Income-Related Inequality Changes in Osteoarthritis First-Line Interventions: A Cohort Study.

OBJECTIVE: To examine income-related inequality changes in the outcomes of an osteoarthritis (OA) first-line intervention. DESIGN: Retrospective cohort study. SETTING: Swedish health care system. PARTICIPANTS: We included 115,403 people (age: 66.2±9.7 years; females 67.8%; N=115,403) with knee (67.8%) or hip OA (32.4%) recorded in the "Swedish Osteoarthritis Registry" (SOAR). INTERVENTIONS: Exercise and education. MAIN OUTCOME MEASURES: Erreygers' concentration index (E) measured income-related inequalities in "Pain intensity," "Self-efficacy," "Use of NSAIDs," and "Desire for surgery" at baseline, 3-month, and 12-month follow-ups and their differences over time. E-values range from -1 to +1 if the health variables are more concentrated among people with lower or higher income. Zero represents perfect equality. We used entropy balancing to address demographic and outcome imbalances and bootstrap replications to estimate confidence intervals for E differences over time. RESULTS: Comparing baseline to 3 months, "pain" concentrated more among individuals with lower income initially (E=-0.027), intensifying at 3 months (difference with baseline: E=-0.011 [95% CI: -0.014; -0.008]). Similarly, the "Desire for surgery" concentrated more among individuals with lower income initially (E=-0.009), intensifying at 3 months (difference with baseline: E=-0.012 [-0.018; -0.005]). Conversely, "Self-efficacy" concentrated more among individuals with higher income initially (E=0.058), intensifying at 3 months (difference with baseline: E=0.008 [0.004; 0.012]). Lastly, the "Use of NSAIDs" concentrated more among individuals with higher income initially (E=0.068) but narrowed at 3 months (difference with baseline: E=-0.029 [-0.038; -0.021]). Comparing baseline with 12 months, "pain" concentrated more among individuals with lower income initially (E=-0.024), intensifying at 12 months (difference with baseline: E=-0.017 [-0.022; -0.012]). Similarly, the "Desire for surgery" concentrated more among individuals with lower income initially (E=-0.016), intensifying at 12 months (difference with baseline: E=-0.012 [-0.022; -0.002]). Conversely, "Self-efficacy" concentrated more among individuals with higher income initially (E=0.059), intensifying at 12 months (difference with baseline: E=0.016 [0.011; 0.021]). The variable 'Use of NSAIDs' was not recorded in the SOAR at 12-month follow-up. CONCLUSION: Our results highlight the increase of income-related inequalities in the SOAR over time.