Among Johnston County Osteoarthritis Project participants with radiographic knee or hip OA, associations of race with pain and function varied by joint site. African Americans with only knee OA had significantly higher WOMAC total, pain, and function subscale scores than Caucasians, even when controlling for the presence of moderate or severe radiographic OA and demographic factors including also included age, gender, and education level. However, when also controlling for BMI and /or depressive symptoms, these association became non-significant. Racial differences in pain have been observed in a number of other populations, but underlying mechanisms are not well understood (22
). It is noteworthy that an objective measure of disease severity (i.e., the presence of moderate or severe radiographic disease) did not explain racial differences in pain. Results of this study suggest that among patients with knee OA, BMI and depressive symptoms may be important factors explaining racial differences in pain, as well as physical function.
In models of WOMAC function scores among patients with only knee OA, the WOMAC pain subscale had a stronger association than any other variable, confirming previous research regarding the important association of pain severity with function (20
). The model including pain explained substantially more of the variance in function scores than a model that include all other covariates but not pain. Further, addition of WOMAC pain scores into the model resulted in a substantial decrease in the association of race with WOMAC function scores, and in this model, the least squared mean WOMAC function scores were actually slightly lower for African Americans than Caucasians. Results of this study suggest pain may be a key factor underlying racial differences in function among individuals with knee OA; longitudinal studies would be helpful to more clearly examine these interrelationships.
Our findings regarding racial differences in the only knee OA group differ from a previous study that observed no racial differences in WOMAC scores veterans with OA even in unadjusted analyses (5
). There are two likely contributors to these differences. First, the previous study involved a sample with mixed knee and hip OA, and our results indicate racial differences in pain may be more pronounced in only knee OA. Second, the study of veterans with OA was restricted to individuals with moderate to severe pain, perhaps suggesting that racial differences in symptoms may be more pronounced when including patients with a broader spectrum of disease severity. It is also noteworthy that the veteran sample comprised primarily men, whereas our sample was over half women. Further, females had worse pain and function than males in this sample.
In contrast to findings among the group with only knee OA, there were no racial differences in WOMAC total score or subscale scores in the group with only hip OA or the group with both knee and hip OA, even in unadjusted analyses. One possible reason for the lack of racial difference among participants with only hip OA may have been that this group had lower symptom severity and variability than the only knee OA group, as well as less severe radiographic disease. Among those with only hip OA, 7% and 4% had K-L grade 3 or 4 OA in the right and left hips, respectively. In comparison, among the only knee OA group 42% and 41% had K-L grade 3 or 4 OA in the right and left knees, respectively. It is possible that among a sample of individuals with more severe radiographic hip OA, racial differences in pain and function may be more pronounced. Since there were racial differences in pain and function among participants with only knee OA, it is interesting that no racial differences were observed in the group with both knee and hip OA. African Americans in the group with both knee and hip OA actually had lower WOMAC scores than their counterparts with only knee OA, whereas the converse was true among Caucasians (). Reasons for these patterns are not clear and warrant further investigation.
There are several important strengths to this study. The data come from a large community-based sample with a substantial proportion of African Americans that included both men and women. We observed the full spectrum of radiographic OA severity. Radiographs were completed on both knees and hips, which allowed us to conduct analyses separately according to affected joint groups, including those with both knee and hip OA.
There are also some limitations. While we attempted to include a broad spectrum of potential confounders of the associations of race with pain and function, based on previous research (15
), there may be other explanatory factors not examined in this study. We examined overall radiographic severity, defined by total K-L grade, but did not assess specific radiographic features. We also did not assess radiographic changes at the patellofemoral joint, which can be associated with pain and disability (25
). In other analyses of the Johnston County Osteoarthritis Project cohort, African Americans were not more likely than Caucasians to have isolated patellofemoral joint osteophytes (26
). The Johnston County Osteoarthritis Project sample is from a relatively rural North Carolina community, and additional research is needed to examine whether these results generalize to other groups.
In summary, this study found that in a community sample, African Americans and Caucasians with only hip OA or both hip and knee OA did not differ in self-reported pain or function. In unadjusted analyses among participants with only knee OA, African Americans had greater self-reported pain and functional limitations than Caucasians, but these differences were explained by other clinical factors. In particular, pain was an important factor explaining racial differences in function, and BMI and depressive symptoms were important factors explaining racial differences in both pain and function. Therefore improving management of obesity and depressive symptoms may be key avenues for reducing racial disparities in symptom severity among patients with knee OA. Additional research is needed to understand differences in the associations of race with pain and function according to joint site(s) affected by OA.