Associations between single and combined occupational mechanical exposures and surgery for subacromial impingement syndrome: a nationwide Danish cohort study

OBJECTIVE: This study aimed to evaluate whether the risk of surgery for subacromial impingement syndrome (SIS) increases with the number of combined occupational mechanical exposures compared with single exposure. METHODS: We reanalyzed data from a register-based cohort study of the entire Danish working population (N=2 374 403) with 14 118 events of surgery for SIS (2003–2008). Exposure information in 10-year windows was obtained by combining occupational codes with a job exposure matrix. For single and combined mechanical exposures, we created three exposure variables of the number of years with specific exposure intensities with or without co-existing mechanical exposures. We used logistic regression as survival analysis. RESULTS: We found exposure–response relations for duration and intensity of each single mechanical exposure except for repetition. The single effect of arm elevation >90º reached a maximum adjusted odds ratio (OR(adj)) of 1.7 [95% confidence interval (CI) 1.5–2.0], which increased to 1.8 (95% CI 1.5–2.0), 2.0 (95% CI 1.9–2.2), and 2.2 (95% CI 2.0–2.5) when combined with repetition, force, and both. When combining repetition with arm elevation >90º, force, and both, OR(adj) increased from 1.5 (95% CI 1.3–1.8) to 2.1 (95% CI 1.8–2.4), 2.5 (95% CI 2.4–2.9), and 2.7 (95% CI 2.4–3.0). For force, OR(adj) increased from 2.5 (95% CI 2.1–2.9) to 2.6 (95% CI 2.3–2.8), 2.8 (95% CI 2.4–3.2), and 3.0 (95% CI 2.6–3.4). CONCLUSION: We found an increased risk of surgery for SIS with the number of combined exposures; the risk was especially pronounced when the combined exposures included force.