Comparison of two self-reported measures of physical work demands in hospital personnel related to lower back pain
From: BMC Musculoskelet Disord. 2008 Apr 29;9(1):61 [Epub ahead of print]
Lower back pain is a frequent health complaint among health care personnel. Several work tasks and working postures are associated with an increased risk of lower back pain. The aim of this study was to compare two self-reported measures of physical demands and their association with lower back pain.
Musculoskeletal pain is a common health complaint in the general population and a significant part of all musculoskeletal pain is related to unspecific lower back pain. It is estimated that 44-54% of the 30-50 year old Nordic population have experienced back pain at least once during a one-year period. It is generally found that lower back pain is more frequent among nursing personnel compared to many other occupational groups.
Several physical exposures in the working environment have been linked to an increased risk of lower back pain, and a number of these are present in the working environment of hospital personnel. In a report by the National Research Council and Institute of Medicine (US) it is concluded that there is a clear relationship between back disorders and physical load imposed by lifting and/or carrying loads, frequent bending and twisting, physically heavy work, and whole-body vibration.
The majority of studies in this area are based on self-reported questionnaire data. Despite methodological flaws, self-reported exposure measurement has several advantages compared to technical exposure measurement and laboratory studies. The feasibility of questionnaire-based instruments is generally high in epidemiological studies and several instruments have been developed in order to measure the perceived physical demands in the working environment. These instruments can serve to unveil risk factors for negative health outcomes and to identify high risk jobs and work tasks.
The physical demands in the hospital sector can be measured with a generic questionnaire including several general questions on the relative frequency of different working postures and carrying of loads or by means of a single question related to the frequency of one specific but complex work tasks, e.g. the number of patient handling task during a normal work day.
Since both generic questionnaires with several items as well as specific questions are widely used, the overall aim of this study was to compare these two types of measures in a group of hospital personnel. First, we wanted to compare the accordance between the two types of self-reported exposure measurements and more objective measures of exposure as type of hospital ward and job category. Secondly, we wanted to investigate and compare the associations between each of the two types of self-reported exposure measurements and the occurrence of lower back pain.
The instrument based on several items was expected to imply less non-differential misclassification than a single item. In general, non-differential misclassification would lead to a bias toward the null value. Compared to subjective appraisal of the relative frequency of work postures, asking specifically about the number of patient handling tasks was expected to lead to less differential misclassification. This may also yield smaller risk estimate. Therefore, we hypothesized that the association between lower back pain and a generic questionnaire with several items including subjective appraisals would be stronger than the association between lower back pain and a specific question on number of patient handling tasks.
In this cross-sectional study the one-year prevalence of lower back pain was associated with work related physical demands among hospital personnel. The association was strongest when employing the daily number of patient handling task as a measure of exposure to physical demands. Although the physical load index is constructed as a measure of the added compressive forces on the lower lumbar spine during a normal work day this index turned out to be less exact in capturing the physical demands that increased the risk of reporting lower back pain. One reason could be that each patient handling situation implies a high risk of accidents due to sudden, unexpected loading. The physical load index, however, was a more sensitive measure of work-related physical demands in job groups and wards where the frequency of patient handling tasks was low.
Because of the high correlation between the two types variables measuring physical demands, the estimate for the association between patient handling tasks and lower back pain will also to some extent reflect the exposure to awkward postures and vice versa.
Several other studies have shown a relationship between patient-lifting frequency and low back problems. According to Jensen, each patient handling involves an increased risk of a back injury especially when something unexpected happens (e.g. the patient slips) and in a meta-analysis of 6 epidemiologic studies the prevalence of low back problems among nursing personnel who frequently handled patients was 3.7 higher than the prevalence among personnel performing patient handling patients less frequently. In a prospective study of nurses’ aides the odds ratio for intense lower back pain was 1.63 when positioning patients in bed 5-9 times per day compared to zero times a day (the risk decreased when doing the same task 10 times or more per day). In another cross-sectional study, however, working for long periods with head, arms or body in awkward positions or working while bent or twisted at the waist were generally more strongly associated with back musculoskeletal disorders (OR 3.4-4.9) than physical demands which specifically involved patients (OR 2.0-2.8).
Also psychosocial factors are found to be of importance, especially in relation to the course of lower back pain from an acute to a chronic state, and to the degree of disability caused by lower back pain. The estimates of the association between physical demands and lower back pain have therefore been controlled for differences in psychosocial work factors. Including several job categories in the study population advantageously increased the contrast in exposure but did also enhance the risk of residual confounding. We did not control for job category in the adjusted analyses since job category is highly correlated with workrelated demands and further adjustment would consequently weaken the study’s ability to investigate the effect of different physical exposure measurements. Also the employment of (only) two wards increased the risk of residual confounding since we were not able to control for e.g. dimensions of work culture at the two wards.
The cross-sectional study design has limitations related to selection bias in terms of the healthy worker effect. This tends to yield conservative estimates of the association between physical demands and lower back pain. However, we expect this bias to equally influence the estimates for both measures of physical demands. Moreover, we can not determine causal relationships between physical demands and lower back pain in this study. On the other hand, this study can provide basis for decisions regarding exposure measurement in large scale follow-up studies on causal risk factors for lower back pain in hospital personnel.
Differential misclassification could be a source of bias in the present study, yielding spurious associations between exposure and outcome. Results of studies of validity and reliability of self-assessed physical demands point into different directions. We assume that the number of patient handling tasks is a more “objective” measure which implies a lesser degree of individual interpretation than the frequency (in relative terms) of different working postures. We also found job category and ward to explain more of the variation in the daily patient handling than in the physical load index. Thus, the variable with the strongest association with lower back pain was also to a higher degree explained by more objective, though probably less precise, measures of exposure. These results indicate that differential misclassification between lower back pain cases and non-cases is not a major source of bias, even though differential misclassification can not unequivocally be ruled out.
This study shows that among hospital personnel the frequency of patient handling tasks seems to be more strongly associated with lower back pain than a generic instrument estimating the total mechanical load on the lower lumbar spine. A single question on frequency of patient handling tasks therefore has advantages as a screening instrument both for practical reasons (e.g. the risk of missing data when asking several questions) and because of accuracy. It can be discussed whether these results can be generalized to other work places in the health care sector. At both the orthopedic and psychiatric ward the patients are highly selected with a relatively limited range of disabilities. It can be hypothesized that if the physical challenge involved in each patient handling task is even more diversified depending on the varying capacity and cooperation of each client the frequency of patient handling will be too unspecific as a measure of exposure.
