According to current estimates, 27 million adults in the United States—or more than 10% of the adult population—have clinical osteoarthritis, a condition that ranks among the leading causes of hospitalization. Recent estimates show that approximately 9.9 million adults have symptomatic osteoarthritis (OA) of the knee. With life expectancy rates continuing to rise, it is anticipated that the prevalence of knee OA will increase in the coming years. The number of people older than age 65 is expected to increase to more than 77 million by the year 2040. The burden of OA of the knee is largely attributable to the effects of disability, comorbid disease, and the expense of treatment. OA is the most frequent cause of disability among U.S. adults, and the burden is increasing due to the rising disease prevalence as well as higher expectations among patients to receive effective treatment. OA is the leading indication for joint replacement surgery. About 905,000 knee and hip replacements were performed in 2009, costing the healthcare system $42.3 billion. These estimates factor together direct medical, long-term medical, home modification, and nursing home costs.