Epidemiology
reprinted from Primer on the Rheumatic Diseases, edition 12

Osteoarthritis (OA) is by far the most common joint disorder in the United States and throughout the world, and is one of the leading causes of disability in the elderly (1). People with OA account for one of every eight days of restricted activity among the elderly in the United States. Although the disease commonly affects the cervical and lumbar spine, most epidemiologic studies report that it has a predilection for weight-bearing joints in the leg and certain joints in the hand. The prevalence of OA in all joints correlates strikingly with age. One-third of people aged 65 years and older have knee OA that is evident by radiograph. Before the age of 50, men are more likely to have OA than women, but after age 50, it is women who are more likely to be affected.

Not all people with radiographic OA develop symptomatic OA. The discrepancy between radiographic and symptomatic OA accounts for differing results of epidemiologic studies designed to define risk factors; most studies have surveyed only asymptomatic radiographic disease. These investigations have implicated systemic factors (age, sex, race, heredity, and obesity) and local factors (certain physical activities, injury, and developmental deformity) as risks for developing radiographic OA (2).

Local Factors

Excess Weight
Population-based studies of OA consistently have shown that overweight people are at greater risk of developing knee OA than average-weight controls. Obese women are four to five times more likely to have knee OA than persons of average weight. Longitudinal studies suggest that obese people with knee OA are at greater risk than thinner people for disease progression. Weight reduction is likely to lessen the symptoms of knee OA.

Although the association with body weight is not as strong for hip OA as it is for knee OA, overweight people appear to be at increased risk of OA in all weight-bearing joints, including hip OA. Surprisingly, there is also a positive association between obesity and OA in the hand, suggesting that obesity is a systemic risk factor for OA.

Injury and Occupation
Major acute knee injuries, including cruciate ligament and meniscal tears, are common causes of knee OA. Osteoarthritic changes have been reported in up to 89% of people after meniscectomy (3). Most people who have experienced complete anterior curciate ligament rupture will develop knee OA. OA is associated with a variety of  sport activities, including marathon running (hip OA), soccer playing (knee and hip OA), and American football playing (knee OA). However, there are conflicting data, because information on joint injury in this population often is unavailable. Standing, bending, walking long distances over rough ground, lifting, and moving heavy objects appear to increase the high risk of hip OA. Occupations associated with high rates of OA include farmer (hip OA), jackhammer operator (elbow), miner (knee and spine OA), and cotton mill worker (hand OA).

Developmental Deformities
Anatomic abnormalities of the knee and hip that are present at birth or that develop during childhood may result in accelerated or premature OA. These abnormalities include genu varum, genu valgum, congenital hip subluxation, slipped capital fermoral epiphysis, Legg-Calvé-Perthes disease, and acetabular dysplasia.

Other Local Factors
Strength of the lesser quadriceps, after adjustment for body weight, age, and sex, is predictive of both radiographic and symptomatic OA of the knee. Knee laxity, defined as abnormal displacement or rotation of the tibia with respect to the femur, may increase the risk of OA and contribute to progression. Damage to other local cartilage also can leasd to OA.

Systemic Factors

Sex Hormones
Osteoarthritis occurs more frequently in women over the age of 50 than in age-matched men. Epidemiologic studies of women who take estrogen replacement therapy report that these women are less likely to have OA than women not taking estrogen (4).

Genetic Susceptibility
Many studies have demonstrated that genetic factors influenced the incidence of OA. Heritability of primary OA of the hands has been reported to be as high as 65% (5). The existence of familial forms of certain OA, such as hand OA, implies the involvement of one or more genetic factors. Early studies focused on the alterations in type II and IX collagen, but no significant abnormalities were detected, except for mutations in COL2A1 in some families that had a phenotype distant from the common primary OA. More recent studies on other matrix components, such as aggrecan or the vitamin D receptor, have revealed no significant genetic abnormality in people with primary OA. In fact, twin and familial studies have shown OA to be a multigenic trait, with several genes involved (6).

Racial Differences
There is conflicting evidence as to whether African Americans have different rates of OA than Caucasians. The incidence of knee OA could be higher in African-American women (7).

Other Systemic Factors
Low vitamin D and vitamin C intakes are associated with increased risk of knee OA progression. Metabolic and endocrine disorders are associated with secondary forms of OA.