Osteoarthritis Supplement Epidemiology of Osteoarthritis and Associated Comorbidities Pradeep Suri, MD, MS, David C. Morgenroth, MD, David J. Hunter, MBBS, PhD Abstract: Osteoarthritis (OA) is the most common cause of walking-related disability among older adults in the United States, and the prevalence and incidence of OA are increasing rapidly. Systemic and local risk factors for knee OA have been identified, and obesity and joint injury appear to be the strongest risk factors that are both modifiable and have the potential for substantial impact on a population level. The risk factors for functional decline and disability in persons with symptomatic OA have been examined in relatively few studies. The course of functional decline in persons with symptomatic OA on a population level is generally one of stable to slowly deteriorating function, but on an individual level, many patients maintain function or improve during the first 3 years of follow-up. Obesity stands out as one of few modifiable risk factors of OA that also is a potentially modifiable predictor of functional decline. Physical activity also appears to have a substantial protective impact on future OA-related disability. Further epidemiologic studies and randomized controlled trials are needed to prioritize prevention through targeting these modifiable risk factors for OA and related disability. PM R 2012;4:S10-S19 INTRODUCTION Knee and/or hip osteoarthritis (OA) are the most common causes of walking-related disability among older adults in the United States [1]. OA of the hand also is associated with substantial upper extremity disability [2]. The prevalence of OA in the United States has increased sharply in the past 2 decades, and the public health consequences of OA and OA-related disability are expected to acquire still greater urgency as a result of the increasing incidence of obesity and the aging of the population [3]. An understanding of the distribu- tion and determinants of OA, as well as associated comorbidity, forms the basic framework within which OA prevention and the management of OA-related disability occurs. This narrative review focuses primarily on knee and hip OA, with limited discussion of hand OA, because of the extensive literature regarding these sites of OA and their impact on disability at the population level. This review will preferentially draw upon evidence from longitudinal cohort studies and randomized controlled trials (RCTs), with an emphasis on U.S. population-based studies. DEFINITIONS AND GRADING OF OA Although various definitions of OA exist, the definitions of both radiographic OA and symp- tomatic OA are used widely and are clinically pertinent. The most commonly used radiologic grading system for knee OA is the Kellgren-Lawrence (K-L) grade, which determines the severity of radiographic OA on the basis of the presence and degree of osteophytosis, joint-space narrowing (JSN), sclerosis, and deformity affecting the tibiofemoral joint, irrespective of clinical symptoms [4]. Radiographic OA of the knee usually is defined as a K-L grade of 2 or higher. K-L grading also is used for the hip, hand, and other joints. Symptomatic OA is defined as the presence of radiographic OA in combination with characteristic symptoms attributable to OA, including pain, aching, or stiffness in the affected joint. Because not all persons with radiographic OA experience joint symptoms, symptomatic OA definitions are necessarily more restrictive than radiographic OA defini- P.S. Division of PM&R, VA Boston Healthcare System–JP Campus, 150 S Huntington Ave, Boston, MA 02130; Department of PM&R, Harvard Medical School; Spaulding Rehabili- tation Hospital; and New England Baptist Hospital, Boston, MA. Address correspon- dence to: P.S.; e-mail: pradeep.suri@va.gov Disclosure: 8B, NIH (grant #K12 HD001097) D.C.M. RR&D Research Center, VA Puget Sound Healthcare System; and Department of Rehabilitation Medicine, University of Wash- ington, Seattle, WA Disclosure: 8, VA Career Development Award D.J.H. Northern Clinical School, The University of Sydney, Sydney, New South Wales, Austra- lia Disclosure: 4A, royalties on knee brace; 8, NIH, Australian Research Council Future Fel- lowship Disclosure Key can be found on the Table of Contents and at www.pmrjournal.org PM&R © 2012 by the American Academy of Physical Medicine and Rehabilitation 1934-1482/12/$36.00 Vol. 4, S10-S19, May 2012 Printed in U.S.A. DOI: 10.1016/j.pmrj.2012.01.007 S10