The Gazel cohort
The GAZEL cohort was established in 1989 from the employees of Electricité de France (EDF) and Gaz de France (GDF), the French national utility for energy production and distribution. The company employs approximately 150,000 people of various trades and socioeconomic status throughout France, and EDF-GDF employees hold civil servant–like status that entails job security. At baseline in 1989, the cohort included 20,625 volunteers (men aged 40 to 50 years and women aged 35 to 50 years). In the January of each successive year, participants received a general questionnaire about their lifestyle, health, and occupational situation [11]. Medically-certified sickness absence data were available from company records.In this study, we focused on the 14,502 participants who had responded to the 2006 questionnaire (Figure 1). The questionnaires sent in 2006 and 2012 contained questions concerning the information of interest in this study: in the following text, 2006 is called the “baseline” phase and 2012 is called the “follow-up” phase.
The Cosali cohort
This prospective study was based on two successive surveys in a large sample of workers in the Loire Valley area of West Central France, the Cosali cohort [Cohorte des Salariés Ligériens[12]]. The diversity of the regional economic structure (5.6% of the French workforce) is very similar to that of the national workforce [13].
At the time of the first survey, all French employees, including temporary and part-time workers, were required to undergo an annual health examination by an occupational physician (OP) in charge of the medical surveillance of a group of companies. The 83 OPs who volunteered for the study (without compensation) randomly selected workers aged between 20 and 59 years from those undergoing one of these annual health examinations between 2002 and 2005. Subjects filled in self-administered questionnaires before the OP’s physical examination [Surveillance network of musculoskeletal disorders [14]]. This first phase is called the “baseline” phase in the following text.
A follow-up questionnaire was sent to the 3,710 participants in 2007 [12] (Figure 1). In the case of non-response, two successive reminder letters were sent. This second phase is called the “follow-up” phase in the following text.
Study population
Workers who were underweight (BMI < 18.5 kg/m2) were excluded from the analyses in this study because this may correspond to a variety of other medical conditions [15].
In order to have homogeneous measures of occupational exposure, members of the Gazel cohort who were not retired at baseline or who were deceased before the follow-up phase were excluded. The members of the Cosali cohort were all active workers at baseline.Workers who were not followed up were excluded from the analyses in both cohorts (Figure 1).
Outcome
Participants completed the standardized Nordic questionnaire for several musculoskeletal symptoms including KP [16]. KP was defined as at least 1 day of KP during the preceding year. Persistent cases were those who reported KP both in the baseline phase and in the follow-up phase. The KP was described according to pain duration and period in two classes: long-lasting pain (>30 days during the preceding year) and other pain (1 to 30 days during the preceding year or only during the preceding week).
Risk factors
Personal factors
Age at the time of the baseline questionnaire was divided into 5 classes: 20–39 years, 40–49 years, 50–59 years, 60–64 years and 65–69 years.
Weight and height were self-reported in the baseline questionnaire in the both cohorts. The Body Mass Index (BMI) at baseline was divided into three classes: normal weight/missing (18.5 to 25 kg/m2 or missing), overweight (25 to 30 kg/m2) and obese (>30 kg/m2).
The Gazel cohort members were asked about their history of knee injury before 2006 (Yes/No). Sickness absence for depression during the whole career (ever/never) was taken into account, since pain may be associated with depression.
Characteristics of KP at baseline
The intensity of KP from the Nordic questionnaire at baseline was dichotomized into slight pain (1 to 4 on the 8-degree scale in Gazel, 1 to 5 on the 10-degree scale in Cosali) and severe pain (5 to 8 in Gazel, 6 to 10 in Cosali). The Gazel cohort members had to describe the intensity of the pain in the last episode and the Cosali cohort members had to answer only if they had pain at the time of the questionnaire.
The baseline questionnaire also investigated nine other musculoskeletal symptoms (fingers, hands, elbows, shoulders, neck, upper back, lower back, hips and ankles). A composite variable called “Pain in other areas” focusing on pain in joints in the body other than the knees was divided into three classes: no pain in other areas, pain in hips (with or without other areas) and pain experienced in at least one of the eight other areas.
The Gazel cohort members were asked to self-assess the origin of KP with four pre-coded categories: tendinitis, meniscus disorders, arthrosis, other origin. For the analyses, three categories were considered: degenerative osteoarthritis, other origin (tendinitis, meniscus disorders or other), not completed.
Occupational factors
The socio-professional category at 35 years was available for all Gazel cohort members and at baseline for the Cosali cohort members. We divided this characteristic into 4 classes: manual workers, white collar, associate professionals/technicians and executives/others (craftsman, shopkeeper, business owner, managers, executives or not completed).
In the 2006 questionnaire, the Gazel cohort members reported the cumulative duration of exposure to three biomechanical constraints during their careers, i.e. carrying heavy loads (more than 10 kg), working in kneeling/stooping position and walking up more than 10 flights of stairs every day. Four answers were possible for each type of exposure: never, less than 10 years, 10–20 years, and longer than 20 years. We divided exposure into three classes: none (never or missing), short (less than 10 years) and long (more than 10 years).
In the 2002–2005 questionnaire, the Cosali cohort members reported current exposure during a typical working day to two types of biomechanical exposure: handling heavy loads (more than 4 kg) and working in a kneeling position. The response categories for occupational exposure were initially presented on a 4-point Likert-type scale as never or almost never, rarely (less than 2 hours a day), often (2 to 4 hours a day) and always (more than 4 hours a day). We divided the exposure categories into 3 classes: none (never or almost never or not completed), moderate (<2 hours a day) and severe (2 to 4 hours a day or more than 4 hours a day). Handling loads was also available at the follow-up questionnaire. The occupational changes since 2002 were reported at follow-up, in four categories (no change; change of job in the same company; change of company; not actively employed).
Length of exposure was also divided into short or long/moderate or severe exposure vs no exposure (none) in both cohorts in order to have sufficient statistical power in the analyses.
Statistical analyses
All analyses were performed separately for each cohort and sex and the results are described separately.
The annual recovery rate from KP in each cohort was approximated by the rates of recovery from KP during the follow-up period multiplied by the estimated proportion of recoveries one year after baseline. This last figure was estimated from the percentage of Gazel members that did not declare shoulder/elbow/hip/knee pain in the annual questionnaires during the follow-up period. The natural history of KP was described according its duration at baseline and at follow-up. As the participants lost to follow-up were excluded from the analyses, a sensitivity analysis on the persistence of KP was performed assuming two extreme scenarios on the missing values: a pessimistic scenario, in which all missing individuals had KP at follow-up and an optimistic scenario, in which no missing individuals had KP at follow-up.
The main analyses consisted of evaluating associations between risk factors and the persistence of KP (i.e. reporting of KP in the baseline and the follow-up questionnaires). Univariate and multivariate logistic models were used to estimate the odds ratios (ORs) for persistence of KP. Wald tests were used to compare ORs between categories, in particular to test whether there were some dose-effect relationships for occupational exposure.
Two multivariate models were used to evaluate the relationship with carrying/handling heavy loads or kneeling with adjustment on:
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Age and BMI (called model 1)
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Age, BMI, intensity of pain, pain in other areas, self-assessed origin of KP (Gazel only), past history of knee injury (Gazel only) (called model 2)
The second-order interactions between the associations of KP with occupational factors and indicators of severity of KP were explored in model 2.
Data analyses for this study were generated with SAS 9.4 software (SAS Institute Inc., Cary, NC, USA). Statistical significance was defined as a p-value lower than 0.05.
Ethics approval
Each cohort was approved by the French National Data Protection Committee (CNIL, Commission Nationale de l’Informatique et des Libertés) and all participants have given their consent to be entered in the cohorts.