To what extent do education and physical work load factors explain occupational differences in disability retirement due to knee OA? A nationwide register-based study in Finland

OBJECTIVES: To examine the association of education and physical work load factors on the occupational differences in disability retirement due to knee osteoarthritis (OA). DESIGN: Longitudinal study. SETTING: Linkage of several nationwide registers and a job exposure matrix in Finland. PARTICIPANTS: A total of 1 135 654 Finns aged 30–60 years in gainful employment were followed from 2005 to 2013 for full disability retirement due to knee OA. PRIMARY AND SECONDARY OUTCOME MEASURES: We calculated age-adjusted incidence rates and examined the association of occupation, education and physical work load factors with disability retirement using competing risk regression model. Disability retirement due to other causes than knee OA, old-age retirement and death were treated as competing risk. RESULTS: A total of 6117 persons had disability retirement due to knee OA. Women had a higher age-adjusted incidence rate than men (72 vs 60 per 100 000 person-years, respectively). In men, a very high risk of disability retirement was found among construction workers, electricians and plumbers (HR 16.6, 95% CI 12.5 to 22.2), service workers (HR 12.7, 95% CI 9.2 to 17.4) and in women among building caretakers, cleaners, assistant nurses and kitchen workers (HR 15.5, 95% CI 11.7 to 20.6), as compared with professionals. The observed occupational differences were largely explained by educational level and noticeably mediated by physical work load factors in both genders. CONCLUSION: Our observational study suggests that the risk of disability retirement among manual workers is strongly attributed to the physically heavy work.