Published Online: October 06, 2009
Roland W. Moskowitz, MD
Osteoarthritis (OA), the most common form of arthritis, is a potentially devastating joint disease, affecting 27 million US adults. Its pathophysiology is marked by a gradual degenerative process accompanied by low-grade inflammation, and, although there is a strong correlation between age and OA risk, the abnormal changes that occur in the articular cartilage of people with OA differ notably from the typical changes associated with joint aging in several important ways. Risk factors for OA are multiple and span a variety of risk domains, such as lifestyle issues (eg, obesity and engagement in manual labor), genetic predisposition, sex and ethnicity (risk is higher in women and African Americans), and comorbidities. Clinical outcomes for people with OA typically involve pain, limitations of daily living activities, and overall diminution of quality of life (QOL). The need to evaluate the degree of this burden, as well as to determine treatment approaches and measure their success, requires instruments for measuring QOL. The 2 most commonly used instruments to measure QOL in OA are the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and the Medical Outcomes Study 36-Item Short-Form Health Survey. Both provide useful global information to the clinician and researcher alike about pain and function in patients with OA, although the WOMAC is more often used in the clinical setting as it is self-administered. A number of other pain and function-specific measures are also available that may provide additional insight into patient status when used in combination with global QOL instruments.
(Am J Manag Care. 2009;15:S223-S229)
Osteoarthritis (OA) is the most common form of arthritis, affecting 27 million adults in the United States.1 OA typically occurs in the hands, knees, spine, and hips, although it may be seen in any of a variety of joints.2 Clinical diagnosis is based on observed symptoms, radiographic changes, or both, whereas differential diagnosis is normally supported through the use of laboratory studies. Although OA is often characterized as a degenerative disease, low-grade inflammation actually constitutes an important aspect of OA's pathologic process.3,4
OA is strongly correlated with aging: the risk of OA increases considerably with each decade after the age of about 45 years.1 Nevertheless, aging is not inevitably associated with OA. In fact, several pathophysiologic changes that occur in osteoarthritic cartilage differ notably from that associated with age-related changes in cartilage.5 That said, such age-related changes do play an important role in OA pathogenesis and, at a minimum, predispose individuals to the disease.6
Other than increasing age, there are a number of risk factors for OA, including comorbidities both related and unrelated to musculoskeletal conditions.7 The presence of other joint diseases is the most common musculoskeletal comorbid risk factor, whereas obesity is among the most common nonmusculoskeletal comorbidities associated with OA.7 Lifestyle variables, such as a history of manual labor and cigarette smoking, as well as sex- and phenotype-related conditions-such as age at menarche and joint hyperlaxity in men-can also play a role in conferring risk of OA.8 The genetic component of OA risk, while still being studied at present, has been partially elucidated in recent years as genome-wide scan studies have identified genetic variants associated with OA.9
Ultimately, it is the burden of suffering experienced by people with OA that is of primary concern, and that burden can be significant. Pain and functional impairment are the key domains of that burden, and taken together they often exert a significant reduction in quality of life (QOL).10-13 The present review will briefly describe the pathophysiology, prevalence, and typical outcomes of OA before addressing the issue of QOL in OA and the best means in which to measure it.
Pathophysiology of OA
Cartilage remodeling involves balanced interactions of synthesis and degradation to achieve homeostasis of the extracellular matrix (ECM).14 In OA this process becomes unbalanced, leading to pathologic changes in the affected joint.15 The articular cartilage cells, chrondocytes, are responsible for maintaining homeostasis of the ECM by producing its major components, collagen and proteoglycan, in response to deterioration. Changes in the chrondocytes are associated with abnormal anabolic and catabolic activities as well as abnormal proliferation and apoptosis.
In the early stages of OA, loosening of the collagen network as well as proteoglycan loss occur in the upper cartilage zones and may still, at that point, be reversible.15 Over time, these changes occur within deeper cartilage zones, reducing the elasticity of the cartilage and making a return to homeostasis increasingly difficult to achieve. Chrondocyte senescence-which is associated with increasing age-also appears to play a part in a reduced capacity for cartilage repair and contributes to OA progression.16
Recent data support the notion that changes in subchondral bone are also a factor in cartilage degradation.17 The subchondral bone, which is in immediate proximity of cartilage, may contribute to cytokines, growth factors, and prostaglandins escalating-perhaps initiating-the degenerative process.
Prevalence and Incidence of OA
Collecting prevalence data from multiple sources, including the Third National Health and Nutrition Examination Survey (NHANES III), the Framingham Osteoarthritis Study, and the Johnston County Osteoarthritis Project, the National Arthritis Data Workgroup arrived at a prevalence figure for 2005 of 26.9 million US adults (aged >25 years) with some form of OA.1 This constitutes a growth of approximately 6 million cases from 1995, more than a one-fourth increase in just 10 years. This likely reflects, in part, an aging of the US population, although increases in other related factors, such as obesity, as well as increases in methods of OA detection, may play a role in this observed prevalence.
Nevertheless, the prevalence of OA does increase dramatically with age. Data from the Framingham study showed that the prevalence of knee OA nearly doubled in patients aged 45 years or older compared with those 26 years or older.1 The proportion of women with OA also increases relative to men as age increases. Whereas the Framingham study found that 4.9% of women at least 26 years of age had knee OA compared with 4.6% of men, the gap increased to 7.2% versus 5.9%, respectively, in the 45 years or older group. This gap was replicated in the Johnston County study, although a higher rate of knee OA for both men and women aged 45 years or older was observed.1 In that study, the rate of knee OA was 18.7% for women compared with 13.5% for men. The Johnston County study further observed a higher rate of hip OA in women 45 years or older (9.3%) compared with men in the same age group (8.7%).
The observation that the OA gap increases between men and women as they age is consistent with incidence data from the Framingham Osteoarthritis Study focusing on individuals aged 63 to 91 years (mean age, 70.8 years).18 Among these older subjects, the age-adjusted relative risk (RR) of women experiencing radiographically determined knee OA compared with men was 1.79 (95% confidence interval [CI], 1.08-2.94).18 Symptomatic knee OA was almost twice as likely (RR, 1.96; 95% CI, 1.01-3.82) for women compared with men.18 Estimates of the rate of increase for knee OA in women was approximately 2% per year for radiographically determined disease.18 Prevalence data regarding hand OA in more than 1000 study subjects aged 71 to 100 years from the original Framingham study (years analyzed: 1992-1993) again found a much higher rate of disease in women (26.2%) compared with men (13.4%). The Figure shows the distribution of OA symptoms at various joints in the hands for both men and women.19
Ethnicity also plays a role in OA risk. According to prevalence data from NHANES III (1991-1994), radiographic knee OA was observed in 17.7% of African American participants aged 60 years or older compared with 14.8% of Mexican American and 11.9% of white participants in the same age group (both differences P <.01).20 Of note, the dual elevated risk of being both African American and female was observed with a prevalence of radiographic knee OA of 60.2% (95% CI, 52.8-67.5).20
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