Arthritis: Additional joint sites lead to at least additively worse outcomes

The prevalence, impact, and interaction of affected joints are not well described for arthritis, the most common cause of disability. To address this gap, we utilized data from the Arthritis Conditions Health Effects Survey, a random digit-dialed telephone survey conducted in 2005-06 of US adults ≥45 years. Affected joints were grouped into 6 joint sites, reflecting single and multiple joints affected: knee only (KO: reference; 8.8%), one extremity other than knee (1-O; 13.4%), knee plus one extremity (K+1; 22.0%), two/three extremities other than knee (2/3-O; 17.3%), knee plus two extremities (K+2; 23.3%), or knee plus three extremities (K+3; 15.2%). Effects of arthritis in knee and multiple joints on impact (symptom severity, function, and work limitations) were assessed, independently. Analyses were conducted using generalized estimated equations that account for complex sample design and sampling weights. Of 1,392 adults with doctor-diagnosed arthritis with complete demographic and joint site information, impact measures were similar among the single joint sites (KO vs. 1-O) examined. The effects of having arthritis in multiple joints were at least additive for severe symptoms and function, and the effect was multiplicative for work limitation, e.g., working age (45-64 years) people who had arthritis at 1 (1-O), 2 (K+1), 2-3 (2/3-O and K+2), and 4 (K+3) joint sites were, respectively, 1, 3, 5-6, and 16 times more likely to have work limitations than those with arthritis in just the knee (KO). Early diagnosis and interventions that prevent the onset of symptoms in additional joints is important for reducing arthritis impact.