 1998; Krause et al., 1997a; Krause, Ragland, Greiner, Syme, & Fisher, 1997b). However, the causal and independent contribution of the work environment on the incidence of musculoskeletal disorders is still debated, especially with regard to psychosocial factors (Frank et al., 1995). Two recent comprehensive reviews on low back (Davis & Heaney, 2000) and neck pain (Ariëns et al., 2001) have pointed out that the majority of the studies in this field are cross-sectional in design, which severely limits the application of causal inference. This problem is further heightened by the fact that most studies have measured both the psychosocial environment as well as the outcome by self-report, which makes their results vulnerable to common method bias (Kasl, 1998). In addition, only a few studies have tested specific psychosocial exposure models based on stress theory. Finally, the two reviews show that studies often fail to control adequately for exposure to physical demands at the workplace. In fact, the first company-based prospective study which showed a strong association between the psychosocial work environment and low back injury (Bigos et al., 1991) has been criticized for its failure to assess physical workload on an individual basis (Bernard, 1997; Frank et al., 1995).

In an earlier study, we analyzed prospective data of a 1983–1985 cohort of San Francisco public transit operators. We found substantial independent effects on spinal injuries for both physical and psychosocial stressors (Krause et al., 1998). However, no differentiation was made between neck and back injuries in this study. In addition, the outcome measures relied on self-report (Krause et al., 1997b) or insurance company ANSI codes of ‘body part injured’ (Krause et al., 1998). The use of ANSI codes compared to ICD-9 codes is likely to introduce a conservative bias because of misclassification (Oleinick, Gluck, & Guire, 1996).

The goal of the present study is to address and overcome the limitations of previous studies, including our own, by using data from a new cohort (1993–1995) of San Francisco transit operators. We prospectively investigated if exposure to an adverse psychosocial work environment increases the risk of neck and low back injuries, while adjusting for objective and subjective indicators of physical workload. Injuries were ascertained objectively based on medical diagnoses by a physician, and analyses controlled for pre-existing low back and neck pain. Psychosocial exposure was defined in accordance with the demand–control–support model by Karasek et al., 1998) and Theorell and Karasek (1996), which is based on theoretical considerations from psychology, sociology, and stress research. The model allows for the calculation of specific exposure constellations capturing mismatches between high psychological demands and low job control (“job strain”), and high demands, low control and low social support at the workplace (“iso-strain”). Both job strain and iso-strain have been found to be predictive of cardiovascular disease in several cohort studies (Belkić, Landsbergis, Schnall, & Baker, 2004) but have seldom been tested prospectively for other health outcomes.

Section snippets

Study design and population

This is a 7.5-year prospective cohort study of 1974 transit vehicle operators employed by the San Francisco Municipal Railway (Muni), who completed a physical examination and extensive medical history forms required for commercial driver's license renewal between August 1993 and September 1995. Seventy-three subjects were excluded because a review of their data showed that they certainly or very likely held other jobs in the company (e.g., supervisors). Sixty additional subjects were excluded

Frequency of injuries

Among the 1841 transit operators eligible for participation in the study, a total of 3688 workers’ compensation claims were observed during the 7.5-year follow-up period. ICD-9 codes were missing for 497 of the claims (13.5%), leaving a total number of 3191 claims. Of these claims, 910 (28.5%) had at least one ICD-9 code that was indicative of definite low back diagnosis and 807 (25.3%) had at least one ICD-9 code that was indicative of definite neck diagnosis.

Based on all available ICD-9 codes

Summary of findings

The purpose of this study was to examine the impact of the psychosocial work environment on the incidence of low back and neck injuries. The main results are: (1) For low back injuries, drivers with high job strain and high iso-strain (based on tertiles of the subcomponents) showed increased HR of 1.30 and 1.41, respectively, although these results were not statistically significant. No dose–response relationships between psychosocial exposure and the hazard rate for low back injuries were

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