Musculoskeletal disorders and associated factors among dockers and handlers in the cotonou port companies in 2020

Public Health Unit, Faculty of Health Sciences, University of Abomey-Calavi, Cotonou, Republic of Benin. Occupational Health and Environment Unit, Faculty of Health Sciences, University of Abomey-Calavi, Cotonou, Republic of Benin. Population and Health Unit, Center for Training and Research in Population, University of Abomey-Calavi, Cotonou, Republic of Benin. Regional Institute of Public Health, University of Abomey-Calavi, Republic of Benin.


INTRODUCTION
Health risks in the workplace remain a real public health problem and often lead to functional disability in developing countries (De Carvalho et al., 2016). As far as material handlers are concerned, they are constantly exposed to Musculoskeletal Disorders (MSD), making MSDs a priority for prevention at international level (Caroly et al., 2012). Several factors such as physical strain, heavy loads, and postures favour the occurrence of MSDs in these workers. In 2015, a study carried out in Brazil showed that the prevalence of MSDs was 37.4% among dockworkers, with lumbar spine diseases predominating in 22.5% of cases (De Carvalho et al., 2016). Another study conducted in 2018 in Brazil shows that the prevalence of low back pain among casual dockworkers is 69.8%, and that of back pain is 50.9% (Cezar-Vaz et al., 2018). In Africa, several studies demonstrate the relevance of the topic. In Cameroon, the prevalence of MSDs was 87.9% among dockers working in the port of Douala (Dieuboue et al., 2020). In Ghana, at the port of Tema, a study showed that 38% of dockers suffer from musculoskeletal diseases (Ivan, 2015). In Benin, to our knowledge, no specific and limited data are available on MSDs among dockworkers. This study aimed to assess the prevalence and factors associated with MSDs in these workers.

Study area
The study took place at Société Béninoise de la Manutention Portuaire (SOBEMAP) directorates and at the captaincy department of the autonomous port of Cotonou (PAC). The two companies are located in Cotonou, the economic capital of Benin country.

MATERIALS AND METHODS
A cross-sectional survey was conducted from May to November 2020. The study population was dockers and handlers from the two selected port companies. Included participants are SOBEMAP dockers with at least two years of experience and PAC dockworkers with at least 1 year of experience. A total of 576 dockers and 38 dockworkers or handlers were included in the study. The selection of dockworkers and dockers was done by non-probability sampling for convenience. Two types of survey tools were used: the MSD questionnaire from INRS (Cail et al., 2010) and the Karasek questionnaire, SUMMER 2003 survey version (Conners, 2006).
The variables studied were socio-demographic (age, sex, marital status, level of education), socio-professional (length of service at work, type of position, stress at work felt or not), clinical, and psychosocial. The data were analysed using R 3.6.1 software with the RStudio environment. The weight of the association between the MSD and exposure factors was measured by the Odds ratio. Logistic regression is performed to remove some confounding factors. All the tests used were interpreted with a significance level of 5%.  Table 1).

Prevalence of musculoskeletal disorders
The spine (neck, upper back, lower back) and the shoulders were the most affected, with 37.9% for the neck; 39.4% for the upper back; 88.6% for the lower back; 40.4% for the right shoulder and 39.1% for the left shoulder. Lower back pain is the most frequent complaint; almost all (88.6%) of the workers suffer from it ( Table 2). Of the 614 subjects, 549 (89.4%) were exposed to job strain.

Factors associated to musculoskeletal disorders
According to univariate analysis, the occurrence of MSDs in the neck was influenced by: age, seniority, stress, psychological demand, job strain and iso strain. We note that the longer the seniority was, the higher was the occurrence of neck MSDs; intense stress influenced also the occurrence of MSDs (p<0.001) as did high psychological demand (p<0.001), (Table 3). The occurrence of upper back MSDs was influenced by age, seniority, stress, psychological demand and job strain, iso strain (Table 4). The occurrence of low back MSDs is influenced by age, seniority, level of education, job type and stress (Table 5). The occurrence of shoulder MSDs was influenced by age, seniority, level of education, stress, psychological demand, job and iso strain (Tables  6 and 7). After multivariate analysis, the occurrence of MSDs was influenced by seniority, stress, and psychological demand for the neck; by age, seniority, stress and job strain for the upper back; by seniority, level of education, type of job and stress for the lower back; and by seniority, stress, and psychological demand for the shoulders (Tables 3 to 7).

Prevalence of MSDs
We found 37.9% of neck pain in our study. This prevalence is lower than that of Mikponhoue (2013) which was 43.3% in a port company in Cotonou (Mikponhoue et al., 2017). Our prevalence of low back pain is similar to that of the general population in Canada in 2019 (L'Estrie Centre De Santé Communautaire, 2019). But it is higher than that found in France in a study of low back pain in the general population which found 13.7%  for men and 17.2% for women (Plouvier et al., 2010). It is also higher than that found by De Carvalho et al. (2016) in Mexican dockworkers in 2016 which was 4.7%. These low prevalences observed in some developed countries may be explained by the fact that they have better means of prevention. "In the current study, the TMS located on the lower back prevalence is lower (39.4%) than that observed by Boton (Boton, 2010) among a port handling company in Cotonou in Benin (48.5%)." In our study, the spine is the most affected part, especially the lumbar spine. The French data available are in agreement with what is found in almost all studies, namely that low back pain is more frequent (Buchbinder et al., 2018). This finding was also made in China by Ye et al. (2017)

Age
In our study, we found that MSDs of the upper back and      (Kacem et al., 2016). This could be explained by the pathophysiology of MSDs, as MSDs are the consequence of prolonged exposure over time to different risk factors.

Occupational stress
In our study, 89.4% (549 workers) have occupational stress. Ghailan et al. (2020) found similar results when studying occupational stress and MSDs among women in artisanal fisheries in 2020. Ben Aissa has shown that stress at work has direct consequences on the individual including MSDs (Ben and Galindo, 2016). Stress would therefore play a key role in the occurrence of MSDs since it increases basal muscle tension, which leads to a more rapid onset of pain.

High psychological demand
We found an association between high attentional demand from work and MSDs of the neck, upper back and shoulders. A Quebec survey of 5000 workers on working conditions studied the relationship between MSDs and high psychological demand (Stock, 2011). Multivariate analyses of the data from this survey concerning spinal MSDs confirm the contribution of risks associated with high psychological demand. Mechergui et al. (2018) also identified high psychological demand as a risk factor for MSDs, estimated at 37.1% in 2018.

Limitation of the study
This study explored the factors associated with musculoskeletal disorders of several parts of the human body. However, the tasks or jobs that most often induce these MSDs were not investigated. The identification of these tasks could facilitate advocacy with the managers of these structures in order to improve the working conditions and thus the well-being of dockers and handlers.

Conclusion
The study aimed to evaluate the prevalence and associated factors to MSDs among dockers and handlers in the Cotonou port companies. The spine (neck, upper back, lower back) and the shoulders were the most affected, with 37.9% for the neck; 39.4% for the upper back; 88.6% for the lower back; 40.4% for the right shoulder and 39.1% for the left shoulder. The prevalence of low back pain (MSD of the lower back) in this population is the highest. The main risk factors for MSDs were: age, seniority, perceived stress, high psychological demand and job strain. The multifactorial risks associated to MSDs must guide the prevention measures to be implemented in these companies.