Educational inequalities in all-cause and cause-specific mortality among patients with osteoarthritis in southern Sweden

Abstract

Purpose: To assess the association between socioeconomic status, measured by education, and all-cause and cause-specific mortality among patients with osteoarthritis (OA) in southern Sweden. Methods: A register-based open cohort study was conducted. The Region of Skåne, which is the southeast part of Sweden, was the geographical area of interest. All residents aged ≥45 years in the region with doctor-diagnosed OA (ICD-10 codes: M15-M19) between the 1st of January 1998 and the 31st of December 2013 (n=123,981) were followed until death, relocation outside the Skåne region, or the end of 2014, whichever occurred first. Individual-level data was obtained from the Skåne Healthcare Register (SHR), Statistics Sweden and the National Board of Health and Welfare´s Cause of Death Register. The Slope index of inequality (SII) and the Relative index of inequality (RII), which measure the absolute and relative educational inequality, respectively, were estimated. The Aalen´s additive hazard model was used to estimate the SII and Cox model was used to estimate the RII. Two cause-of-death attribution approaches were used to estimate the RIIs: “underlying cause” and “weighted multiple-cause”. All estimates were adjusted for age, sex, marital status and whether the subject had a Swedish or foreign background. Results: During a mean follow-up of 7.1 years, we identified 24,493 deaths. Age-standardized all-cause mortality rates increased as the educational level decreased (Figure 1). The RII showed that all-cause mortality rate was 1.54 (95% CI: 1.47, 1.62) times higher in low vs. high educated OA patients (Table 1). Among causes of death, the greatest relative inequality was seen in deaths caused by diseases in circulatory system (RII 1.88, 95% CI: 1.73, 2.04). Higher mortality rates were observed in low educated for all causes except for mental and behavioural disorders & diseases of the nervous system. Regarding the results for the subgroups based on sex, age and OA-type, there were a higher all-cause mortality in low educated patients. A higher cause-specific mortality in favour of high educated was also generally seen in these groups.Table 1Relative index of inequality (RII) and slope index of inequality (SII) in mortality.Disease group (ICD-10 codes)All (≥45 years)SexesAge groupsOA-typeMenWomen45-74 years≥75 yearsKnee OA (M17)Hip OA (M16)RIISII*RIISII*RIISII*RIISII*RIISII*RIISII*RIISII*All-cause1.54 ( 1.47, 1.62)904 (779, 1030)1.55 (1.44, 1.67)1270 (1060, 1480)1.54 (1.44, 1.65)924 (767, 1080)1.72 (1.59, 1.86)700 (600, 800)1.44 (1.35, 1.54)1350 (1050, 1650)1.52 (1.41, 1.64)874 (680, 1070)1.53 (1.39, 1.67)1300 (990, 1610)Diseases of the circulatory system (I00-I99)1.88 (1.73, 2.04)555 (475, 635)1.85 (1.64, 2.08)732 (596, 868)1.86 (1.67, 2.08)542 (443, 641)2.25 (1.94, 2.60)325 (269, 381)1.68 (1.53, 1.85)981 (775, 1190)1.78 (1.58, 2.01)515 (389, 641)1.90 (1.64, 2.19)840 (642, 1040)Malignant neoplasms (C00-C97)1.20 (1.09, 1.32)77.6 (11.7, 143)1.30 (1.13, 1.49)193 (80.7, 305)1.19 (1.04, 1.37)86 (5.1, 167)1.27 (1.12, 1.45)127 (64.5, 190)1.11 (0.96, 1.29)-4.7 (-136, 127)1.19 (1.02, 1.39)66.3 (-32.3, 165)1.21 (1.01, 1.45)137 (-21.6, 296)Mental and behavioural disorders (F00-F99) & Diseases of the nervous system (G00-G99)1.12 (0.93, 1.34)4.8 (-30.8, 40.5)0.98 (0.71, 1.35)-6.7 (-59.5, 46)1.17 (0.93, 1.47)7.6 (-40.3, 55.4)1.47 (1.04, 2.08)24.7 (0.59, 48.8)0.97 (0.78, 1.20)-31.5 (-123, 60.4)1.14 (0.85, 1.52)0.8 (-54.1, 55.6)0.95 (0.69, 1.32)-38.3 (-125, 48.7)Diseases of the respiratory system (J00-J99)1.58 (1.28, 1.93)53 (20.3, 85.7)1.70 (1.25, 2.32)101 (44, 158)1.54 (1.16 2.06)55.1 (15.5, 94.7)2.45 (1.73, 3.48)63.9 (40.2, 87.6)1.30 (1.01, 1.68)42.9 (-38.4, 124)1.66 (1.18, 2.34)51.5 (4.7, 98.3)1.48 (1.01, 2.19)63.6 (-13.4, 141)Other causes1.70 (1.52, 1.91)234 (178, 290)1.55 (1.31, 1.84)258 (164, 352)1.81 (1.55, 2.12)258 (187, 329)1.93 (1.61, 2.32)163 (119, 207)1.60 (1.38, 1.85)375 (239, 511)1.69 (1.42, 2.01)228 (141, 315)1.68 (1.35, 2.09)327 (191, 463)* per 100,000 person-years. 95 % Confidence intervals are shown in parenthesis. Models were adjusted for sex, marital status, and background. Open table in a new tab There were 904 (95% CI: 779, 1030) more all-cause deaths per 100,000 person-years in the low educated compared with high educated OA patients (Table 1). Diseases of the circulatory system and mental and behavioural disorders & diseases of the nervous system contributed the most and least, respectively, to the absolute inequalities in all-cause mortality in the whole study population and all subgroups (Table 2).Table 2Contribution (%) of specific causes of death to the absolute inequality in mortality.Disease group (ICD-10 codes)All (≥45years)SexesAge groupsOA-typeDiseases of the circulatory system (I00-I99)MenWomen45-74 years≥75 yearsKneeHipDiseases of the circulatory system (I00-I99)60.057.357.146.272.059.863.2Malignant neoplasms (C00-C97)8.415.19.118.1-0.37.710.3Mental and behavioural disorders (F00-F99) & Diseases of the nervous system (G00-G99)0.5-0.50.83.5-2.30.1-2.9Diseases of the respiratory system (J00-J99)5.77.95.89.13.16.04.8Other causes25.320.227.223.227.526.524.6Total mortality99.9100.0100.0100.1100.0100.1100.0 Open table in a new tab When we used the weighted multiple-cause approach, the RII for diseases of the circulatory system increased, while the RII for malignant neoplasms decreased. In addition, the RII for respiratory system was no longer statistically significant when the weighted multiple-cause approach was used. Conclusions: We found an inverse association between educational level and all-cause mortality in OA-patients. The cause-of-death attribution approaches generally led to similar conclusion. These findings suggest that educational level is an important factor to consider in OA-patients, especially if they suffer from cardiovascular diseases. Further studies are required to explain the inequality observed in our study.