Defining arthritis and measuring functional status in elders: methodological issues in the study of disease and physical disability.

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Abstract
This study explores the degree to which the association of knee osteoarthritis with physical disability changes with different definitions of osteoarthritis in 1,416 Framingham Study elders. When all categories of radiographic osteoarthritis were aggregated into a single "arthritis" variable and dependence upon human assistance in one or more functional activities combined into a single variable, elders with osteoarthritis had only moderately increased odds for dependence after controlling for age or sex (OR = 1.25). A definition of osteoarthritis based only upon symptoms produced larger odds for dependency (OR = 1.85). Elders with mild osteoarthritis and infrequent knee pain had no significantly elevated risk for dependence in any of the seven functional activities. Elders with infrequently symptomatic, moderate to marked radiographic osteoarthritis were at increased risk for dependence in stair climbing, walking a mile, housekeeping, and carrying bundles. Elder with radiographic osteoarthritis accompanied by frequent pain had an increased odds of dependence in stair climbing, walking a mile, and housekeeping. Using a generic definition of "arthritis" and aggregating functional activities may underestimate the impact of osteoarthritis on physical disability in the elderly and obscure the task-specific nature of that relationship.