Figure 2. Taxonomy of Osteoarthritis (OA).

We propose a new ‘taxonomy of OA’ based on the standardized nomenclature of disease (made up of molecular, anatomic and physiologic components, domains or disease elements) and illness (panel A). As illustrated here, a clinical threshold would be anticipated that would result in the transition from disease to illness. This taxonomy anticipates the development of composite indices of OA (arrow) that by analogy to the Disease Activity Score (DAS) in rheumatoid arthritis, would encompass all three-disease domains (molecular, anatomic and physiologic) and illness that could be useful for clinical evaluation and trials. Varying weights might be ascribed to the different elements in the composite score (illustrated by the terms lower and higher within the arrow). Osteoarthritis specific examples for each domain are included in panel B.