The characteristics of the complete sample with all non-missing explanatory variables (n = 3,522) are displayed in Table . The sample sizes are given in Figure , and the percentages for the OA definitions are based on all participants (unilateral OA included). The mean age was 60.9 ± 10.5; 61.7% were female; 39.8% had BMI of at least 30; 21.6% were current smokers of tobacco; 19.1% reported a past knee injury; and 46.9% reported frequent occupational activities (lifting, standing, squatting, walking). In terms of our main variables of interest, SES variables, 33.7% had low educational attainment (less than 12 years), 56.2% had non-managerial occupations at their longest job, and 22.4% lived in block groups with household poverty rates over 25%. Radiographic knee OA was noted in 29.5% of the sample, with 14.7% having bilateral radiographic knee OA, 17.7% with symptomatic knee OA, and 6.3% with bilateral symptomatic knee OA.
Characteristics of the sample overall
The odds ratios for the unadjusted associations between the three SES measures with each of the covariates with the four knee outcome variables are displayed in Table . Low educational attainment, a non-managerial occupation, and living in a neighborhood with higher rates of household poverty were all significantly (P < 0.05) associated with radiographic, bilateral radiographic and symptomatic OA (ORs ranging from 1.39 to 2.29). In addition, low educational attainment (OR = 1.89, 95% CI 1.43, 2.49) and a non-managerial occupation (OR = 1.43, CI 1.08, 1.90) were both significantly associated with bilateral symptomatic OA. The adjusted analyses for each of the SES variables considered in separate models are also shown in Table . The magnitudes of the ORs for the SES variables were attenuated when compared to the bivariate results, but each SES variable remained statistically significant for radiographic, bilateral radiographic, and symptomatic knee OA (association of non-managerial occupation and bilateral radiographic knee OA was of borderline significance, OR = 1.24, 95% CI 0.99, 1.55, P = 0.056). Those with low educational attainment were 50% more likely than those with higher educational attainment to have radiographic knee OA and bilateral radiographic knee OA and 75% more likely to have symptomatic knee OA. Those living in high poverty areas were almost twice as likely as those not living in such areas to have radiographic knee OA, 64% more likely to have bilateral radiographic knee OA, and 44% more likely to have symptomatic knee OA. Compared to those in managerial occupations, those with non-managerial occupations were about 25% more likely to have radiographic knee OA and bilateral radiographic knee OA, and 33% more likely to have symptomatic knee OA. None of the SES variables were significantly associated with bilateral symptomatic OA, although associations were in the expected direction, and the association between low educational attainment and bilateral symptomatic knee OA was of borderline significance (OR = 1.32, 95% CI 0.97, 1.79, P = 0.08). The low frequency of bilateral symptomatic knee OA is likely responsible for the lack of statistical significance.
Associations between each independent SES variable analyzed singly with each OA outcome.
The results of the fully adjusted logistic model, containing all three SES variables simultaneously, and adjusted for all covariates, are displayed in Table . Both low educational attainment (OR = 1.44) and high community poverty rate (OR = 1.83) were independently and significantly associated with radiographic OA. Those with low educational attainment were 44% more likely to have radiographic OA after adjusting for occupation, community poverty and covariates. And, individuals living in a community with a high poverty rate were 83% more likely to have radiographic OA after adjusting for educational attainment and occupation. Both low educational attainment and living in a community with a high poverty rate were independently associated with bilateral knee symptomatic OA as well. Compared to individuals with at least 12 years of schooling, those with low educational attainment were 43% more likely to have bilateral radiographic knee OA and 66% more likely to have symptomatic knee OA after adjusting for occupation, community poverty and other covariates. And, those individuals living in a community with a high poverty rate were 56% more likely to have bilateral radiographic OA and 36% more likely to have symptomatic OA compared to those in communities with low poverty rates, after adjusting for education, occupation and other covariates. Having a non-managerial occupation was not associated with any outcome when adjustments were made for educational attainment and community poverty, in addition to the covariates.
Associations between all three SES variables analyzed simultaneously with OA outcomes