Our study was designed to confirm and extend observations of associations between measures of individual SES with disability and pain in persons with hip rOA or symptomatic hip OA, while adjusting for known predictors of disability. In addition, we examined the associations of community SES with disability in our sample of 708 African American and Caucasian Johnston County residents with hip rOA, of whom 251 also had symptoms in the same hip that had rOA. Individuals with less than 12 years of schooling were more likely to have significantly higher HAQ and WOMAC scores, reflecting more disability and pain, than those with higher educational attainment, even after adjusting for age, gender, BMI, race, occupational physical activity score, comorbidities, and the presence of knee symptoms.
Significant associations were also noted between individuals with non-managerial occupations and higher WOMAC scores, but not with HAQ scores, in participants with hip rOA or the subgroup with hip symptomatic OA. In participants with rOA, residing in a Census block group with a community poverty rate of 25% or greater was associated with increased disability, measured by the HAQ, although the magnitude of these findings were weaker than for those with low educational attainment.
In multivariable analyses introducing all three SES variables simultaneously as main explanatory variables for disability, educational attainment was the only significant independent SES predictor of HAQ disability in both the rOA group and symptomatic OA subgroup and of WOMAC function and WOMAC total in rOA. However, the parameter estimate for the association between education and WOMAC function in the symptomatic hip OA sample is sizable; the small sample size (n=244) may make the determination of significance more difficult. Occupation was the only significant SES predictor of WOMAC pain in both radiographic and symptomatic hip OA.
Looking at magnitudes of the parameter estimates across Tables and , without regard to the significance, it may be that education is most associated with HAQ and WOMAC function, whereas occupation tends to be stronger in its association with WOMAC pain and WOMAC total. In rOA alone, living in a community with the highest poverty rate tends to be associated with HAQ.
As would be expected, variables that are known to be associated with disability and pain in hip OA, including gender, BMI and knee symptoms were strongly associated with disability in our population with hip OA. However, even after adjusting for these covariates, there was still a significant association of educational attainment with HAQ scores and educational attainment and occupation with WOMAC scores.
Our study of a Caucasian and African American population in the United States confirmed findings from the three European studies identifying educational attainment as a significant factor associated with WOMAC function in individuals with hip OA [14
]. In those studies, educational attainment was not associated with hip pain, but we extended the work of these European studies to examine other SES variables, occupation and community poverty, with WOMAC. Interestingly, occupation was linked significantly with WOMAC pain but not function for persons with both rOA and symptomatic OA in the multivariable analyses. We also included another commonly used measure of arthritis disability, the HAQ. Our study, like the Finnish study [14
], included knee symptoms which, as stated earlier, involve pain, aching or stiffness that could ultimately be responsible for a participant’s measured disability even if that participant exhibited hip rOA or symptomatic OA. The knee symptoms were a strong contributor to disability, but even after controlling for these symptoms and other important risk factors, SES variables contributed significantly. Our study was also the first study to include an area-level measure of SES, community poverty, in addition to individual level SES measures, education and occupation. Measuring community SES by community poverty level may be rather non-specific, but it is an all-encompassing single variable that may represent both the physical community environment as well as the physical and mental health of its residents. For example, this could include poorer access to exercise facilities, public transportation, or medical care, as well as fewer health-conscious neighbors or residents of the same household.
The strengths of this investigation lie in its well-described bi-racial study population of men and women, its inclusion of both persons with radiographic and symptomatic OA instead of only persons with hip pain or other symptoms when considering disability, as well as its adjustment for knee symptoms. The study population was also enriched with groups at higher risk for OA outcomes and had a high rate of participation. All participants underwent identical clinical and in-home interviews. We expect that these estimates will generalize to our target population of civilian, non-institutionalized, African Americans or Caucasians, aged 45 years or older, residing in a rural community.
Additionally, this study uses a population in the United States, as opposed to similar studies that took place in Europe and entailed a remarkably different patient population; the populations in these European studies are cared for under a different healthcare system than in the United States. The limitations of our study include the lack of other potentially important variables of individual SES, such as income, and the crude measure of community SES we are using by employing Census block group poverty level. Although it would be desirable to include other measures, these measures are ones that are easily obtainable and appear to give a good reflection of both individual and community SES.