Annals of Occupational Hygiene Advance Access originally published online on January 15, 2007
Annals of Occupational Hygiene 2007 51(3):337-344; doi:10.1093/annhyg/mel080
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Predictive Factors for Incident Musculoskeletal Disorders in an In-Plant Surveillance Program
1 INSERM, U687 Saint-Maurice, F-94410, France
2 Université Paris XI, IFR69 Villejuif, F-94807, France
3 Occupational health department, Poincaré teaching hospital AP-HP Garches, F-92380,France
4 Laboratory of Ergonomics and Occupational Health, Angers F-49000, France
5 Division of General Medical Sciences, Washington University School of Medicine St Louis, MO 63110, USA
6 Occupational Health Service of Anjou, Angers F-49000, France
*Author to whom correspondence and reprints should be addressed. Alexis Descatha MD, INSERM, U687, HNSM, 14 rue du Val d'Osne, 94415 Saint-Maurice Cedex, France, Tel: +33 47 10 77 54; fax: +33 47 10 77 68; e-mail: alexis.descatha{at}rpc.aphp.fr
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIX 1. QUESTIONS ABOUT...ACKNOWLEDGEMENTSREFERENCES |
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Objectives: A surveillance program for upper-limb work-related musculoskeletal disorders (UWMSD) based on assessment of health and risk factors was implemented between 1996 and 2000 in a large shoe factory with overall high levels for biomechanical exposure. The study aimed to identify workers with an increased risk of UWMSD incidence. Methods: In 1996, 1997 and 2000, 166 workers filled out a questionnaire and underwent a standardized physical examination. Factors from the 1996 questionnaire (general, personal and occupational factors) associated with UWMSD incidence in 1997 were selected. The predictive role of these variables was studied with a logistic model, taking into account also gender and age. The performance of a risk score based on this model was studied in 2000, using the Wilcoxon test and ROC curves. Results: In 1997, 28 incident cases of UWMSD were observed (N = 107, 26.2%). Work pace and prior history of UWMSD were the only factors significantly associated with UWMSD incidence in 1997 (respectively 33% versus 13%, P = 0.02 and 58% versus 22%, P = 0.01). Psychological distress (36% versus 21%, P = 0.10), physical fatigue (35% versus 22%, P = 0.14), repetitiveness (30% versus 18%, P = 0.17) and task precision (33% versus 21%, P = 0.16) were also included in the logistic model for 1997 UWMSD incidence. Controlling for these variables, prior history of UWMSD remained associated with incidence in 1997 (OR = 5.5, 95% CI = 1.4–21.8). In the period from 1997 to 2000, 24 incident cases were observed (N = 102, 23.5%). The risk score, based on variables from the 1997 model, was significantly higher for incident cases (median = 6 in incident cases versus 4.5 for healthy subjects, P = 0.02). ROC curves indicated that the highest agreement reached 67% for sensitivity and 59% for specificity. Among subjects who did not change their task (N = 71, 18 incident cases), performance reached 66% for specificity with the same sensitivity. Conclusion: These results suggest that surveillance programs of UWMSD at a company level are possible even with overall high levels for biomechanical exposure and should take into account occupational and personal factors, including prior history of UWMSD.
Keywords: ergonomics (musculoskeletal diseases, upper extremity, predictive factors, surveillance, physical exposures) • health surveillance • measurement strategy musculoskeletal injury
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIX 1. QUESTIONS ABOUT...ACKNOWLEDGEMENTSREFERENCES |
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One dimension of Upper-Limb Work-related Musculoskeletal Disorders (UWMSD) prevention is implementation of surveillance systems. Some authors have proposed a multilevel model for surveillance of UWMSD and their risk factors, such as a first level using questionnaires and checklists, for a rapid assessment, and a second level including clinical examinations and in-depth job analysis by trained health care providers (Hagberg et al., 1995; Scientific committee for musculoskeletal disorders of the ICOH, 1996; Ricci et al., 1998). However, very few evaluations of this surveillance system have been performed (Hagberg et al., 1995; Burdorf et al., 1997; Silverstein et al., 1997).
A surveillance program for UWMSD based on assessment of health and risk factors was implemented between 1996 and 2000 in a large shoe factory (Roquelaure et al., 2001, 2002, 2004). This in-plant surveillance program was performed in order to study the ability of various factors to predict incident cases of UWMSD. The factor derived from the incidence of UWMSD in the first year of surveillance were more reliable in predicting the risk of UWMSD in the following 3 year period than those based on prevalence data or on exposure assessment (Roquelaure et al., 2004). However, these analyses were based on job categories rather than individual data for exposure and other risk factors.
The main purpose in the present study was to complement the previous study of in-plant surveillance with an evaluation of prediction based on individual level data. In order to reach this goal, individual and occupational factors associated with the incidence of UWMSD between 1996 and 1997 were studied. In a next step, a score based on these variables was built and its validity was evaluated on health data from 2000.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIX 1. QUESTIONS ABOUT...ACKNOWLEDGEMENTSREFERENCES |
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Design
The UWMSD surveillance program implemented in the firm between 1996 and 2000 has been described in previous papers (Roquelaure et al., 2001, 2002, 2004). In 1996, workers were examined by the occupational physician in charge of health surveillance in this company and completed a questionnaire about their working conditions. Workers were examined again by the same occupational physician in 1997 and in 2000.
Population
Six of the 12 production units of the shoe factory, employing about 2000 workers, were randomly selected and 20% of the blue-collar workers from each of these production units were randomly selected using the payroll rosters. In 1996, 253 blue-collar workers were included in the study and 191 of them were re-examined in 1997. At the second follow-up in 2000, 166 of the 191 workers examined both in 1996 and 1997 were examined again. Our study population comprised these 166 workers.
Personal and exposure factors
Personal and exposure factors were assessed by self-administered questionnaire in 1996.
Personal factors were gender, age in two categories with a threshold at 45 years, present smoker, past history of diabetes mellitus, hypothyroidism, menopause, and obesity (BMI over 30). Psychological status was assessed by the General Health Questionnaire (12 items) (Goldberg et al., 1997). Physical strain and occupational factors (repetitiveness, work pace, force, task precision, awkward postures, local mechanical stress, vibration exposure, psychological distress and physical fatigue) were self-assessed using a 6-point scale (Appendix 1) ranging from very low (0) to very high (5) for each factor [except for vibration assessed on a 5-point scale, ranging from never (0) to all the time (4)]. They were divided into two categories as follows: high level/ low level with a threshold at 3 (>3 versus 
3). Job changes between 1996 and 2000 were also recorded.
Health outcomes
All the workers were interviewed during the annual compulsory medical visit (1996, 1997 and 2000), and were examined using the same procedure using a structure examination by the same occupational physician in the course of its normal work activity (Roquelaure et al., 2001, 2002, 2004). In this paper, the health outcome considered was having at least one of the nine UWMSD under review, i.e. tension neck syndrome, rotator cuff syndrome, lateral epicondylitis, medial epicondylitis, cubital tunnel syndrome, radial tunnel syndrome, carpal tunnel syndrome, Guyon's Tunnel syndrome and hand-wrist tendonitis.
Incident cases in the 1996–1997 period were defined as workers with at least one UWMSD in 1997 without any disorder in 1996; for the 1997–2000 period, incident cases were workers with at least one UWMSD in 2000 without any disorder in 1997. Prior history of UWMSD was based only on interview made by the occupational physician in 1996 for the analyses of factors associated with incidence in the 1996–1997 period. For the analyses dealing with incidence in 1997–2000, prior history of UWMSD was defined as at least one UWMSD in 1996, without any disorder in 1997.
Statistical analyses
Statistical Analysis Software was used for all analyses (SAS v8.2, SAS institute Inc, Mary, NC, USA).
Associations between risk factors in 1996 and 1996–1997 incidences were studied by bivariate analyses (Chi2 and Fisher exact tests). In a next step, associations were described with a logistic model controlling for age and gender, with a P level at 0.20 for keeping a variable in the model. A risk score quantifying the probability to be an incident case was built from the model. The score was defined for one worker as:
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In 2000, bivariate and multivariate analyses using similar methods were performed using the 1996 exposure items. A logistic model was built also for the subsample of subjects who did not change their task. The performance of the risk score described above was analysed in 2000: ranks based on this score among the 1997–2000 UWMSD incident cases were compared with ranks among the 1997–2000 UWMSD nonincident cases, using the Wilcoxon test. Sensitivity and specificity were also studied using receiver operating characteristic (ROC) curves (Goodenough et al., 1974).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIX 1. QUESTIONS ABOUT...ACKNOWLEDGEMENTSREFERENCES |
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Between 1996 and 2000, 166 workers could be followed. Among them, 65 were men (39%); the mean age was 40.6 years (range 23–53), and the mean number of years at job was 15.2 years in 1996 (range 0–35). The study population and the comparison between those lost to follow-up and the rest of the sample were described in previous papers (Roquelaure et al., 2001, 2002, 2004). A comparison between these two groups is presented in Table 1. High exposure to repetitiveness was the only variable which differed between the two groups, but it was less frequent for the workers lost to follow-up.
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Incidence rate of carpal tunnel syndrome was 14% between 1996 and 1997 and 13% between 1997 and 2000, rotator cuff syndrome 8 and 7%, respectively, lateral epicondylitis 3 and 7%. Incidence rates for the other nine disorders were <5% in both periods.
Table 2 summarizes factors associated in bivariate analyses with UWMSD incidence between 1996 and 1997 (analyses based on 107 workers since 59 suffered from at least one UWMSD in 1996). In 1997, 28 incident cases of UWMSD were observed (26.2%). Work pace and prior history of UMSD were the only factors significantly associated with 1996–1997 UWMSD incidence. Psychological distress, physical fatigue, repetitiveness and task precision were also kept for the logistic model considering a P level at 0.20. Gender and age were also included. In this model, only prior history of UWMSD remained significantly associated with 1996–1997 UWMSD incidence (Table 4). In 2000, 24 incident cases since 1997, among the 102 workers free from disorders in 1997, were observed (23.5%). A similar procedure (bivariate and multivariate analyses using the same variables) was performed on 1997–2000 UWMSD incidence: only psychological distress and prior history of UWMSD were associated with 1997–2000 UWMSD incidence (Table 3). Only prior history of UWMSD remained significantly associated with 1997–2000 UWMSD incidence in multivariate analysis of the whole sample (Table 4). However, similar analysis restricted to workers who did not change their task showed that psychological distress was also associated with incidence (elevated OR but limit for significance, Table 4).
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The risk score from the 1996–1997 model, which is presented in Table 5, was significantly higher for incident cases (median = 6 in incident cases versus 4.5 for healthy subjects, P = 0.02). ROC curves indicated that the highest agreement reached 67% of sensitivity and 59% of specificity, corresponding to 85% of negative predictive value but 33% on positive predictive value (risk score at 5.25, Fig. 1). Among the subjects who did not change their task (N = 71 with 18 incident cases, 25.4%), performance reached 66% for specificity with 67% of sensitivity, 85% of negative predictive value and 40% on positive predictive value for the same threshold. Areas under curves were 0.69 and 0.73, respectively. In the purpose of surveillance, the risk score performance could be considered as rather good.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIX 1. QUESTIONS ABOUT...ACKNOWLEDGEMENTSREFERENCES |
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This study suggests that it is possible to build an individual risk model to identify workers with an elevated risk of UWMSD, even in the setting of high physical exposures. Prior studies of this plant showed no large association between mechanical exposures among workers and elevated prevalence and incidence of UWMSD (Roquelaure et al., 2001, 2002, 2004). The low variation in mechanical exposures between workers make it difficult to identify predictive factors of UWMSD, where usual risk factors (repetitiveness, work pace, task precision, force, awkward postures ...) could not be considered as discriminating factors. Actually, some known risk factors for UWMSD were found in our study, such as repetitiveness, work pace and task precision (Hagberg et al., 1995; Bernard, 1997). However, none of these factors were significantly associated with UWMSD incidence, with fluctuant ORs in the two study periods. Some other known risk factors, such as force and awkward posture, and individual factors, were not associated at all with UWMSD incidence. In addition to the low variability in exposures between workers, this might be due to low statistical power of the study with a large fluctuation of the odds ratios and the components of the risk score or the heterogeneous aspect of UWMSD, since all UWMSD do not have the same risk factors (Hagberg et al., 1995; Bernard, 1997; Leclerc et al., 1998; Fredriksson et al., 1999). A score based on factors from the literature on large surveys on UWMSD could be theoretically more stable, but less discriminative considering low contrast in levels of exposure.
Self-assessment of exposure is also a limitation considering the possibility of a lack of precision for risk factor assessment (Li and Buckle 1999; Heinrich et al., 2004). However, there is no consensus about the best methods for exposure evaluation (Leclerc, 2005; Stock et al., 2005), and the optimal exposure assessment methods are probably different for different types of studies including surveillance (Hagberg et al., 1995). Previous work on the 1996–1997s data from this company showed that self-reported exposures via questionnaire may be more accurate than observational methods in identifying subjects at high risk for UWMSD (Descatha et al., 2006). The risk score described above was thus adapted to one specific plant, with overall high levels for biomechanical exposure.
The study period was a potential source of limitation. Jobs changes could have occurred in the 4 years of the study period. However, worker turn over was low (Roquelaure et al., 2002, 2004). In 4 years, only 30% of the workers without any UWMSD (N = 32) declared that their tasks had changed. The risk score was evaluated according to presence or absence of a disorder 3 years later, which is a long period of time: UWMSD could occur and be cured between 1997 and 2000. For instance, in a similar population, exposed to high level of repetitiveness, the recovery rate of medial elbow disorders (medial epicondylitis and ulnar nerve entrapment at the elbow) was over 80% in a 3 year period (Descatha et al., 2003, 2004). However, the only variable significantly associated with 1996–1997 UWMSD incidence in the logistic model (i.e. prior history of UWMSD) was also associated with UWMSD incidence between 1997 and 2000, and variables not significantly associated with incidence in the 1996–1996 period were not either significantly associated with incidence in the1997–2000 period.
Selection effects must be discussed considering the 34% of workers lost to follow-up. However, the major cause of loss to follow-up was economic reasons, and the selection for health reasons between the three physical examinations was minimal during the 4-year follow-up period (Roquelaure et al., 2004). Furthermore, the only difference between those lost to follow-up and the rest of the sample in 1996 was that those lost to follow-up had a lower level of repetitiveness. These elements implied probable low selection effects from lost to follow-up. Healthy worker effect could also been discussed, considering the elevated age of workers (34% older than 45 years old). Although the mean age was similar to other studies in the clothing and shoe industry (Li et al., 1995; Kaergaard and Andersen, 2000), workers in our cohort had a very high number of years on the job.
The very high proportion of workers with prior UWMSD confirmed the recurrent aspect of these disorders, and was a reason to consider prior UWMSD as a specific risk factor. It could have been considered as overadjustment. However, in a model without this variable the associations between the other risk factors and incidence were not changed. A prospective study among 598 workers exposed to repetitive work also found that prior history of one UWMSD was associated with incidence of ulnar nerve entrapment at the elbow (Descatha et al., 2004). Fredriksson et al. (1999) found that neck symptoms earlier in life were associated with recurrent disorders in a large range of occupations study.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIX 1. QUESTIONS ABOUT...ACKNOWLEDGEMENTSREFERENCES |
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In conclusion, this study suggests that surveillance programs of UWMSD at company level should take into account occupational and personal factors at an individual level. Prior history of UWMSD is an important element in surveillance programs in high exposures settings.
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APPENDIX 1. QUESTIONS ABOUT PHYSICAL STRAIN AND OCCUPATIONAL FACTORS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIX 1. QUESTIONS ABOUT...ACKNOWLEDGEMENTSREFERENCES |
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Do you think your work is repetitive? (repetitiveness).
Not at all 
/ 0 / 1 / 2 / 3 / 4 / 5 / extremely.
Is work pace: (work pace).
Slow / 0 / 1 / 2 / 3 / 4 / 5 / very fast.
The force needed in your work is? (force).
Very low / 0 / 1 / 2 / 3 / 4 / 5 / very high.
In your work, do you have precision tasks? (task precision).
Never / 0 / 1 / 2 / 3 / 4 / 5 / always.
In your work, have you awkward posture? (awkward posture).
Never / 0 / 1 / 2 / 3 / 4 / 5 / always.
In your work, have you to press tight objects in your hand or between your thumb and your index? (local mechanical stress).
Never / 0 / 1 / 2 / 3 / 4 / 5 / always.
Are you exposed to vibrations (vibrations).
Not at all / 0 / 1 / 2 / 3 / 4 / extremely.
Do you think you work is physically tiring? (physical fatigue).
Not at all / 0 / 1 / 2 / 3 / 4 / 5 / extremely.
Do you think you work is psychologically tiring? (psychological distress).
Not at all / 0 / 1 / 2 / 3 / 4 / 5 / extremely.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONCONCLUSIONSAPPENDIX 1. QUESTIONS ABOUT...ACKNOWLEDGEMENTSREFERENCES |
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The study was supported by the French Health Insurance Fund for Prevention of Occupational risks (CNAM-CRAM Pays-de-la-Loire).
Received September 30, 2006; in final form November 28, 2006
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