Background/Purpose:  

Given the heterogeneity of knee OA impact, it is important to identify persons at risk for poor outcome.  A better understanding of differences between risk groups may lead to development of more effective prevention strategies.  Losina et al recently demonstrated losses in quality-adjusted life years due to knee OA and obesity, disproportionately high in black and Hispanic women (Ann Intern Med 2011).  The impact within racial groups of greater than healthy body weight on more proximal outcomes is not well understood.  We tested the hypothesis that African-Americans have a greater risk (vs. Caucasians) of poor baseline-to-4-year function outcome within each strata: women with high BMI, women with large waist circumference, men with high BMI, and men with large waist circumference.  The OAI cohort study, enriched with individuals above a healthy body weight, provided an ideal setting.

Methods:  

Using WOMAC function, 20 meter walk, and chair stand performance, poor outcome was defined as moving into a worse function group or remaining in the 2 worst groups over 4 years.  Logistic regression was used to evaluate the hypothesized relationships between racial groups and outcomes within each stratum, adjusting for age, education, and income, and then further adjusting for BMI, comorbidity, depressive symptoms, physical activity, knee pain, and OA severity. 

Results:  

In 3,695 persons with or at higher risk for knee OA, high BMI and large waist circumference were each associated with poor outcome.  As shown in the table, among women with high BMI and among women with large waist circumference, African-Americans were at greater risk for poor outcome by every measure, adjusting for age, education, and income.  From fully adjusted models (not shown), potential explanatory factors included income, comorbidity, depressive symptoms, pain, and disease severity.  Findings were less consistent for men (see table), emerging for the 20 meter walk or chair stand outcomes, and potentially explained by age and knee pain. 

Conclusion:  

Among women with high BMI and among women with large waist circumference, African-Americans were at greater risk than Caucasians for poor 4-year outcome by each measure evaluated, adjusting for age, education, and income.  Modifiable factors that may in part explain these findings in women in the OAI include comorbidity, depressive symptoms, and knee pain.  Targeting such factors, while supporting weight loss, may help to lessen the outcome disparity between African-American and Caucasian women.

Table. Odds of poor 4-year outcome associated with race by gender, BMI and waist circumference groups, adjusting for age, education, and income (AA = African-American, C = Caucasian, significant results bolded)