Causal effect of deteriorating socioeconomic circumstances on new-onset arthritis and the moderating role of access to medical care: A natural experiment from the 2011 great east Japan earthquake and tsunami

Socioeconomic disadvantage is a risk factor for arthritis, but its causal relationship remains unclear. This study examined the causal relationship between socioeconomic circumstances and new-onset arthritis by taking advantage of the "natural experiment" that resulted from the Great East Japan Earthquake and Tsunami. The baseline survey was conducted in August 2010, 7 months before the disaster. Self-reported questionnaires were mailed to all eligible residents of Iwanuma City in Miyagi Prefecture. The earthquake and tsunami occurred on March 11, 2011. The follow-up survey was conducted in October 2013, as well as the gathering of information about disaster damage (housing damage and subjective deterioration of economic circumstances) and health-related information. We used a two-stage least squares instrumental variable model to analyze 2360 survivors who did not have arthrosis at baseline, of whom 95 (4.0%) developed arthritis over the 2.5-year follow-up period. We used the linear probability model for the estimations. Our results revealed that both the subjective deterioration of economic circumstances and housing damage were associated with the development of arthritis (95% confidence interval CI, 0.08 0.03-0.12 and 0.02 0.01-0.04, respectively). In addition, we also found that the disruption of access to orthopedics was associated with the development of arthritis. Our findings added robust evidence of the causal relationship between worsening economic circumstances and the development of arthritis. Our study emphasized the importance of recovery as well as the establishment of the post-disaster orthopedic medical system in the aftermath of a disaster.