Incidence of shoulder and neck pain in a working population
Incidence of shoulder and neck pain in a working population: effect modification between mechanical and psychosocial exposures at work? Results from a one year follow up of the Malmö shoulder and neck study cohort
From: J Epidemiol Community Health. 2005 Sep;59(9):721-8
Work related musculoskeletal disorders and complaints constitute an important health problem in many industrialised countries, as they account for a large number of working days lost and considerable workers compensation and disability payments. For a long time, low back pain has been the dominant problem. However, pain from the shoulder and neck region now seems to occur more frequently. The prevalence of shoulder and neck symptoms is highest in the 45–65 year age bracket, as well as among women, manual workers, and certain ethnic groups.
However, its aetiology is still incompletely understood. Mechanical exposure at work and psychosocial conditions within and without the workplace, in addition to lifestyle and individual variables (age, previous symptoms, etc) are frequently discussed as causal factors in the literature.
Shoulder and neck symptoms have been linked to jobs with highly repetitive work, static work, and work above shoulder level. However, mechanical exposure explains only part of these complaints. The role of psychosocial factors in the workplace has therefore received increasing attention. On the job pressure, monotonous work, and a high perceived workload have also been associated with musculoskeletal symptoms just as much as working situations characterised by high psychological demands, low decision latitude, and low social support.
Hence, an aetiological model explaining shoulder and neck symptoms could be based on the assumption of an interaction between mechanical and psychosocial factors at work. There is, however, a need to clarify the interplay between these risk factors.
A number of shortcomings in previous research into the causes of back and neck pain have been recognised. Firstly, few studies have separated low back and neck pain, which would seem to be an important distinction because of potentially different epidemiological patterns and assumedly different risk factors. Secondly, studies regarding the role of psychosocial risk factors in these outcomes seldom have adequately controlled their risk estimates with respect to potential confounding from mechanical exposure.
Moreover, few studies have been performed on populations having a sufficient variety of both mechanical and psychosocial exposures. The presumed high correlation between the two risk factors cited can only be weighed adequately in a large study sample in which diverse job tasks are represented.
Finally, few attempts have been made to quantify the multidimensionality of mechanical exposure. Nor have such measures been used in relation to psychosocial ones—particularly not in a prospective study design.
This study has attempted to address all the mentioned issues. Based on a large general population cohort, its intentions are to analyse the importance of both mechanical and psychosocial exposures in the workplace on the incidence of shoulder and neck complaints, and evaluate the possible effect modification between these two exposures.
We found that mechanical exposure (for men) and job strain (for women)—in other words, the combination of high job demands and low job decision latitude—were the factors most strongly associated with a higher risk for developing shoulder and neck pain during the one year follow up period.
We also found evidence for a synergistic effect of these two factors in heightening the risk of developing shoulder and neck pain among women, but not among men.
Our results may have been biased by selection, misclassification, and confounding. It is probable that the most vulnerable people had left their jobs and thus were excluded from the cohort in this study—either because they were no longer pursuing a physically demanding vocation, or because they were already ill at the time of the baseline assessment. This would bias the risk estimates towards the null.
Furthermore, it is known that people who are ill have less of a tendency to participate in studies. A higher non-participation rate among people complaining of shoulder and neck pain during the follow up study could also lead to an underestimation of the true association between mechanical and psychosocial exposures and the incidence of shoulder and neck pain. In actuality, the high participation rate in our follow up study (86.6%) would render this possibility a negligible consideration.
Bias attributable to dependent misclassification should not influence the results of this prospective study, as the exposures were determined at baseline, and case status at follow up. However, another possibility of misclassification could be present, namely, between mechanical and psychosocial exposures. It is possible that exposure to one of these factors might affect the person’s assessment of the other. However, the correlation coefficient between these two exposures were rather moderate, which ought to exclude the possibility that the results were very much influenced by this type of bias.
Another bias of importance could be confounding. One potential confounder could be age, but adjustment for this factor in the multivariate analysis only resulted in a very marginal change in the ORs.
The most important confounder to account for was job strain regarding the estimated impact of mechanical exposure, and mechanical exposure regarding the impact of job strain. However, including these variables in the model only decreased the age adjusted ORs slightly, without changing their statistical significance. It can, thus, be concluded that the impact of mechanical exposure and job strain on shoulder and neck pain exhibits little or no confounding by each others’ effect.
Further potential confounding factors were marital status and country of origin. Once again, inclusion of these variables in the multivariate models hardly changed the ORs. Occupational status was not included in the multivariate analysis because of its close association with both mechanical and the psychosocial exposures. For this reason, the inclusion of occupational status in the multivariate model would most probably result in an over-adjusted model, or problems of multicolinearity. Educational level was therefore chosen as the variable denoting socioeconomic status in the confounding analysis. But even when this variable was added to the multivariate model, its effect on the risk estimate was moderate.
A number of previously published studies concerning shoulder and neck pain have included both mechanical and psychosocial exposures. Most of these have been performed within occupationally homogenous groups, such as newspaper workers, transit operators, forestry workers, carpenters, car workers, homecare workers, aluminium smelters, students, medical secretaries, and female nursing staff. However, there are a few studies that have used groups drawn from the general population.
Most of the aforementioned studies were cross sectional, and one was a case-referent study. In these investigations, the effect of mechanical exposure was controlled for psychosocial exposure, and vice versa. However, only two studies assessed possible effect modification between the two types of exposure. Both of these were cross sectional, and were based on occupationally homogenous samples (newspaper workers and female homecare workers).
Most of the studies mentioned used a kind of demand/control instrument to assess work related psychosocial exposure, while a great variety of measures were used for assessing mechanical workload. These ranged from time spent in a certain occupation to observational assessments of workload and postures. A wide array of instruments was also used in these studies to determine shoulder and neck pain. In some instances, a version of the standardised Nordic questionnaire was used.
In conclusion, there seems to be a lack of previous prospective studies that have been undertaken on large, general, population based samples (or at least samples representing a wide variety of occupations/work tasks), and that use well recognised instruments for assessing mechanical and psychosocial exposure. Such standards are required to optimally address the question of whether one or both of these exposures can be convincingly linked to shoulder and neck pain. The preceding caveat may especially hold true if the objective is a valid analysis of effect modification.
In a recently published review article concerning neck pain, in which 22 cross sectional, two prospective, and one case-referent study were evaluated, the authors concluded that awkward work postures could be linked to neck disorders with a reasonable degree of certainty. This coincides with our findings that mechanical exposure (as assessed by an index primarily based on awkward work postures) is associated with an increased risk of developing shoulder and neck pain during the follow up period, independent of psychosocial exposure.
In the case of women, we also found a statistically significant association between work related psychosocial factors such as job strain and the heightened risk of developing shoulder and neck pain during the follow up period, independent of mechanical exposure. Similarly, in some cross sectional studies, high psychosocial job demands were found to be associated with shoulder and neck pain, once again, independent of mechanical exposure. Several studies found low decision latitude associated with this outcome again, independent of mechanical exposure. Other studies, however, failed to find such an association. One prospective study, however, did find high decision latitude associated with an increased risk of shoulder and neck pain.
The most important finding in this study was that mechanical exposure has an impact on shoulder and neck pain in men and women, and so have work related psychosocial factors in women even when taking into account confounding for each others’ effect. Furthermore, evidence for the existence of a synergistic relation between these two types of exposures among women in a vocationally active, urban, middle aged, general population.
